Katrin Stephan

Roland Wagner

Ralf Jaumann

Rogern N Clark

Dalep P Cruikshank

Roberth H Brown

Bernd Giese

Thomas Roatsch

Gianrico Filacchione

Dennis Matson

Cristinadalle Ore

Fabrizio Capaccioni

Kevinh H Baines

Sebastian Rodriguez

Norbert Krupp

PhilD Bonniej J Buratti

Nicholson

Cristina Dalle Ore

Phil D Nicholson

Cassini's geological and compositional view of Tethys

Keywords: Cassini, VIMS, Tethys, surface composition, geology

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

A C C E P T E D M A N U S C R I P T 1 Introduction

Since 2004 Cassini has been orbiting the Saturnian system with its instruments detecting the chemical and physical properties of Saturn's atmosphere, magnetosphere, satellites, and rings.

The VIMS instrument is the first imaging spectrometer that operates in the Saturnian system, enabling not only the identification of the surface composition but also the mapping of its distribution on the surfaces of Saturn's satellites. Spectral analyses of Dione, Rhea, Iapetus, Hyperion [START_REF] Clark | Compositional mapping of Saturn's satellite Dione with Cassini VIMS and implications of dark material in the Saturn system[END_REF][START_REF] Clark | The surface composition of Iapetus: Mapping results from Cassini VIMS[END_REF][START_REF] Cruikshank | Surface composition of Hyperion[END_REF][START_REF] Stephan | Dione's spectral and geological properties[END_REF][START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF] and Enceladus [START_REF] Jaumann | Distribution of icy particles across Enceladus' surface as derived from Cassini-VIMS measurements[END_REF] have made it possible to relate location and extension of spectral units to specific geological and/or geomorphological surface features or characteristics of the space environmentwhich is essential to resolve the origin of the surface compounds and thus provides valuable information to describe the evolution of the satellite. Although the VIMS spectra of the icy satellites' surfaces are mostly dominated by H 2 O-ice, its distribution and physical characteristics differ distinctly from one satellite to the other [START_REF] Clark | Compositional mapping of Saturn's satellite Dione with Cassini VIMS and implications of dark material in the Saturn system[END_REF][START_REF] Clark | The surface composition of Iapetus: Mapping results from Cassini VIMS[END_REF][START_REF] Jaumann | Distribution of icy particles across Enceladus' surface as derived from Cassini-VIMS measurements[END_REF][START_REF] Stephan | Dione's spectral and geological properties[END_REF][START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF]. Global hemispherical differences are mostly related to the satellite's position in the Saturnian system, i.e., the distance to Saturn and/or its E ring, with particles trapped in Saturn's magnetosphere and/or ice grains of the E ring impacting their surfaces [START_REF] Clark | Compositional mapping of Saturn's satellite Dione with Cassini VIMS and implications of dark material in the Saturn system[END_REF][START_REF] Stephan | Dione's spectral and geological properties[END_REF][START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF]. Thanks to VIMS mapping capabilities, compositional variations on a regional and local scale can be identified and related to the surface geology. In particular, young or reactivated impact craters and tectonic featuresrevealing "fresh" (unaltered) surface material -offer a unique view into the crustal properties of the satellite [START_REF] Stephan | Dione's spectral and geological properties[END_REF][START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF]. Furthermore, the comparison of the physical and chemical surface properties of impact craters emplaced at In this paper we present the analysis of the spectral properties of Tethys in comparison to its geology, which is unique in several ways and completes our investigation of Saturn's inner major satellites. Tethys is 1,062 km in diameter and exhibits a very low density (~0.985 g/cm 3 ) [START_REF] Thomas | Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission[END_REF] suggesting that the satellite is composed almost entirely of H 2 O ice plus a small amount of rock.

Tethys is marked by the huge impact basin Odysseus, which is expected to have globally affected surface and internal properties. Odysseus is still discussed as being responsible for the formation of Tethys' prominent tectonic system, Ithaca Chasma [Moore and Ahern, 1983; [START_REF] Moore | Large impact features on middle-sized icy satellites[END_REF]. Surface ages, however, indicate that Ithaca Chasma is geologically older than Odysseus [START_REF] Giese | Tethys: Lithospheric thickness and heat flux from flexurally supported topography at Ithaca Chasma[END_REF] speaking against this relationship. Further, the impact crater statistics, at the basis of Tethys' surface age calculations, are unusual. The distribution of small impact craters (1 to 10 km in diameter) is distinctly different from the other Saturnian satellites because the number of these small impact craters is much higher relative to the larger ones [START_REF] Schmedemann | The Signature of Secondary Cratering on 4 Vesta and Tethys[END_REF].

J M
Finally Tethys has a high albedo value of about 0.8 in the visual spectral range [START_REF] Verbiscer | Enceladus: Cosmic Graffiti Artist Caught in the Act[END_REF] indicating a composition largely made of H 2 O ice. Possibly, the high reflectivity is enhanced because Tethys collects Saturn's E-ring H 2 O-ice particles generated by geysers on Enceladus. Tethys orbits Saturn at a distance of 294,660 kilometers, between the orbits of Enceladus and Dione, and is thus embedded in the E ring environment. Consequently, the comparison of the compositional aspects of all Saturnian satellites provides valuable information for the evolution of the Saturnian system. Key questions about Tethys upon which this investigation is focused to unravel include:

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A N U S C R I P T 6 1. How are Odysseus and Ithaca Chasma spectrally characterized? How are they related to each other in general? Do the spectral surface properties provide any hint of a common origin and their relation to the history and formation of the body? 2. Are there any fresh impact craters on Tethys? What are the similarities and differences in the spectral properties of fresh surface features on Tethys compared to fresh impact craters on Dione (Creusa) and Rhea (Inktomi)?

3. What are the differences in spectral properties between any fresh surface material and the surroundings? What are the resurfacing dynamics and space weathering effects of the satellite? 4. Are there any hemispherical changes in the spectral properties similar to Dione and Rhea related to the interaction with Saturn's magnetosphere? How do they change from one satellite to the other? What information do they give about the properties of Saturns magnetosphere?

5. How do the E ring particles interact with Tethys, which orbits relatively close to the E ring?

We start the presentation of our work with an overview of the data set we used (section 2), including the methods used for the spectral and geological analysis. Sections 3 and 4 present the achieved results. In section 3 the identified major geological units are described in detail.

In section 4 Tethys spectral properties and the distribution of spectrally different units across Tethys' surface as well as their possible associations to specific geologicaland/or geomorphological features or location on Tethys's surface are discussed. The possible implications for the origin/formation of Tethys' surface compounds and Tethys' geological evolution are discussed in section 5 and summarized in section 6.

A C C E P T E D M A N U S C R I P T 2 Cassini data basis

Since 2004, the Cassini spacecraft has been orbiting the Saturnian system exploring Saturn, its magnetosphere and its satellites with numerous flybys at Tethys (Tab. 1). Cassini performed 10 (1 targeted and 9 non-targeted) flybys during its nominal mission until 2008, with an altitude between 1500 and about 120,000 km. In particular, Cassini's orbit 47 offered the unique possibility to observe Ithaca Chasma, Tethys' most prominent tectonic features (see below) from a distance of only 16,000 km. Throughout the first extended mission, the Cassini Equinox Mission, from 2009 and 2010, more data were collected during 11 flybys with a closest approach of about 37,000 km during orbit 136, which mainly observed Tethys' trailing hemisphere. In the current second extended mission, the Cassini Solstice Mission, which will continue until 2017, already 7 flybys within distances between about 7600 and 69,000km were successfully performed so far, with orbit 168 offering the possibility to investigate Tethys' largest impact crater Odysseus.

ISS observations

Spatially highly resolved Clear filter images acquired by the Imaging Science Subsystem (ISS) provided not only the geological context but provided the possibility to associate any changes in the spectral properties to specific geographic location, as well as individual geological and/or geomorphological surface features.

A digital global mosaic in a simple-cylindrical map-projection of Tethys produced with Clear filter images of the Cassini ISS instrument with a map scale of 0.29 km/pixel, as presented in [START_REF] Roatsch | High-resolution Atlases of Mimas, Tethys, and Iapetus derived from Cassini-ISS images[END_REF], was used for the global geological mapping procedure as well as the comparison to Tethys' global spectral surface properties. For this purpose we rescaled the mosaic to a lower map scale of 1 km/pixel, which is sufficient for comparison with the global VIMS map since the latter have generally a lower pixel ground resolution than the ISS

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A N U S C R I P T 8 images. This is the same approach as used for our geologic mapping of Dione and Rhea [START_REF] Stephan | Dione's spectral and geological properties[END_REF][START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF]. A map scale of 1 km/pixel is also sufficient to identify global geologic units, individual units within major impact structures. In order to constrain the location of geologic boundaries and in order to characterize a geologic unit as well as to support the investigation of local geological and spectral properties of Tethys, however, we also used the original higher-resolution imaging data to support the mapping procedure. Furthermore, in order to prevent a misinterpretation of VIMS signal due to viewing conditions [START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF] at the time of the acquisition, simultaneously acquired ISS images were used for comparison.

Although not exclusively designed for the development of digital terrain models (DTMs), individual images provide the opportunity to produce DTMs of regional areas using the stereo-technique like the DTM of Ithaca Chasma already presented in the work of [START_REF] Giese | Tethys: Lithospheric thickness and heat flux from flexurally supported topography at Ithaca Chasma[END_REF]. Previous works [START_REF] Stephan | Small fresh impact craters on asteroid 4 Vesta: A compositional and geological fingerprint[END_REF][START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF] showed the potential in the possibility to associate spectral properties to local topographic changes like slopes, etc., especially in tectonically formed regions.

VIMS observations

VIMS consists of two instruments that measure sunlight reflected from the surfaces of the satellites. They generate image cubes in which each pixel represents a spectrum consisting of 352 contiguous spectral channels [START_REF] Brown | The Cassini Visual And Infrared Mapping Spectrometer (Vims) Investigation[END_REF]. The VIMS V instrument possesses 96 spectral channels that measure radiation between 0.35 and 1.05 mm, while the VIMS IR instrument operates between 0.86 to 5.2 mm and collects light in 256 spectral channels. The spectral sampling intervals of VIMS V and VIMS IR is 8 and 16 nm, respectively, which is sufficient to analyze absorptions (their wavelength position, shape and band depth) of H2O-ice

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A N U S C R I P T 9 at 1.04, 1.25, 1.5 and 2 µm. The dynamic range of 12 bit (2 12 =4096 values) makes it possible to measure relatively small variations in the band depth of the specific absorptions.

Possessing an Instantaneous Field Of View (IFOV) of 0.5 mrad in the nominal operation mode enables spectral images with resolutions sufficient for conducting detailed spatial analyses of the spectral characteristics of the surface [START_REF] Brown | The Cassini Visual And Infrared Mapping Spectrometer (Vims) Investigation[END_REF][START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF].

VIMS observations were chosen depending on coverage, pixel ground resolution and illumination conditions. These parameters differ significantly during each VIMS observation sequence. Table 1 summarizes the Cassini flybys offering VIMS data incorporated into our analysis. VIMS observations with pixel ground resolutions of better than 100 km per pixel cover most of Tethys' surface except the northern polar region (Fig. 1). Pixel ground resolutions up to 50 km enable at least a global view of Tethys' geological and spectral properties, such as hemispherical differences. VIMS observations exhibiting pixel ground resolution between 50 and 20 km per pixel offer the analysis of any regional relationships.

VIMS data for Ithaca Chasma region, acquired during Cassini's 47 th orbit in June, 2007, exhibit pixel ground resolution of better than 20 km/pixel and cover the canyon system between 60°S and 80°N. Furthermore, during several flybys (Cassini orbits 94, 119, 136, 164 and214) in the "Equinox" mission (2009 -2010) and in the recent Cassini Solstice Mission (2010-2017) VIMS has acquired data showing us a unique view of Tethys' trailing hemisphere. During orbit 168 (Cassini Solstice Mission) VIMS observations revealed details of Tethys' impact crater Odysseus (32.8°N/128.9°W) on the satellite's leading hemisphere, which, with a diameter of 445 km is the largest impact crater on this body. Unfortunately, being at the terminator, Odysseus was not fully illuminated during the acquisition. As a consequence of this, the western portion of the crater is hidden on the night side. Most of the

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A N U S C R I P T 10 VIMS observations exhibit phase angles smaller than 60° (Fig. 1), which are sufficient for the spectral analysis, cover most of Tethys surface (see below).

[Figure 1]

[Table 1]

Processing and Spectral Analysis of VIMS data

VIMS data were radiometrically calibrated, i.e., converted from raw data numbers (DN) into physical values of reflectance (I/F) by the VIMS team using the latest instrument's spectroradiometric response function [START_REF] Brown | The Cassini Visual And Infrared Mapping Spectrometer (Vims) Investigation[END_REF][START_REF] Stephan | Dione's spectral and geological properties[END_REF]. In order to attribute variations in the spectral properties to the corresponding geographic locations and to combine the VIMS data set with the Cassini ISS base map [START_REF] Roatsch | High-resolution Atlases of Mimas, Tethys, and Iapetus derived from Cassini-ISS images[END_REF] and individual ISS images, each individual VIMS cube is geometrically re-projected, converted into a mapprojected VIMS cube following the procedure described in detail by [START_REF] Jaumann | High-resolution CASSINI-VIMS mosaics of Titan and the icy Saturnian satellites[END_REF],

and finally combined into a global Tethys VIMS mosaic that fits the global ISS base map.

Since our applied mosaicking procedure also allows arranging the incorporated individual observations depending on coverage, spatial resolution and quality (including S/N ratio, phase angle and/or saturation effects) influences of VIMS observations with lower spatial resolution (>60 km per pixel), phase angles larger than 60° as well as saturation effects, which frequently occurred in the wavelength region between 1 and 1.4µm onto the quality of the spectral maps are minimized.

Like most of the Saturnian satellites, Tethys' surface is substantially covered by H 2 O-ice, the main focus of this study. Its major absorption bands at 1.04, 1.25, 1.5, 2, 3 and 4.5µm

A C C E P T E D M A N U S C R I P T
11 dominate every VIMS spectrum. These H 2 O-ice absorptions are known to be sensitive to abundance, size and crystallinity of the ice particles, affecting the slope of spectral continuum, the depth and wavelength position of these absorptions. Our goal is to measure the individual spectral parameters of H 2 O-ice in the VIMS spectra and map their variations across Tethys' surface on a pixel by pixel basis in order to investigate the observed pattern with respect to possible associations to a specific location and/or geological and morphological surface features, which is essential for resolving the processes responsible for their formation. We concentrate the spectral analysis on specific spectral parameters of the H 2 O-ice spectrum, which were already successfully used for the spectral analysis of icy satellites' surfaces [START_REF] Clark | Compositional mapping of Saturn's satellite Dione with Cassini VIMS and implications of dark material in the Saturn system[END_REF][START_REF] Jaumann | Distribution of icy particles across Enceladus' surface as derived from Cassini-VIMS measurements[END_REF][START_REF] Stephan | Dione's spectral and geological properties[END_REF][START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF]. Because the absorptions at 3 and 4.5µm are always spectrally saturated we focus on the H 2 O-ice absorption bands occurring between 1 and 3µm. In order to compare the spectral properties of Tethys to the previous analyses done for Enceladus, Dione, and Rhea we use the same tools for identifying and mapping existing absorption features, i.e. their spectral parameters like wavelength position and depth [START_REF] Jaumann | Distribution of icy particles across Enceladus' surface as derived from Cassini-VIMS measurements[END_REF][START_REF] Stephan | Dione's spectral and geological properties[END_REF][START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF], which we briefly summarize in the following.

The band depths (BDs) of the individual H 2 O-ice absorptions at 1.5 and 2µm were found to be a good indicator for the relative abundance of H 2 O-ice. Although, the depths of these absorptions are also influenced by the size of the H 2 O-ice particles, this effect is mostly masked by the varying abundance of H 2 O-ice when an additional absorbing compound like the dark material on Dione and Rhea is mixed with the H 2 O ice [START_REF] Stephan | Chemisch-physikalische Zusammensetzung der Ganymedoberfläche: Zusammenhänge mit geologischen Strukturen und deren Gestaltungsprozessen[END_REF][START_REF] Stephan | Ganymede's spectral properties: implications for further investigations in a future mission to Jupiter and its satellites[END_REF][START_REF] Stephan | Dione's spectral and geological properties[END_REF][START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF].

The BDs of the absorptions at 1.04 and 1.25µm are rather dominated by the size of the ice particles [START_REF] Mccord | Discussion of the 1.04-μm water ice absorption band in the Europa NIMS spectra and a new NIMS calibration[END_REF]. The BDs of all absorptions were measured according to [START_REF] Clark | Imaging spectroscopy: Earth and planetary remote sensing with the USGS Tetracorder and expert systems[END_REF]. A good approximation of changes in the sizes of the ice particles without

A C C E P T E D M A N U S C R I P T 12
extensive spectral modeling can be achieved by comparing the band depth ratios (BDRs) of the H 2 O-ice absorptions measured across Tethys to model spectra of H 2 O ice [START_REF] Stephan | Chemisch-physikalische Zusammensetzung der Ganymedoberfläche: Zusammenhänge mit geologischen Strukturen und deren Gestaltungsprozessen[END_REF][START_REF] Stephan | Comparison of variations in ice particle sizes across Callisto's and Ganymede's surface[END_REF][START_REF] Stephan | Ganymede's spectral properties: implications for further investigations in a future mission to Jupiter and its satellites[END_REF]. Although, the depths of the individual H 2 O-ice bands are influenced by the abundance as well as the size of the H 2 O-ice particles, only the particle size appears to affect the ratio of the BDs of two of the H 2 O-ice absorptions. This assumption is valid for all incompletely saturated H 2 O-ice absorptions and in case of a nonice, visually dark contaminant, which is spectrally neutral in the near-infrared wavelength range. Since Tethys exhibits a high geometric albedo in visible light [START_REF] Verbiscer | Enceladus: Cosmic Graffiti Artist Caught in the Act[END_REF] only small amounts of a dark contaminant are expected on the satellite's surface. No absorptions exclusively associated to the dark non-ice material could be detected in the nearinfrared spectral range so far [START_REF] Clark | Compositional mapping of Saturn's satellite Dione with Cassini VIMS and implications of dark material in the Saturn system[END_REF][START_REF] Emery | [END_REF][START_REF] Stephan | Dione's spectral and geological properties[END_REF][START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF]. Changes in the physical properties of H 2 O ice, like the sizes of the individual particles are of special interest, because they are very sensitive to surface ages and weathering effects [START_REF] Stephan | Ganymede's spectral properties: implications for further investigations in a future mission to Jupiter and its satellites[END_REF][START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF]. The BDR values derived from the VIMS data have been compared to BDR values of model spectra calculated based on the algorithm published by [START_REF] Hansen | Calculation of single-scattering albedos: Comparison of Mie results with Hapke approximations[END_REF] and the optical constants offered by [START_REF] Grundy | The temperature-dependent near-infrared absorption spectrum of hexagonal <formula>H2O ice[END_REF] for a surface temperature of 80K. Model spectra of H 2 O ice based on the algorithm of [START_REF] Hansen | Calculation of single-scattering albedos: Comparison of Mie results with Hapke approximations[END_REF] were already successfully used in previous works [START_REF] Hansen | Amorphous and crystalline ice on the Galilean satellites: A balance between thermal and radiolytic processes[END_REF][START_REF] Hansen | Water ice abundance and CO2 band strength on the saturnian satellite Phoebe from Cassini/VIMS observations[END_REF][START_REF] Jaumann | Distribution of icy particles across Enceladus' surface as derived from Cassini-VIMS measurements[END_REF]. Since the BDR values were not yet calculated in detail before for Dione and Rhea, they have been also calculated for the VIMS data presented in [START_REF] Stephan | Dione's spectral and geological properties[END_REF]; [START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF] and included in this work in order to compare the resulting BDR maps to the BDR map of Tethys.

In addition fresh H 2 O-ice not covered yet by submicron particles show a distinct Fresnel reflection peak at 3.1µm, which also changes its shape depending on the crystallinity of H 2 O ice [START_REF] Hansen | Amorphous and crystalline ice on the Galilean satellites: A balance between thermal and radiolytic processes[END_REF]. The crystallinity of H 2 O ice is also expected to influence the wavelength positions of the H 2 O-ice absorptions, in particular in case of the absorption at

A C C E P T E D M A N U S C R I P T 13
2µm, shifting the band to longer wavelengths when the H 2 O-ice transforms from amorphous to crystalline H 2 O ice [START_REF] Clark | The surface composition of Iapetus: Mapping results from Cassini VIMS[END_REF].

It is well known that H 2 O-ice is completely transparent at 0.5µm without any significant absorption by the H 2 O-ice [START_REF] Dozier | Remote Sensing of snow in visible and near-infrared wavelengths[END_REF]. Therefore, the geometric albedo at 0.5µm is a good indicator of how the abundance of H 2 O-ice varies across Tethys' surface. We performed a photometric correction of the VIMS channel at 0.5µm using the function algorithm that the Cassini ISS team [START_REF] Porco | Cassini Imaging Science: Instrument Characteristics And Anticipated Scientific Investigations At Saturn[END_REF][START_REF] Roatsch | High-resolution Atlases of Mimas, Tethys, and Iapetus derived from Cassini-ISS images[END_REF] applied to their Clear/Green-filter images, which shows Tethys as seen at a wavelength of about 0.568µm. Finally, spectral slopes [START_REF] Filacchione | Saturn's icy satellites investigated by Cassini-VIMS[END_REF][START_REF] Filacchione | Saturn's icy satellites and rings investigated by Cassini-VIMS: III -Radial compositional variability[END_REF][START_REF] Filacchione | Saturn's icy satellites investigated by Cassini-VIMS: I. Full-disk properties: 350-5100 nm reflectance spectra and phase curves[END_REF] like the slope from the visible spectral range into the ultraviolet spectral range (VIS/UV ratio: I/F 0.5µm /I/F 0.35µm ) changes with increasing contribution of dark non-ice material, as identified on Dione and Rhea [START_REF] Clark | Compositional mapping of Saturn's satellite Dione with Cassini VIMS and implications of dark material in the Saturn system[END_REF][START_REF] Stephan | Dione's spectral and geological properties[END_REF][START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF] but has also been shown to visualize regions on the satellite's surface which have endured enhanced space weathering effects, i.e. impacting dust and particles from Saturn's magnetosphere and E Ring [START_REF] Schenk | Plasma, plumes and rings: Saturn system dynamics as recorded in global color patterns on its midsize icy satellites[END_REF][START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF].

Overview of Tethys' geological units

The pre-Cassini view of the geology of Tethys

Voyager images of Tethys, taken in the flybys in November 1980 (Voyager-1) and August 1981 (Voyager-2), revealed a generally densely cratered landscape and two major landmarks: the graben system of Ithaca Chasma and the 400-km large impact structure Odysseus.

Voyager-2 returned images with the highest resolution primarily of the Saturn-facing hemisphere. The southern latitudes of the leading and trailing hemispheres as well as the south polar terrain could not be imaged by Voyager at sufficient resolution. Voyager-based

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A N U S C R I P T 14 geology of Tethys was described by [START_REF] Smith | Encounter with Saturn: Voyager 1 Imaging Science Results[END_REF], [START_REF] Smith | A New Look at the Saturn System: The Voyager 2 Images[END_REF], and Moore and Ahern [1983] and is briefly reviewed here.

The densely cratered terrain shows little variation at Voyager resolution and was mapped as a more or less global unit. Most of the cratered terrain is hilly and rugged with a large number of highly degraded craters [Moore and Ahern, 1983;[START_REF] Smith | A New Look at the Saturn System: The Voyager 2 Images[END_REF]. Odysseus is the largest, basin-sized impact feature 445 km across. Voyager frames show a rim consisting mostly of arcuate scarps, partly double, a central peak complex, and radial crater chains reminiscent of secondary craters. Distinct continuous ejecta could not be discerned.

Approximately antipodal to Odysseus, a variety of smoother, less densely cratered plains occurs near the large impact structure Penelope. Moore and Ahern [1983] interpreted these plains to have formed from a resurfacing event which could have been linked to the formation of Odysseus. Unlike the rugged or "weird" terrain which occurs on, e.g., Mercury antipodal to the major basin Caloris [START_REF] Neukum | Geologic evolution and cratering history of Mercury[END_REF], however, Tethys does not show such terrain.

The smooth plains at this locale were interpreted to have been resurfaced by material forming the smooth plains [Moore and Ahern, 1983].

Troughs, scarps and lineaments of Ithaca Chasma span approximately 270˚ around the satellite [START_REF] Smith | A New Look at the Saturn System: The Voyager 2 Images[END_REF]. This tectonic structure is up to 100 km wide and has been shown to be several kilometers deep, inferred from shadow lengths [START_REF] Smith | Encounter with Saturn: Voyager 1 Imaging Science Results[END_REF][START_REF] Smith | A New Look at the Saturn System: The Voyager 2 Images[END_REF]. The morphologically most pronounced part of the structure is located at 0˚ W and trends approximately north-south.

Voyager images show two remarkable albedo and color features. The first one described is a longitudinal dark strip centered at 270° W with an approximate North-South trend. This feature is about 15% lower in albedo and redder with a lower violet/orange ratio than the surrounding terrain [START_REF] Buratti | Albedo and color maps of the Saturnian satellites[END_REF][START_REF] Smith | Encounter with Saturn: Voyager 1 Imaging Science Results[END_REF]. The second feature is an elliptical

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15 equatorial band in the leading hemisphere which is darker and bluer (higher violet/orange ratio) than the surroundings [START_REF] Buratti | Albedo and color maps of the Saturnian satellites[END_REF][START_REF] Schenk | Plasma, plumes and rings: Saturn system dynamics as recorded in global color patterns on its midsize icy satellites[END_REF][START_REF] Stooke | Tethys: Volcanic and Structural Geology[END_REF][START_REF] Stooke | Tethys and Dione: New Geological Interpretations[END_REF]; the feature is depicted but not explicitly described by [START_REF] Smith | Encounter with Saturn: Voyager 1 Imaging Science Results[END_REF] and [START_REF] Smith | A New Look at the Saturn System: The Voyager 2 Images[END_REF] in their preliminary albedo and topography maps.

Description of the mapped geological units

As described in our previous papers [START_REF] Stephan | Dione's spectral and geological properties[END_REF][START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF], geologic units are treated as material units rather than terrain types that (a) were formed by a specific process, or (b) were significantly resurfaced by one or even more processes. Surface morphology, therefore, is an important category to subdivide geologic units, in addition to albedo (or color) as the primary category to discriminate geologic units [START_REF] Wilhelms | Geologic Mapping[END_REF].

Based on these criteria, we identified the following sets of units: (1) cratered plains materials and (2) impact-related materials, including craters and basins.

Also following the procedure in [START_REF] Stephan | Dione's spectral and geological properties[END_REF] and [START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF], craters and basins are classified by their erosional state into at least three units, a technique that has been widely used for geologic maps published by the U. S. Geological Survey [START_REF] Hansen | Water ice abundance and CO2 band strength on the saturnian satellite Phoebe from Cassini/VIMS observations[END_REF][START_REF] Wilhelms | Geologic Mapping[END_REF]]. This procedure is described below in more detail.

Although crater counts were not explicitly carried out for this work, crater size distributions were measured on Tethys by some of us and have been published elsewhere [START_REF] Giese | Tethys: Lithospheric thickness and heat flux from flexurally supported topography at Ithaca Chasma[END_REF]Wagner et al., 2013a;Wagner et al., 2013b]. For stratigraphic purposes, we will use and discuss these results in this paper. Absolute cratering model ages for geologic units, when cited here, are derived from two cratering chronologies, (a) a chronology model with a more or less lunar-like time-dependence of the cratering rate [START_REF] Neukum | Cratering Chronologies and Ages of the Major Saturnian Satellites[END_REF][START_REF] Plescia | Impact Cratering History of the Saturnian Satellites[END_REF], and (b) a chronology with a constant cratering rate [START_REF] Zahnle | Cratering rates in the outer solar system[END_REF]. For a

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16 detailed discussion of these models the reader is deferred to [START_REF] Dones | Icy Satellites of Saturn: Impact Cratering and Age Determination Saturn from Cassini-Huygens[END_REF]. The final geological map is presented in Figure 2. In the following the mapped geological units will be described in detail.

[Figure 2]

Cratered plains materials a) Densely cratered plains

The most abundant geologic unit on Tethys are densely cratered plains (dcp) whose general characteristics could be defined from Voyager images [Moore and Ahern, 1983;[START_REF] Smith | Encounter with Saturn: Voyager 1 Imaging Science Results[END_REF][START_REF] Smith | A New Look at the Saturn System: The Voyager 2 Images[END_REF]. Here, this unit has been designated as dcp and represents a group in a lithostratigraphic sense [START_REF] Wilhelms | Geologic Mapping[END_REF]. A typical locality is seen east of the basin Penelope, in the quadrangles Ste-9 Penelope and Ste-10 Salmoneus of the Tethys atlas [START_REF] Roatsch | High-resolution Atlases of Mimas, Tethys, and Iapetus derived from Cassini-ISS images[END_REF], which is shown in Figure 3. The unit is gently hilly, undulating, in parts rugged, and generally densely cratered. The hilly appearance results from numerous degraded impact features. The largest of the impact craters could already be discerned in Voyager data [Moore and Ahern, 1983;[START_REF] Smith | Encounter with Saturn: Voyager 1 Imaging Science Results[END_REF][START_REF] Smith | A New Look at the Saturn System: The Voyager 2 Images[END_REF]. Some of these degraded features are members of crater groups, but the high degradation state does not allow us to exactly define the number of craters in these groups.

Cassini ISS data covering regions not very well imaged by Voyager complete a global survey of impact basins on the order of 100 km and larger in diameter in unit dcp. Remarkably, these large impact features, except Odysseus, are concentrated on the trailing hemisphere in the regions south of the equator. In addition, higher-resolution ISS images show smaller, fresher

A C C E P T E D M A N U S C R I P T
and therefore younger craters. As has been observed on Enceladus and Dione [START_REF] Wagner | Clusters of craters on the Saturnian satellite Dione: morphology and size distribution[END_REF], pairs or clusters of craters at all diameters and degradation states are similarly common on Tethys.

Fine-scale tectonic features are a characteristic of unit dcp, which is revealed only at higher Cassini ISS resolution. In the local east of the basin Penelope (Fig. 3), subparallel, narrow, comparatively fresh fractures (white arrows) cut an older set of more degraded and wider troughs, but in turn are superimposed by fresher craters up to ~ 10 km in diameter. In other localities of unit dcp, these narrow fractures as well as wider troughs resemble chains of circular or elliptical pits instead and probably are of impact rather than tectonic origin. The geologic boundaries of unit dcp are clear-cut with respect to all impact crater forms and to tectonically resurfaced units. However, boundaries between unit dcp and other cratered plains materials are difficult to define in some parts, as described in the following.

[Figure 3]

b) Smooth cratered plains

A less rugged, smoother but still densely cratered variety of the cratered plains is located between Ithaca Chasma and the basin Penelope, centered approximately at the trailing hemisphere. This variety was described for the first time by [START_REF] Smith | A New Look at the Saturn System: The Voyager 2 Images[END_REF] after the Voyager 2 encounter with Saturn and termed plains terrain by Moore and Ahern [1983]. [START_REF] Smith | A New Look at the Saturn System: The Voyager 2 Images[END_REF] report a lower crater frequency compared to the more rugged and hilly densely cratered plains dcp. Furthermore, the spatial extent of this unit was not revealed in Voyager images due to incomplete coverage (see, e.g., the geologic sketch map in Moore and Ahern [1983]).

A C C E P T E D M A N U S C R I P T 18

In this work, we termed this unit smooth cratered plains (scp). As with unit dcp, the smooth cratered plains scp represent a rock-stratigraphic group. Despite the much higher resolution of Cassini ISS images, the boundary between unit dcp and scp is not sharp but appears to be gradational over a few kilometers or even tens of kilometers, as described by Moore and Ahern [1983]. On lower resolution images, the boundary is easier to locate and can be transferred to a higher-resolution map by the largest craters along its path. An example of a low-resolution ISS NAC frame, obtained in orbit 069TE, is shown in Figure 4a.

The gradational boundary between the two units dcp and scp, and the characteristics of unit scp, are shown at higher resolution in Figure 4b. Five large craters several tens of kilometers in diameter are located within scp: Phemius, Polyphemus, Icarius, Ajax, and an unnamed crater between Icarius and Polyphemus. The intercrater plains in scp, also between smaller craters, are comparably smooth but the transition to the hilly, rugged densely cratered plains dcp is clearly gradational and lacks a sharp distinct contact. Penelope, a basin of type b 2 (unit description see section 3.2.3) superimposes the boundary between the two units. We could not identify a scarp between dcp and scp, as inferred by Moore and Ahern [1983], but southeast of crater Ajax the boundary between the two units seems to be enhanced by a very subdued rim. The impression of a scarp in some parts could be an artifact resulting from numerous overlapping crater rims in the hilly terrain of unit dcp which are little discernable due to the lower-resolution Voyager images.

A color asymmetry at the trailing hemisphere, characterized by a high IR/UV ratio, coincides more or less with the smooth cratered plains scp but seems to be more extended than this unit. This color feature has been described by [START_REF] Schenk | Plasma, plumes and rings: Saturn system dynamics as recorded in global color patterns on its midsize icy satellites[END_REF]. In Figure 4c, a color ratio of the trailing hemisphere is shown, including the boundary between dcp and scp with a dotted line.

The boundary between high IR/UV albedo and the surrounding terrain with lower IR/UV

c) Lightly cratered plains

A specific variety of cratered plains found in only one location was not distinguishable at Voyager resolution. This variety is less densely cratered at medium and small crater sizes compared to units dcp or scp. We mapped and termed this unit as lightly cratered plains, designated lcp. Its exact characterization, as well as mapping its spatial extent, is problematical, however.

The type locality south of Odysseus of unit lcp is shown in Figure 5. The boundary of this unit with respect to the densely cratered plains (dcp) is very sharp and well defined by an abrupt change in crater frequency from dcp to lcp. The unit is characterized by a rolling, undulating topography with heavily degraded craters several tens of kilometers across.

Furthermore, the unit appears depleted in crater frequency at craters smaller than ~ 20 km.

The sharp boundary is approximately linear (seen in an equidistant map projection used for the geologic map in Fig. 2) with more or less rectangular changes in direction reminiscent of a segmented fault.

Except in the area south of Odysseus, the boundary of this unit is difficult to identify and to locate. This is due to the fact that the region between Odysseus, Ithaca Chasma and the 100km large crater Anticleia (whose location is shown in Fig. 6) was imaged at varying viewing conditions, sun elevations and spatial resolutions, as it is documented in the DLR ISS basemap [START_REF] Roatsch | High-resolution Atlases of Mimas, Tethys, and Iapetus derived from Cassini-ISS images[END_REF] and in color mosaics [START_REF] Schenk | Plasma, plumes and rings: Saturn system dynamics as recorded in global color patterns on its midsize icy satellites[END_REF]. Towards the south pole, imaging at more or less constant viewing conditions and spatial resolutions is also not

A C C E P T E D M A N U S C R I P T 20 complete.
Therefore, mapping of unit lcp between Odysseus, Melanthius in the south, Ithaca Chasma in the east, and Anticleia in the north has to be regarded as preliminary. Additional images at 200 to 300 m/pxl spatial resolution taken at medium sun elevation as well as more stereo coverage is required to exactly map and characterize the spatial abundance of unit lcp.

The dcp/lcp boundary south of Odysseus is in parts marked by a topographic ridge, possibly an outer ring linked to the formation of Odysseus. This is described and shown in more detail in section 3.2.3.

[Figure 5]

d) Fractured cratered plains

The graben system Ithaca Chasma is a major landmark on Tethys and was first seen in the Voyager 1 encounter images [START_REF] Smith | Encounter with Saturn: Voyager 1 Imaging Science Results[END_REF]. The structure is described as a branching trough featuring terraces more than 1000 km long and 50 to 100 km wide [START_REF] Giese | Tethys: Lithospheric thickness and heat flux from flexurally supported topography at Ithaca Chasma[END_REF]Moore and Ahern, 1983]. Ithaca Chasma follows a great circle, predominantly trends NNE-SSW and is approximately centered at the sub-Saturnian hemisphere, transgressing the equator near a longitude of 0° W. Southward, it nearly extends to the south pole. Northward, the troughs are superimposed by the, with a diameter of 92 km, relatively large crater Telemachus (53.5°N/339°W). The major part of the Ithaca Chasma structure on the sub-Saturnian hemisphere is shown in the global ISS map of Figure 2. Here, Ithaca Chasma is approximately 100 km wide and branches into two narrower, ~ 50 km wide troughs southward of 10° S. [START_REF] Giese | Tethys: Lithospheric thickness and heat flux from flexurally supported topography at Ithaca Chasma[END_REF].

A C C E P T E D M A N U S C R I P T 21
We identified and mapped two facies of this unit. The first one, fractured cratered plains 1 (fcp 1 ), is the most common type and consists of a major terraced trough featuring numerous sub-parallel interior minor scarps and troughs. The type locality of fcp 1 south of the large crater Telemachus is depicted in Figure 6. In a digital elevation model (DEM) derived by [START_REF] Giese | Tethys: Lithospheric thickness and heat flux from flexurally supported topography at Ithaca Chasma[END_REF], the bounding troughs are topographically up to 6 km high with respect to the densely cratered plains dcp outside Ithaca Chasma which the authors attributed to flexural uplift. The depth of Ithaca Chasma was found to be on the order of 2 -3 km, based on this DEM.

Ithaca Chasma may have been caused by expansion of internal liquid water as it froze into ice after the surface had already frozen. An alternate theory is that the impact that created the Odysseus Crater also generated forces that created Ithaca Chasma, especially since the chasm is on the opposite side of Tethys from the Odysseus Crater. The chasm and surrounding area are heavily cratered, indicating that it was formed long ago.

Results from crater counts discussed by [START_REF] Giese | Tethys: Lithospheric thickness and heat flux from flexurally supported topography at Ithaca Chasma[END_REF] infer that Ithaca Chasma is younger than the densely cratered plains but older than the basin Odysseus, rejecting a possible link between its formation and the Odysseus impact as has been suggested by some authors [Moore and Ahern, 1983]. Higher-resolution images, as those shown in detail in Figure 6, strongly imply the comparably great age of Ithaca Chasma, documented in the majority of craters within Ithaca Chasma superimposing the tectonic structures [Wagner et al., 2013a;Wagner et al., 2013b]. Furthermore, crater counts performed in these higher-resolution data confirm this stratigraphic finding, as well as previous results by [START_REF] Giese | Tethys: Lithospheric thickness and heat flux from flexurally supported topography at Ithaca Chasma[END_REF], Wagner et al. [2013a] and Wagner et al. [2013b] (see section 5).

A second facies was mapped as fractured cratered plains 2 (fcp 2 ) northeast of Telemachus where the Ithaca Chasma structure approximately trends west-east. In this part, the trough and the scarps are much more degraded and seem to "fade into" tectonically unmodified densely

Crater materials

The three erosional classes of craters c 1 , c 2 , and c 3 roughly represent an overlapping age sequence, meaning that c 1 craters are generally, but not necessarily older than c 2 craters (same for c 2 versus c 3 craters) [START_REF] Hansen | Water ice abundance and CO2 band strength on the saturnian satellite Phoebe from Cassini/VIMS observations[END_REF]. No overlap in time exists for c 3 versus c 1 craters, however. In terms of lithostratigraphy, crater materials can be considered as a supergroup, with the individual erosional classes c 1 , c 2 , c 3 representing a rock-stratigraphic group each [START_REF] Wilhelms | Geologic Mapping[END_REF]. The same subdivision for crater materials was used in [START_REF] Stephan | Dione's spectral and geological properties[END_REF] and [START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF].

a) Heavily degraded crater materials

A great number of craters on Tethys are characterized by a high state of degradation. This type is termed heavily degraded crater materials, designated c 1 . Type examples are shown in Figure 7a. Rims of these craters are mostly degraded. In some craters, only crater ruins with ring arcs are visible (mapped with dotted lines in the global geological map in Fig. 2). No ejecta are visible. Central peaks are generally absent. Numerous smaller craters superimpose the floors of craters of type c 1 . In some parts, groups of heavily degraded craters occur.

A C C E P T E D M A N U S C R I P T b) Partly degraded crater materials

Another rather common type of impact features are partly degraded craters materials, designated c 2 . Type example is crater Euanthes (33 km diameter, 7.9° N/ 238.9° W) north of Penelope (Fig. 7b). Rims in this type are preserved but may show some degradation. Like in type c 1 , central peaks are mostly absent, except in larger craters (e.g, Ajax, see Fig. 7b).

Continuous ejecta are generally obliterated by superimposed smaller craters. Larger examples of c 2 -craters, such as Euanthes, do have remnants of continuous ejecta, however. As type c 1 , craters of type c 2 also occur in clusters or groups.

c) Fresh crater materials

A comparably small number of craters, especially at larger diameters (> 10-20 km) are mapped as fresh crater materials, designated c 3 . These craters have pristine rims, welldeveloped continuous ejecta and generally central peaks. The floors and continuous ejecta display a low frequency of superimposed smaller craters indicating the relative young age of c 3 -craters. Type example is Telemachus (92 km diameter, 54° N/ 339° W) shown in Figure 7c.

Another type example is crater Icarius (54.5 km diameter, 6° S/ 306° W; Fig. 4b). At the given resolution 1 km/pixel of the geologic map (Fig. 2), these two craters are the only two larger features included in the map, plus a comparably small crater near Penelope with a strong H 2 O ice signature (see section 4). At higher resolution, more craters of type c 3 are discernible.

From crater counts carried out on floor and continuous ejecta of Telemachus, cratering model ages of 3.6 ± 0.1 Ga (model by [START_REF] Neukum | Cratering Chronologies and Ages of the Major Saturnian Satellites[END_REF]), 0.74 Ga (Case A; uncertainties: 2.3 Ga (high), 0.2 Ga (low)), and 0.12 Ga (Case B; uncertainties: 0.46 Ga (high), 0.03 Ga (low)) (model by [START_REF] Zahnle | Cratering rates in the outer solar system[END_REF]) were derived (see section 6).

A C C E P T E D

M A N U S C R I P T 24 [Figure 7]

Basin materials

An impact basin is a large impact feature that generally forms early in the history of a planet or satellite by the impact of a massive body [START_REF] Spudis | The Geology of Multi-ring Impact Basins[END_REF]. The boundary diameter separating craters from basins is set approximately at 300 km on the Earth's moon, and such large impact structures are characterized by (a) a complex morphology, (b) extensive ejecta blankets including secondary crater chains and radial sculptures, and (c) may display several concentric rings of ridges, scarps or graben, therefore are designated as multi-ring basins [START_REF] Spudis | The Geology of Multi-ring Impact Basins[END_REF].

Large impact structures are also abundant on the icy satellites of Saturn but lack a distinct multi-ring morphology. At least two rings, however, may be common, especially in basins on Iapetus on which most of these large impact features were observed to occur [START_REF] Giese | The topography of Iapetus' leading side[END_REF][START_REF] Jaumann | Icy Satellites: Geological Evolution and Surface Processes[END_REF]. Unlike the rocky silicate surfaces of the terrestrial planets, complex crater morphologies and ring structures on icy satellites can be distinguished in impact features smaller than ~300 km but larger than ~100 km, especially on the Galilean satellites of Jupiter [START_REF] Stephan | Geology of Icy Bodies[END_REF]. Therefore, we set the boundary between craters and basins on icy moons at a threshold of 100 -150 km to separate basin from crater units. On Tethys, only seven impact structures significantly larger than 100 km are found. Like the crater units, these six features are classified according to their erosional state, and are designated as b 1 , b 2 , b 3 , from older to younger.

In general, impact basins, as shown in detailed geologic maps, are characterized by a set of material units. Especially in those cases where several units can be discerned within a basin at

A C C E P T E D M A N U S C R I P T
25 the map scale of 1 km/pixel, we follow the convention established for geologic mapping of the moon to consider each set of deposits associated with a specific basin as a named or unnamed group in a rock-stratigraphic sense, with each individual unit in the basin representing a formation, in turn named or unnamed [START_REF] Spudis | The Geology of Multi-ring Impact Basins[END_REF][START_REF] Wilhelms | Geologic Mapping[END_REF].

Each basin and its associated unitssimilarly a major craterprovides a stratigraphic datum to be selected as a time-stratigraphic base for a subdivision of the geologic history of a body into systems and series [START_REF] Wilhelms | Geologic Mapping[END_REF]. A suggestion for Tethys' time-stratigraphic column and a sequence of geologic events is presented in Figure 2 and discussed in section 5. Anaglyph images of Penelope (Fig. 9) clearly show a significantly deeper interior compared to the surrounding cratered plains, mostly likely due to little relaxation. The anaglyphs also reveal remnants of an inner ring approximately 50 km in diameter [Wagner et al., 2013a;Wagner et al., 2013b]. The northern and southeastern arc of this inner ring is best preserved.

a) Heavily degraded impact basin materials

In both anaglyphs, the location of a landslide at the interior wall is indicated by a long white arrow. Short white arrows mark fractures cutting through the rim. Another tectonic feature, possibly a fault, transects the basin floor but seems to be absent outside Penelope (black arrows in left anaglyph).

[Figure 8]

[Figure 9] A C C E P T E D M A N U S C R I P T 27 c

) Fresh basin material

The youngest basin unit, fresh basin material, b 3 , only has one representative on Tethys. This basin, Odysseus, 445 km in diameter and located at 32.8° N and 128.9° W was discovered in the first Voyager flyby [START_REF] Smith | Encounter with Saturn: Voyager 1 Imaging Science Results[END_REF]. In Figure 8d it is shown in comparison with the other basin units. Odysseus' diameter represents almost two-fifths of Tethys itself. Usually such an impact could have shattered a solid body suggesting that the internal composition of Tethys was still partially molten at the time of the impact event. The crater's rim and central peak have largely collapsed, leaving a shallow crater, and this also suggests a terrain that was elastic enough to change shape. The subdued features of Odysseus are in contrast to many steep cliffs elsewhere. This again suggests that the ancient terrain that was still elastic enough to change shape. Even at a map scale of 1 km/pixel, several units associated with this basin can be identified. In detail, these units described below are shown in Figure 10. These units can be considered as rock-stratigraphic formations of the Odysseus group.

Basin rim and continuous ejecta

Images of the Odysseus basin at spatial resolutions of ~ 500 -700 m/pixel show a pronounced undulating rim (Fig. 10). The rim is mostly fresh and sharp except in the eastern and southern part of the basin where it appears slightly degraded, as seen in the stereo anaglyph (Fig. 10, top right). Outside the basin rim, no clear-cut continuous ejecta blanket can be discerned (Fig. 10, detail 1 and 2).

Terrace material

A C C E P T E D M A N U S C R I P T 28
Odysseus features a terraced interior (Fig. 10, detail 1 and 2) which was mapped as terrace material (bot). The terraces form gentle to steep-scarped lobes extending towards the basin center. The lobes and scarps are located approximately at a fraction of 75% of the basin rim diameter.

Floor material

At low sun under which most of the highest-resolution images (500 -700 m/pxl) have been taken so far from Odysseus, the floor (unit bof) appears rough, rugged or blocky and features narrow arcuate fractures (Fig. 10, detail 1 and 2).

Central peak material

Odysseus is characterized by an elevated complex, including a central peak surrounded by a horseshoe-shaped mountain (Fig. 10, top right, and detail 1). The complex was named Scheria Montes [START_REF] Roatsch | High-resolution Atlases of Mimas, Tethys, and Iapetus derived from Cassini-ISS images[END_REF]. The central peak and the horseshoe-shaped feature were mapped as central peak material, bocp. The horseshoe-shaped mountain has a diameter of approximately 100 km.

[Figure 10]

Discontinuous ejecta materials (secondaries)

Towards the northeast, east, and southeast, chains of kilometer-sized craters radiate away from the basin rim (Fig. 10, top right, detail 3). These chains were mapped as discontinuous Moore, and Ahern, J. L., 1983]. Cassini images do not reveal any chains or discontinuous ejecta west of Odysseus.

Smooth deposits

Cassini images reveal several smooth, patchy areas associated with the radial crater chains.

These features were mapped as smooth deposits (bcsd). The origin is unknown, but these features could represent patches of impact melt. The boundaries with respect to the densely cratered plains (dcp) are in many places not sharp.

Tethys' spectral properties

Although every VIMS spectrum of Tethys' surface is dominated by H 2 O-ice, both global and regional/local spectral variations could be identified in the maps of the individual spectral parameters (Fig. 11 [START_REF] Buratti | Voyager photometry of Rhea, Dione, Tethys, Enceladus and Mimas[END_REF]. Photometric measurements of Cassini data showed that the leading hemisphere of Tethys is about 10-15% brighter than the trailing hemisphere [START_REF] Buratti | Albedo and color maps of the Saturnian satellites[END_REF][START_REF] Buratti | Near-Infrared Photometry of the Saturnian Satellites during Ring Plane Crossing[END_REF][START_REF] Emery | [END_REF]. Slight changes in the VIS/UV ratio (Fig. 12) are often correlated to the existence of additional non-ice compounds. No absorptions of surface compounds other than H 2 O ice like observed on Dione and Rhea [START_REF] Stephan | Dione's spectral and geological properties[END_REF][START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF]] could be identified.

Like the BDs of the individual absorptions, also the variations in the BDRs are small (BDR 2/1.5µm : 1.1±0.1 -1.4±0.1; BDR 2/1.25µm : 7.1±0.3 -13.4±0.3); BDR 1.5/1.25µm : 5.5.±0.3 -9.4±0.3). The BDR combinations including the absorption at 1.04µm could not be measured on a pixel-by-pixel basis with sufficient accuracy because of the strong dominance of the BD 1.04µm by noise signal. Nevertheless, in comparison to the H 2 O-ice model spectra developed by [START_REF] Hansen | Calculation of single-scattering albedos: Comparison of Mie results with Hapke approximations[END_REF] based on optical constants derived by [START_REF] Grundy | The temperature-dependent near-infrared absorption spectrum of hexagonal <formula>H2O ice[END_REF] for a surface temperature of 80K BDRs measured on Tethys indicate changes in the average radius of the H 2 O-ice particles between less than 1µm and 10 µm, which will be shown and discussed in the following sections.

Distribution of Tethys' spectral classes

Spectral variations occur either in relation to Tethys' geology, i.e. associated with specific geological/geomorphological surface features or geological units, or they mirror global/hemispherical differences. Based on variations in the spectral parameters across Tethys' surface that can be observed in the VIMS maps of Figure 11 and 12 six global and local regions of unique spectral properties can be distinguished, as described in detail below.

The corresponding VIMS average spectra of the described regions are presented in Figure 13.

A C C E P T E D M A N U S C R I P T a) Geologically young impact features

The strongest H 2 O-ice absorptions are associated with few small morphologically fresh impact craters or crater walls of larger impact craters like Telemachus (Fig. 11), which is similar to what has been observed on Tethys' neighboring satellites Dione and Rhea [START_REF] Stephan | Dione's spectral and geological properties[END_REF][START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF]. On Tethys, however, the frequency of fresh impact craters is quite low, corresponding to Tethys' geologically old surface (see section 3).

BDR maps do not show any hint of these fresh impact craters (Fig. 12). This is contrary to what is seen on Dione and Rhea where fresh impact craters like Creusa and Inktomi also exhibit relatively large H 2 O-ice particles compared to their surroundings (Fig. 12) [START_REF] Stephan | Dione's spectral and geological properties[END_REF][START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF]. However, this might still be valid also for fresh impact craters on Tethys. Here these impact craters lie already within larger regions of relative low BDRs i.e. large particle sizes (discussed below) making their identification in the global BDR map difficult.

The smallest fresh impact crater (less than 10 km in diameter) with the "iciest" VIMS spectrum, i.e. exhibiting the strongest H 2 O ice absorptions (spectrum #1 in Fig. 13; BD 2µm =0.77±0.01; BD 1.5µm =0.60±0.01; BD 1.25µm =0.11±0.03; BD 1.04µm =0.07±0.03), is located at 16°S and 267°W close to the older and larger impact crater Penelope (16°S/267°W, Fig. 14). Although, the pixel ground resolution is very low in comparison to the corresponding ISS image, the "icy" spot in the VIMS data, which occurred in every VIMS observation of this region, could be easily associated to this small impact crater. It is characterized by the most pronounced, i.e. freshest, crater morphology (designated as c3 in geological map of Fig. 2) compared to other impact craters in this densely cratered terrain. The corresponding BDR map (Fig. 14) also shows a slight decrease in the BDR value associated to this impact crater, which is not visible in the global BDR map (Fig. 12), confirming the assumptions that fresh surface material displays larger ice particle than their geologically older regions. Spectrum #1 is the

A C C E P T E D M

A N U S C R I P T 32 only one, where the Fresnel reflection peak at 3.1µm can be clearly identified supporting the interpretation of larger ice particles associated to this impact crater.

The VIMS spectrum of Telemachus (spectrum #2 in Fig. 13; BD 2µm =0.71±0.01; BD 1.5µm =0.54±0.01; BD 1.25µm =0.07±0.03; BD 1.04µm =0.04±0.03) appears less icy than the described small fresh crater and appears rather similar to the VIMS spectrum of Odysseus (discussed in the next section). However, spectrum #2 includes the whole impact features including the crater floor, which is probably already covered by regolith. Only the steep crater walls still show the icy signature (Fig. 11).

b) Odysseus

Although also rich in H 2 O ice (spectrum #3 in Fig. 13; BD 2µm =0.72±0.01; BD 1.5µm =0.56±0.01; BD 1.25µm =0.10±0.03; BD 1.04µm =0.05±0.03), the H 2 O-ice absorption in the vicinity of the large impact structure of Odysseus (33°N/132°W) are slightly weaker than measured for the fresh impact craters but still deeper than measured for most parts of the surrounding areas (Fig. 11).

Similar measurements fortify Odysseus' older geological age (see section 3). Despite the complex nature of this impact structure (Fig. 10), no significant spectral variations could be measured within Odysseus. Either Odysseus indeed exhibits uniform spectral characteristics, or any existing variations are not detectable due to the much lower pixel ground resolution of the VIMS data compared to the spatially highly resolved ISS images.

A C C E P T E D M A N U S C R I P T 33

Even though, the BDs of individual VIMS observations incorporated in the BD map of Figure 11 sometimes appear to decrease toward the northwestern portion of the impact structure, it cannot be excluded that this effect is due viewing conditions. The coverage of Odysseus by VIMS observations is very limited. And, especially during Cassini's orbit 126, which offers a full coverage and the spatially best resolved VIMS data, this portion of Odysseus was hidden in darkness on the night side. Nevertheless, major parts of the crater are included in and/or exhibits the same BDs like a N/S oriented band on the leading hemisphere running from the northern to the southern polar region of the satellite, which is discussed in detail below.

Interestingly, Odysseus shows also similar BDR values, i.e. similar H 2 O-ice particle sizes not only like the N/S oriented band but also like the relatively fresh impact craters on Tethys (Fig. 12).

c) Ithaca Chasma

No exclusive spectral signature can be associated to Ithaca Chasma contrary to what has been observed in case of the tectonic surface features on the neighboring satellites Dione and Rhea.

On Dione and Rhea the major tectonic structures located on their trailing hemispheres show a higher abundance [START_REF] Stephan | Dione's spectral and geological properties[END_REF][START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF] of relatively large H 2 O ice particles (Fig. 12). In contrast, it is difficult to identify Ithaca Chasma in the global VIMS maps of Tethys (Fig. 11 and12). Although it is well pronounced in the ISS data (Figure 2), it is almost not distinguishable from the neighborhood on a global scale. Ithaca Chasma was best observed by VIMS in Cassini's orbit 47. The VIMS spectrum (#4) in Figure 13, which corresponds to the portion of the Chasma located south of Telemachos (geologic unit fcp1 in the geological map of Fig. 2), rather implies that Ithaca Chasma belongs to the less icy, i.e.

geologically older and weathered regions on Tethys' surface (BD 2µm =0.69±0.01; BD 1.5µm =0.49±0.01; BD 1.25µm =0.05±0.03; BD 1.04µm =0.03±0.03). Figure 15 shows the resulting

A C C E P T E D M A N U S C R I P T 34
VIMS maps overlapped onto the DTM produced by [START_REF] Giese | Tethys: Lithospheric thickness and heat flux from flexurally supported topography at Ithaca Chasma[END_REF], which offers the unique possibility to investigate the spectral properties in comparison to the local topography (Fig. 16). Although changes in the spectral properties are apparent in the region with a light deepening of the H 2 O-ice absorption at 1.5µm and decreasing of the BDR of the H 2 O-ice absorptions at 2 and 1.5µm in the vicinity of the Chasma, these characteristics rather spectrally separate the region westwards of Ithaca Chasma (including the Chasma istself) from the region eastwards of the Chasma. Because of the Chasma's location around 0°W it exactly lies in the area of transition from the leading to the trailing hemisphere. Thus, spectral variabilities possibly mirror hemispherical differences, which exist independent of the Chasma, rather than spectral properties caused by geological surface processes (discussed below).

[Figure 15] [Figure 16]

d) Oval albedo feature

Voyager images already showed an oval shaped surface feature in the equatorial region of Tethys' leading hemisphere characterized by a slightly darker visible albedo [START_REF] Stooke | Tethys and Dione: New Geological Interpretations[END_REF].

This surface feature can also be seen in Cassini images [START_REF] Schenk | Plasma, plumes and rings: Saturn system dynamics as recorded in global color patterns on its midsize icy satellites[END_REF] and extends in E/W direction between 20°N and 20°S almost across the entire leading hemisphere (Fig. 2).

No effects can be recognized in the BD map of the H 2 O-ice absorption at 1.25µm (Fig. 11) and the BDR map (Fig. 12) excluding particle size effects as the cause of this feature. On the other hand, the VIMS observations reveal a slight decrease in the BD 1.5µm values in comparison to its northern and southern neighboring region and a less steep slope VIS/UV-

A C C E P T E D M

A N U S C R I P T 35 slope (Fig. 11) associated to the oval shaped dark albedo feature seen in the Cassini ISS map (Fig. 2). Although, this implies a contamination of the H 2 O ice by an non-ice and absorbing species in this region (Fig. 11) as predicted by [START_REF] Schenk | Plasma, plumes and rings: Saturn system dynamics as recorded in global color patterns on its midsize icy satellites[END_REF], no absorption feature could be identified in the corresponding VIMS spectrum (spectrum #4 in Fig. 13) that is exclusively associated with the visually dark material.

e) N-S oriented bands

As already indicated above, VIMS maps of Tethys show two N/S trending bands of a slightly more pronounced H 2 O-ice signature in the central portions of Tethys' trailing and leading hemisphere, which could be neither observed on Dione nor on Rhea [START_REF] Stephan | Dione's spectral and geological properties[END_REF][START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF] (Fig. 12). These bands are apparent in both BD maps of Figure 11 and stretch from the northern to the southern polar region (Fig,[START_REF] Brown | The Cassini Visual And Infrared Mapping Spectrometer (Vims) Investigation[END_REF]. They appear narrower close to the equator and widen toward the poles. The band on the leading hemisphere, which is slightly suppressed where it is overlapped by the dark oval feature described above, extends at low latitudes from 80° to 130°W in the BD map of the H 2 O-ice absorption at 1.5µm. In contrast, the band on the trailing hemisphere reaches from 250° to 290°W when measured close to the equator. Usually, higher BD values indicate a higher abundance of H 2 O ice (N/S band on the leading hemisphere: BD 2µm =0.73±0.01; BD 1.5µm =0.54±0.01; BD 1.25µm =0.07±0.03; BD 1.04µm =0.05±0.03; N/S band on the trailing hemisphere: BD 2µm =0.72±0.01; BD 1.5µm =0.55±0.01; BD 1.25µm =0.09±0.03; BD 1.04µm =0.07±0.03), which corresponds to a higher visual albedo. The ISS albedo map (Fig. 2) as well as the VIS/UV slope map (Fig. 11), though, do not show any hint of these bands. The corresponding VIMS spectra (spectrum #5 and #6) only reveal a slightly more pronounced ice signature, i.e. deeper H 2 O ice absorptions and a steeper slope from 0.5 to 2µm similar to the spectral signature of Odysseus (Fig. 13).

An explanation could be that the changes in the abundance of H 2 O ice are very small and that changes in the particle size of H 2 O becomes more dominant in the VIMS signal. Indeed, in

A C C E P T E D M A N U S C R I P T

36 the BDR maps these bands are characterized by a lower BDR value indicating larger H 2 O-ice particles in these regions (Fig. 12). In all BD and BDR maps Odysseus is included in the N/S band on the leading hemisphere. On the contrary, Penelope, the geologically older impact basin (section 3.2.3) is excluded from the N/S band in the BD maps. In the BDR map this band appears to be rather independent of Penelope. The transition away from the band in eastern direction cuts the impact basin in N/S direction. The small fresh impact crater described above cannot be distinguished from the N/S band anymore in the global BDR map implying a similar particle size. These bands appear in numerous VIMS observations acquired during several Cassini flybys verifying that these effect is real and not an artefact of unusual observation conditions.

f) Hemispherical differences

Hemispherical differences in the spectral surface properties of Tethys' like the ones known from the neighboring satellites Dione and Rhea [START_REF] Stephan | Dione's spectral and geological properties[END_REF][START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF] can be only identified, when regional spectral differences, as described above, are excluded.

These underlying hemispherical differences indicate that the major H 2 O-ice absorptions are slightly weaker on the trailing hemisphere (spectrum #8 in Fig. 13; BD 2µm =0.67±0.01; BD 1.5µm =0.41±0.01; BD 1.25µm =0.05±0.03; BD 1.04µm =0.02±0.03). Indeed in the areas W and E of the N/S band on Tethys' trailing hemisphere show the weakest H 2 O-ice absorptions measured on Tethys' surface demonstrating the lowest amount of H 2 O-ice in the regions. This also indicated by a slightly lower visible albedo in these regions (Fig. 2).

In contrast both hemispheres show a similar relatively high VIS/UV ratio of about 1.35 (Fig. 11). The shapes of these regions, however, are different than the N/S bands of larger H 2 O-ice particles. Thus, these two effects are not related to each other and two processes

A C C E P T E D M

A N U S C R I P T 37 independently working are responsible for these spectral characteristics. The variations in the VIS/UV ratio across Tethys' surface, however, are similar to have been observed by the Cassini ISS camera [START_REF] Schenk | Plasma, plumes and rings: Saturn system dynamics as recorded in global color patterns on its midsize icy satellites[END_REF].

No clear hemispherical differences could be measured for the BDR 2/1.5µm across Tethys' surface. Although, smallest BDR values interpreted as small H 2 O-ice particles (<1µm) could be measured on the trailing hemisphere, Odysseus and the N/S oriented bands of larger H 2 Oice particles could mask any existing hemispherical differences. Although, the values of the BDR 2/1.5µm measured for Dione and Rhea show a domination of their trailing hemispheres by relatively small particles (Fig. 12), the maps also show how the fresh impact craters Creusa

Discussion -Tethys stratigraphy and geological evolution

The geological units identified on Tethys' surface, their characteristics, the nature of their contact to neighboring geological units as well as their measured geological age enable us to reconstruct the stratigraphic relationships between the geological units and thus to shed light into the geological evolution of Tethys. The association of the spectral properties to either geological surface features on a local scale or to hemispheres on a global scale offers a unique possibility to investigate the processes responsible for the observed spectral properties and to further our understanding about the geological history as well as the space environment of the satellite.

A C C E P T E D M A N U S C R I P T 38

The oldest geologic unit on Tethys are the densely cratered plains (dcp) which in this work were not further subdivided at the image scale of 1 km/pxl used for geologic mapping (Figure 2). This unit features numerous craters as well as basins > ~ 150 km in diameter of all degradation types. Figure 17 shows a cumulative distribution measured in a sample of unit dcp east of Ithaca Chasma, compared to other units [Wagner et al., 2013a;Wagner et al., 2013b]. Of the four geologic units depicted in this diagram, the cumulative frequency of the densely cratered plains dcp is highest next to that of unit fcp 1 , indicating the age of the fractured cratered plains has an age comparable to that of unit dcp. Cratering model ages for dcp are on the order of > 4 Ga, more or less independent of the cratering chronology used [START_REF] Kirchoff | Impact cratering records of the mid-sized, icy saturnian satellites[END_REF][START_REF] Neukum | Cratering Chronologies and Ages of the Major Saturnian Satellites[END_REF]Wagner et al., 2013a;Wagner et al., 2013b;[START_REF] Zahnle | Cratering rates in the outer solar system[END_REF].

[Figure 17] Although spectral properties indicate no unique surface composition of the dcp, this geological unit is included in the extended portion of Tethys's surface that shows the weakest H 2 O-ice signature and very small H 2 O-ice particles with radii <1µm (Fig. 12). These submicron sized particles are characteristic for all geologically old surfaces on Tethys, which have been exposed to weathering processes for billions of years (see below). Similar results could be measured on Dione and Rhea (Fig. 12 and19). Sub-micron sized H 2 O-ice particles on Saturn's icy satellites already have been detected and discussed by [START_REF] Clark | The surface composition of Iapetus: Mapping results from Cassini VIMS[END_REF] supporting our results.

Impact crater and basin units c1, b1 and b2 date back to the formation of this oldest geologic unit, based on their high state of degradation as well as their superimposed crater frequencies,

A C C E P T E D M

A N U S C R I P T 39 especially on units b1 and b2. In terms of large impact structure density, Tethys is characterized by the unique fact that basins are preferentially concentrated on the southern hemisphere, except Odysseus. The number or frequency of large craters and/or basins on the other mid-sized satellites of Saturn is either too limited (e.g., Mimas, Dione) to infer a spatially preferential occurrence, or they are more or less uniformly distributed across the surface (e.g., Rhea, Iapetus) [START_REF] Giese | The topography of Iapetus' leading side[END_REF][START_REF] Kirchoff | Impact cratering records of the mid-sized, icy saturnian satellites[END_REF][START_REF] Stephan | Dione's spectral and geological properties[END_REF][START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF]. Due to the fact that basins of type b1 and b2 are still distinct despite their high degradation state, it is apparent that they were formed preferentially on Tethys' southern hemisphere. A complete degradation of pre-existing basins on the northern hemisphere is unlikely since no significant difference in morphology and/or crater frequency between northern and southern hemisphere is detectable. Furthermore, measured crater frequencies on Tethys do not show an equilibrium distribution or saturation that could have been responsible for erasing age differences [START_REF] Schmedemann | The Signature of Secondary Cratering on 4 Vesta and Tethys[END_REF]. Spectrally, these impact craters and basins cannot be distinguished from their surroundings, which supports their high geological age (Fig. 11 and12).

The smooth cratered plains unit scp, like dcp, is interpreted as an old unit, but has a lower crater frequency according to [START_REF] Smith | A New Look at the Saturn System: The Voyager 2 Images[END_REF]. In crater counts performed by [START_REF] Kirchoff | Impact cratering records of the mid-sized, icy saturnian satellites[END_REF] on Tethys, no distinction was made between the densely cratered plains dcp and the smooth cratered plains scp in their measurements. Because of the antipodal location of unit scp with respect to the Odysseus basin, J M Moore, and Ahern, J. L. [1983] discussed the possibility that the formation of this major impact basin also caused the formation of the smooth plains. However, no pervasive resurfacing affecting the high density of craters in scp caused by the Odysseus impact event is detectable. Seismic shaking from the impact event was cited as the most likely cause for the origin of the smooth plains [J M [START_REF] Moore | Topography of endogenic features on Saturnian midsized satellites[END_REF], resulting in a process of "smoothing" the former rugged and hilly terrain of unit dcp.

Other processes such as cryovolcanic resurfacing by liquid material inferred as a possible

A C C E P T E D M

A N U S C R I P T 40 cause for smooth terrain formation [START_REF] Schenk | Eruptive Volcanism on Saturn's Icy Moon Dione[END_REF] can be ruled out since the boundary between units dcp and scp is in most parts gradational. A similar high geological age supported by the fact that both units, i.e the scp and dcp, do not show any differences with respect to their spectral signature (Fig. 12 and13). Any differences are probably masked and/or eliminated due to uppermost surface regolith as a result of billions of years of impacts and space weathering processes.

We interpret the prominent tectonic feature Ithaca Chasma, mapped in two varieties of fractured cratered plains (units fcp 1 and fcp 2 ) to have formed early in Tethys' geologic history.

The state of degradation of the scarps and troughs and the high density of craters mainly of type c 1 and c 2 superimposing these tectonic structures in Ithaca Chasma infer a time or time period of origin prior to Odysseus. A formation as a consequence of impact deformation by the Odysseus impact event, as suggested by, e.g., [START_REF] Smith | Encounter with Saturn: Voyager 1 Imaging Science Results[END_REF], seems less likely according to measured crater size frequency distributions [Wagner et al., 2013a;Wagner et al., 2013b]. Older, even Voyager-based measurements of the crater distribution support these results [START_REF] Giese | Tethys: Lithospheric thickness and heat flux from flexurally supported topography at Ithaca Chasma[END_REF][START_REF] Plescia | Crater densities and geological histories of Rhea, Dione, Mimas and Tethys[END_REF]. It could be argued that the high density of craters superimposed on scarps and troughs of Ithaca Chasma was caused by extensive secondary cratering from the Odysseus impact event. However, this can be more or less excluded since the superimposed craters in turn show wide varieties of degradation states and therefore were formed over a prolonged period of time rather than by a single event.

Apart from a supposed impact-related origin Ithaca Chasma could have formed from two sources of stress: (1) Freeze expansion of H 2 O ice [Moore and Ahern, 1983] or (2) tidal stress due to a high excentricity [START_REF] Chen | Implications from Ithaca Chasma for the thermal and orbital history of Tethys[END_REF]. [START_REF] Giese | Tethys: Lithospheric thickness and heat flux from flexurally supported topography at Ithaca Chasma[END_REF] performed stereo analysis using Cassini ISS images in order to determine the lithospheric thickness and heat flux in Ithaca Chasma. Their results are compatible with Ithaca Chasma being an old surface feature, supporting an age from crater counts derived with lunar-like cratering chronology [START_REF] Neukum | Cratering Chronologies and Ages of the Major Saturnian Satellites[END_REF][START_REF] Plescia | Crater densities and geological histories of Rhea, Dione, Mimas and Tethys[END_REF], which is on the order of 4.0 Ga [START_REF] Giese | Tethys: Lithospheric thickness and heat flux from flexurally supported topography at Ithaca Chasma[END_REF]Wagner et al., 2013a;Wagner et al., 2013b]. A younger formation age for Ithaca Chasma from the application of the constant cratering rate model by [START_REF] Zahnle | Cratering rates in the outer solar system[END_REF], order of ~3 Ga, has been considered unrealistic, based on a plausible thermal history model and heat flux of Tethys [START_REF] Giese | Tethys: Lithospheric thickness and heat flux from flexurally supported topography at Ithaca Chasma[END_REF][START_REF] Multhaup | Stagnant lid convection in the mid-sized icy satellites of Saturn[END_REF].

A C C E P T E D M A N U S C R I P T 41 models
On the other hand, an age younger than ~ 4 Ga could be possible if Ithaca Chasma originated from tidal stress associated with a higher eccentricity in a more recent past (more recent than ~ 4 Ga) due to a 3:2 orbital resonance with Dione [START_REF] Chen | Implications from Ithaca Chasma for the thermal and orbital history of Tethys[END_REF]. The present-time eccentricity is close to zero and Tethys therefore is tidally inactive, despite a resonance between Tethys and Mimas and a resonance between Enceladus und Dione respectively.

According to [START_REF] Chen | Implications from Ithaca Chasma for the thermal and orbital history of Tethys[END_REF] it is not certain how the orbit of Tethys evolved into this present-time orbital configuration from a higher-eccentricity paleo-resonance with Dione possibly accounting for the formation of Ithaca Chasma. In a more recent work of [START_REF] Zhang | Late-stage impacts and the orbital and thermal evolution of Tethys[END_REF] imply that the impact of Odysseus could have broken a 3:2 Tethys-Dione paleo-resonance.

In the VIMS data, the major tectonic surface features on Dione and Rhea show a distinct H 2 Oice signature and a distinct morphology similar to fresh impact craters on these satellites pointing to a geologically younger age [START_REF] Stephan | Dione's spectral and geological properties[END_REF][START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF]. The spectral signature of Ithaca Chasma on Tethys (geologic unit fcp), however, is almost nonexistent anymore and is probably masked by spectral signatures of the surface caused by younger surface (exogenic) processes (discussed below). On the contrary, Odysseus, as stated above, shows a distinctly icier, i.e. fresher H 2 O ice signature. Thus, our measurements support the thesis that the formations of Ithaca Chasma took place early in Tethys' history but before the Odysseus impact event happened. Therefore, both events are unrelated to each other, supporting the results of [START_REF] Giese | Tethys: Lithospheric thickness and heat flux from flexurally supported topography at Ithaca Chasma[END_REF].

A C C E P T E D M A N U S C R I P T 42
The interpretation and definition of the stratigraphic position of the geologic unit mapped as lightly cratered plains (lcp) is problematic, mostly due to insufficient imaging, as described above. The boundary between this unit and unit dcp is linear and very sharp south of Odysseus, inferring a tectonic influence, as shown in Figures 5 and18. The subdued morphology of most large craters as well as the low density of smaller craters (≤ 20 -30 km in diameter) indicates a resurfacing process of so far unknown origin. The spatial extent of this unit is also unknown due to the reasons cited above. In the images available at present, Ithaca Chasma seems to about against the eastern boundary of this unit. Northward and southward, the boundaries of lcp with respect to other units must still be considered as preliminary. The fact that both the modified cratered plains units scp and lcp are located in proximity to Ithaca Chasma suggests that these two units, as well as the fractured cratered plains fcp 1 /fcp 2 of Ithaca Chasma could have a common origin related to endogenous processes. This needs further study, based on improved ISS image coverage that might possibly become available toward the end of the Cassini mission.

Odysseus is the youngest of the Tethyan basins and was formed after the cratered plains varieties. However, the resurfacing process in unit lcp that erased most of the mid-sized and smaller craters could have taken place after the Odysseus impact event. Measurements of crater frequencies verify that the basin is younger than Ithaca Chasma, based on the observation that the majority of the craters superimposes the tectonic structures within Ithaca Chasma and assuming that they are primary craters and not Odysseus secondaries [START_REF] Giese | Tethys: Lithospheric thickness and heat flux from flexurally supported topography at Ithaca Chasma[END_REF]Wagner et al., 2013a;Wagner et al., 2013b]. Cratering model ages of Odysseus from crater counts are on the order of 3.8 Ga in the model by [START_REF] Neukum | Cratering Chronologies and Ages of the Major Saturnian Satellites[END_REF], versus 2.1 Ga (Case A) and 400 Ma (Case B) in the model according to [START_REF] Zahnle | Cratering rates in the outer solar system[END_REF]] [Wagner et al., 2013a;Wagner et al., 2013b].

A C C E P T E D M A N U S C R I P T [Figure 18]
The close proximity of obvious secondary crater chains (unit bocs) to the basin rim (boc) pointing radially away and the apparent lack of an extended continuous ejecta blanket suggests a low impact energy, possibly from a large projectile impacting at a comparably low velocity. [START_REF] Zhang | Late-stage impacts and the orbital and thermal evolution of Tethys[END_REF] discuss that the Odysseus projectile might have had its origin within the Saturnian system and possibly represented a small Saturnian satellite, impacting at low ~ 500 m/sec, which implies a planetocentric (Saturno-centric) impactor. The origin of the Odysseus unit designated as bosd is uncertain. Here it is interpreted as smooth deposits, possibly impact melt. Areas of bosd are spatially close to the secondary chains (bocs). Their boundaries are mostly indistinct. Higher-resolution images are necessary for studying these features in more detail.

Several geologically old impact basins were observed with Odysseus the only one with a significantly icier spectral signature, pointing to a younger geologic age than most parts of the surface including Ithaca Chasma. Usually, surface features formed due to excavation of subsurface material like during an impact event and tectonic activity are assumed to exhibit a similar spectral signature if the space environment (discussed below) is not much different in these locations. Interestingly, the size of the H 2 O-ice particles is relatively large and similar to fresh icy material on Tethys (ice particle radius of about 5µm) and observed in case of fresh impact craters and tectonically resurfaced regions on Dione and Rhea (Fig. 19). Thus, Odysseus' spectral characteristics are interpreted to be a remnant of its primordial surface composition. On the contrary, Odysseus shows the same signature than the N/S oriented icy band, which is located together with Odysseus on Tethys leading hemisphere indicating a possibly relationship with respect to their formation. However, the N/S band cannot be associated to any surface feature and rather points to originate due to an exogenic effect. After the formation of the cratered plains varieties including the large basins and degraded basins, the majority of impact craters were emplaced and continuously degraded by space weathering as well as micrometeorite bombardment. These craters are mostly of types (units) c1 and c2. Individually, larger craters of these types may be dated by counting smaller superimposed craters in higher-resolution ISS images in order to define their relative position in the stratigraphic column. Only the youngest craters of unit c3 show a strong correlation with H 2 O ice absorption features in Cassini VIMS data as, e.g., the impact crater east of basin Penelope (Fig. 14), or Telemachus, the largest c3 crater on Tethys (Fig. 4). Telemachus was dated by crater counts. Its cratering model age is ~ 3.6 Ga according to the Neukum et al. [2003] model [Wagner et al., 2013a;Wagner et al., 2013b]. These youngest surface features are usually characterized by clean H 2 O ice of relatively large particle sizes with particle radii of about 5µm (Fig. 19). Interestingly, H 2 O-ice particle sizes measured of fresh impact craters on other Saturnian satellites like Inktomi on Rhea and Creusa on Dione are similar (Fig. 13 and 19). Assuming this material is freshly excavated subsurface material, the relatively large H 2 O ice particles imply that the crustal material of these satellites is similarly chemically and physically composed. However, this cannot explain the particle sizes of the H 2 O ice in the N/S band, which lies in the vicinity of Odysseus. This aspect is discussed below.

Weathering of the surface material due to impacting micrometeorites, dust and/or H 2 O-ice particles from Saturn's magnetosphere and rings are still actively altering the chemical and physical properties of Tethys' surface material. Thus, the hemispherical differences, the E-W-

A C C E P T E D M

A N U S C R I P T 45 as well as N-S-trending albedo and color features in the low latitudes on the leading hemisphere and on the center of the trailing hemisphere may have formed recently, or, more likely, they have been persisting for a long (>> 1 Ga) time and are subject to a constant removal and re-deposition of material.

Differences in the strength of the H 2 O-ice absorptions between the leading and trailing hemisphere on Tethys are expected to mirror the influence of the space environment on Tethys' surface properties. Subdued BDs of the H 2 O-ice absorptions at 1.5 and 2µm due to visually dark material on the satellites' trailing hemisphere were also measured on Dione and Rhea [START_REF] Clark | Compositional mapping of Saturn's satellite Dione with Cassini VIMS and implications of dark material in the Saturn system[END_REF][START_REF] Stephan | Dione's spectral and geological properties[END_REF][START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF]. Dust particles in Saturn's magnetosphere are known to impact their trailing hemispheres and to cause sputtering of H 2 O-ice particles, which can result in a tenuous exosphere of O 2 like detected at Rhea [START_REF] Teolis | Cassini finds an oxygen-carbon dioxide atmosphere at Saturn's icy moon Rhea[END_REF], whereas bombardment by small E-ring grains brightens the leading hemispheres.

The subdued albedo and BDs of the equatorial E-W trending band on Tethys' leading hemisphere corresponds to the dark bluish band observed in Cassini ISS data interpreted to be formed by irradiation of the surface by incident MeV electrons, which are expected to rotate in a direction opposite to the main plasma flow [START_REF] Schenk | Plasma, plumes and rings: Saturn system dynamics as recorded in global color patterns on its midsize icy satellites[END_REF]. Weathered regions on Tethys, especially the anti Saturnian hemisphere, are dominated by sub-micron sized H 2 O-ice particles (Fig. 19), which confirm studies by [START_REF] Clark | The surface composition of Iapetus: Mapping results from Cassini VIMS[END_REF] and probably results from an undisturbed deposition of micrometeorite-gardened micron to sub-micron particles [START_REF] Hansen | Ice Grain Size Distribution: Differences Between Jovian and Saturnian Icy Satellites From Galileo and Cassini Measurements[END_REF].

On the contrary to what to expect for surface areas experiencing an enhanced bombardment, the N-S trending icy bands show unusual large ice particles similar to fresh impact craters (see above). Usually, relatively large ice particles only occur in regions that did not experience significant space weathering (either there are geologically young or are shielded from micrometeoritic and/or radiation bombardment) [START_REF] Clark | Compositional mapping of Saturn's satellite Dione with Cassini VIMS and implications of dark material in the Saturn system[END_REF][START_REF] Stephan | Dione's spectral and geological properties[END_REF] 46 al., 2012] or occur in regions of higher surface temperature [START_REF] Jaumann | Distribution of icy particles across Enceladus' surface as derived from Cassini-VIMS measurements[END_REF][START_REF] Stephan | Ganymede's spectral properties: implications for further investigations in a future mission to Jupiter and its satellites[END_REF]. Neither explanation works for the N-S bands on Tethys.

; Stephan et A C C E P T E D M A N U S C R I P T
Because of the similar spectral signature like Odysseus, a formation of these bands related to the Odysseus impact event cannot be ruled out. Several authors discussed the influence of the Odysseus impact event on Tethys, which might have caused a temporary non-synchronous rotation of Tethys that has now become synchronous again. No evidence of any surface signature related to an at least temporarily non-synchronous rotation like cycloid linear surface features, as observed on the Jovian satellite Europa [START_REF] Hurford | Cycloidal cracks on Europa: Improved modeling and non-synchronous rotation implications[END_REF], could be identified so far. [START_REF] Zhang | Late-stage impacts and the orbital and thermal evolution of Tethys[END_REF] only propose an ancient episode of heating and deformation on Tethys, which has been attributed to its passage through a 3:2 resonance with Dione and possibly was broken by the Odysseus impact event [START_REF] Zhang | Late-stage impacts and the orbital and thermal evolution of Tethys[END_REF].

Since these N-S trending bands occur mostly independent of geological features, they are interpreted as relatively recently formed. Tethys exhibits some thermal anomalies, with higher surface temperatures corresponding to the low-albedo feature close to the equator of Tethys' leading hemisphere [START_REF] Howett | PacMan returns: An electron-generated thermal anomaly on Tethys[END_REF]. No thermal characteristic can be connected to the N-S bands. The location and overall shape of these bands also do not fit the areas expected to endure enhanced impacts of particles from Saturn's magnetosphere as well as from the E ring.

However, Tethys orbits Saturn relatively close to the E ring as well as in a region of dense plasma of Saturn's magnetosphere, which might also cause the higher porosity of Tethys' surface compared to the other Saturnian satellites derived from UV measurements [Royer and Hendrix, 2014]. Thus, the interaction between the surface material and the space environment might be more complex here and different from that seen on Dione and Rhea [START_REF] Stephan | Dione's spectral and geological properties[END_REF][START_REF] Stephan | The Saturnian satellite Rhea as seen by Cassini VIMS[END_REF].

A C C E P T E D M

A N U S C R I P T 47 6 Summary Tethys' surface shows geological and spectral surface characteristics, whose appearance, nature and spatial distribution mirror those identified on Tethys' neighboring satellites but partly are unique in the Saturnian system. The geological units are in places associated with the spectral variations of relatively large pure H 2 O ice particles in particular at fresh impact craters. The very few fresh impact craters observed on Tethys as well as Dione and Rhea exhibit similar particle sizes implying similar chemical and physical properties of the subsurface material of these satellites. Geologically old, weathered regions instead are dominated by very small submicron-sized particles.

Our geological investigations confirm that Odysseus with its pronounced morphology and ejecta cannot belong to the oldest features. It is clear that the Odysseus impact event globally affected the satellite. The results of the combined analysis of spectral and imaging data, however, contradict the theories that the Odysseus impact event caused the formation of Ithaca Chasma.

But it is not certain yet if the Odysseus impact event is connected to the formation N-S trending 'icy' bands that are consistent with Odysseus in abundance and particle size of H 2 O ice. These bands cannot be correlated to expected pattern due to recent radiation effects and/or variations in surface temperature. Although, it cannot be excluded that the Odysseus impact event led to their existence, further investigations are needed to identify the processes responsible for their formation and their possible relationship to either Odysseus or the interaction of Tethys' surface material with Saturn's rings and magnetosphere.

A C C E P T E D M

A N U S C R I P T 48

Table Captions

Table 1: Tethys Tour Event Summary of non-Targeted (nT) Cassini Flybys at Tethys offering VIMS observations with pixel ground resolutions of better than 60 km/pixel sufficient for spectral and spatial analysis. Note that the distance between the spacecraft and Tethys surface and phase angle is given for the time at closet approach. The dcp/lcp boundary is shown here as a curved line due to the orthographic projection but is more or less linear in cylindrical map projections as used in DLR basemap (Fig. 2). See also Figure 19: H 2 O-ice particle radii of various terrains on Tethys (described in section 4), Dione and Rhea derived from the BDR values measured in the corresponding VIMS spectra (Fig. 13) and theoretical H 2 O-ice model spectra of [START_REF] Hansen | Calculation of single-scattering albedos: Comparison of Mie results with Hapke approximations[END_REF] representing the spectral properties of H 2 O ice at a temperature 80K based on the optical constants derived by [START_REF] Grundy | The temperature-dependent near-infrared absorption spectrum of hexagonal <formula>H2O ice[END_REF] . 10 Figures

Figure Captions

Tables

  to study the alteration of the surface material due to space weathering processes.

  distinct than the dcp/scp boundary, somehow fading into the lower IR/

  A C C E P T E D M A N U S C R I P T 22 cratered plains dcp. The difference in the state of degradation is clearly visible; therefore a separation between the two facies is justified. [Figure 6]

  Figure 8c. Bright fresh ice is exposed at the western interior wall. The eastern rim and interior

  secondary craters), bocs. The chains are also distinguishable in Voyager images [J M

  and 12). Spectral variations occur especially with respect to the BDs of the H 2 O-ice absorptions and how the BDs of the individual H 2 O-ice absorptions vary to each other. Due to the dominance of H 2 O ice the individual absorption bands at 1.04, 1.25, 1.5 and 2µm are mostly distinct and more or less relatively deep in every VIMS spectrum of Tethys' surface confirming more or less pure H 2 O ice. BDs vary only from 3.3±0.3 to 6.7±0.3% (BD 1.04µm ), 5.0±0.3 to 13±0.3% (BD 1.25µm ), 42.2±0.2 to 65.1±0.2% (BD 1.5µm ) and 67.0±0.1 to 77.2±0.1% (BD 2µm ) across the entire surface.As mentioned above the geometric albedo at 0.5µm shows any existing changes in the abundance of H 2 O ice without the influences of particle ice changes. Existing base maps of Tethys show only slight changes in the visible albedo. It reaches up to 0.9, corresponding to

[

(

  Dione) and Inktomi (Rhea) and their extended ejecta globally affect the BDR values of the leading hemispheres causing a generally larger average H 2 O-ice particle size. On all three satellites the largest BDR values reach above the BDR value derived for our smallest model particle size of 1µm (see section 5) indicating particle sizes in the sub-micron range. If the correlation of the BDR values and the H 2 O-ice particle size is still valid in the sub-micron region with larger BDR values indicating smaller H 2 O-ice particles, Dione could exhibit the smallest H 2 O-ice particles followed by Rhea and Tethys, respectively.

[

  2006] model, or ~ 700 Ma (Case A) and ~ 100 Ma (Case B) according to the Zahnle et al.

Figure 1 :

 1 Figure 1: Coverage of Tethys by VIMS observations summarized inTable 1 depending on:

Figure 2 :

 2 Figure 2: Global base map of Tethys acquired by the Cassini ISS instrument and published in

Figure 3 :

 3 Figure 3: Type locality of the geologic unit densely cratered plains (dcp) east and northeast of

Figure 4 :

 4 Figure 4: (a) Low resolution image from orbit 069TE, ISS NAC frame N1590391036, oblique

Figure 5 :

 5 Figure 5: Type locality of unit lightly cratered plains (lcp) and its boundary with unit dcp

FigureFigure 6 :

 6 Figure 17 for more details.

Figure 7 :

 7 Figure 7: Crater material units, mapped according to their erosional state (see text for

Figure 8 :

 8 Figure 8: Impact basin material units, mapped according to their erosional state (see text for

Figure 9 :

 9 Figure 9: Anaglyph images of basin Penelope (type locality of unit b 2 ). Height exaggeration is

Figure 10 :

 10 Figure 10: Geologic units mapped in Odysseus, the only representative of basin type b 3 . Top

Figure 11 :

 11 Figure 11: Global VIMS maps showing the variations in the band depth (BD) of the H 2 O-ice

Figure 12 :

 12 Figure 12: Global VIMS maps showing the variations in the band depth ratio (BDR) of the

Figure 13 :

 13 Figure 13: Average VIMS spectra of spectrally different regions described in section 4, which

Figure 14 :

 14 Figure 14: Close up view of the unnamed small fresh impact crater located at 16°S and

Figure 15 :

 15 Figure 15: Close-up view onto Ithaca Chasmacombination of VIMS data with ISS images

Figure 16 :

 16 Figure 16: Topographic profiles of the Ithaca Chasma region combined with VIMS results,

Figure 18 :

 18 Figure 18: Geology of the surrounding of Odysseus. Left: ISS Nac frame N1481443192 (orbit

Figure 1 A

 1 Figure 1

Table 1

 1

	Mission	Date	Cassini Flyby	Closest Approach [km]	Phase Angle [°]	Number of VIMS cubes	Pixel ground resolution
							[km/pixel]
		2005-03-09	4	83,000	64	17	2.5 -52
	Nominal	2005-05-02	7	52,000	110	15	31 -40
		2006-07-23	26	120,000	29	8	29 -63
		2006-11-09	32	212,300	37	11	49 -59