Sonia Fornasier
email: sonia.fornasier@obspm.fr

B E Clark

E Dotto

A Migliorini

M Ockert-Bell

M A Barucci

Spectroscopic survey of M-type asteroids 1

Keywords: Asteroids, Spectroscopy, Meteorites

, are medium albedo bodies supposed to have a metallic composition and to be the progenitors both of differentiated iron-nickel meteorites and enstatite chondrites. We carried out a spectroscopic survey in the visible and near infrared wavelength range (0.4-2.5 µm) of 30 asteroids chosen from the population of asteroids initially classified as Tholen M-types, aiming to investigate their surface composition. The data were obtained during several observing runs during the years 2004-2007 at the TNG, NTT, and IRTF telescopes. We computed the spectral slopes in several wavelength ranges for each observed asteroid, and we searched for diagnostic spectral features. We confirm a large variety of spectral behaviors for these objects as their spectra are extended into the near-infrared, including the identification of weak absorption bands, mainly of the 0.9 µm band tentatively attributed to orthopyroxene, and of the 0.43 µm band that may be associated to chlorites and Mg-rich serpentines or pyroxene minerals such us pigeonite or augite. A comparison with previously published data indicates that the surfaces of several asteroids belonging to the M-class may vary significantly.

We attempt to constrain the asteroid surface compositions of our sample by looking

Introduction

Within the taxonomic system of [START_REF] Tholen | Asteroid taxonomy from cluster analysis of photometry[END_REF], M asteroids have featureless spectra and red spectral slope over the visible wavelength range (0.34 -1.04 µm) and medium visual albedos (0.1 -0.3). [START_REF] Tholen | Asteroids taxonomy[END_REF] identify approximately 40 M-type asteroids using the Tholen (1984) class definition. The M-types are presumed to be the progenitors both of differentiated iron-nickel meteorites and enstatite chondrites [START_REF] Gaffey | Spectral reflectance characteristics of the meteorite classes[END_REF]Cloutis et al. 1990;[START_REF] Gaffey | Mineralogical variations within the S-type asteroid class[END_REF]. Some authors have proposed alternative meteorite linkages, such as CH, CB (Bencubbinite) [START_REF] Hardersen | Near-IR spectral evidence for the presence of iron-poor orthopyroxenes on the surfaces of six M-type asteroids[END_REF], or other carbonaceous chondrites (Vilas 1994). M-class albedos generally match those of nickel-iron meteorites, leading some to propose that these asteroids are the collisionallyproduced fragments of differentiated metal cores [START_REF] Bell | Asteroids: The big picture[END_REF]). This interpretation requires parent bodies heated to at least 2000 • C to produce iron meteorites [START_REF] Taylor | Solar system evolution: a new perspective. an inquiry into the chemical composition, origin, and evolution of the solar system[END_REF]. However, based on recent findings, these interpretations are being challenged. Polarimetric measurements [START_REF] Lupisko | On the surface composition of the Mtype asteroids[END_REF]) and radar observations [START_REF] Ostro | Asteroid 1986 DA: Radar evidence for a metallic composition[END_REF]2000;[START_REF] Magri | A radar survey of main-belt asteroids: Arecibo observations of 55 objects during 1999[END_REF][START_REF] Shepard | A radar survey of M-and X-class asteroids[END_REF]) of selected M-type asteroids have revealed a large variety of surface properties that are in some cases inconsistent with only a metallic composition. [START_REF] Shepard | A radar survey of M-and X-class asteroids[END_REF], in a radar survey of 14 M-type asteroids, find that only four objects ( 16Psyche, 216 Kleopatra, 758 Mancunia, and 785 Zwetana) have radar albedos high enough to be dominantly metallic.

The first estimations of the bulk density of M-asteroids 16 Psyche (1.4-1.8 g/cm 3 ) and 22 Kalliope (2.4 g/cm 3 ) gave values surprisingly lower than expected (Viateau 2000;Kocheva 2003;[START_REF] Britt | Asteroid density, porosity, and structure[END_REF][START_REF] Margot | A low-density M-type asteroid in the mainbelt[END_REF]. These low densities are not consistent with predominantly metal composition, and require very high values for the bulk porosity. However, these values have been disputed and recently [START_REF] Shepard | A radar survey of M-and X-class asteroids[END_REF] estimates a very high bulk density for 16 Psyche (7.6±3.0 g/cm 3 ), more consistent with a metal composition. [START_REF] Descamps | New determination of the size and bulk density of the binary Asteroid 22 Kalliope from observations of mutual eclipses[END_REF] find a bulk density of 3.35±0.33 g/cm 3 for 22 Kalliope, higher than the first estimation but still implying a high bulk porosity.

Of note is the evidence for hydrated minerals on the surfaces of some Mtype asteroids [START_REF] Jones | The composition and origin of the C, P, and D asteroids: Water as a tracer of thermal evolution in the outer belt[END_REF][START_REF] Rivkin | 3-m spectrophotometric survey of M-and E-class asteroids[END_REF]2000;2002), inferred from the identification of an absorption feature around 3 µm typically due to the first overtone of H 2 O and to OH vibrational fundamentals in hydrated silicates. [START_REF] Rivkin | 3-m spectrophotometric survey of M-and E-class asteroids[END_REF]2000) proposed splitting the M-types into wet W-class asteroids (those which show the 3 µm band), and dry M-class asteroids (those which do not show the 3 µm band). If the 3 µm band on M-type asteroids is due to hydrated minerals, then these objects might not be anhydrous and igneous as previously believed and the thermal scenarios for the inner main belt may need a revision (e.g. [START_REF] Bell | Asteroids: The big picture[END_REF]. Gaffey et al. (2002) propose alternative explanations for the 3 µm band: materials normally considered to be anhydrous containing structural OH; troilite, an anhydrous mineral that shows a feature at 3 µm, or xenolithic hydrous meteorite components on asteroid surfaces from impacts and solar wind implanted H.

Recent spectroscopic observations in the visible and near infrared region show that the spectra of M asteroids are not uniformly featureless as previously believed. In addition to the 3 µm band, faint absorption bands in the 0.9 µm region have been identified on the surfaces of some M-type asteroids [START_REF] Chapman | Spectral reflectances of the asteroids[END_REF][START_REF] Hardersen | Near-IR spectral evidence for the presence of iron-poor orthopyroxenes on the surfaces of six M-type asteroids[END_REF]Clark et al. 2004a;[START_REF] Ockert-Bell | Observations of X/M asteroids across multiple wavelengths[END_REF], and attributed to orthopyroxene. [START_REF] Busarev | Spectral Features of M-Asteroids: 75 Eurydike and 201 Penelope[END_REF] detected weak features tenta-tively attributed to pyroxenes (0.51 µm) and oxidized or aqueously altered mafic silicates (0.62 and 0.7 µm) in the spectra of the M-type asteroids 75 Eurydike and 201 Penelope [START_REF] Busarev | Spectral Features of M-Asteroids: 75 Eurydike and 201 Penelope[END_REF].

In this paper we present new spectroscopic data obtained in the visible and near infrared wavelength range (0.4-2.5 µm) of 30 asteroids sampled from the population of objects whose visible wavelength and albedo properties satisfy the definition of the Tholen M-class. In this work, we (1) identify faint absorption bands which may help reconstruct surface composition, (2) search for meteorite spectral analogues, and (3) model surface composition with geographical mixtures of selected minerals when a meteorite match is not found.

Data Acquisition [HERE TABLE 1]

The data presented in this work were obtained during 2 runs (February and November 2004) at the italian Telescopio Nazionale Galileo (TNG) of the European Northern Observatory (ENO) in la Palma, Spain, and 3 runs (May 2004, August 2005, and January 2007) at the New Technology Telescope (NTT) of the European Southern Observatory (ESO), in Chile. Six asteroids, investigated in the visible range at the NTT and TNG telescopes, were separately observed in the near infrared at the Mauna Kea Observatory 3.0 m NASA Infrared Telescope Facility (IRTF) in Hawaii during several runs in 2004-2008 (Table 1). Our survey was devoted to the investigation of asteroids belonging to the E, M and X classes, following the [START_REF] Tholen | Asteroid taxonomy from cluster analysis of photometry[END_REF] taxonomy. Here we present the results of our observations sampling within the Tholen M-class. The results from Tholen Etype asteroids were published in [START_REF] Fornasier | Visible and near infrared spectroscopic investigation of E-type asteroids, including 2867 Steins, a target of the Rosetta mission[END_REF] and the results from Tholen X-type observations will be presented in a separate paper.

Observations at the TNG Telescope

At the TNG telescope, for visible spectroscopy we used the Dolores (Device Optimized for the LOw RESolution) instrument equipped with the low resolution red grism (LR-R) covering the 0.51-0.95 µm range with a spectral dispersion of 2.9 Å/px (see http : //www.tng.iac.es/instruments/lrs). Most of the objects were also observed with the low resolution blue grism (LR-B, dispersion of 2.8 Å/px) in February 2004, covering the 0.4-0.8 µm range, and with the medium resolution blue grism (MR-B, dispersion of 1.7 Å/px, 0.4-0.7 µm range), for the November 2004 run. The 'red' and 'blue' spectra in the visible range were separately reduced and finally combined together to obtain spectral coverage from 0.40 to 0.95 µm.

The Dolores detector is a Loral thinned and back-illuminated CCD with 2048 × 2048 pixels, with a pixel size of 15 µm and a pixel scale of 0.275 arcsec/px. Like most of the Loral CCDs, the Dolores chip is affected by moderate-to-strong fringing at red wavelengths. Despite taking as much care as possible in the data reduction process, some asteroid spectra show residual fringing that impedes identification of absorption bands in the 0.9 µm region.

For the infrared spectroscopic investigation at the TNG telescope we used the near infrared camera and spectrometer (NICS) equipped with an Amici prism disperser. This equipment covers the 0.85-2.40 µm range during a single exposure with a spectral resolution of about 35 [START_REF] Baffa | NICS: The TNG Near Infrared Camera Spectrometer[END_REF]). The detector is a 1024 × 1024 pixel Rockwell HgCdTe Hawaii array. The spectra were acquired nodding the object along the spatial direction of the slit, in order to obtain alternating pairs (named A and B) of near-simultaneous images for the background removal. For both the visible and near infrared observations we utilized a 1.5 arcsec wide slit, oriented along the parallactic angle to minimize the effect of atmospheric differential refraction.

Observations at the NTT Telescope

At the NTT telescope, visible spectra were acquired using the EMMI instrument, equipped with a 2x1 mosaic of 2048×4096 MIT/LL CCD with square 15µm pixels (D'odorico, 1990). We used the grism #1 (150 gr/mm) in RILD mode to cover the wavelength range 4100-9600 Å with a dispersion of 3.1 Å/px at the first order. For the near infrared spectroscopy, we utilized the instrument SOFI (Son OF Isaac) in the low resolution mode. The blue and red grisms, covering respectively the 0.95-1.64 µm (dispersion of 6.96 Å/px) and the 1.53-2.52 µm range (dispersion of 10.22 Å/px) were used [START_REF] Moorwood | SOFI sees first light at the NTT[END_REF]. The acquisition technique was identical to that used for the TNG observations, however we used a larger slit (2 arcsec), always oriented along the parallactic angle. The two spectral regions of the blue and red grism observations were combined matching the overlapping region in the 1.53-1.64 µm range.

Observations at the IRTF Telescope

At the IRTF telescope we used the SpeX instrument, equipped with a cooled grating and an InSb array (1024 x 1024) spectrograph at wavelengths from 0.82 to 2.49 µm. [START_REF] Rayner | SpeX: A Medium-Resolution 0[END_REF]. Spectra were recorded with a slit oriented in the north-south direction and opened to 0.8 by 15 arcseconds. A dichroic lens reducing the signal below 0.8 µm was used for all observations. Objects were consistently observed near the meridian to minimize airmass, but total integration time varied from ∼ 6 to 30 minutes, depending on the strength of the signal relative to sky lines.

Data Reduction

Visible and near-infrared spectra were reduced using ordinary procedures of data reduction with the software packages Midas and IDL as described in Fornasier et al. (2004a,b). For the visible spectra, the procedure includes the subtraction of the bias from the raw data, flat-field correction, cosmic ray removal, sky subtraction, collapsing the two-dimensional spectra to one dimension, wavelength calibration, and atmospheric extinction correction. The reflectivity of each asteroid was obtained by dividing its spectrum by that of the solar analog star closest in time and airmass to the object. Spectra were finally smoothed with a median filter technique, using a box of 19 pixels in the spectral direction for each point of the spectrum. The threshold was set to 0.1, meaning that the original value was replaced by the median value if the median value differs by more than 10% from the original one.

For observations in the infrared range, spectra were first corrected for flat fielding, then bias and sky subtraction was performed by producing A-B and B-A frames. The positive spectrum of the B-A frame was shifted and added to the positive spectrum of the A-B frame. The final spectrum is the result of the mean of all pairs of frames previously combined. The spectrum was extracted and wavelength calibrated. For the NTT-SOFI spectra, this last step was performed acquiring the spectrum in the blue and red grisms of a Xenon lamp, and comparing the observed lines with those of a reference table, obtaining the dispersion relation. For the TNG-NICS spectra, due to the very low resolution of the Amici prism, the lines of Ar/Xe lamps are blended and cannot be easily used for standard reduction procedures. For this reason, the wavelength calibration was obtained using a look-up table which is based on the theoretical dispersion predicted by ray-tracing and adjusted to best fit the observed spectra of calibration sources. Finally, the extinction correction and solar removal was obtained by division of each asteroid spectrum by that of the solar analog star closest in time and airmass to the object.

For the IRTF data, after the normal data reduction procedures of flat-fielding, sky subtraction, spectrum extraction, and wavelength calibration, each spectrum was fitted with the ATRAN atmospheric model for telluric absorption features [START_REF] Lord | A New Software Tool for Computing Earth's Atmospheric Transmission of Near-and Far-Infrared Radiation[END_REF]Bus et al. 2002a[START_REF] Bus | Characterizing the visible and near-IR spectra of asteroids using principal component analysis[END_REF][START_REF] Sunshine | Highcalcium pyroxene as an indicator of igneous differentiation in asteroids and meteorites[END_REF]. This procedure required an initial estimate of precipitable water in the atmospheric optical path using the zenith angle for the observation and the known τ-values (average atmospheric water) for Mauna Kea. This initial guess was iterated until the best fit between predicted and observed telluric band shapes was obtained, and an atmospheric model spectrum was generated [START_REF] Bus | Characterizing the visible and near-IR spectra of asteroids using principal component analysis[END_REF]. Following this, each asteroid spectrum was divided by the atmospheric model and then ratioed to each star spectrum, similarly reduced, before normalization at 1.2 µm. The software and procedures used for data reduction are described in [START_REF] Demeo | An extension of the Bus asteroid taxonomy into the near-infrared[END_REF]. The final spectra we report are averages of 3-5 asteroid/star ratios, calculated to minimize variations due to standard star and sky variability. Usually, 2-5 different standard stars were observed on any given night at the telescope. We used only solar standard stars, and each standard star was observed 1-3 times per night.

The stellar and asteroid spectra were cross-correlated and, if necessary, subpixel shifts were made before ratioing the asteroid to the star. This reduction step is needed to reduce the noise and/or the changes in the final asteroid slope due to small changes in the wavelength dispersion between asteroid and star observations, introduced by instrumental flexure.

The infrared and visible spectral ranges of each asteroid were finally combined by overlapping the spectra, merging the two wavelength regions at the common wavelengths and utilizing the zone of good atmospheric transmission to find the normalization factor between the two spectral parts. For the TNG and IRTF data the overlapping region goes from ∼ 0.86 to 0.94 µm, and we took the average value over the 0.89-0.91 µm region of the visible spectrum to normalize the infrared spectrum. For the NTT data, the overlapping region is very small so we took the average value over the 0.95-0.96 µm region to normalize the spectra. The spectra of the observed asteroids, all normalized at 0.55 µm, are shown in Figures 12345.

Observational conditions are reported in Table 1. [Here Figs 1,2,3,4,5]

Results: Spectral Analysis and Absorption Features

[HERE TABLE 2 and3] We present new spectra of 30 asteroids whose visible wavelength properties and albedos fall within the boundaries of the M-class as defined by Tholen (1984). We obtained new visible spectra for 6 objects, and new visible and nearinfrared spectra for 24 objects (Figures 12345).

To analyze the data, spectral slope values were calculated by a linear fit to different wavelength regions: S cont is the spectral slope in the whole range (continuum slope) observed for each asteroid, S VIS is the slope in the 0.55-0.80 µm range, S NIR1 is the slope in the 1.1-1.6 µm range, and S NIR2 is the slope in the 1.7-2.4 µm range. Values are reported in Table 2, together with the asteroid's physical and orbital parameters, and the Bus-DeMeo classifications (Bus &Binzel 2002b;[START_REF] Demeo | An extension of the Bus asteroid taxonomy into the near-infrared[END_REF]) derived respectively from the visible and VIS-NIR spectrum.

Band centers and depths were calculated for each asteroid showing an absorption feature, following the [START_REF] Gaffey | Mineralogical variations within the S-type asteroid class[END_REF] method. First, a linear continuum was fitted at the edges of the band, that is at the points on the spectrum outside the absorption band being characterized. Then the asteroid spectrum was divided by the linear continuum and the region of the band was fitted with a polynomial of order 2 or more. The band center was then calculated as the position where the minimum of the spectral reflectance curve occurs, and the band depth as the minimum in the ratio of the spectral reflectance curve to the fitted continuum (see Table 3).

The infrared data reveal the silicatic nature of 516 Amherstia. Its spectrum exhibits the ∼ 1 and 2 µm bands typically associated with olivine and pyroxene (Table 3). On this basis, we re-classify 516 as an S-type (it belongs to the Sq class in the Bus-DeMeo classification). For this reason, 516 will not be included in the following discussion. The asteroids in our sample show different spectral behaviors (Figs 1-5). Sixteen of them have spectra with at least one absorption feature (Table 3). The main feature identified is in the 0.9 µm region, as already reported by several authors (Clark et al. 2004a;[START_REF] Hardersen | Near-IR spectral evidence for the presence of iron-poor orthopyroxenes on the surfaces of six M-type asteroids[END_REF]2006a,b;2007a,b;[START_REF] Ockert-Bell | Observations of X/M asteroids across multiple wavelengths[END_REF], and it is attributed to low-Fe, low-Ca orthopyroxene minerals. The band center ranges from 0.86 µm (417 Suevia) to 0.97 µm (216 Kleopatra), with a band depth of 1-5 % as compared to the continuum. Some asteroids (16 Psyche,22 Kalliope,69 Hesperia,216 Kleopatra,338 Budrosa,and 498 Tokio) show an additional faint feature at ∼0.43 µm (Table 3). This band is not easily attributable: it might be associated with chlorites and Mg-rich serpentines as suggested by [START_REF] King | Spectral characteristics of chlorites and Mgserpentines using high resolution reflectance spectroscopy[END_REF] for enstatite chondrites; with pyroxene minerals such us pigeonite or augite as suggested by [START_REF] Busarev | Spectral Features of M-Asteroids: 75 Eurydike and 201 Penelope[END_REF] for M-asteroids; or to an Fe 3+ spin-forbidden transition in the iron sulfate jarosite, as suggested by Vilas et al. (1993) for low-albedo asteroids. Within our sample, [START_REF] Rivkin | 3-m spectrophotometric survey of M-and E-class asteroids[END_REF]2000) detected the 3 µm absorption band on 22 Kalliope, 55 Pandora, 110 Lydia, 129 Antigone, 135 Hertha, 201 Penelope (they proposed the W wet class for these bodies), and they suggested that this feature can be attributed to the presence of hydrated phases on their surfaces (Table 3). We detect the 0.43 µm band only on one W asteroid (22 Kalliope). If this feature is actually due to the presence of chlorites-Mg rich serpentines and/or to jarosite, this might support the interpretation of the 3 µm band as due to aqueous alteration products.

The other W asteroids do not show the 0.43 µm feature, while it is present in the spectra of two asteroids not having the 3 µm absorption (16 Psyche and 216 Kleopatra). We conclude that there is no correlation between the 0.43 and 3 µm bands.

Asteroid 132 Aethra shows a faint band centered at ∼ 0.49 µm that resembles the absorption seen on some E-type (subclass EII) asteroids [START_REF] Fornasier | Are the E-type asteroids (2867) Steins, a target of the Rosetta mission, and NEA (3103) Eger remnants of an old asteroid family?[END_REF]2008;Clark et al. 2004b), where it is attributed to sulfides such as oldhamite and/or troilite [START_REF] Burbine | The detection of troilite (FeS) on the surfaces of E-class asteroids[END_REF]2002a;2002b). We obtained only one spectrum in the visible range and no other observations are available in the near infrared nor in the 3 µm region. Bus and Binzel (2002b) gave 132 Aethra an Xe designation, and its IRAS albedo is 0.2 (Table 4).

Asteroid 135 Hertha has a faint band at ∼ 0.51 µm, narrower than that seen on EII asteroids or Aethra, and similar to the Fe 2+ spin-forbidden crystal field transitions in Earth and lunar pyroxene [START_REF] Burns | Spectral evidence for Cr 3+ , Ti 3+ , and Fe 2+ rather than Cr 2+ , and Fe31 in lunar ferromagnesian silicates[END_REF][START_REF] Adams | Interpretation of visible and near-infrared reflectance spectra of pyroxenes other rock-forming minerals[END_REF][START_REF] Hazen | Effects of compositional variation on absorption spectra of lunar pyroxenes[END_REF]. This band has been previously detected by [START_REF] Busarev | Spectral Features of M-Asteroids: 75 Eurydike and 201 Penelope[END_REF] on the surface of two other M asteroids, 75 Eurydike and 201 Penelope (however our visible spectrum of 201 does not show any feature detectable within the noise of the data). 125 Liberatrix was observed 3 times on November 20, 2004. The three spectra are shown in Fig. 1, and were obtained ∼ 1.5 hours apart, covering ∼ 75% of the Liberatrix rotational phase (period of 3.97 hours). The spectra are very similar (Fig. 1) so, in this region of the rotational phase, the body appears to have a homogeneous surface composition. There is a hint of an absorption band in the 0.9 µm region, also seen by [START_REF] Hardersen | Near-IR spectral evidence for the presence of iron-poor orthopyroxenes on the surfaces of six M-type asteroids[END_REF], however we cannot clearly identify it due to the aforementioned fringing in this region. The features at 0.48 and 0.52 µm are spurious residuals of the flat field. [START_REF] Hardersen | Near-IR spectral evidence for the presence of iron-poor orthopyroxenes on the surfaces of six M-type asteroids[END_REF] report the presence of absorption bands in the 0.9 µm region on 16 Psyche, 69 Hesperia, 110 Lydia and 216 Kleopatra (and an absence of absorption features on 325 Heidelberga, within the scatter of their data). Our spectra confirm the presence of an absorption feature in the 0.9 µm region, but our band center values are higher than those reported by [START_REF] Hardersen | Near-IR spectral evidence for the presence of iron-poor orthopyroxenes on the surfaces of six M-type asteroids[END_REF].

Comparison with Existing Literature Data

They found the band also on 125 Liberatrix and 201 Penelope, which we observed only in the visible region with the TNG telescope, and which show an indication of a faint absorption feature (Fig 1). Nevertheless, due to the fringing problems and low sensitivity of the detector in this wavelength region, we cannot clearly confirm that an absorption band is present, nor can we quantify it. The difference in the band center location might be due to different methodologies in data analysis (choice of continuum removal or imperfect removal of the telluric water vapor feature), but we cannot exclude that it is real and associated with variations in the orthopyroxene content over the asteroid's surface. [START_REF] Birlan | Spectral properties of nine M-type asteroids[END_REF] observed 9 M-type asteroids, and they did not report the presence of absorption features. Three of their objects belong also to our survey.

Our spectra of 325 Heidelberga and 860 Ursina seem in agreement with the data of [START_REF] Birlan | Spectral properties of nine M-type asteroids[END_REF], while for 558 Carmen we obtained only a spectrum in the visible range. 558 Carmen was observed also by Hardersen et al. (2006a) through about 1/3 rd of a full rotation, and their spectra exhibit a weak 0.9 µm band, attributed to low-Fe pyroxenes, with band centers ranging from 0.91 to 0.94 µm. The [START_REF] Birlan | Spectral properties of nine M-type asteroids[END_REF] spectrum is featureless, and our visible spectrum ends at 0.91 µm so it can not help with band identification (the faint absorption around 0.83 µm is due to incomplete removal of telluric bands). It is possible that asteroid 558 Carmen has an inhomogeneous surface composition.

325 Heidelberga was observed also by [START_REF] Takir | The Near-Infrared Spectroscopy of Two M-Class Main Belt Asteroids, 77 Frigga and 325 Heidelberga[END_REF] who found in its spectrum a weak (1-2% depth) feature center at 0.9 µm, while our spectrum, that of [START_REF] Birlan | Spectral properties of nine M-type asteroids[END_REF], and of [START_REF] Hardersen | Near-IR spectral evidence for the presence of iron-poor orthopyroxenes on the surfaces of six M-type asteroids[END_REF] are featureless. This asteroid may have a heterogeneous surface composition and further high S/N ratio observations covering the whole rotational period are needed for a full investigation. Hardersen et al. (2006a) detected the ∼ 0.9 µm absorption feature also on 216 Kleopatra (center at 0.91 µm, depth 3%), and on 347 Pariana (center varying from 0.88-0.93 µm with depth 3%, in spectra obtained through more than one asteroid full rotation). We confirm the detection of the band on these two asteroids (Table 3) with similar band parameters. Our data agree with the Hardersen et al. (2006a) finding also for 97 Klotho (featureless spectra in both case, and also for the Klotho data presented in Ockert- [START_REF] Ockert-Bell | Observations of X/M asteroids across multiple wavelengths[END_REF]), but disagree for 498 Tokio and 129 Antigone. In fact, for 498 Tokio, Hardersen et al. (2006a) reported a blue featureless spectrum, while our data show an increasing slope up to ∼ 0.7 µm, and then a flat spectral behavior in the near-infrared, with two faint absorption bands centered at 0.43, and 1.16 µm. On the other hand, for 129 Antigone Hardersen et al. (2006a) found absorption features at 0.76, 0.9, 1.07, and 1.39 µm, attributed to the presence of antigorite, which has absorption bands consistent with those of the asteroid. Our spectrum has a poor S/N ratio in the near-infrared range. We do not see the 0.76 or 0.9 µm bands, but a broad feature in the 0.93-1.14 µm region. The spectrum is peculiar: it has a convex shape below 0.9 µm, then it is almost flat up to ∼ 1.8 µm with a steep increase of the spectral slope in the 1.8-2.0 µm region. Ockert- [START_REF] Ockert-Bell | Observations of X/M asteroids across multiple wavelengths[END_REF] observed Antigone at different aspects, and they found the 0.9 µm band and spectral variations in continuum slope above 20% for different rotational aspects. It is thus possible that this asteroid has an inhomogeneous surface composition. Hardersen et al. (2007a;2007b;2006b) observed several asteroids reported in this survey (135 Hertha,224 Oceana,250 Bettina,369 Aeria,516 Amherstia,and 872 Holda). For 250 Bettina, our spectrum is steeper in the infrared compared to their data, but both spectra show a band in the 0.9 µm region. For 369 Aeria, Hardersen et al. found absorption bands at 0.9 and 1.9 µm, while our spectrum shows only the 0.9 µm feature and a reddish spectral behavior in the NIR with a steeper spectral slope than Hardersen et al. (2007a). 224 Oceana shows similar featureless spectra and 135 Hertha has an absorption at 0.9 µm in both surveys.

516 Amherstia shows similar ∼ 1 and 2 µm features, but they are deeper in our spectrum compared to Hardersen et al (2006b). 872 Holda shows a broad and weak feature in both surveys centered at ∼ 0.96 µm. Hardersen et al. (2006b) found that Holda is spectrally similar to synthetic troilite [START_REF] Cloutis | The spectral properties of troilite/pyrrhotite and implication s for the E-asteroids[END_REF], and proposed for the surface composition of this asteroid a mixture of NiFe metal and troilite.

Four M-types (16 Psyche, 110 Lydia, 216 Kleopatra, and 785 Zwetana) of the Clark et al. (2004a) survey of the X complex were re-studied in this work. The spectral behavior of 110 Lydia and 216 Kleopatra looks quite similar, while for 16

Psyche our data show a linear NIR spectrum with an higher spectral gradient than that reported in Clark et al. (2004a), and for 785 Zwetana our NIR spectrum is almost linear and featureless, while Clark et al. (2004a) reported a convex shape spectrum and detected a feature at ∼ 0.9µm. Psyche was observed also by [START_REF] Binzel | Rotationally Resolved Spectra of Asteroid 16 Psyche[END_REF] over its rotation, and they did not find any significant variation in its visible reflectance spectrum within their measurement precision of 1%.

Zwetana was observed also by Hardersen et al. (2007a), who detected in its spectrum a broad and faint absorption beyond 1.3 µm, but no features in the 0.9-1.0 µm region, and by Ockert- [START_REF] Ockert-Bell | Observations of X/M asteroids across multiple wavelengths[END_REF], who presented a featureless spectrum with no significant slope variations at different aspects.

755 Quintilla shows a very peculiar spectrum with several absorption bands (Fig 5, Table 3). The bands at 0.9 and 1.86 µm can be attributed to orthopyroxene, however the peculiar features at 1.37 and 1.61 µm are not easily interpreted. It is worth noting that Fieber-Beyer et al. ( 2006) find three absorption features in a Quintilla spectrum but at different central wavelengths. They interpreted bands at 0.93, and 1.7 µm as due to spinel, while the nature of the 1.59 µm band was unknown.

In sum, comparing our spectra with those in the existing literature, we suggest that asteroids 129 Antigone, 325 Heidelberga, 498 Tokio, and 785 Zwetana may display surface variability as they show different spectral behaviors throughout independent observations. Some differences in the spectral slope values or band center positions are seen also for asteroids 16 Psyche, 250 Bettina, 369 Aeria, 516 Amherstia, 558 Carmen, and 755 Quintilla, but we cannot clearly state if they are due to real variation or simply to observational uncertainty and/or differences in data acquisition and reduction. Additional high S/N ratio observations are needed to investigate possible surface inhomogeneities for these asteroids.

From the Tholen to the Bus-DeMeo taxonomy [HERE TABLE 4]

Our targets were classified as belonging to the Tholen M class on the basis of their spectra from 0.4 to 1 µm and their moderate albedo values. Our new spectral observations enrich the available physical information for these objects and allow us to apply the Bus-DeMeo classification recently published [START_REF] Demeo | An extension of the Bus asteroid taxonomy into the near-infrared[END_REF].

The Bus-DeMeo system is based on the asteroid's spectral characteristics over the wavelength range 0.45 to 2.45 µm without taking the albedo into consideration. Indeed, the observed Tholen M-type asteroids show different spectral behaviors in the near infrared range. We therefore re-classified our 24 targets observed in the V+NIR range according to the Bus-DeMeo taxonomy and the corresponding classes are summarized in Table 2.

Thirteen of the 24 Tholen M-type asteroids we investigated are Xk-types in the Bus-DeMeo taxonomy. Asteroids belonging to this class have spectra characterized by a shallow absorption feature that appears as a concavity over the range 0.8 to 1.0 µm, in what is otherwise a generally linear spectrum. Indeed, all 13 asteroids classified as Xk (Table 2) show a band in the 0.9 µm region (Table 3) attributed to low-Fe, low-Ca orthopyroxene minerals. Their albedo value varies between 0.12 and 0.21, except for 2 asteroids, where it is higher (250 Bettina has an albedo of 0.25 and 55 Pandora of 0.30). Some of the Xk asteroids investigated show additional features as summarized in Table 4. A 0.43 µm band is seen in the Xk asteroids 16, 22, 69, 216, and 338. Considering the asteroids' moderate albedo values, this band might be due to chlorites and Mg-rich serpentines or pigeonite or augite minerals.

For 755 Quintilla, the classification as Xk type seems to be questionable since Quintilla presents other features at 1.37, 1.61, and 1.87 µm, as previously mentioned (Table 4). Finally, 135 Hertha also shows a faint feature at 0.51 µm probably due to Fe 2+ spin-forbidden crystal field transitions in pyroxene.

Five of the observed asteroids (Table 2) have a slightly reddish spectrum, featureless in the 0.45-2.4 µm range, and belong to the Xc-type in the Bus-DeMeo taxonomy. Among them only 498 Tokio has a low albedo value compatible with carbonaceous chondrite analogs. It is the only Xc type that also show a feature at 0.43 µm, that we interpret as due to the iron sulfate jarosite, and a broad absorption in the 1-1.3 µm region usually seen in C-type asteroids (Table 4). The Xc type 129 Antigone has a band centered at 1.03 µm possibly due to the phyllosilicate antigorite, as suggested by Hardersen et al. (2006a).

The four asteroids 325, 441, 785, and 860 have featureless spectra and follow the X class. Their albedo values vary between 0.11-0.16.

According to these new near infrared observations 516 Amherstia must be reclassified as an S-type in the Tholen classification and an Sq-type in the Bus-DeMeo system. The asteroid 849 Ara has a steep spectral slope and falls in the D class according to the Bus-DeMeo taxonomy (Table 2), but its albedo is very high (0.27), excluding a surface composition of organic-rich silicates, carbon, and anhydrous silicates as commonly expected on low-albedo Tholen D type asteroids.

Meteorite Spectral Matches

Methodology

To constrain the possible mineralogies of our asteroids (those with both visible and near infrared spectra), we conducted a search for meteorite and/or mineral spectral analogs. We used the publicly available RELAB spectrum library [START_REF] Pieters | Strength of mineral absorption features in the transmitted component of near-infrared reflected light: First results from RELAB[END_REF], which consisted of nearly 15,000 spectra in November of 2008. For each spectrum in the library, a filter was applied to find relevant wavelengths (0.4 to 2.45 µm). Then, a second filter was applied to reject spectra with irrelevant albedo values (i.e. brightness at 0.55 µm too high or too low). This process produced a list of approximately 4000 RELAB spectra of appropriate brightness and wavelength coverage for comparison to the asteroid. RELAB spectra were normalized to 1.0 at 0.55 µm, and then a Chi-squared value was calculated relative to the normalized input asteroid spectrum. We used the asteroid wavelength sampling to resample the laboratory spectra by linear interpolation. This allowed a leastsquares calculation with the number of points equal to the wavelength sampling of the asteroid spectrum. The RELAB data files were sorted according to Chisquared, and then visually examined for dynamic weighting of spectral features by the spectroscopist. Given similar Chi-squared values, a match that mimicked spectral features was preferred over a match that did not. We visually examined the top 50 Chi-squared matches for each asteroid. No specific grain-size sampling filter was applied, and many of the best matches found were of slab or sanded slab (or rough-cut) surfaces.

The importance of our second filter is that we constrained the search for analogs to those laboratory spectra with brightness (reflectance at 0.55 µm) roughly comparable to the albedo of the asteroid. For the asteroid albedo value, we gen-erally used the IRAS albedo published by [START_REF] Tedesco | The supplemental IRAS minor planet survey[END_REF]. For the darker asteroids (below 10% albedo), this is a fairly well-known number. The uncertainty for brighter asteroid albedos (above 10% albedo) is relatively larger. This is because at equivalent geocentric and heliocentric distances, a darker asteroid will have stronger thermal flux, allowing a more precise and more accurate albedo determination. To account for this, we filtered out lab spectra if brightness differed by more than ± 3% in absolute albedo for the darker asteroids in our sample (albedo less than 10%), and we filtered out lab spectra if brightness differed by more than ± 5% in absolute albedo for the brighter asteroids (albedos greater than 10%). For example, if an asteroids albedo was 20%, we searched over all lab spectra with 0.55 µm reflectance between 15% and 25%. If an asteroid's albedo was 7%, we searched over all lab spectra with reflectance between 4% and 10%. Once the brightness filter was applied, all materials were normalized before comparison by least-squares. We note that mismatches between asteroid albedo and meteorite 0.55 µm reflectance occurs for a few well-known meteorite-asteroid pairings, such as Vesta (albedo about 40%) and HEDs (many of which have 0.55 micron reflectances of 25-30%). The last filter to be applied was a "no lunar sample" filter, and this will be explained further in the next section. Our search techniques effectively emphasized the spectral characteristics of brightness and shape, and de-emphasized minor absorption bands and other parametric characteristics.

Meteorite Analogs [HERE TABLE 5]

[ HERE FIGURES 6,7 and 8] The best matches between our observed sample and meteorites (from the RE-LAB database) are reported in Table 5 and in Figs 6, 7, and 8. Most best matches resulted in iron meteorites (IM), pallasites (Pall), or enstatite chondrites (EC), as suggested in the literature for the M-type asteroids. Some meteorite matches are quite good, while for some asteroids (16 Psyche, 22 Kalliope, 69 Hesperia, 216 Kleopatra 755 Quintilla, 872 Holda) the best meteorite match we found does not satisfactorily reproduce both the visual and near infrared spectral behavior.

The RELAB spectral library does not contain CB meteorite spectra, so the suggested analogue of CB Bencubbinites [START_REF] Hardersen | Near-IR spectral evidence for the presence of iron-poor orthopyroxenes on the surfaces of six M-type asteroids[END_REF]) remains spectrally untested. In contrast, the RELAB spectral library contains many samples of other carbonaceous chondrites, only one of which was found as a best match in this study (498 Tokio was best fit with a CV carbonaceous chondrite, but its albedo is anomalously low (7%) within the M class). This would seem to provide evidence against the suggestion of Vilas (1994) that carbonaceous chondrites are analogues for the M-type asteroids. In fact, CV chondrites are usually associated with K-type asteroids (Mothé-Diniz et al. 2008;[START_REF] Clark | Spectroscopy of K-complex Asteroids: Parent Bodies of Carbonaceous Meteorites?[END_REF], or with B-type asteroids [START_REF] Clark | Metal Silicate Mixtures: Spectral Properties and Applications to Asteroid Taxonomy[END_REF].

The only asteroid to result in an enstatite chondrite best match is 161 Athor.

Neither the asteroid nor the enstatite chondrites (in general) show absorption features, so the match is purely based on spectral slope and albedo.

Iron meteorites matched both featureless asteroids (69 Hesperia, 97 Klotho, 224 Oceana, 325 Heidelberga, 849 Ara) and asteroids with bands (16 Psyche, 216 Kleopatra, 110 Lydia, 135 Hertha, 338 Budrosa, 347 Pariana, 369 Aeria, 860 Ursina, and 872 Holda). In particular, we note that RELAB file c1mb46 (of the iron meteorite Landes which has silicate inclusions) was selected as a best fit for 5 different asteroids (135 Hertha,224 Oceana,347 Pariana,22 Kalliope,and 872 Holda). This spectrum shows a faint absorption band at 0.9 µm, probably due to the silicate inclusions. Of concern, of course, is the fact that this sample was measured as a slab, not as a particulate.

Pallasites were the best matches for 785 Zwetana (no bands), 110 Lydia, 250 Bettina, 22 Kalliope, and 55 Pandora (all with bands). Several asteroid matches resulted in different RELAB spectral files (c1mb43, ckmb43, and cmmb43) of the same meteorite, the stony-iron pallasite Esquel. The differences between these spectra are that c1mb43 is of a sanded metal area on an Esquel slab, measured from one incidence angle, while cmmb43 is of the same sanded area measured from two different incidence angle directions (averaged), and ckmb43 is a metalrich powder of the meteorite with a grain size less than 63 µm. In a recent paper, [START_REF] Cloutis | Reflectance spectra of iron meteorites: Implications for spectral identification of their parent bodies[END_REF] study in detail the physical reasons for the spectral differences observed as a function of sample preparation. This example illustrates a typical issue with a library database search. The same sample can look spectrally different depending on measurement illumination and grain size circumstances. Such spectral differences result not from compositional differences, but from measurement circumstances. This indicates that the spectral differences between the asteroids found to match these meteorites (110 Lydia, 22 Kalliope, and 55 Pandora), are insignificant according to the spectral library.

We note that the peculiar object 516 Amherstia (which we have suggested be reclassified as an S-type) was found to be similar to a laser-irradiated ordinary chondrite (OC). Similarly, asteroid 441 Bathilde was best fit with a laser-irradiated sample of olivine. Unlike 516 Amherstia, 441 Bathilde does not show absorption features, so the spectral match is based on slopes, shape, and brightness only. Nevertheless, we note with amusement that highly space weathered materials cannot be ruled out as a surface composition for 441 Bathilde, based on the excellent agreement between the asteroid and the laboratory spectra of a simulated weathered olivine.

M-type asteroid spectra, being largely featureless, were very often fit with a lunar soil sample during the automated part of our RELAB search. For example, 55 Pandora, which has a band at 0.9 µm, was well fit with RELAB file csls65, which is a spectrum of Apollo program sample 61221,114, a lunar highland soil from the Apollo 16 landing site. Like M-type asteroids, lunar soils tend to be very steeply spectrally sloped. When they are most mature (highly space-weathered), lunar soils tend to show only very minor silicate absorption features, like M-type asteroids. Thus, without applying a "no lunar sample" filter in our search, we would have been led to the conclusion that some M-type asteroids are covered with lunar-soil-like materials.

In sum, our method of searching for meteorite analogues emphasized the spectral characteristics of brightness and shape, and de-emphasized minor absorption bands. Automated Chi-squared algorithms will always result in such an emphasis.

We attempted to account for this by selecting fits with good band mimickry over poor band mimickry when Chi-squared values were comparable, however there are some cases where weak (1-2%) bands are not faithfully matched (e.g. 110 Lydia). Only spacecraft mission sample returns will ultimately allow us to answer the question "Could a 'featureless' iron meteorite be considered an analogue of an asteroid which has a band?"

Statistics

[HERE TABLE 6] [HERE FIGURE 9]

We ran a Spearman Rank Correlation [START_REF] Spearman | The proof and measurements of associations between two things[END_REF] to search for possible correlations between our sample's spectral characteristics and their orbital and physical parameters. The Spearman correlation function gives a two-element vector containing the rank correlation coefficient (ρ) and the two-sided significance of its deviation from zero (P r ). The value of ρ varies between -1 and 1; if |ρ| is close to zero, then there is no correlation and if |ρ| is close to 1, then a correlation exists.

The significance (P r ) is a value in the interval 0 < P r < 1. A small value indicates a significant correlation. We consider a strong correlation to have P r < 0.01 and |ρ| > 0.6, and a weak correlation to have P r < 0.08 and 0.3 < |ρ| < 0.6.

Our sample includes 29 asteroids observed in the visible range. We eliminated 516 Amherstia from all further analysis because spectral properties diverge completely from all others. In fact, it displays strong 0.9 and 1.9 µm features, putting it into the Sq-class in the Bus-DeMeo system. 23 asteroids were observed both in the visible and near infrared range. We analyse the whole sample as well as the subsamples we consider most likely to be the most metal rich (those without the 0.9 µm absorption band) and those containing silicates, presenting the 0.9 µm band, and belonging to the Xk type in the Bus-DeMeo taxonomy.

Among our Spearman Rank Correlation test results, we find weak anti-correlations between the slopes S cont , S NIR1 , and S NIR2 , and the asteroids' rotational period (Table 6). [START_REF] Belskaya | Physical properties of M class asteroids[END_REF] found that M-asteroids have a faster mean rotational period compared to S and C type asteroids. They interpreted this as an indication of an higher mean density that allows M-asteroids to survive energetic collisions without disruption, which increases their rotational angular momentum. One possible explanation is that fast rotators have a higher density, so they have a composition metal rich, which is characterized by steeper spectral slope. The slow M-type rotators may have a smaller mean density and so a smaller amount of metals in their composition, which is characterized by a less steep spectral slope.

From our Spearman Rank Correlation tests, there also appear to be significant correlations between slope and semi-major axis, and albedo (Table 6). The continuum slope (S cont ) and the 1.1-1.6 µm slope (S NIR1 ) are correlated with semi-major axis a, while the slope in the visible range (S VIS ) is only weakly correlated with a (Fig. 9). Asteroids in our sample with the 0.9 µm feature show a correlation between S cont versus a, and a weaker corelation between S NIR1 versus a. It must be noted that our sample size is small.

Considering all 29 asteroids in our sample, a weak anti-correlation exists

between the S VIS and the albedo, in particular the anti-correlation becomes stronger for asteroids not showing the silicatic band at 0.9 µm. So the visual slope tends to increase with decreasing albedo.

The correlation of the continuum slope to the semi-major axis together with the anti correlation of the visible slope to albedo indicates that our sample derived from Tholen M-type asteroids tend to be dark in albedo and red in near-infrared slope with increasing semi-major axis. This is true in particular for 'featureless' subset of our sample that we consider most likely to be iron rich. Darkening of albedo plus reddening of the slope is a typical effect of space weathering seen on lunar samples and silicate rich asteroids. Using statistical arguments, [START_REF] Lazzarin | Space Weathering in the Main Asteroid Belt: The Big Picture[END_REF] have shown that space weathering probably occurs on all asteroid types. Nevertheless, space weathering effects have not been fully investigated for metallic rich surfaces. If space weathering acts on M-type asteroids as it does with S type asteroids, then asteroids that are closer to the Sun might be expected to show stronger spectral changes because of the higher abundance of solar wind ions. Our results indicate an opposite trend. A few measurements exist on the effect of space weathering on enstatite chondrite and iron and stony iron meteorites (e.g. [START_REF] Vernazza | Plausible parent bodies for enstatite chondrites and mesosiderites: Implications for Lutetia's fly-by[END_REF]). An irradiated sample of the Eagle enstatite chondrite meteorite revealed a slight reddening of the spectral slope and no more than 3% darkening of the visual albedo. For the Vaca Muerta mesosiderite meteorite [START_REF] Vernazza | Plausible parent bodies for enstatite chondrites and mesosiderites: Implications for Lutetia's fly-by[END_REF] found again a reddening in the spectral slope and a darkening of the visual albedo. Grain size effects can play an important role in the slope changes as well. [START_REF] Britt | Bidirectional reflectance properties of ironnickel meteorites[END_REF] studied the bidirectional reflectance properties of some iron-nickel meteorites for different surface roughness. They found that the spectra of M-type asteroids show good agreement with those of iron meteorites with surface features in the range of 10 µm to 1 mm, that is larger that the wavelength of incident light. Meteorites with these roughness values are diffuse reflectors and show the classic red slope continuum of iron, with pratically no geometric dependence on reflection. On the other hand, a decreasing of the meteorite surface roughness changes the reflectance characteristics: complex scattering behaviour is seen for roughness in the 0.7-10 µm range, while for roughness values < 0.7 µm the reflectance is characterised by two distinct components, the specular one which is bright and red sloped, and the nonspecular one which is dark and flat. Moreover, Cloutis et al. (1990) found that the spectra of three iron meteorites reddened with increasing grain size.

Thus, the correlation of the continuum slope to the semimajor axis together with the anti correlation of near-infrared slope to albedo are not easily interpreted. They may reflect a different surface composition of different M-type asteroids, and/or a combination of surface composition, grain size and space weathering effects that

ACCEPTED MANUSCRIPT

are not yet fully understood.

Discussion

There are 30 asteroids having M-type visible wavelength and albedo properties that were investigated in this study. One was removed due to its similarity to an S-type asteroid (516 Amherstia). Of the remaining 29 asteroids, 16 have detectable faint absorption features. The shape and slopes in the continuum, visible, and near infrared varies greatly when Tholen M-class objects have their spectral measurements extended. All were found to be classifiable in the extended wavelength taxonomy defined by DeMeo et al. (2009). Our search for meteorite analogs concluded that the shape of the spectra could be modeled by a variety of iron and stony iron meteorites, as well as pallasites, enstatite chondrites, and irradiated mixtures. However, this method of search de-emphasizes the small absorption features. To investigate the significance of the shallow absorption features found in our sample, we used a linear (geographical) spectral mixing model.

One possible avenue for investigation is the mixture of the totally featureless, red-sloped metallic meteorites with orthopyroxenes, which are thought to create the weak 0.9 µm feature. To constrain the surface composition of our sampled asteroids and the abundance of silicate material needed to reproduce the weak 0.9 µm band seen on some spectra, we created spatially segregated (geographical) mixtures of several terrestrial and meteoritic materials in different grain sizes. We considered endmembers from among all the samples included in the US Geological Digital Spectral Library http : //speclab.cr.usgs.gov/spectrallib.html) and in the RELAB database. The synthetic spectra that we created were compared with the asteroid spectra using the known IRAS albedo and the V+NIR spectral behavior and slope as constraints. A few percent (less than 2%) of orthopyroxenes or hydrated silicates (goethite) added to iron meteorites allowed us to reproduce the weak spectral feature around 0.9 µm. Unfortunately, the synthetic spectra did not match both albedo, spectral slope, and band depth and center of the observed asteroids. Moreover, synthetic spectra do not give unique results, as we can obtain similar spectra by adding more components or changing the grain size properties of minerals.

[ HERE FIGURE 10] In Fig. 10 we show an example of two different mixtures proposed for the asteroid 22 Kalliope. The solid line represents a mixture with 98% pallasite (RE-LAB file ckmb43) and 2% goethite, the dashed line shows a mixture of the same meteorite enriched with 1% of orthopyroxene (grain size of 25-45 µm). The spectral behaviour of both mixtures does not completely match all the spectral characteristics of 22 Kalliope. Figure 10 emphasizes our results that a) it is not easy to fully reproduce the 0.9 µm band, asteroid albedo and spectral slope simply enriching an iron meteorite with some silicates, b) the 0.9 µm band could be reproduced both with a small amount of anhydrous silicates such as orthopyroxene, but also with some hydrated silicates like goethite. This last possibility cannot be completely ruled out. It would support the hypothesis of the presence of hydrated materials on some M-type asteroids, as inferred by [START_REF] Rivkin | 3-m spectrophotometric survey of M-and E-class asteroids[END_REF]2000) with the detection of the 3 µm band. Cloutis et al. (1990) identified the 0.9 µm feature in the spectra of some enstatite chondrites. However, the shape of the spectra in our study is similar to iron meteorites, which are featureless. [START_REF] Feierberg | Spectroscopic evidence for undifferentiated S-type asteroids[END_REF] have shown that the physical state of the surface plays an important role in the relative contribution of metal to the spectrum of a silicate-metal mixture. If the surface is spotty with separate areas of pure silicate and pure metal, the spectrum differs very little from that of pure silicate, even in the case of 50% metal content. But if metal and pyroxene particles were mixed, the presence of 50% metal almost completely suppresses the pyroxene bands and the spectrum appears to be indistinguishable from that of the pure metal. Thus, our spectra could have a fairly high concentration of silicate materials, as long as the surface composition is well-mixed.

The results of our search for meteorite analogs confirm that these objects originally in the Tholen M-class could be the parent bodies both of iron (or stony-iron) meteorites and enstatite chondrites. Note that the enstatite chondrites were supposed to have formed close to the Sun (1-1.4 AU) [START_REF] Shukolyukov | Manganesechromium isotope systematics of enstatite meteorites[END_REF]). Together with aubrites, enstatite chondrites are the only group of meteorites that have the same oxygen isotopic composition as the Earth-Moon system [START_REF] Clayton | Oxygen isotopic compositions of enstatite chondrites and aubrites[END_REF]. It is excluded that the parent bodies of enstatite chondrites still reside so close to the Sun, but they may have been scattered into the main belt (∼ 2.2-3.0 AU), as described by the [START_REF] Bottke | Iron meteorites as remnants of planetesimals formed in the terrestrial planet region[END_REF] migration scenario. The same hypothesis has been proposed by [START_REF] Bottke | Iron meteorites as remnants of planetesimals formed in the terrestrial planet region[END_REF] for iron meteorites, that is iron meteorite parent bodies that formed in the terrestrial planet region with fast accretion times. This allowed small planetesimals to melt early in Solar System history by the decay of short-lived radionuclides, then be scattered in the main belt.

Conclusions

We observed and analyzed spectra obtained in the visible and near infrared wavelength range (0.4-2.5 µm) of 30 asteroids selected according to their original classifications as M-types, following the Tholen (1984) taxonomy. The results of our investigation are the following:

• Tholen M-type asteroids show a large variety of spectral behaviors when their spectral coverage is extended into the near-infrared.

• Several weak absorption bands have been identified in extending the spectral range for objects defined as Tholen M-types, showing that their surface compositions are not exclusively metallic. In particular on several asteroids we identify a weak feature (1-5 % depth) in the 0.9 µm wavelength region attributed to low-Fe, low-Ca orthopyroxene minerals. All these asteroids belong to the Xk class in the Bus-DeMeo taxonomy. Some asteroids show a band at 0.43 µm (16 Psyche, 22 Kalliope, 69 Hesperia, 216 Kleopatra, 338 Budrosa, and 498 Tokio) that can be attributed to chlorites and Mg-rich serpentines, or to pigeonite or augite, or to an Fe 3+ spin-forbidden transition in the iron sulfate jarosite. 132 Aethra shows a 0.49 µm band typical of the Xe type (following the Bus& Binzel (2002b) taxonomy) probably due to sulfides such as oldhamite and/or troilite. 135 Hertha has a faint band at ∼ 0.51 µm similar to the Fe 2+ spin-forbidden crystal field transitions in Earth and lunar pyroxene. The spectrum of 755 Quintilla is peculiar with four absorption bands: 0.9 and 1.9 µm features could be attributed to pyroxene silicates, but the origins of the 1.37 and 1.61 µm bands are unknown.

• A comparison with the published literature reveals that at least 4 asteroids (129 Antigone, 325 Heidelberga, 498 Tokio, and 785 Zwetana) display surface variability. Seven additional objects here investigated (16 Psyche,201 Penelope,250 Bettina,369 Aeria,516 Amherstia,558 Carmen,and 755 Quintilla) have slightly different spectral slope values or band center positions as compared to the data already published in the literature. These variations may be real but we cannot exclude that they are simply due to the observational uncertainties or to different data acquisition and reduction processes.

• We performed a search of meteorite and/or mineral spectral matches between our asteroid sample and the RELAB database. We confirm that these objects could be the parent bodies both of iron (or stony-iron) meteorites and enstatite chondrites, as suggested in the literature. Only 498 Tokio, which has an anomalous low albedo value (7%) within the Tholen definition for the M-type population, is best matched by a CV carbonaceous chondrite.

• Our attempt to model the asteroid spectra with mixtures of iron/stony iron meteorites enriched with a few percent silicates did not match the albedo and spectral characteristics of our sample. The amount of silicates needed to reproduce the weak 0.9 µm feature seen on some asteroids is very small, less than 2%.

• A statistical analysis of our sampled asteroids' spectral characteristics versus orbital elements and physical parameters indicated a possible correlation between the continuum slope and semi-major axis, and two anti-correlations between visible slope to albedo and continuum slope to rotational period.

Our asteroid sample derived from the Tholen M-type tends to be dark in albedo and red in near infrared slope with increasing value of the semimajor axis. Fast M-type rotators tend to be redder than slow rotators.

Table 1. Observational circumstances for the observed sample, for which their visible wavelength spectra and albedos all satisfy the definition of M-type in the Tholen taxonomy. Solar analog stars named "hip" come from the Hipparcos catalogue, "la" from the Landolt photometric standard stars catalogue, and "HD" from the Henry Draper catalogue Table 3: Band center, depth and width for the features detected on the asteroid spectra. The results of the 3 µm band investigation from [START_REF] Rivkin | 3-m spectrophotometric survey of M-and E-class asteroids[END_REF][START_REF] Rivkin | The nature of M-class asteroids from 3-m observations[END_REF] are also reported. M with the 0.9 µm band (Xk) M without the 0.9 µm band

Figure captions Fig. 1 -Fig. 2 -

 captions12 Figure captions

Fig. 3 -

 3 Fig. 3 -Observed spectra for Tholen M-type asteroids extended into the near-infrared.

Fig. 4 -

 4 Fig. 4 -Observed spectra for Tholen M-type asteroids extended into the near-infrared.

Fig. 5 -

 5 Fig. 5 -Observed spectra for Tholen M-type asteroids extended into the near-infrared.

Fig. 6 -

 6 Fig.6-Spectral matches between the featureless observed asteroids and meteorites from the RELAB database (see Table5for details on the meteorites samples).

Fig. 7 -

 7 Fig. 7 -Spectral matches between the observed asteroids showing the 0.43 µm feature and meteorites from the RELAB database (see Table 5 for details on the meteorites samples). In the Bus-Demeo taxonomy all these asteroids, except 498 Tokio (Xc-type), belong to the Xk-type and show also a band in the 0.9 µm region Fig. 8 -Spectral matches between the observed asteroids having absorption features beyond 0.8 µm and meteorites from the RELAB database (see Table 5 for details on the meteorites samples). All asteroids belong to the Xktype in the Bus-Demeo taxonomy, except 129 Antigone (Xc-type), and 516

Fig. 9 -

 9 Fig. 9 -Visual and NIR1 slopes versus the semi-major axis, albedo and rotational period for the observed asteroids. Black bullet represent the asteroids showing a band in the 0.9 µm region and belonging to the Xk-type in the Bus-DeMeo taxonomy. Fig. 10 -Geographycal mixtures proposed to interpret the surface composition of 22 Kalliope: 98% pallasite and 2% goethite (continuous line, mixture albedo = 0.14); 99% pallasite and 1.0% orthopyroxene (dashed line, mixture albedo = 0.14).

Figures

  Figures

  Figure 9:

Table 2 :

 2 Physical and orbital parameters of the M asteroids observed and selected on the basis of the Tholen taxonomy[START_REF] Tholen | Asteroid taxonomy from cluster analysis of photometry[END_REF]. The Bus and Bus-DeMeo classifications are also reported, together with the spectral slopes value (S

	µm
	VIS calculated for the 0.55-0.8

Table 4 :

 4 Summary of the features observed on the asteroids spectra and possible mineralogical interpretation.The Bus-DeMeo (BD) taxonomy is also reported. CFT=crystal field transitions.

	Asteroid	Band center (µm) Depth (%) Width (µm) 3 µm band
	16 Psyche	0.949±0.008	2.9	0.898-1.020	No
	16 Psyche	0.430±0.004	1.6	0.418-0.442	No
	22 Kalliope	0.903±0.008	2.5	0.756-1.017	Yes
	22 Kalliope	0.434±0.005	1.2	0.407-0.456	Yes
	55 Pandora	0.910±0.010	1.7	0.790-0.970	Yes
	69 Hesperia	0.951±0.009	2.6	0.830-1.080	-
	69 Hesperia	0.430±0.004	1.5	0.417-0.443	-
	110 Lydia	0.942±0.008	2.0	0.857-1.033	Yes
	125 Liberatrix	-	-	-	No
	129 Antigone	1.028±0.010	1.5	0.929-1.136	Yes
	132 Aethra	0.498±0.004	2.0	0.456-0.537	-
	135 Hertha	0.905±0.008	1.9	0.771-1.042	Yes
	135 Hertha	0.515±0.005	1.2	0.497-0.531	Yes
	161 Athor	-	-	-	No
	201 Penelope	-	-	-	Yes
	216 Kleopatra	0.969±0.008	3.2	0.815-1.104	No
	216 Kleopatra	0.429±0.004	3.2	0.411-0.446	No
	250 Bettina	0.885±0.010	2.0	0.819-0.972	-
	338 Budrosa	0.876±0.010	2.1	0.766-0.981	-
	338 Budrosa	0.425±0.004	4.2	0.407-0.446	-
	347 Pariana	0.871±0.008	2.4	0.780-0.950	-
	369 Aeria	0.884±0.008	1.9	0.772-0.987	No?
	498 Tokio	0.430±0.005	1.6	0.413-0.449	-
	498 Tokio	1.159±0.008	2.5	0.905-1.384	-
	516 Amherstia	0.965±0.008	6.8	0.750-1.440	-
	516 Amherstia	1.949±0.010	6.1	1.702-2.193	-
	755 Quintilla	0.904±0.010	4.9	0.760-1.010	-
	755 Quintilla	1.864±0.010	3.7	1.699-2.062	-

Table 5 :

 5 The best matches between the 24 asteroid sample observed in the visible and near infrared range and meteorites from the RELAB database. χ 2 values

noted with an asterisk indicate asteroids for which the best RELAB fit matches only the slope and albedo but not the 0.9 µm feature.

	χ 2 value	6.46667e-04 *	1.84672e-03	4.15916e-03	8.74344e-04 *	5.14856e-04	4.52068e-04 *	3.05211e-03	8.47345e-04	1.46184e-03	1.70951e-03	1.26472e-03	1.18340e-03 *	7.68864e-04	1.31101e-03	9.04683e-04	1.98982e-03 *	2.18014e-03	1.27790e-03	5.49437e-04	1.15605e-02 *	8.76718e-04	4.08895e-03	6.02053e-04	2.03065e-03 *
	Grain Size (µm)	particulate	slab	sanded slab	particulate	<300	<63	45-90	slab	<300	<25	slab	-	filings	particulate	slab	particulate	>125	<100	>300	<25	<63	thin section	particulate	slab
	Met. Albedo	0.13	0.16	0.29	0.12	0.24	0.14	0.11	0.17	0.17	0.11	0.16	0.25	0.15	0.13	0.17	0.13	0.12	0.08	0.14	0.12	0.14	0.22	0.13	0.16
	Met. Class	IM	IM	Pall	IM	IM	Pall	Iron-Nickel	IM	EC	IM	IM	Pall	IM	IM	IM	IM	Laser Irr Ol	CV	OC*	IM	Pall	Iron-Nickel	IM	IM
	Met. name	MET101A	Landes	Esquel	MET101A	Babb's Mill	Esquel	AWA101	Landes	Pillistfer	DRP78007	Landes	Y-8451,20	Brown (filings)	MET101A	Landes	MET101A	-	Vigarano	Tsarev	DRP78007	Esquel	Metal in ALH84019	MET101A	Landes
	RELAB file	eac/sc/s1sc99	txh/mb/c1mb46	txh/mb/cmmb43	eac/sc/lasc99	mjg/mr/cgp017	txh/mb/ckmb43	eac/sc/c1sc14	txh/mb/c1mb46	mjg/mr/mgn021	txh/mb/cfmb47	txh/mb/c1mb46	txh/mb/r4mb39	cmp/mi/c1mi08	eac/sc/lasc99	txh/mb/c1mb46	eac/sc/c1sc99	atb/ma/c1ma47	txh/mb/ncmb59	atb/ma/c1ma53	txh/mb/cfmb47	txh/mb/ckmb43	tjm/tb/c1tb56	eac/sc/c1sc99	txh/mb/c1mb46
	Albedo	0.12	0.14	0.30	0.14	0.23	0.18	0.16	0.14	0.20	0.12	0.17	0.26	0.11	0.18	0.18	0.19	0.14	0.07	0.16	0.16	0.12	0.27	0.17	0.22
	Asteroid	16 Psyche	22 Kalliope	55 Pandora	69 Hesperia	97 Klotho	110 Lydia	129 Antigone	135 Hertha	161 Athor	216 Kleopatra	224 Oceana	250 Bettina	325 Heidelberga	338 Budrosa	347 Pariana	369 Aeria	441 Bathilde	498 Tokio	516 Amherstia	755 Quintilla	785 Zwetana	849 Ara	860 Ursina	872 Holda

Table 6 :

 6 Significant Spearman Correlations (the most significant ones are in bold).

	ρ	P r	n

S NIR1 vs rot (all) -0.482213 0.0197941 23

	S NIR1 vs rot (with the 0.9µm band)	-0.0274725 0.929013 13
	S NIR1 vs rot (without the 0.9µm band) -0.466667	0.173939 10
	S NIR2 vs rot (all)	-0.516926 0.0115455 23
	S NIR2 vs rot (with the 0.9µm band)	-0.478022 0.0984901 13
	S NIR2 vs rot (without the 0.9µm band) -0.547115	0.101678 10
	S VIS vs alb (all)	-0.486513 0.00744684 29
	S VIS vs alb (with the 0.9µm band)	-0.329670	0.271335 13
	S VIS vs alb (without the 0.9µm band)	-0.635294 0.00818205 16
		52

Acknowledgments

We thank Prof R. Binzel and an anonymous referee for their very useful comments that help us improving the paper. We thank Dr. F. DeMeo for her help in the classification of the observed asteroids in her taxonomy. Taxonomic-type results presented in this work were determined in part using a Bus-DeMeo Taxonomy Classification Web tool developed by Stephen M. Slivan at MIT with the support of National Science Foundation Grant 0506716 and NASA Grant NAG5-12355. This research utilizes spectra acquired with the NASA RELAB facility at Brown University. This paper is in part based on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Centro Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. BEC thanks the Observatoire de Paris for kind hospitality during her sabbatic from Ithaca College in the Spring of 2009.


Maria Luisa Moriconi
email: m.moriconi@isac.cnr.it

J I Lunine

A Adriani

E D'aversa

A Negrão

G Filacchione

A Coradini

Characterization of Titan's Ontario Lacus region from Cassini/VIMS observations

Keywords: Titan, Satellite, surfaces, Infrared observations, Image processing

Liquid hydrocarbons were long predicted on Titan's surface before the RADAR instrument onboard Cassini detected lakes poleward of 70°N in July 2006. Before that the Cassini Imaging Science Subsystem (ISS) observed a lake-like feature in the South Pole, named Ontario Lacus, in July 2004. Here we analyze one observation of Ontario Lacus taken by the Visual and Infrared Mapping Spectrometer (VIMS) on 2007 December 5, during the T38 flyby. This is the best spatially resolved image of a Titan lake to date by an imaging spectrometer, and has been previously reported in Brown et al. composition and morphology is developed with the help of ISS and RADAR observations. The simplest model consistent with the data is an optically deep lake surrounded by a region in which ethane, propane, possibly methane, and other, less volatile hydrocarbons and nitriles are present mixed into spectroscopically neutral sediments. The dominance of relatively low vapor pressure organics outside the lake itself suggests a retreat of Ontario Lacus associated with evaporation on seasonal or longer timescales, consistent with analysis of RADAR and ISS images.

 (2008)and in [START_REF] Baines | Shoreline features of Titan's Ontario Lacus from Cassini/VIMS observations[END_REF]. The observing geometry and our data processing will be explained, followed by a discussion of the main characteristics of the image. The analyzed image covers a small portion of Ontario Lacus and shows what appears from RADAR data to be a region of modest slope ("ramp") adjacent to the dark lake itself. Our analysis of 5.0 µm spectral data suggests that the previously reported absorption feature of ethane seen at shorter wavelengths may be produced by damp sediments adjacent to the main liquid basin. The latter appears to be absorbing all of the photons incident upon it in the 5 µm spectral region and shows no discernible absorption features. A characterization of the basin

Introduction

Although both Imaging Science Subsystem (ISS) (Porco et al., 2004) and RADAR (Elachi et al., 2004) observations from the Cassini spacecraft detected the existence of dark surface features interpreted as liquid-filled lakes and seas, only in December 2007 did the Visual and Infrared Mapping Spectrometer (VIMS) [START_REF] Mccord | Cassini VIMS observations of the Galilean satellites including the VIMS calibration procedure[END_REF] provide spectroscopic observations of one of these structures.

Approximately centered at 183° W, 72° S, and named Ontario Lacus for its similarity in size and form to Lake Ontario on Earth, this is now the best-studied lake on Titan thanks to the seasonal phase of Titan which was southern summer during the primary Cassini mission from [2004][2005][2006][2007][2008]. Using specific algorithms based on the NAIF-SPICE tools [START_REF] Acton | Ancillary Data Services of NASA's Navigation and Ancillary Information facility[END_REF] in order to accurately calculate the observational geometries, we have unambiguously determined the portions of the lake seen by VIMS in three consecutive observations of Ontario Lacus by the Cassini Orbiter. In doing so, we have accomplished a co-registration of the sequence of observations at 5.0 µm with the whole lake as imaged by the ISS narrowangle camera (NAC) in the June 2005 encounter. Ample evidence exists that the putative lakes and seas are filled with liquid (Lunine and Lorenz, 2009). Given the thermodynamic conditions, the abundance of methane in the atmosphere, and the production of ethane as the dominant photochemical byproduct of methane, the lake composition should be dominated by methane, ethane, or both [START_REF] Stofan | The lakes of Titan[END_REF], along with a second product of methane photolysis, propane, which is liquid under Titan surface conditions and highly soluble in ethane and methane (Cordier et al., 2009). Spectroscopic detection of these hydrocarbons is complicated both by the large amount of methane in the atmosphere, which effectively blacks out the portions of the spectrum where we expect to see liquid methane, and by the lakes themselves, which are at least many meters deep based on the absorption properties of these materials at centimeter wavelength [START_REF] Paillou | Microwave dielectric constant of liquid hydrocarbons: Application to the depth estimation of Titan's lakes[END_REF]. The optical depth in the absorption bands of both methane and ethane should be very large in the lakes unless they are polluted with a large amount of suspended debris. The weak absorption feature of ethane in the 2.0 µm atmospheric window reported by [START_REF] Brown | The identification of liquid ethane in Titan's Ontario Lacus[END_REF], is difficult to understand unless the lake is extremely shallow [START_REF] Clark | Coupling photochemistry with haze formation in Titan's atmosphere, Part II: Results and validation with Cassini/Huygens data[END_REF]. We have carried out an analysis of VIMS data in the 5.0 µm methane atmospheric spectral window and conclude that the region adjacent to the lake contains ethane, which might be responsible for the weak feature seen at shorter wavelengths.

Then, we hypothesize that sediments adjacent to the lake basin itself are soaked in ethane, methane, and other less volatile hydrocarbons, exposed by progressive evaporation of the lake on seasonal or longer timescales. Four cubes covering the Ontario region were acquired during the flyby, but only three of them -those reported in Table 1 -in our opinion, show a spectral and spatial featuring, particularly in the CH 4 spectral window at 5 µm, suitable to carry out a satisfactory match with the ISS camera image.

Observations and mapping

The three selected VIMS cubes observe approximately the same surface region, but at increasing spacecraft distances (i.e.

Fig. 1 at decreasing spatial resolution ranging from 1.5-6 km for the nearest to 3.5-45 km for the farthest observation) and moving in a northwestward direction. In Table 1 some of their average characteristics are listed (mean distance, viewing angle, subspacecraft point position, average spatial resolution) and in Fig. 2 the sequence of VIMS observations is reported at the top.

The data have been geometrically calibrated using ad hoc algorithms based on NAIF-SPICE tools [START_REF] Acton | Ancillary Data Services of NASA's Navigation and Ancillary Information facility[END_REF]. In view of a spectral analysis, connected to the removal of the atmosphere by means of radiative transfer calculations, we radiometrically calibrated the images in spectral radiance (W m -2 nm -1 ster -1 ) using the RC17 calibration pipeline [START_REF] Mccord | Cassini VIMS observations of the Galilean satellites including the VIMS calibration procedure[END_REF]Filacchione, 2006;[START_REF] Adriani | The de-striping of the VIMS-V images and the observations of HCN limb emission in the Titan atmosphere at 3 µm[END_REF].

The georeferencing allows the bright region in the upper right corner of the third cube to be interpreted as the southeast limb of Titan, consistent with the spacecraft's direction of motion and the instrumental line of sight, as shown in Fig. 1. This feature allowed us to correctly position the VIMS images and match them with the ISS one.

Image processing techniques have been applied to the 5.0 µm sequence at the top of Fig. 2 in order to simplify the successive mapping procedures. In the image processing procedures applied here we did not perform any averaging among the VIMS spectral channels to maintain the possibility of getting some information about the composition of different regions by analyzing the spectral shape in the observed wavelength range.

A destriping algorithm, specifically developed for VIMS [START_REF] Adriani | The de-striping of the VIMS-V images and the observations of HCN limb emission in the Titan atmosphere at 3 µm[END_REF], has been applied to the three cubes, followed by a low-pass filtering based on Gauss kernel [START_REF] Van Denboomgaard | Gaussian Convolutions Numerical Approximations Based on Interpolation, Scale-Space and Morphology in Computer Vision[END_REF]. The result of this processing is reported at the bottom of Fig. 2.

A limb distortion can be noted in the third panel at the bottom.

We interpret it as a side-effect of the long exposure time used to improve the signal-to-noise ratio. As a consequence, the image footprint on the surface changes substantially during the time interval needed for the full image acquisition, producing the observed distortion. A radial component can be seen, suggesting a rotational component around the z axis in the spacecraft motion. This effect can be detected as well in the other two cubes by adding the geographical grid to them, as shown in the first two panels at the bottom of Fig. 2.

Similarly a mapping of the pixel radiances in any projection will produce a distortion in the shape of the lake with respect to the image seen by VIMS, as can be seen in Fig. 3.

To produce the maps of Figure 3 the irregular gridded pixel values, obtained by the geometrical calibrations, went through a Delauney triangulation procedure (Lee and Schachter, 1980) and bilinearly interpolated to a regular grid.

Among the material publicly accessible on the Cassini/ISS website (http://ciclops.org/ir_index_main/Cassini), a particular image, showing Ontario Lacus imaged by the NAC during the sequence ISS_009TI_COMPMAP002_CIRS the June 6 2005, has been selected for comparison with VIMS (Fig. 4), because of its well detailed and contrasted appearance.

The ISS-NAC original data on the PDS website for this image consisted of a sequence of similar observations, already containing suitable kernels information for a correct georeferenciation by means of the NAIF-SPICE tool. These observations are characterized by the use of a variety of filters combination. We selected for our co-registration the N1496745450_1.IMG image, filtered by an IR polarizer in conjunction with a narrowband, centered at 938 nm, that assured a better penetration through the Titan haze and viewing of surface spatial features with respect to other combinations.

Then we co-registered and overlapped the VIMS and ISS-NAC data by means of the correspondence of their geographical coordinates. The same satellite projection has been selected to map both the VIMS pixel radiances and the ISS-NAC pixel units, assuming a vertical perspective of the satellite position, Looking particularly at the 5b, 5c and 5d panels the blue region stands for the lowest radiance value in the adopted color table, consistent with a sink for the photons. This visualization seems to indicate that the Ontario southern border had moved partially inward (toward the lake center) with respect to the shoreline of the shape imaged by ISS at 938 nm. A progressive loss of shape and color definition can be noticed passing from panel b) to panel d). This effect can be ascribed to the growing pixel deformation, due to the corresponding increase of the viewing angle.

Figure 5 Interpretation and spectral analysis VIMS can observe Titan's surface through the methane spectral windows at 2.0, 2.8 and 5.0 µm. For wavelengths < 2.0 µm the thick atmospheric layer of organic aerosols extinguishes most of the solar radiation before it can reach the surface in and out of the methane windows. In the spectral windows between 0.9 and 2.0 µm the surface can be distinguished at very small viewing angles, but multiple scattering, particularly at high air masses, hides most of the details in the 2.0 and 2.8 µm windows. Thus in the observations we have analyzed, characterized by low viewing angles and grazing incidence of solar radiation, the surface features are truly well defined only in the 5.0 µm spectral window. Motivated in part by RADAR altimeter observations [START_REF] Lorenz | Ontario Lacus: Brilliant Observations of a Titan Lake by the Cassini Radar Altimeter[END_REF] showing a modest slope in the ramp adjacent to the western side of Ontario Lacus, which then steepens further to the west, we hypothesize that there are three distinct physiographic regions from the 5.0 µm spatial frame in the first panel of Fig. 2: 1. A liquid reservoir (the darker area in the image) where the solar radiation is essentially fully extinguished along a physical distance through the liquid small compared to the lake depth.

2. A relatively uniform area, perhaps sedimentary deposits or exposed lakebed, seen as the dark grey region surrounding the reservoir. We call this the "ramp", based on radar altimetry.

3.

An irregular ridge, suggesting non-uniform elevation, morphology, or both, seen as the brighter area in the image.

Discrimination among the regions has been also made on a radiance basis, co-adding the 13 channels in the 5 µm methane window (from 4.87 µm to 5.07 µm) and using it for a spectral average. From this result three regions representative of the physiographic areas have been selected on the base of their radiance values:

• lake bed ≤ 1.e-07 W/m 2 /nm/ster,

• 1.e-07< ramp ≤ 2.e-07 W/m 2 /nm/ster,

• 2. e-07< ridge W/m 2 /nm/ster. Not all the pixels meeting these conditions have been selected; rather only a visual selection representative of the regions chosen for the analysis has been made.

These three regions are reported in red, cyan and green respectively in Fig. 6.

The hypothesis, then, is that Ontario Lacus is a basin with an optically deep liquid surrounded by an area of sediments, soaked with ethane and other organics. Because of its long pathlength, we assume that the spectra acquired of the lake basin itself are only due to the atmosphere contribution (both scattering and fluorescent sources), motivated also from the calculations reported in [START_REF] Brown | The identification of liquid ethane in Titan's Ontario Lacus[END_REF], explained in the caption of Figure 3. In other words, the lake area will be considered a sink for the radiation at 5.0 µm. The latter hypothesis is consistent with other VIMS observations -to which we do not refer explicitly here -viewing the limb of Titan (see e.g. Baines et al., 2006). In Fig. 7 The generally lower intensity in the nadir viewing spectra can be attributed to the different radiation pathways in the two than to the atmospheric scattering, as can be seen from Fig. 7.

Here the CH 4 , the CH 3 D and the CO contributions can be recognized in the plotted spectral behaviors. Because the analyzed region is limited both in geographical extension and in viewing angles, we can assume that the atmospheric contribution evaluated on the lake area should also be the same for the regions where L 0 ≠0. Therefore the average lake spectrum (or in other words the atmosphere contribution, as the lake has been considered to absorb all the radiation that reaches it) has been subtracted from the ramp-and ridge-average spectral radiances, simulating the removal of atmosphere scattering and emission. This operation enhanced some spectral features in the remaining surface spectra, reducing the rest of the spectral range to white noise (Fig. 8).

In order to make some hypotheses about candidate materials that could be responsible for the spectral signatures, we have considered a list of alkanes and nitriles taken from the paper recently published by [START_REF] Clark | Reflectance spectroscopy of organic compounds: 1. Alkanes[END_REF]. On the basis of evidence elsewhere on Titan (Lunine and Lorenz, 2009) The Spectral Angle Mapper (SAM) is a physically-based spectral classification that uses a n-D angle to match pixels to reference spectra, where n is the number of selected bands and D is a shorten form for dimension (Kruse et al.,1993;Kruse, 2007). SAM compares the angle between the end member spectrum vector and each pixel vector in n-D space. Smaller angles represent closer matches to the reference spectrum and higher probability of being a target. The maximum angle threshold for a SAM analysis can be specified. Decreasing the threshold usually results in fewer matching pixels (better matches to the reference spectrum). Increasing the threshold may result in a more spatially coherent image; however, the overall pixel matches will not be as good as for the lower threshold. In the present analysis a quite conservative threshold of 0.15 radians has been applied, as reported in Table 2 together with a summary of the operations carried out in our SAM analysis. The results of this target detection algorithm application are reported in Fig. 9.

These analyses have been applied to the original image without any image processing (Fig. 2) in order to avoid any possible spectral contamination among adjacent pixels. The SAM classification would seem to suggest that ethane and methane (possibly with butane and/or hydrogen cyanide) can be recognized in what we interpret as the " retreated lakebed", namely the region that in the analyzed VIMS image fills the right portion of the frame

Discussion and Conclusions

In the "ramp" region adjacent to the lake propane, butane, and acetylene seems to predominate. This result is consistent with the hypothesis that propane is dissolved in the lake as a significant component along with ethane and methane (Cordier et al., 2009). Butane and acetylene dissolve in the lake liquid as well, but could also be present in the bottom sediments. As the lake shrunk over the southern summer, these components may have become exposed by the retreating liquid, or precipitated out, or both. All three have similar volatilities (Figure 10). However, it remains to be explained why the much more volatile methane and ethane are present over a portion of the lake boundary as well.

Both may be seeping into the surrounding soil from the lake itself, but methane in particular might also be recharged by springtime precipitation [START_REF] Turtle | Cassini imaging of Titan's high latitude lakes, clouds and south polar surface changes[END_REF]. This is supported by its greater abundance in the ramp region than that of ethane.

The absence or very sparse presence of any of these constituents far from the lake reinforces our view that we are seeing material whose deposition is related to the immediate lake environment, or in fact the retreat of the lake itself.

Hydrogen cyanide appears to be present in only three locations in the VIMS image, one of which is so close to the edge of the image that its presence there (as well as that of other species in those same pixels) could be an artifact. One photochemical model [START_REF] Clark | Coupling photochemistry with haze formation in Titan's atmosphere, Part II: Results and validation with Cassini/Huygens data[END_REF] predicts its net stratospheric production rate to be comparable to that of acetylene-the two being the most abundant products of the photochemistry which are solid at Titan's surface. Other models (e.g., [START_REF] Wilson | Current state of modeling the photochemistry of Titan's mutually dependent atmosphere and ionosphere[END_REF]) predict lesser production rates of HCN relative to acetylene. Its relative scarcity might also be due to, or exacerbated by, its much lower volatility than the other identified components (Figure 10). The absence of benzene, which is a product of the (exothermic) cyclization of acetylene and is seen by VIMS elsewhere on Titan [START_REF] Clark | Coupling photochemistry with haze formation in Titan's atmosphere, Part II: Results and validation with Cassini/Huygens data[END_REF], argues that the acetylene seen in the data has been exposed or come out of solution from the lake relatively recently--again consistent with a scenario in which the lake is currently shrinking. Elsewhere on Titan, where lakes are not present, acetylene has had plenty of time to be exposed to the low cosmic ray flux which might induce it to convert into benzene (Zhou et al., 2010).

The terrestrial analogy to Ontario Lacus might be that of a salty lake which is retreating, thereby exposing sand dampened by water and coated with salts along its periphery. In the Titan case the methane substitutes for water and compounds like acetylene function as the salt. The analogy is made imperfect by the interesting complication that multiple liquid hydrocarbons of widely varying volatility appear to make up the bulk composition of Ontario Lacus on Titan. The alkane reflectances reported by [START_REF] Clark | Coupling photochemistry with haze formation in Titan's atmosphere, Part II: Results and validation with Cassini/Huygens data[END_REF] refer to a two-way transmission of a very thin liquid film of methane and ethane, with effective pathlength of 100 µm. In a geophysical scenario in which Ontario Lacus is a basin full of liquid hydrocarbons some meters or tens of meters deep, as inferred by the Cassini/RADAR tracking [START_REF] Lorenz | Ontario Lacus: Brilliant Observations of a Titan Lake by the Cassini Radar Altimeter[END_REF], the solar radiation at 5 µm would be totally extinguished in the relevant wavelength bands. This is confirmed by the average VIMS spectrum of the lake's region which does not show obvious absorption features of methane or ethane. However, an indirect determination, obtained by observing what are presumed to be sediments of ice or possibly refractory organics immediately adjacent to the lake, indicates the presence of ethane and propane of sufficiently low pathlength that the spectral signatures are weak.

Our result supports the conclusion of [START_REF] Brown | The identification of liquid ethane in Titan's Ontario Lacus[END_REF] that Ontario Lacus contains ethane and methane, but adds to it the possibility that propane is present as predicted by thermodynamic models. Other data, from the Huygens probe, buttress the idea that Titan's sediments can hold measurable amounts of methane and other hydrocarbons. At the Huygens landing site, methane and ethane began evaporating from the surface once contact with the Huygens probe's warm mass spectrometer inlet was made [START_REF] Niemann | The composition of Titan's atmosphere from the GCMS on the Huygens probe, and implications for the origin of nitrogen and methane[END_REF]Lorenz et al., 2006). Propane was not seen in the Huygens data, indicating either surface heterogeneity or a volatility effect having to do with the maximum temperature of the heated probe and the lower vapor pressure of propane relative to ethane and methane. In summary, our analysis of the VIMS data suggest that the region adjacent to Ontario Lacus is a shoreline in which ethane, propane, several other organic species, and possibly methane itself, saturate sediments as the lake undergoes its seasonal retreat. The spatial extent of this region -of orders kilometers -is consistent with the amount of summertime evaporation of Ontario Lacus seen through comparison of RADAR and ISS images taken four years apart (Lunine et al., 2009), and from analysis of VIMS data by [START_REF] Baines | Shoreline features of Titan's Ontario Lacus from Cassini/VIMS observations[END_REF]. Astrophys. J., 715, in press.

Figure Captions

  On 2007 December 5 VIMS observed Titan's South Polar Region during the T38 Cassini flyby. The Ontario Lacus observations were acquired during a fast spacecraft passage over Titan's South Pole, near orbital perihelion. A geometrical reconstruction of the whole sequence is shown in Fig. 1, with the aim of identifying what portion of the lake would have been seen by VIMS, where the projection of the Cassini trajectory is denoted by a green line and the terminator by a red one.

Figure 2

 2 Figure2

Figure 3

 3 Figure 3

Figure 4 centered

 4 Figure 4

  , the radiance spectra, averaged over the regions of interest, are compared with a VIMS limb spectrum at 60 km tangent altitude, namely the distance from the surface of the line of sight (LOS) relative to the observed pixel, and 75 ° S, with the Sun behind the observer. It can be seen that the spectral shapes in nadir viewing appear very similar to the case of limb-viewing geometry shortward of 4.87 µm (channel 241 of VIMS-IR).

  Figure 6

Figure 9

 9 Figure 9

Figure 10

 10 Figure 10

Figure 1 :

 1 Figure 1: Geometrical reconstruction of the sequence of

Figure 2 :

 2 Figure 2: The sequence of the original 5 µm VIMS images of

Figure 3 :

 3 Figure 3:The 5 µm VIMS images V1575506843,

Figure 4 :

 4 Figure 4: The June 6, 2005 observation of the ISS narrow-

Figure 5 :

 5 Figure 5: Overlapping of the three cube maps of Figure 3 on

Figure 6 :

 6 Figure 6: In the three panels the main morphological

Figure 7 :

 7 Figure 7: The average spectral behaviors for the lake's (red),

Figure 8 :

 8 Figure 8:The average spectral behaviors for the ramp's

Figure 9 :

 9 Figure 9:Tentative surface composition of the Ontario's

Table 2

 2 Target detection summary. The bands range from 241-253 (4.87-5.07 m). Six target spectra were selected (listed under target name in the table) A forward MNF transform (3 bands) was run. The spectral angle mapper (SAM) was used.

	Rule Threshold Table:
	Target Name	Method Rule Threshold Revealed
	C2H6	SAM	0.150	yes
	C3H8	SAM	0.150	yes
	CH4	SAM	0.150	yes
	C2H2	SAM	0.150	yes
	C4H10	SAM	0.150	yes
	C6H6	SAM	0.150	no
	HCN	SAM	0.150	yes

Acknowledgments

We thank F.Tosi, by IFSI-INAF, for his comments regarding the surface classification. We thank the VIMS and ISS teams for planning and operations. This research has been supported by the Italian Space Agency (ASI), the Cassini Project (NASA), and the program "Incentivazione alla mobilita' di studiosi stranieri e italiani residenti all'estero."

concentrations of each single end member. At the right of each image a mapped version in satellite projection has been added. [START_REF] Tokano | Limnological structure of Titan's hydrocarbon lakes and its astrobiological implication[END_REF]. The black horizontal line divides components according to their volatility: below the line, vapor pressures are too low to permit significant evaporation of the liquid over a Titan year, regardless of the availability of solar energy. For relevant temperatures at Ontario Lacus, only methane contributes to a reduction in lake level; propane and the other constituents are left behind. Ethane is a marginal case; some evaporation of ethane does occur seasonally, but its behavior is more like propane than like methane. Vapor pressures from [START_REF] Brown | Advances in Cryogenic Engineering[END_REF], [START_REF] Perry | The vapor pressures of solid and liquid hydrogen cyanide[END_REF], and the NIST Chemistry Webbook http://webbook.nist.gov/chemistry/.