Jan Raack

Susan J Conway

Thomas Heyer

Valentin T Bickel

Meven Philippe

Harald Hiesinger

Andreas Johnsson

Marion Massé

Present-day gully activity in Sisyphi Cavi, Mars -Flow-like features and block movements

Keywords: Mars, Mars, surface, Geological processes, Photometry, Mars, climate, Mars, polar geology, Ices

The continuously increasing number of multi-temporal high-resolution images from the surface of Mars offers the possibility for detailed studies of present-day surface activity. In this study we investigated all gullies in the Sisyphi Cavi region (355° E, 71° S) of the south polar region of Mars. This region is influenced by the seasonal deposition of a decimeters-thick translucent slab ice in late autumn/winter and its subsequent sublimation in spring. We mapped all gullies (n = 17.760) and measured their orientations. We also identified gullies with contemporary activity (n = 35) using multi-temporal HiRISE images for martian years (MY) 28 to 34. We observed two different kinds of activity: (1) dark flow-like features, and (2) movement of blocks. For both, sediment was transported from the source region (alcove and/or flanks of channels) down the gully. Using image data from HRSC, CTX, and HiRISE, we monitored the general defrosting of the study region. We also analyzed the maximum daytime surface temperatures of the complete study region based on TES data from MYs 24 to 26. To identify the origin and triggering mechanism of the observed activity, we used: (1) detailed topographic investigations (e.g., slope angles) of two extensively gullied slopes based on two HiRISE-DTMs, (2) identification of small scale displacements with Digital Image Correlation (DIC), and (3) orientation measurements of active gullies and comparison to non-active gullies.

We found that for the active gullies studied, activity happens at the end of spring between LS ~225° and ~250°. This is consistent with the timing of final stages of defrosting in the region. At this time, some surfaces are already defrosted while others still host the seasonal slab ice cover. For the surfaces with slab ice, dark defrosting spots (and flows, if the surface is inclined) are observed on dark dunes as well as on gully aprons and in gully channels. These spots form when, triggered by basal sublimation generated overpressure, sediment entrained in CO2-gas is transported through cracks within the ice and redeposited onto the frosted surface. We compared and linked both morphologic features (dark dune spots and dark flow-like features) and concluded that these features have comparable or even the same triggering mechanisms. Based on this extensive study, the most plausible mechanism for ongoing gully activity can be divided into two steps: 1) accumulation of material within gully channels via small dry flows on top of the slab ice (comparable to dark dune spots/flows), 2) when a critical mass is reached, the sediment flows down the still frosted gully on top of the sublimating ice or as a mixture of dry material and ice in a catastrophic flow. The triggering factor for the movement of blocks remains unclear, as their timing could not be constrained with the available data. We identified headwall erosion in one gully in the study region, whereas a discrete source could not be identified for the other sites, suggesting multiple failure mechanisms could be active in such gullies. Finally, through volume balance calculations we show that active gullies in Sisyphi Cavi could have been formed within decades to several tens of thousands of MY, but gully-morphology indicates a much-longer period for formation of the entire gully-landform.

Introduction

Gullies on Mars are features comprised of an alcove, channel, and debris apron and can range up to several kilometers in length [START_REF] Malin | Evidence for Recent Groundwater Seepage and Surface Runoff on Mars[END_REF]; Figure 1). They are found on steep slopes in the mid to high latitudes of Mars and have a wide range of morphologies (e.g., [START_REF] Dickson | Martian gullies in the southern mid-latitudes of Mars: Evidence for climate-controlled formation of young fluvial features based upon local and global topography[END_REF][START_REF] Dickson | The formation and evolution of youthful gullies on Mars: Gullies as the late-stage phase of Mars' most recent ice age[END_REF][START_REF] Conway | Enhanced runout and erosion by overland flow at low pressure and sub-freezing conditions: Experiments and application to Mars[END_REF][START_REF] Conway | The comparison of topographic long profiles of gullies on Earth to gullies on Mars: A signal of water on Mars[END_REF][START_REF] Harrison | Global documentation of gullies with the Mars Reconnaissance Orbiter Context Camera and implications for their formation[END_REF]. They were initially thought to originate from erosion and deposition of sediments by flowing liquid water, in a similar fashion to gullies on Earth [START_REF] Malin | Evidence for Recent Groundwater Seepage and Surface Runoff on Mars[END_REF].

An alternate hypothesis, proposed shortly after the discovery of gullies, is that CO2 could play a role in triggering sediment motion that forms gullies [START_REF] Musselwhite | Liquid CO2 Breakout and the Formation of Recent Small Gullies on Mars[END_REF][START_REF] Hoffman | Active polar gullies on Mars and the role of carbon dioxide[END_REF].

CO2 ice is deposited on the martian surface every winter, forming a continuous layer down to ~50° in both hemispheres and a discontinuous layer down to ~30° latitude [START_REF] Piqueux | Sublimation of Mars's southern seasonal CO2 ice cap and the formation of spiders[END_REF][START_REF] Schorghofer | Seasonal surface frost at low latitudes on Mars[END_REF]Vincendon et al., 2010aVincendon et al., , 2010b;;[START_REF] Pilorget | Dark spots and cold jets in the polar regions of Mars: New clues form a thermal model of surface CO2 ice[END_REF] mirroring the latitudinal distribution of gullies. The CO2 hypothesis was initially proposed based on the presence of gullies located near the south pole of Mars where annual CO2 frost deposits would be at their thickest [START_REF] Hoffman | Active polar gullies on Mars and the role of carbon dioxide[END_REF][START_REF] Ishii | Formation of recent martian gullies by avalanches of CO2 frost[END_REF][START_REF] Cedillo-Flores | CO2 gas fluidization in the initiation and formation of martian polar gullies[END_REF] and where surface temperatures are too low to support the presence of liquid water. The mechanism proposed by [START_REF] Hoffman | Active polar gullies on Mars and the role of carbon dioxide[END_REF] was that liquid CO2 outbursts were responsible for gullies here, but the stability of liquid CO2 on Mars means its presence is even more unlikely than liquid water (Stewart and Nimmo, 2002). [START_REF] Ishii | Formation of recent martian gullies by avalanches of CO2 frost[END_REF] and [START_REF] Ishii | Constraints on the formation of gullies on Mars: a possibility of the formation of gullies by avalanches of granular CO2 ice particles[END_REF] proposed that gullies could be formed by CO2 ice avalanches and [START_REF] Hugenholtz | Frosted granular flow: a new hypothesis for mass wasting in Martian gullies[END_REF] slightly modified this idea by suggesting CO2 ice coated grains could avalanche by vaporization of the CO2 frost. [START_REF] Cedillo-Flores | CO2 gas fluidization in the initiation and formation of martian polar gullies[END_REF] proposed a mechanism by which dust or sand superposed onto a CO2 ice surface would be mobilized by sublimation of the underlying CO2 ice. [START_REF] Pilorget | Formation of gullies on Mars by debris flows triggered by CO2 sublimation[END_REF] suggested that martian gullies could result from basal CO2 sublimation of an ice layer and subsequent movement of material underneath the CO2 ice layer.

Ongoing observations of gully activity at the present-day have continued to raise questions, not only regarding the formation mechanisms of gullies, but also the triggering mechanisms of the activity [START_REF] Malin | Present-Day Impact Cratering Rate and Contemporary Gully Activity on Mars[END_REF]McEwen et al., 2007a;[START_REF] Diniega | Seasonality of presentday Martian dune-gully activity[END_REF][START_REF] Dundas | New and recent gully activity on Mars as seen by HiRISE[END_REF][START_REF] Reiss | Evidence for present day gully activity on the Russell crater dune field[END_REF][START_REF] Dundas | Seasonal activity and morphological changes in martian gullies[END_REF][START_REF] Jouannic | Morphological and mechanical characterization of gullies in a periglacial environment: The case of the Russell crater dune (Mars)[END_REF][START_REF] Dundas | Long-term monitoring of martian gully formation and evolution with MRO/HiRISE[END_REF][START_REF] Raack | Present-day seasonal gully activity in a south polar pit (Sisyphi Cavi) on Mars[END_REF][START_REF] Pasquon | Present-day formation and seasonal evolution of linear dune gullies on Mars[END_REF][START_REF] Dundas | The formation of gullies on Mars today[END_REF]Pasquon et al., 2019aPasquon et al., , 2019b)). Recent laboratory experiments under martian atmospheric conditions have shown that unstable water on the martian surface could have an influence on the formation of recent and present-day mass wasting features [START_REF] Raack | Water induced sediment levitation enhances downslope transport on Mars[END_REF][START_REF] Herny | Downslope sediment transport by boiling liquid water under Mars-like conditions: experiments and potential implications for Martian gullies[END_REF], but the climatic conditions (e.g., low surface temperatures) at times when gully activity occurs on Mars makes water-triggered activity very unlikely. As a consequence, the focus of research has moved from H2O-to CO2-related mechanisms when investigating contemporary gully activity, but all three possible triggering factors (H2O, CO2 and dry or possible combinations) should continue to be considered for active gullies on Mars.

This study focuses on polar pit gullies, which were the focus of the initial studies referring to CO2 frost as the mechanism behind martian gullies. Our objective is to exploit all available imaging data to identify and catalogue the types, timing, and frequency of present-day activity in these gullies with the aim to provide insights into the processes driving this activity. We also investigate whether the present-day processes could account for the presence and morphology of all the gullies in the south-polar pits or whether other mechanisms need to be invoked. Furthermore, the polar location of the gullies investigated in this study combined with the polar orbits of the main imaging orbiters means that there is a higher frequency of observations compared to gullies at lower latitudes, providing a unique opportunity to study the activity of gullies in detail. [START_REF] Thomas | The Colour and Stereo Surface Imaging System (CaSSIS) for the ExoMars Trace Gas Orbiter[END_REF] of a gullied slope within the study region. The gullies are the same as those presented in Figure 14.

Bright surfaces correspond to the annually deposited CO2 surface frost cover. This image was acquired at the end of spring when the general defrosting of the study region was already welladvanced. CO2 surface frosts are only visible on the large dark dune, which is completely covered with a bright ice cover, and on gully aprons / alcoves. Dark spots on the dune show slight flowlike streaks where the slope is highest (on the lower part of the dune). Image (MY34_003464_256_1) was acquired in MY 34 at LS 242°, flow direction is from top to bottom.

(b) Context image of the complete gullied region (Region 5, see Figure 2). Image was acquired in MY 30 at LS 201° (CTX-image G06_020600_1115) where surface frost covers the complete region and first dark spots on the dune and defrosting of gully alcoves at the north-facing (equatorfacing) slope are visible. The black frame represents the location of the CaSSIS image.

The study region comprises the complete Sisyphi Cavi region, an area with numerous polar pits (depressions) up to ~1 km deep [START_REF] Tanaka | Geologic map of the polar regions of Mars[END_REF]. Sisyphi Cavi is named after "Sisyphus", the ancient king of Ephyra. In Greek mythology, Sisyphus was punished in the underworld after his death. He had to roll a large boulder up a steep hill, but each time just before reaching the top of the hill, the boulder would roll down again and he would have to start again. This frustrating punishment including rolling boulders turns out to be surprisingly relevant to this work and shows the name was well-chosen (see Section 3.1.2).

The formation of these irregular steep-sided polar pits in general and specifically in this region is not well understood. The pits are incised into relatively smooth upland surfaces of Amazonian age [START_REF] Tanaka | Geologic map of the polar regions of Mars[END_REF]) and some layering is visible at their slopes. There are two large regions with polar pitted terrain (Angusti Cavi at ~290° E and ~77° S; Sisyphi Cavi at ~355° E and ~71° S) near the south pole, which were first noticed on Mariner 9 images and described by [START_REF] Murray | Geological framework of the south polar region of Mars[END_REF] and [START_REF] Sharp | Mars: South polar pits and etched terrain[END_REF] as eolian erosion (denudation) of sedimentary rocks of eolian or volcanic origin. Later, [START_REF] Howard | Etched plains and braided ridges of the south polar region of Mars: Features produced by basal melting of ground ice? NASA Technical Memorandum[END_REF] raised the hypothesis that basal melting of ground ice formed these pits, which was supported by [START_REF] Thomas | Polar deposits of Mars[END_REF] who reported that the region consists of volatile-rich polar cap deposits, and by [START_REF] Head | Tests for ancient polar deposits on Mars: Assessment of morphology and topographic relationships of large pits (Angusti and Sisyphi Cavi) using MOLA data[END_REF] and [START_REF] Ghatan | Candidate subglacial volcanoes in the south polar region of Mars: Morphology, morphometry, and eruption conditions[END_REF]. The basal melting is believed to have been brought about by increased geothermal heat flux in the vicinity of subglacial volcanoes located in the eastern part of Sisyphi Cavi [START_REF] Ghatan | Candidate subglacial volcanoes in the south polar region of Mars: Morphology, morphometry, and eruption conditions[END_REF].

No matter of their origin, the pits of the Sisyphi Cavi form steep slopes in the polar regions. Gullies form preferentially on steep slopes [START_REF] Conway | New slope-normalized global gully density and orientation maps for Mars[END_REF]) so this is one of two polar concentrations of gullies in the southern hemisphere, the other being Angusti Cavi. We defined our study region from -66° to -76° S and from 340° to 10° E in which nearly all the pits in Sisyphi Cavi were covered (Figure 2). [START_REF] Raack | Present-day seasonal gully activity in a south polar pit (Sisyphi Cavi) on Mars[END_REF] were the first to study the activity of gullies in Sisyphi Cavi in detail. One of their gullies showed signs of activity every Mars year (MY) during their observation phase between MYs 29 and 31 and was analyzed in detail. [START_REF] Raack | Present-day seasonal gully activity in a south polar pit (Sisyphi Cavi) on Mars[END_REF] found that sediments were transported down the gully and deposited over and next to the apron. Gully activity appeared during mid-spring when surface temperatures began to rise and the seasonal surface ice sublimated. Based on spectral observations and numerical modeling, the surface ice was identified as CO2 translucent slab ice with minor contaminations of H2O ice. The most plausible mechanism for the observed activity were dry flows mobilized over the sublimating CO2 ice surface (e.g., [START_REF] Cedillo-Flores | CO2 gas fluidization in the initiation and formation of martian polar gullies[END_REF]. The material for the dry flows was most likely to originate from the slopes of the gully channel, triggered by sublimation. Our here presented work includes the active gully analyzed by [START_REF] Raack | Present-day seasonal gully activity in a south polar pit (Sisyphi Cavi) on Mars[END_REF] and adds new insights based on newly available high-resolution datasets. [START_REF] Dundas | Seasonal activity and morphological changes in martian gullies[END_REF][START_REF] Dundas | Long-term monitoring of martian gully formation and evolution with MRO/HiRISE[END_REF][START_REF] Dundas | The formation of gullies on Mars today[END_REF]) also included the Sisyphi Cavi gullies. [START_REF] Dundas | Seasonal activity and morphological changes in martian gullies[END_REF][START_REF] Dundas | Long-term monitoring of martian gully formation and evolution with MRO/HiRISE[END_REF][START_REF] Dundas | The formation of gullies on Mars today[END_REF] noted activity in them but made no detailed specific morphologic observations. In a study of the topographic long profiles of gullies on Mars, [START_REF] Conway | The comparison of topographic long profiles of gullies on Earth to gullies on Mars: A signal of water on Mars[END_REF] found that gullies in the polar pits had shallower slopes than those elsewhere and that they had generally a more pronounced curvature. Gullies in Sisyphi Cavi can be named to be "classic gullies" with a distinct alcove, channel, and apron system. because these multi-temporal datasets were acquired after the begin of our study. These footprints (25) can be found in the middle of the study region. These numbers represent the 25 individual multi-temporal covered study regions which were investigated. Gully activity was found in seven of them (5, 10, 11, 12, 14, 18, 20, see yellow dots). Red dots represent mapped gullies based on CTX imagery (n = 17,760), blue dots represent mapped gullies on multi-temporal HiRISE imagery (n = 2,067), yellow dots represent contemporary active gullies (n = 35) identified on multitemporal HiRISE images.

Studies of gully activity by

Data and methods

Datasets

We used different visual and thermal datasets for our study of active gullies and the defrosting within the study region. For a general overview, context, and topographic information, we used Thermal Emission Imaging System daytime infrared (THEMIS-IR) [START_REF] Christensen | The Thermal Emission Imaging System (THEMIS) fort he Mars 2001 Odyssey mission[END_REF] data combined with a Mars Orbiter Laser Altimeter digital terrain model (MOLA DTM) [START_REF] Zuber | The Mars Observer laser altimeter investigation[END_REF]. MOLA polar gridded data (460 m/pix) were used for slope orientation measurements. Furthermore, 28 High Resolution Stereo Camera (HRSC) images (at approximately 12.5 m/pixel, [START_REF] Jaumann | The high-resolution stereo camera (HRSC) experiment on Mars Express: Instrument aspects and experiment conduct from interplanetary cruise through the nominal mission[END_REF] were also used for a general overview of the study region and for frost identification during spring, as a validation of observations with the Context Camera (CTX). CTX images [START_REF] Malin | Context camera investigation on board the Mars Reconnaissance Orbiter[END_REF] were used for mapping and orientation measurements of all gullies and gullied slopes, as well as for identification of defrosting in the entire study region. We used 382 CTX images with a resolution of ~5 m/pixel. Furthermore, we used a CTX mosaic provided by [START_REF] Dickson | A global, blended CTX mosaic of Mars with vectorized seam mapping: A new mosaicking pipeline using principles of nondestructive image editing[END_REF] as a basis for the mapping of all gullies in the study region. For detailed and multi-temporal investigations, we analyzed 212 multi-temporal (with a coverage of at least two images of same area) High Resolution Imaging Science Experiment (HiRISE) images with a resolution between 0.25 and 1 m/pixel (McEwen et al., 2007b). Two Colour and Stereo Surface Imaging System (CaSSIS) images [START_REF] Thomas | The Colour and Stereo Surface Imaging System (CaSSIS) for the ExoMars Trace Gas Orbiter[END_REF] were used for a color overview of frostcovered gullied slopes.

Visual datasets were identified using the MUTED webtool [START_REF] Heyer | The Multi-Temporal Database of Planetary Image Data (MUTED): A webbased tool for studying dynamic Mars[END_REF], which provides a direct download link to the PDS Geoscience Node (http://pdsgeosciences.wustl.edu/dataserv/mars.html). CTX and HiRISE images were downloaded as a preprocessed product for direct integration into an ESRI ArcGIS project. Possible necessary adjustments of the exact location and perfect co-registration of multi-temporal datasets were done manually in ArcGIS. We identified 25 different regions with multi-temporal coverage (see Figure 1). In seven of these regions, we identified contemporary gully activity (active mass wasting features within gullies). Furthermore, we undertook detailed analyses of the timing of defrosting within these seven regions and measured the orientations of active mass wasting features on HiRISE images.

For thermal investigations, we used data from the Thermal Emission Spectrometer (TES) with a resolution of about 3 km/pixel [START_REF] Christensen | Mars Global Surveyor Thermal Emission Spectrometer experiment: Investigation description and surface science results[END_REF]. We used all available daytime surface temperatures, which were collected between ~13:00 and ~15 to 26) than the identified contemporary activity (MY 28 to 34). However, we assume that large variations of surface temperatures within the observation time of our investigations and in the study region did not occur and that annually temperatures did not show any large variations. TES data were processed and downloaded directly from the TES webpage at the Arizona State University (ASU) (http://tes.asu.edu/data_tool/).

Methods

Mapping of gullies and slopes

We have mapped all gullies within the study region based on CTX images in order to place our detailed observations in context and assess the representativeness of the HiRISE sampling. In total, 17,760 single gullies were identified and marked. Individual gullies were identified by clearly visible alcoves associated with depositional fans, which are the most prominent features of a gully.

Channels are more difficult to identify because sometimes their widths are below the resolution of the CTX data used. Hence, some of the identified sites may be relict gullies, lacking channels. We also identified and marked all gullies that were present within the multi-temporal HiRISE imagery (n = 2,067).

Multi-temporal analysis

For the identification of areas with both gully activity and high spatial and temporal repeat coverage of high-resolution images, we used the Multi-Temporal Database of Planetary Image Data (MUTED; [START_REF] Heyer | The Multi-Temporal Database of Planetary Image Data (MUTED): A webbased tool for studying dynamic Mars[END_REF]. The web-based tool enables users to identify orbital images covering the same surface area on Mars based on user-defined spatial and temporal constraints. In particular, the database was used to find multi-temporal images with a spatial resolution better than 15 m/pix (for CTX images) within the area of interest.

After the import of multi-temporal HiRISE imagery into an ArcGIS project, we manually georeferenced particular images to get the best visual overlap to identify changes between the images. In a first step, we focused in the comparison of images with similar timings within a year (comparable solar longitudes) and similar solar incidence angles. This was done so as to have comparable shadows on the surface, which makes it easier to identify changes associated with activity and reduces mistakes, e.g., misidentification of shadows at different angles as mass wasting activity. In a second step, we tried to narrow down the timing of activity to a minimum by analyzing images within a year. If these images are available, (sometimes only a few images within several different years are available), we identified the image "before" and "after" the activity happened, which gives us the maximum timing within a year. For this study, we used the layerbased ArcGIS structure to click manually through all available HiRISE images (Figs. 34567).

Slope measurements and topography

Slope measurements of six active gullies based on HiRISE-Digital Terrain Models (HiRISE-DTMs) were conducted (in red, Figure 8). We used one publicly released HiRISE-DTM (HiRISE stereo pair ESP_013097_1115 and ESP_013585_1115) in region 5 (Figure 8b) which was downloaded from the UA-HiRISE webpage (https://www.uahirise.org/dtm/) and we processed another HiRISE stereo pair, ESP_023290_1090 and ESP_048608_1090, in region 11 (Figure 8a)

with the standard routines in the USGS Integrated Software for Imagers and Spectrometers (ISIS3)

and SOCET SET [START_REF] Kirk | Ultrahigh resolution topographic mapping of Mars with MRO HiRISE stereo images: Meter-scale slopes of candidate Phoenix landing sites[END_REF]. Both DTMs were used to visualize the slope angles of the six active gullies (Figure 8: A, B, F, G, L, N) in comparison with non-active adjacent gullies (Figure 8: C, D, E, H, I, J, K, M, O, P). Profile measurements followed the path from the beginning of their alcoves, through their channels, and across their aprons to the termini of the slopes (Figure 8).

Vertical exaggeration is about 2.7 in the displayed figures. For a better statistical analysis of all gullies, we calculated the average slope angle of each individual gully separated in segments of 100 m (Figure 8 and Supplementary Table 02). With this method, we were able to identify broad trends in the slopes of the gully profiles.

Orientations

We have undertaken four different measurements of orientations, binned into 10° azimuth intervals: [START_REF] Diniega | A new dry hypothesis for the formation of martian linear gullies[END_REF] the number of pixels of all gullied slopes identified on CTX imagery (n = 8,303) based on an aspect map derived from a 21-pixel moving window applied to the MOLA polar gridded data at 460 m/pix, (2) all individual gullies (n = 17,760) found on CTX imagery, (3) the number of pixels of all gullied slopes identified on HiRISE imagery (n = 1,430) also based on the same aspect map used for gullied slopes identified on CTX imagery, and (4) all active gullies (n = 35) based on multi-temporal HiRISE imagery (Figure 9). Individual gully orientation measurements were derived from the mapped gully alcoves (as points) and sampling the value directly underneath the point representing the alcoves of the underlying MOLA aspect map.

Orientations of active gullies were derived from the long profiles of the gullies they were hosted in and, if possible, from the direct visible path of active dark flow-like feature. The analysis of the gullied and non-gullied slopes was carried out on all pixels with slopes > 6° because this value was the lowest value that reliably captured all the gullied and non-gullied slopes without including parts of the plateaux or pit-floors. Please note that the resolution of the MOLA aspect map smoothed out the gully-alcoves so that it represents the general orientation of the hillslope.

The number of pixels of all gullied slopes (1) and ( 3) were divided by the orientation of all pixels of all slopes (gullied and non-gullied CTX-identified slopes: 43,632 pixels in total; gullied and non-gullied HiRISE-identified slopes: 1,648 pixels in total) to account for the influence and biases of the general pre-existing topography, resulting in normalized data. Densities for all gullies (2) and active gullies (4) were calculated by (2) dividing the number of all gullies by the total area with gullies based on CTX imagery (in pixels) for each orientation, and (4) dividing the number of all active gullies by the total area with gullies based on multi-temporal HiRISE imagery (in pixels) for each orientation, respectively.

Slow small-scale displacement detection

To investigate the possibility of small-scale movements like creep as precursors to gully activity, which can only be observed over multi-annual time scales, we applied a method called digital image correlation (DIC). We produced four HiRISE-DTMs of region 11 to investigate the flow event that occurred between MY32 and 33 (Figure 4) as well as to study general gully activity in the region. We used five HiRISE-images: PSP_005621_1090, ESP_023290_1090, ESP_030859_1090, ESP_039747_1090 and ESP_048608_1090 to cover the time between MY 28 and 33. DTM production was conducted using the Ames Stereo Pipeline (ASP, [START_REF] Beyer | The Ames Stereo Pipeline: NASA's Open Source Software for Deriving and Processing Terrain Data[END_REF].

Governed by the available HiRISE images, all produced DTMs are either pre-event or co-event DTMs, i.e., no post-event DTM could be produced. The change in surface appearance and texture due to the recent flow feature that occurred between MY 32 and 33 is minimal and did not influence the quality of the image matching during stereo correlation. All images were bundle adjusted and matching was performed with a Bayes EM-weighted affine adaptive window correlator to ensure the highest possible DTM quality. Due to the good correlation quality, the point cloud files could be meshed to DTMs with very high spatial resolutions ranging from 0.5 to 3 m/pixel. Despite the low noise content, all DTMs feature distinct striped inter-CCD seam artifacts that are common for HiRISE-based DTMs. Tab. 2 in the Appendix reports additional relevant details about the produced and used DTMs.

Orthorectification was performed using the Ames Stereo Pipeline, where DTMs from timeframes MY 28 to 30 and MY 28 to 33 have been used to orthorectify HiRISE images PSP_005621_1090, ESP_023290_1090, and ESP_048608_1090 (Figure 10). This was necessary because digital image correlation (DIC) needs highly accurate orthorectification to extract small-scale horizontal surface displacements.

DIC is an image processing technique that allows for the quantification of coherent, in-CVP (camera viewing plane) surface displacements in multi-temporal imagery [START_REF] Ayoub | Co-Registration and correlation of aerial photographs for ground deformation measurements[END_REF][START_REF] Heid | Evaluation of existing image matching methods for deriving glacier surface displacements globally from optical satellite imagery[END_REF][START_REF] Bickel | Quantitative assessment of digital image correlation methods to detect and monitor surface displacements of large slope instabilities[END_REF]. The technique is limited by its ability to co-register pixel neighborhoods within a specified search window, i.e., incoherent or too large surface displacements between two image acquisitions will cause a loss of signal. This usually limits the applicability of DIC to monitoring of continuous and slow displacements, e.g., of glaciers [START_REF] Heid | Evaluation of existing image matching methods for deriving glacier surface displacements globally from optical satellite imagery[END_REF], volcanic edifices [START_REF] Casu | Deformation Time-Series Generation in Areas Characterized by Large Displacement Dynamics: The SAR Amplitude Pixel-Offset SBAS Technique[END_REF], and slow moving landslides [START_REF] Bickel | Quantitative assessment of digital image correlation methods to detect and monitor surface displacements of large slope instabilities[END_REF]. This limitation is particularly restrictive for space-and airborne sensors, as revisit rates are usually lower and the displacement of objects between two acquisitions might be larger, and potentially, too large. On Mars, DIC has successfully been applied to monitor dune migration and sand fluxes using HiRISE imagery, e.g., in Nili Patera [START_REF] Bridges | Earth-like sand fluxes on Mars[END_REF] although it was unsuccessful for coseismic displacement of faults in Cerberus Fossae on Mars [START_REF] Grindrod | The Search for Active Marsquakes Using Subpixel Coregistration and Correlation: Best Practice and First Results[END_REF].

The three above mentioned orthorectified HiRISE images have been selected for the DIC analysis for two reasons: 1) Their maximum temporal baseline, as a larger difference in time will allow for the detection of smaller displacements (PSP_005621_1090 of MY 28 and ESP_048608_1090 of MY 33); and 2) As they could potentially indicate pre-event displacement in the gully of interest (PSP_005621_1090 of MY 28 and ESP_023290_1090 of MY 30). Prior to DIC ingestion, a dynamic contrast enhancement has been applied (Wallis filter) and all orthorectified images were co-registered with sub-pixel accuracy, as past studies have found that these steps generally increase the correlation quality and coverage of the displacement field, if present [START_REF] Bickel | Quantitative assessment of digital image correlation methods to detect and monitor surface displacements of large slope instabilities[END_REF].

Subsequently, a frequency domain-based DIC code that is applying a Fast-Fourier-Transform [START_REF] Bickel | Quantitative assessment of digital image correlation methods to detect and monitor surface displacements of large slope instabilities[END_REF] has been used to analyze the potential displacement between both image pairs, using varying template and search window sizes. No post-processing of the results has been performed to maintain the full interpretability of the results.

Thermal investigations and seasonal frost coverage

For thermal investigations, we collected all TES surface temperature datasets from MY 24 to 26 (except some measurements in MY 25 due to the influence of a global dust storm, see above) and binned them into five groups of 2° of latitude from 66° to 76°. We then calculated the mean of all individual measurements at the same time with their associated standard deviation (black lines of red dots in Figure 11). For our thermal studies, we investigated all available surface temperature datasets of all years together (except some data from MY 25 due to a global dust storm). No further division into different MYs was done because annually variations of MY 24 and 26 are small and negligible.

Furthermore, we also detected and investigated the seasonal frost coverage based on CTX and HRSC imagery. We identified seasonal frost coverage by its bright appearance compared to unfrosted regions (well-defined during the sublimation phases in spring by dark defrosting spots)

and the assumption, that the study region is covered by surface frost during the winter, which was shown by [START_REF] Raack | Present-day seasonal gully activity in a south polar pit (Sisyphi Cavi) on Mars[END_REF] in the same study region and substantiated by the general low surface temperatures during winter [START_REF] Piqueux | Sublimation of Mars's southern seasonal CO2 ice cap and the formation of spiders[END_REF]. For investigations of the timings of surface frost, we looked into all available CTX and HRSC images of the study region. We generalized the frost cover and marked an image as "frosted" even when only a small part of the image shows bright ice cover. This results in detailed information on the timing of surface frost in the complete study region based on two different datasets without detailed analyses of local differences of frost coverage such as orientations and altitude.

Results

We have identified 25 regions with multi-temporal HiRISE coverage (see black rectangles in Figure 2), 13 regions include coverage of gullies. In total, 2,067 individual gullies were identified on multi-temporal HiRISE images. In seven regions we identified contemporary active mass wasting features within gullies. Overall, we found only 35 (~1.7 %) individual gullies that showed activity within MY 28 to 34 (yellow dots in Figure 2 and Supplementary Table 01). One large gully showed two independent activities at two different places in the wide alcove, therefore we classified these activities as gully independent activities 32 and 33 (see Supplementary Table 01).

The activity can be divided into flow-like features (n = 22), as well as block movements within gullies (n = 16), where three of them are combined features which makes 35 active gullies in total.

For seasonal investigations, multi-temporal coverage of some images within one martian year are necessary. Due to the greater frequency of HiRISE-coverage of some regions within the study area, we were able to narrow-down the activity of 10 gullies during one martian year.

Present-day mass wasting activity on multi-temporal HiRISE imagery

Present-day activity was defined by clearly visible erosion and/or deposition of material down a gully. Although some gullies could also show seasonally generated albedo differences and dark flow-like features, no sediment transport was detectable in these gullies so we classify these gullies as "non-active". We distinguished between flow-like features (dark toned flow-like forms with visible transport of material) and block movements (movement of blocks down the gully slope).

Flow-like features

The most widespread present-day activity of gullies are flow-like features. We found 22 gullies with active flow-like features or new deposits and erosion forms most likely caused by flow-like activity. Three of them are associated with identifiable block movements. Ten of these 22 active gullies show flow-like activity within only one MY and six of these ten gullies show repeated activity. In detail, three gullies show activity in two separate MYs, two gullies within three different MYs, and one gully shows activity within five MYs (see Supplementary Table 01).

Movements within repeatedly active gullies originated in the same places and covered the same depositional areas of the gully.

Flow-like features have identifiable transport of sediment, which is probably dust and/or sand [START_REF] Raack | Present-day seasonal gully activity in a south polar pit (Sisyphi Cavi) on Mars[END_REF]. The presence of pervasive surface sand/dust is supported by the presence of pervasive surface ripples and the seasonal appearance of dark spots on gully aprons, and next to or within gully channels. These dark spots are likely formed by transport of sand and dust from under the translucent slab ice (e.g., [START_REF] Piqueux | Sublimation of Mars's southern seasonal CO2 ice cap and the formation of spiders[END_REF][START_REF] Kieffer | CO2 jets formed by sublimation beneath translucent slab ice in Mars' seasonal south polar cap[END_REF][START_REF] Thomas | HiRISE observations of gas sublimation-driven activity in Mars' southern polar regions: IV. Fluid dynamics models of CO2 jets[END_REF] as larger particles cannot be mobilized (see further discussion of these features in Section 4.1).

Based on our seasonal observations, we found that the flow-like features occurred episodically, i.e., changes appeared between two images and were not gradual. This was observed as an accumulation of gully channel material in the "before" image, whereas the "after" image displayed distinct flow features over the apron (Figure 3). Sometimes, the deposited material was observed without observable accumulation of material in the channel beforehand. The flow-like features appear dark compared to their surroundings. Over time, the transported material brightens and fades until mid-summer, which could be due to gradual deposition of dust, comparable to the fading of, e.g., dust devil tracks [START_REF] Reiss | Dust Devil Tracks[END_REF] and slope streaks [START_REF] Schorghofer | Three decades of slope streak activity on Mars[END_REF].However, fading of a flow-like feature in Sisyphi Cavi was also observed by [START_REF] Raack | Present-day seasonal gully activity in a south polar pit (Sisyphi Cavi) on Mars[END_REF] and their investigation of the corrected Lambert albedo images showed that the flow-like feature did not brighten, but that the surroundings darkened over time due to seasonal frost sublimation. In summer, the surroundings and the flow-like features have a very similar albedo which can only be observed in the corrected albedo images. This indicates that the brightening is not caused by dust deposition but is a result of frost sublimation. The brightening did not appear to alter the changes in surface relief associated with the dark flow-like features, therefore this study will focus on the formation of these mass movements and not their fading.

In some cases, seasonal observations were not possible and no dark flow-like features were identifiable, but a change of the surface relief caused by sediment transport was still visible. In

Figure 4 an example flow-like feature is shown, with the formation of relatively small new transportation channels (Figure 4b), block movement (Figure 4b), erosion of ripples or superposition of ripples with transported material (Figure 4c), and a general change of the relief of the surface. Thickness and consequently volume calculations cannot be made because it was not possible to measure the height of the newly formed deposits, either on digital terrain models (DTMs) or with shadow measurements (for some volume estimations, see [START_REF] Raack | Present-day seasonal gully activity in a south polar pit (Sisyphi Cavi) on Mars[END_REF]. This indicates that the deposits have a thickness of some decimeters or less.

One gully (#3) in the study region had clearly identifiable headwall erosion and loss of material, which was transported down the gully. This was the only gully where the source region of transported material was clearly visible and measurable (Figure 5). [START_REF] Raack | Present-day seasonal gully activity in a south polar pit (Sisyphi Cavi) on Mars[END_REF] analyzed the same gully but no source region for the identified activity was found as it only became apparent in more recently acquired images. At some localities, the gully-alcove headwall erosion of up to several meters (max. ~5 m) occurred between MY 29 and MY 33. This headwall erosion is the first and only evidence of a clearly identifiable source region for transported material in the study region.

The only noticeable features located upslope of observed sediment transport are small (up to ~15 m), dark flow-like features on the steep flanks of the gully channels (Figure 6). These features were found in some active gullies, as well as in non-active gullies. We infer that sediment erosion and transportation are taking place at these locations, because there dark layers paralleling the crest of the gully channel flanks (Figure 6 d-e), from which the small dark flow-like features emanate.

A second indicator for sediment transport, even if it is not topographically visible, is the accumulation of dark material within gully channels (Figure 6). These accumulations are formed by confluence of the small, dark flow-like features. This was also proposed, but not directly observed by [START_REF] Raack | Present-day seasonal gully activity in a south polar pit (Sisyphi Cavi) on Mars[END_REF], due to the lack of datasets with sufficiently high resolution. ). The large gully alcove was filled with material (rippled surface, presumably ice-rich mantle, e.g., [START_REF] Mustard | Evidence for recent climate change on Mars from the identification of yourthul near-surface ground ice[END_REF][START_REF] Raack | Gullies and their relationships to the dust-ice mantle in the northwestern Argyre Basin, Mars[END_REF], which is eroded, leading to a smaller gully alcove.

Red lines represent clearly identifiable outlines of the headwall of the alcove. Furthermore, gully alcoves uncover an underlying rocky surface, which is otherwise superposed by a mantling deposit in this region (Figure 7). This rocky surface is often the source for blocks, which then move down the slope.

We observed 16 different gullies (three of them also associated with identified dark flow-like features) with identifiable block movements within the study region. Only one block movement could be seasonally investigated (Figure 12), while all other block movements occur in regions with only limited temporal HiRISE-coverage. The number of 16 block movements is a minimum number because block movements are hard to identify and some block movements (particularly short displacements) are likely to have been missed during our investigations. Three of the identified gullies show a combined activity of flow-like features and block movements. Here, blocks moved several meters in the same direction as the flow-like features and/or were superposed by transported material. In the last case it is hard to distinguish between moved blocks or just superposed blocks by loose fine material. Tracks associated with the block movements were not found. Only in one case (Figure 7), the path of the block down the slope is roughly traceable by the displacement of some other blocks.

The distance moved by the blocks varied between ~1 m to 230 m at maximum (Figure 7b). Most blocks moved only 1-2 meters (n = 9), which typically corresponds to a block rolling from one face to another. The other remaining block movements (n = 6) moved between 5 and 40 meters.

In one case, a block seemed to disappear completely, which means it was not found in the 'after'image and therefore the travel distance could not be measured.

Topography, orientation, and digital image correlation

To identify possible mechanisms responsible for the identified present-day activity of gullies in Sisyphi Cavi, we investigated the topography of the active gullies based on HiRISE DTMs, the orientation of all slopes and gullies in the study region, and possible small-scale surface changes / activities based on digital image correlation of HiRISE images.

Topography

We measured the topographic long profiles of selected gullies on slopes of region 11 (Figure 8a) and region 5 (Figure 8b) by tracing the path of the gullies from the top of the alcove to the end of the depositional fan. Gullies marked by red profiles in Figure 8 show present-day activity and the position of the activity is bracketed by black bars with arrows.

In region 11, the shape of all the gullies (6 measured gullies, 3 with present-day activity) is very comparable (Figure 8a (1)), and no systematic differences in the general shape of the long profile between active and non-active gullies could be identified. All gullies have a rectilinear profile with a steeper upper part and shallower lower part, which starts around 400 m downslope, corresponding to the change from exposed rocks to fans. Present-day activity originates at the steeper upper part of the gullies and material is subsequently transported down the gully channel to where slopes are shallower (around 10°-15°).

Furthermore, the average slope angles calculated every 100 meters of gully length (Figure 8a (2)) are broadly similar. The average slope angles in the alcove area reach maxima of between 30° and 45° and diminish to around 20° towards the base of the alcoves. The alcoves show a higher variation in slopes compared to the rest of the profiles. The remaining downslope parts of the profiles have relatively constant slopes of around 10°, which drop to 5° in the last few hundred meters. Two of the three active gullies (A and B) have steeper average slope angles in the first 200

to 300 m, while other non-active gullies only show shallower slope angles. Gully slopes A and B could be not measured in their entirety (the end of the profile is missing) because the DTM does not cover the complete slope.

The profiles of the gullies in region 5 are very comparable to those in region 11. However, they appear a somewhat smoother and have shorter, steeper upper parts (around 200-250 m) than the gullies in region 11 (Figure 8b (3)). Comparable to region 11, gully activity originates on the steeper slopes at the beginning of the gully profile and continues down to shallower sloping regions. Furthermore, the gullies in region 5 are also very comparable to each other, except gully L, which appears smoother at the beginning than the others and has a steeper slope angle at the end (similar to gullies H and M). The smooth beginning is because the top of the profile is not included in the DTM. The increase of average slope angles at the end is caused by the occurrence of a ridge at the base of the slope (Figure 8b (4)). All other profiles show comparable patterns as well as comparable average slope angles. Slope angles decrease from up to ~50° (alcove of gully N) down to ~0° (apron of gully L). Consequently, no differences of active and non-active gullies in region 5 was found.

The average slope angles of all measured gullies of both regions are provided in Supplementary Table 02. The average slope angles for the entire lengths of all measured gullies varied between 16° and 10° (gullies A and B are not taken into account because the profiles do not extend to the terminus of the hillslope and therefore have anomalously higher average slope angles). The average slope angles of for the entire lengths of the active gullies ranges between 15° and 10°, which provides additional confirmation that no difference in topography between active and nonactive gullies is measurable. 2) with semitransparent orthophoto. See also [START_REF] Raack | Present-day seasonal gully activity in a south polar pit (Sisyphi Cavi) on Mars[END_REF] for more information of this specific area. On the right side same profiles as in (a). Active gullies (G, L, N) are also marked in red. Profiles G and H have increasing slopes at the end of the profile due to the presence of a small local depression. However, this has no influence on the gullies because the depression is located beyond the end of the gully deposits.

Orientation

To identify a possible triggering mechanism of gully activity and to find differences between active and non-active gullies, the orientations of gullies, gully densities, and gullied slopes based on CTX and HiRISE imagery of the complete study region were investigated. Four different measurements were performed:

(1) Orientation of pixels identified on MOLA gridded data (460 m/pix) of gullied slopes (slope angle > 6°) found on CTX imagery. In total, 8,303 pixels cover all gullied slopes and show a clear north-south trend (Figure 9a). After normalization against all slopes > 6° in the complete study region (35,329 pixels), the north-south trend is still visible (Figure 9b), but not so distinct compared to non-normalized data. Both the normalized and non-normalized orientation plots show a slightly preferred south-facing (or polefacing) trend for all gullied slopes.

(2) Orientation of all gullies in the study region based on CTX imagery. In total, 17,760 individual gullies were identified and measured. The data show a strong north-south trend with slightly more south-facing gullies than north-facing gullies (Figure 9c). The density of gullies is remarkably uniform with orientation (Figure 9d). The highest peak of gully density is found between 50° and 60° (northeast) and a slightly lower density of gullies is found on south-to west-facing slopes.

(3) Orientation of pixels identified on MOLA gridded data (460 m/pix) of gullied slopes (slope angle > 6°) found on multitemporal HiRISE imagery. In total, 1,430 pixels cover all gullied slopes (Figure 9e), while only 218 pixels cover non-gullied slopes. This is an effect of the intensive monitoring of gullied slopes with the HiRISE camera in this region. After normalization against all slopes > 6° of multitemporal HiRISE-covered slopes (1,648 pixels), a very uniform distribution is visible (Figure 9f).

(4) Orientation of all active gullies based on multi-temporal HiRISE imagery. In total, 35 individual gullies show present-day activity and were measured. The data show a southwest and a northwest facing trend (Figure 9g). The color-coding represents dark flow-like features (in red) and block movements (black). Only two gullies are east facing. The density clearly shows a higher density of active gullies facing northwest (Figure 9h). Same color-coding as in Figure 9g), which shows the highest density of flow-like features orientated towards north-west. Comparison with Figure 9e reveals that the lack of active east-facing gullies could be due to imaging bias, but that the lack of north-facing active gullies is most likely real.

In summary, gullies tend to be oriented north-south in the study region with a slight bias towards south-facing slopes and the highest density towards the north. In contrast, active gullies tend to be oriented towards the northwest and to a lesser extent the southwest, with a lack of north-south facing active gullies. measured in numbers of gullies per pixel (the data in g divided by f), same color-coding as in (g).

Digital image correlation

The correlation of both HiRISE pairs at location 11 that cover the event between MY32 and 33 (see also Figure 4) does not show any displacements in the periods from MY 28 to 30 (pre-event)

and to 33 (co-event), respectively (Figure 10). This could indicate that 1) there is no pre-event displacement of the hillslopes hosting the investigated martian flow-like features, and 2) that there is no slope creep occurring at all or that such creep is smaller than 25 cm within the five observed martian years, i.e., less than ~5 cm per MY on average.

Parts of the displacement fields (Figure 10) show systematic and erroneous displacement values that are the result of either 1) moving shadows that are caused by slightly different illumination conditions for the used images or 2) inter-CCD seam artifacts. These types of artifacts cannot be avoided, which is why the respective areas have been masked to maintain clarity (Figure 10). The

Timing of annual surface ice cover and gully activity, surface temperatures, and comparison to dark spots

Based on our large dataset of CTX, HRSC, and HiRISE images, we were able to correlate seasonal frost coverage with the annual activity of gullies. Using TES data we could correlate these trends with surface temperature. According to [START_REF] Raack | Present-day seasonal gully activity in a south polar pit (Sisyphi Cavi) on Mars[END_REF], this seasonal frost cover is composed mainly of CO2 ice with some minor contaminations of H2O ice and dust. Figure 11 shows the seasonal trend in surface temperature with latitude combined with our observations of visible surface frost. Each latitude band has a similar temperature trend with temperatures stable at around 150 K until defrosting starts, the temperature then climbs relatively quickly and reaches a convexup plateau extending over the summer. For latitudes between 72 and 76° S there is a slight precursor rise in surface temperature before the steep climb. The last image without surface frost does not occur until Ls ~30° after the plateau is reached. For more equatorial latitudes the rise in surface temperature is experienced earlier and is slightly more rapid with the surface frost occurrence showing the same trend. At 75° S, the surface temperature plateau is reached at Ls ~280°, whereas at 67° S it is reached at Ls ~240°.

Figure 12 shows the timing of the gully activity alongside observations of seasonal surface frost cover from HiRISE. In almost all cases, the activity occurs at the very end of the surface defrosting.

The three exceptions are gully 3 in MY 29 and 31, and gully 7 in MY 31, where activity is earlier in the year. These exceptions show the same type of activities compared to the others, but gully 3

is the only active gully in the study region orientated due north, which implies an earlier, more intense sun exposure and earlier defrosting. This is the likely explanation for the relatively early activity compared to the other active gullies. The active gully was investigated by [START_REF] Raack | Present-day seasonal gully activity in a south polar pit (Sisyphi Cavi) on Mars[END_REF] and shows clear headwall erosion of material in the source region (Figure 5). Gully 7 shows an early block movement in MY 31 and this was the only active block movement whose timing could be constrained in time. All other block movements appear in regions where a high-resolution multi-temporal coverage was not available (see Supplementary Table 01). Therefore, whether block movements systematically occur before the flow-like features in the study region could neither be excluded nor confirmed.

Seasonal frost coverage of the study region was easy to identify. During winter and early beginning of spring, the surface appeared relatively bright and dark defrosting spots are visible mainly on dunes. This bright appearance is probably caused by the redeposition of CO2 frost on top of the generally translucent slab ice cover. Later, while the defrosting process is occurring (mid to end of spring), only small bright icy patches on the generally defrosted surface are clearly visible. Dark defrosting spots appear on all dark dunes within the study region. ). The activity of the other 26 identified active gullies in the study region could not be constrained within one martian year. Some of the 10 gullies show periodic activity within different martian years (y-axis). The crosses represent single HiRISE images, as well as the grey question marks of gullies 6 to 8 do. However, the question marks represent images where the identification of changes in previous images could not be confirmed due to lower resolutions, atmospheric dust coverage, adverse insolation, and/or extensive surface albedo changes caused by surface defrosting. The blue bars represent the identified surface frost coverage from the beginning of surface frost (beginning of blue bar) to the first image without any identifiable surface frost (end of blue bar). Black bars represent identified activity from between the "before" and "after" HiRISE images. Detail of the dune crest with several dark defrosting spots as well as visible cracks in the slab ice.

The image was taken at the end of winter/beginning of spring (LS 178°) and the dune appears bright probably due to redeposition of CO2 frost on top on the translucent seasonal CO2 slab ice cover (comparable to dune field in Figure 8). (c) Detail of one of the gully channels and (d) one of the gully channels with adjacent apron. Dark spots are present within the gully channels and on the aprons, while they are absent on the adjacent hillslopes. This could be caused by the lack of fine material on the slopes, a lack of sufficiently sintered slab ice or different thermal contrasts.

HiRISE-image ESP_037729_1115. North is up in all images.

Discussion

Dark spots and flow-like streaks

Prior and during the activity of the flow-features in the gullies in Sisyphi Cavi the appearance of dark spots and flow-like streaks are observed on the surface, which do not change the topography.

To obviate possible confusion we named these small features emanating from dark defrosting spots "dark flow-like streaks", while "dark flow-like features" refer to the present-day activity of gullies with observable sediment transport. The phenomenon of dark spots was first observed by [START_REF] Cantor | Multiyear Mars Orbiter Camera (MOC) observations of repeated Martian weather phenomena during the northern summer season[END_REF] and is well described in the literature (e.g., [START_REF] Piqueux | Sublimation of Mars's southern seasonal CO2 ice cap and the formation of spiders[END_REF][START_REF] Kieffer | CO2 jets formed by sublimation beneath translucent slab ice in Mars' seasonal south polar cap[END_REF][START_REF] Horváth | Analysis of Dark Albedo Features on a Southern Polar Dune Field on Mars[END_REF][START_REF] Hansen | HiRISE observations of gas sublimation-driven activity in Mars' southern polar regions: I. Erosion of the surface[END_REF][START_REF] Pilorget | Dark spots and cold jets in the polar regions of Mars: New clues form a thermal model of surface CO2 ice[END_REF][START_REF] Martínez | The evolution of the albedo of dark spots observed on Mars polar region[END_REF][START_REF] Jouannic | Morphological characterization of landforms produced by springtime seasonal activity on Russel Crater megadune[END_REF]. The dark spots are formed by basal sublimation of seasonal translucent slab-ice (Figure 13; e.g., [START_REF] Piqueux | Sublimation of Mars's southern seasonal CO2 ice cap and the formation of spiders[END_REF][START_REF] Kieffer | CO2 jets formed by sublimation beneath translucent slab ice in Mars' seasonal south polar cap[END_REF]. The basal sublimation leads to overpressure under the ice, leading to the formation of cracks within the ice (see Figure 14b; [START_REF] Portyankina | Polygonal cracks in the seasonal semi-translucent CO2 ice layer in Martian polar areas[END_REF]. Through these cracks degassing occurs and dust/sand particles are entrained and redeposited on the surface of the ice (e.g., [START_REF] Piqueux | Sublimation of Mars's southern seasonal CO2 ice cap and the formation of spiders[END_REF][START_REF] Kieffer | CO2 jets formed by sublimation beneath translucent slab ice in Mars' seasonal south polar cap[END_REF]Portyankina et al., 2012) (Figure 13, 14). If the surface is inclined, the sediment can move downslope forming dark flow-like streaks (e.g., [START_REF] Horváth | Analysis of Dark Albedo Features on a Southern Polar Dune Field on Mars[END_REF][START_REF] Kereszturi | Recent rheologic processes on dark polar dunes of Mars: Driven by interfacial water?[END_REF][START_REF] Kereszturi | Indications of brine related local seepage phenomena of the northern hemisphere of Mars[END_REF][START_REF] Kereszturi | Water ice in the dark dune spots of Richardson crater on Mars[END_REF][START_REF] Gardin | Defrosting, dark flow features, and dune activity on Mars: Example in Russell crater[END_REF][START_REF] Hansen | HiRISE observations of gas sublimation-driven activity in Mars' southern polar regions: I. Erosion of the surface[END_REF][START_REF] Hansen | Observations of the northern seasonal polar cap on Mars: I. Spring sublimation activity and processes[END_REF][START_REF] Jouannic | Morphological characterization of landforms produced by springtime seasonal activity on Russel Crater megadune[END_REF], which are observed in our study region during spring. Within the study region, dark spots are only observed on dark dunes and on gullies (non-active and active ones), particularly within the gully channel and on aprons (Figure 14).

Based on the same timing of activity and very similar morphologic features (Figure 14), we infer that the same formation mechanism is responsible for the dark spots and flows in the study region, on dunes as well as in gullies. Thus, these two substrates may have similar properties that differ from the surrounding terrain in order to explain the fact that the dark spots/flows are only found on the dunes and gullies. This implies that dunes, gully channels, and aprons are formed of fine grained material such as sand and dust, which can be mobilized. It could also imply that these substrates somehow favor the formation of slab ice and/or the basal heating.

Such dark flow-like streaks have been observed on other gullies, notably the linear gullies on the Russell crater megadune [START_REF] Gardin | Defrosting, dark flow features, and dune activity on Mars: Example in Russell crater[END_REF][START_REF] Jouannic | Morphological characterization of landforms produced by springtime seasonal activity on Russel Crater megadune[END_REF] and also on classical gullies in Sisyphi Cavi by [START_REF] Dundas | The formation of gullies on Mars today[END_REF] or in the mid-latitudes by [START_REF] Costard | Current Gully Activity: Dry Avalanches at Seasonal Defrosting as Seen on HiRise Images[END_REF]. [START_REF] Dundas | The formation of gullies on Mars today[END_REF] presented a gully as an example for defrosting flows in a polar pit gully which was also investigated in our study. They did not detect changes in topography after the flow disappeared.

However, our new observations reveal very small topographic changes, probably missed in the [START_REF] Dundas | The formation of gullies on Mars today[END_REF] study due to fewer available HiRISE images.

Dark flow-like features

We have shown that at the very end of defrosting of the seasonal CO2 surface ice, flow-like features occur, which modify the topography of the gullies in Sisyphi Cavi. These flows run out onto slopes of 10-15° (Figure 8), hence require some degree of fluidization. Similar flow-like features have been observed in gullies imaged in the shadows of impact craters in three craters in the midlatitudes (32-38° S; [START_REF] Dundas | The formation of gullies on Mars today[END_REF]. As a consequence of the less frequent monitoring of gully-sites outside the polar regions, changes observed in mid-latitude martian gullies may also have happened over frost. However, this has not yet been documented in images because they were not captured at the right time and cadence. In contrast to the Sisyphi Cavi gullies, in the midlatitudes flow-like features were only observed on south-facing slopes. The timing of activity in Sisyphi Cavi coincides with activity associated with linear gullies on martian sand dunes [START_REF] Pasquon | Present-day formation and seasonal evolution of linear dune gullies on Mars[END_REF][START_REF] Jouannic | Morphological characterization of landforms produced by springtime seasonal activity on Russel Crater megadune[END_REF]. However, in this case the activity seems to originate from the frosted areas and propagate over unfrosted areas [START_REF] Diniega | A new dry hypothesis for the formation of martian linear gullies[END_REF][START_REF] Jouannic | Morphological characterization of landforms produced by springtime seasonal activity on Russel Crater megadune[END_REF] and again this activity is predominantly on south-facing slopes. Classic gullies on sand dunes have a more complex behavior with activity continuing throughout winter [START_REF] Diniega | Seasonality of presentday Martian dune-gully activity[END_REF]Pasquon et al., 2019a,b).

The flow-like features occur when the CO2 seasonal deposit is becoming thin and discontinuous.

This is also visible in the thermal datasets (Figure 11) where the temperature variability of the standard deviation during the sublimation phase is very high. This is the case when ice covered and non-ice covered surfaces are measured at the same time.

The formation mechanism of dark flows via pressurized debris flows underneath the slab ice proposed by [START_REF] Pilorget | Formation of gullies on Mars by debris flows triggered by CO2 sublimation[END_REF] is unlikely in Sisyphy Cavi. [START_REF] Raack | Present-day seasonal gully activity in a south polar pit (Sisyphi Cavi) on Mars[END_REF] showed with spectral and thermal investigations and numerical modelling that the dark flows are on top of The number of active gullies in Sisyphi Cavi is a small proportion of the whole population: 7 of 25 sites are active, i.e., only 35 of 2,067 individual gullies on multi-temporal HiRISE-images (about 1.7 %). This value is even lower when only flow-like features without block movements are included (n = 22, which is about 1 %). Even though some of the gullies are active annually, many only showed one or two events during the time of image acquisition. This shows that contemporary activity of gullies in this region is rare. To-date, about 35 % of all gullies in the study region have multi-temporal coverage. Some of them only have coverage by two multitemporal images, which makes a visual identification of changes even more challenging.

Active gullies in Sisyphi Cavi could have long recurrence times, similar to debris flows on Earth (e.g., [START_REF] De Haas | Earth-like aqueous debris-flow activity on Mars at high orbital obliquity in the last million years[END_REF], but a longer baseline of observation would be needed to substantiate this hypothesis. We found no topographic or other factor (e.g., differential movement of the headwall) that could explain why certain gullies are active (often repeatedly) and others are not, even within those gullies located at the same site. Similarly to debris flows, such events could be limited by the union of sufficient accumulation of loose material (via jetting processes like dark spots/flows?) and rare climatic events that put the system in motion. We now consider whether the activity seen today, integrated over a long time period, could be sufficient to explain the whole gully population in Sisyphi Cavi. The flow-like features that we observed have a sufficient runout to transport material to the base of the gully-fans, hence are capable of explaining the spatial extent of the gullies. Taking into account imaging bias, the flow-like features we observed have a range and variety of orientations that suggest similar events could occur on the whole range of slope orientations in Sisyphi Cavi, although more data would be needed in the under-imaged east-facing orientations to verify this (Figure 9).

For gullies #1, #2, and #3 we calculated the area covered by deposits during the time of observation, which ranged between ~600 and ~2,800 m 2 per MY. Conservatively assuming a 0.5 m thickness for these deposits [START_REF] Raack | Present-day seasonal gully activity in a south polar pit (Sisyphi Cavi) on Mars[END_REF] results in volumes of between ~300 and ~1,400 m 3 per MY. We used the publicly available HiRISE DTM derived from the stereo pair ESP_013097_1115 and ESP_013585_1115 to measure the alcove volumes of the gullies. We removed / masked the values of the DTM inside 19 alcoves and linearly interpolated over the gap then differenced with the original DTM (e.g., [START_REF] Conway | Decameter thick remnant glacial ice deposits on Mars[END_REF] to find alcove volumes between 32,000 and 2,819,000 m 3 (average 956,000 m 3 ). Assuming a constant rate of activity, we find that whole gullies could be formed in decades to several tens of thousands of Mars years (~4,000 Mars years for an average alcove with an average level of activity). As cycles of obliquity have a dominant frequency of 120 ka, eccentricity 95 ka, and precession 51 ka [START_REF] Ward | Climatic variations on Mars:1. Astronomical theory of insolation[END_REF][START_REF] Ward | Present obliquity oscillations of Mars: Fourth-order accuracy in orbitale e and I[END_REF]1992), our estimated gully formation times fall into times when the climate would not have differed substantially from the present-day climate. Hence, it is realistic that processes similar to those active at the present-day could explain the formation of the entire gully-landform without invoking other mechanisms. However, some gully alcoves are filled with presumably aeolian sedimented ice-rich mantle material (e.g., [START_REF] Mustard | Evidence for recent climate change on Mars from the identification of yourthul near-surface ground ice[END_REF][START_REF] Raack | Gullies and their relationships to the dust-ice mantle in the northwestern Argyre Basin, Mars[END_REF]Figure 5). This ice-rich mantle appears uncratered and other studies estimated maximum ages of ~20 Ma for midlatitudinal locations [START_REF] Raack | Gullies and their relationships to the dust-ice mantle in the northwestern Argyre Basin, Mars[END_REF] and ~3-5 Ma near the poles (55-60° S) [START_REF] Willmes | Surface age of the ice-dust mantle deposit in Malea Planum[END_REF]. This suggests that gully formation has been ongoing over much longer timescales than recorded by the most recent active gullies, but could still be dependent on conditions similar to the present-day for their formation (indeed conditions conducive for mantle emplacement seem to be non-conducive for gully-formation).

Block movements

We could only constrain the timing of one of the 16 block movements observed during spring, prior to the activity of the flow-like features. Block movements were sometimes spatially associated with and sometimes independent of the flow-like features we observed. All movements occurred in the alcove areas of the gullies where slopes are typically > 20° (Figure 8) and were usually of a few meters, leaving no track. Sometimes the block just moved ~1-2 meters, consistent with just toppling over. It is tempting to link these movements to the flows associated with the gullies, yet no blocks are found in the deposits and similar block movements have been reported

in areas without gullies. [START_REF] Dundas | The formation of gullies on Mars today[END_REF] found block movements of meters to tens of meters on slopes of typically 25-35° in craters in the northern hemisphere of which the majority occurred in winter and two occurred in summer. [START_REF] Dundas | The formation of gullies on Mars today[END_REF] argued that because similar block movements are not observed at equatorial latitudes, some aspect of the seasonal cycle encourages these types of movement. They suggested that thermal contraction/expansion of underlying ground ice could disturb blocks or even slight changes in the ground ice distribution driven by seasonal temperature changes. They considered loading or other disturbances by the seasonal ice as less likely because some movements were observed in summer. Given that we only have the timing for one of the movements in our dataset, it is currently impossible to move beyond the hypotheses suggested by [START_REF] Dundas | The formation of gullies on Mars today[END_REF].

Conclusions

Based on various visual and thermal remote sensing data, especially high-resolution multitemporal images, we identified and analyzed multiple contemporary active gullies within Sisyphi Cavi. On the basis of our study we conclude:

(1) At least ~1.7 % of all gullies observed with HiRISE show present-day activity (flow-like features and/or block movements). This is a minimum value as some activity could be overlooked and the required number of multi-temporal images for a complete investigation in all martian years is not available for all regions. Nevertheless, the number of active flowlike features (about 1 % of all investigated gullies) is very low which shows that contemporary gully activity in Sisyphi Cavi is generally rare.

(2) All observed gully activity took place during the end of spring at the end of the seasonal defrosting. During this season, the highest surface temperatures for a partially ice-covered surface were observed. During this time, dark defrosting spots on dunes and gullies were most prevalent.

(3) The origin of the transported material are the alcoves and small layers on the flanks of gully channels. We made the first observations of headwall erosion of gully alcoves. Material accumulated within the gully channel and moved down in one catastrophic flow event.

(4) The preferred triggering mechanism for present-day gully activity in Sisyphi Cavi are flows of dry material over seasonal slab ice (comparable to dark spots/flows). It is possible, that the material flowing on top of the ice eroded the underlying ice and formed a mixture of dry material and ice, which was then transported together downslope, fluidizing the flow.

It is estimated that the ice thickness at the time of dark flow-like feature appearance was relatively thin, which is consistent with the observation that this activity is linked to the last defrosting stages. The triggering factor of the movement of blocks remains unclear due to the lack of multi-temporal images.

(5) We found the orientation of active gullies to be broadly consistent with the general population and using volume balance arguments, calculated that some of the active gullies could have been formed within decades to several tens of thousands of MY. This implies that the gullies in Sisyphi Cavi could have been formed in their entirety by processes similar to these active at the present day.

Gully

Figure 1 :

 1 Figure 1: (a) Color and Stereo Surface Imaging System (CaSSIS) image (Thomas et al., 2017) of

Figure 2 :

 2 Figure 2: THEMIS-IR data superposed by a MOLA digital terrain model (DTM) of the study

Figure 3 :

 3 Figure 3: Before and after images of a south-east facing slope of a polar pit (Region 5, see Figure

Figure 4 :

 4 Figure 4: (a) Present-day active gully #11 (MY 28, LS 327°) on a south-west facing slope in

Figure 5 :

 5 Figure 5: Headwall erosion of active gully #3 presented in Raack et al. (2015) (also seen in Figure

  (c) In MY 33 at the beginning of summer (LS 287°) headwall erosion of a few meters could be identified (yellow lines represent the new headwall of the alcove). Especially the uppermost region of the gully alcove shows the highest erosion rate within the four martian years. (d) Detail of the top of the gully alcove in MY 29 and 33. Erosion of a few meters of material is clearly visible. HiRISE-images (a) ESP_029725_1115, (b) ESP_013585_1115, and (c) ESP_048819_1115. North is down in all images as indicated.

Figure 6 :

 6 Figure 6: (a) Example of a gully alcove with identifiable small, dark flow-like features emanating

Figure 7 :

 7 Figure 7: (a) Gully #12 with present-day activity in form of block movement within the alcove in

Figure 8 :

 8 Figure 8: (a) Colorized DTM from HiRISE stereo pair ESP_023290_1090 and

Figure 9 :

 9 Figure 9: (a) Orientations of all sloping pixels with gullies based on MOLA data (n = 8,303) and

  low root mean square error (RMSE) values, which are a measure for the quality of the image correlation, across these affected regions of the images indicate that the rectangular patches in the displacement fields are in fact caused by errors in the DTM used for the HiRISE image orthorectification and are not actual (rapid) displacement.

Figure 10 :

 10 Figure 10: Displacement magnitude and associated correlation RMSE between HiRISE images

Figure 11 :

 11 Figure 11: Schematic diagram of the average annual defrosting of the study region. Solar

Figure 12 :

 12 Figure 12: Diagram of 10 different gullies with contemporary flow-like features activity (except

  01

Figure 13 :

 13 Figure 13: (a) A gullied slope with adjacent dark dune field in MY 33 during defrosting at the

Figure 14 :

 14 Figure 14: Examples of dark defrosting spots in the study region. (a) Overview of an east-facing

  the ice and not underneath it, at least for the most active gully in Sisyphi Cavi and our additional image observations support this earlier work. Instead, a modified Cedillo-Flores et al. (2011) model where relatively hot sediments are transported by sublimation of the CO2 ices underneath the flow seems a more likely scenario. The fact that the deposits are preserved into the summer months leads us to the conclusion that either the underlying CO2 surface frost is relatively thin and/or the dark flows are turbulent and consist of a mixture of some ice and dry material, which erodes the underlying thin surface frost. If large quantities of CO2 ice were located either under or within the transported debris, their subsequent sublimation would disturb and change the morphology of the deposits over time. De Haas et al. (2019) calculated that only very small volumetric fractions of CO2 (<<1%) are required to fluidize a flow, hence a thin surface frost should be adequate to fluidize the flow.

Table 02 :

 02 Slope angles of all investigated gullies in region 5 and 11 from the beginning of their alcoves, through their channels, above their aprons until the end of the general slope (Figure8). Slope angles are reported as calculated over 100 m-long segments of the gully length. The last row of the table shows the overall average angle of the slope following the path of individual gullies. Active gullies are marked in bold.

	2	1.23° /	113°	Dark flow	MY 29	31 sols
		-68.481°			MY 30	91 sols
					MY 31	27 sols
					MY 32	38 sols
					MY 33	26 sols
	Gully 3	1.443° /	352°	Large dark flow	MY 29	27 sols
		-68.537°		(Figure 5 and Raack	MY 30	91 sols
				et al., 2015)	MY 31	17 sols
	Gully 4	1.68° /	321°	Small dark flow	MY 28-31 2012 sols
		-68.512°
	Gully 5	-13.84° /	232°	Small flow	MY 32	42 sols
		-69.48°
	Gully 6	-13.835° /	200°	Very small changes	MY 31	33 sols
		-69.481°			MY 32	42 sols
					MY 33	65 sols
	Gully 7	-13.826° /	224°	Very small changes	MY 31 †	65 sols
		-69.493°		plus block	MY 31 †	16 sols
				movements †	MY 33	54 sols
	Gully 8	-13.786° /	198°	Very small changes	MY 31	33 sols
		-69.512°			MY 32	133 sols
	Gully 9	-13.71° /	196°	Several small	MY 33	44 sols
		-69.525°		changes
	Gully	-13.698° /	208°	Some small changes MY 32	42 sols
	10	-69.527°			MY 33	44 sols
	Gully	1.2° /	216°	Largest activity (see	MY 32-33 673 sols
	11	-70.92°		Figure 4)
	Gully	1.224° /	229°	Very small changes	MY 31-32	673 sols
	12	-70.92°		plus large block	MY 32-33 †	673 sols
				movement † (see
				Figure 7)
	Gully	1.392° /	194°	Medium changes	MY 32-33 673 sols
	13	-70.949°
	Gully	4.292° /	305°	Small dark flow	MY 32-33	575 sols
	14	-70.678°			MY 33	166 sols
	Gully	4.21° /	297°	Medium dark flow	MY 29-32 2160 sols
	15	-70.74°

Table 03 :

 03 List of the HiRISE image pairs used for DTM generation, including their spatial resolution and their mean intersection error as a measure for DTM quality. Some of the listed DTMs have been used to orthorectify the HiRISE imagery used for DIC.

	Timeframe	HiRISE pair	DTM spatial	Mean intersection
			resolution (m/pix)	error (m)
	2007 -2011	ESP_023290_1090	2	0.62
		PSP_005621_1090
	2007 -2016	ESP_048608_1090	1	0.25
		PSP_005621_1090
	2013 -2015	ESP_030859_1090	3	0.87
		ESP_039747_1090
	2015 -2016	ESP_039747_1090	0.5	0.48
		ESP_048608_1090

Acknowledgments

JR and TH are funded by the German Aerospace Agency (DLR) by the Grant #50QM1801. SJC is grateful to the French Space Agency CNES for funding her HiRISE and CaSSIS related work.

AJ is funded by the Swedish National Space Agency SNSA. We thank Dennis Reiss for many fruitful discussions in the initial stage of this survey. The used DIC code is open-source and can be downloaded from: https://github.com/bickelmps/DIC_FFT_ETHZ. We are grateful for very constructive reviews from J. Dickson and an anonymous expert.