 LABORATORY HYPERSPECTRAL STUDY OF ICE AND MARS SOIL SIMULANT ASSOCIATIONS. COMPARISON WITH CRISM OBSERVATIONS OF ICY SURFACES.  Z. Yoldi1, A. Pommerol1, O. Poch2 and  N.  Thomas1,  1Physikalisches  Institut.,  University  of  Bern,  Sidlerstrasse  5,  CH-3012  Bern,  Switzerland (zurine.yoldi@space.unibe.ch), 2Université Grenoble Alpes, CNRS-INSU. Context: Ices are found over a wide range latitudes across the surface of Mars,  whether they are H2O or CO2.  They  undergo  diurnal  [1,  2] and  seasonal  [3] cycles  by sublimating  into  the  atmosphere  and condensing in the atmosphere or at colder parts of the surface.  Depending  on  the  atmospheric  and  surface conditions -mainly temperature  and  partial  pressure-, ices  will  appear  under  various  forms  and  will  be associated in different ways with the substrate (e.g. as frost or slab), either ice or regolith. Spectral or color imaging and reflectance spectrometry stand out as the techniques  of  choice  to  identify  ices  on  the  surface and/or to characterize their association mode with the regolith. Examples of spectrometers and imagers that track  ices  on  the  surface  of  Mars  are  the  OMEGA imaging  spectrometer,  the  stereo  and  color  camera HRSC, the imaging spectrometer CRISM and the color camera HiRISE. As from the first quarter of 2018, a new camera will be added to this list: the Colour and Stereo Surface Imaging System (CaSSIS) [4], onboard ESA's  Trace  Gas  Orbiter  (TGO).  With  four  color bands (BLU: 475 nm; PAN: 650 nm; RED: 850 nm; NIR:  950  nm),  CaSSIS  will  ideally  complement  the datasets collected by other cameras thanks to its high signal-to-noise  ratio  and  the  non-Sun-synchronous orbit  that  TGO  will  follow  around  Mars.  We  thus expect to be able to observe both seasonal and diurnal phenomena  (such  as  H2O  and  CO2 frosts)  in  good conditions. Problematic:  Unfortunately, the  interpretation  of the  observations  of  the  Martian  surface  in  terms  of detection  and  quantification  of  ices  is  not straightforward,  since  many  parameters  control  the shape of the spectra and color of the surface, e.g. shape and  size  of  the  particles,  albedo  of  the  surface, presence of hydrated minerals, relative content of ice and/or type of association of the ice with the surface. In  order  to  help  with the  understanding  of  past  and future  observations,  we  have  conducted  a  series  of experiments  to  study  the  visible  and  near-infrared spectro-photometric  signatures  of  different  ice  and regolith  simulant  associations.  We  have  produced  a wide range of samples to cover parameters of interest, i.e. type of ice and substrate, particle sizes, substrateice association modes and  ice concentration,  and we have  characterized  them  through  hyperspectral imaging.  Next,  we  have  analyzed  the  spectra  and simulated colors of the samples as would be seen with CaSSIS,  to  identify  key  and  discriminative  features. Finally,  as  a  way  of  assessing  the  pertinence  of applying our results to Martian observations,  we have compared  our  laboratory  data  with  CRISM measurements of icy surfaces. Samples and methods: As dry members  for  our associations,  we have used the Martian soil simulant JSCM-1 [5],  that  matches  average  reflectance  of  the bright plains of Mars, and also a darker basalt [6] in order to match terrains that show lower albedo, such as the ones observed where Phoenix landed. The bulk of our ice analogs are made of H2O. Two different water ice size particles (SPIPA-A, 4  µm of mean diameter and  SPIPA-B;  70  µm) have  been  intimately  mixed with the soil simulants (as done in [7]). Atmospheric water has been condensed onto the simulants in order to study frost, and dusty samples have been saturated with  water  and  frozen  to  simulate  frozen  soils.  We have also tested new protocols for  the production of granular carbon dioxide ice and posterior mixing with soil  simulant,  and  we have  conducted  measurements on CO2 ice slabs.  The hyperspectral cubes have been acquired at the Laboratory  for  Outflow  Studies  of  Sublimating  icy materials  (LOSSy)  with the  Simulation Chamber  for Imaging the Temporal Evolution of Analogue Samples (SCITEAS) [8]. This chamber is designed to measure the reflectance  of  samples  from 0.355 to 2.5 µm, at low  temperature  and  pressure  conditions.  We  have characterized the spectra  by studying  and comparing various  spectral  criteria.  For  instance,  we  have measured the reflectance of the continuum at 0.8 µm and compared it to the strength of the absorption band of H2O at 1.5 µm (computed as in [9]). On the other hand, we have convolved the reflectance spectra with the  spectral  response  of  CaSSIS  [10]  in  order  to produce  simulate  color  images  and  compute  color ratios. Figure 1 shows pictures of four JSCM-1 frozen Figure  1.  Left:  RED-PAN-BLU  CaSSIS  color  composite pictures of four JSCM-1 frozen soils after 25h of sublimation. Right: evolution of the reflectance spectra of the area indicated by the yellow square. soils after  25 hours  at  low temperature and pressure conditions.  Figure  1  shows  a  false-color  three-band image composed by the channels RED-PAN-BLU of CaSSIS  as  well  as  the  evolution  of  the  reflectance spectra of the area indicated with a yellow square. On that  area,  the  ice  initially  covering  the  JSCM-1  has sublimated  with  time,  changing  significantly  the reflectance and spectra of the surface. Lastly, we have conducted  the  same  spectral  analysis  on  a  temporal series  of  CRISM  observations  of  a  Martian  polar region  (85º  N  and  0º  E), obtained  during  two consecutive Martian springs (Martian years  (MY) 29 and  30).  An  analysis  of  these  data  was already published  in  [11],  where  this  region  was  informally referred to as "Kolhar". Results  and  discussion: Figure  2  shows  the reflectance  at  0.8  µm  versus  the  strength  of  the absorption band of  water  at  1.5 µm for  different ice and  soil  simulant  associations.  We  see  how  the measurements  corresponding  to  water  frost  samples can  be  differentiated  from  the  ones  involving  other forms  of  water  ice.  This  fact  consolidates  with  the increasing presence of ice in the sample, indicated by the blue arrows. Fig. 2 also evinces that our granular CO2 samples cold-trapped atmospheric water. CRISM measurements fall within our experimental data,  which  suggests  that  our  experiments,  even without  fully  replicating  the  Martian  conditions,  are suitable  for  comparison  with  Martian  observations. The evolution of the surface that can be deducted from the position of the CRISM points in Fig. 2 coincides with the one provided by other studies such as [9, 12, 11]; during early spring the surface of the seasonal cap is  dominated  by  CO2 ice  and  contaminated  by  H2O frost  and  bright  dust.  A  following  increase  in  the reflectance  of  about  50%  suggests  that  dust  is progressively being removed from the ice. After midspring,  the  signature  of  water  ice  peaks,  to  finally reach  a  minimum  around  the  summer  solstice.  In general, CRISM points fall in the vicinity of the points for  laboratory intimate mixtures.  Figure 3 shows the BLU/RED color band ratio of our samples as would be measured with CaSSIS. The  majority  of  the  observations  made  with CRISM  do  not  fall  on  the  trend  drafted  by  the experimental  data,  with the exception of the CRISM points  corresponding  to  the  Martian  summer  (Ls  = 90.04º  and  Ls=93.86º);  we  will  address  this discrepancy  in  detail.  Both  figures  underline  the importance  of  conducting  further  experiments  with CO2 ice and  dust  mixtures,  which  are  being  already conducted at LOSSy. References: [1] Piqueux, S. et al.  (2016)  J. Geophys. Res. Planets, 121, 1174-1189 [2] Smith, P. H. et al. (2009) Science,  325 [3] Leighton,  R.  B and  Murray,  B.C.  (1966) Science,  153, 136.  [4]  Thomas,  N.  et  al  (2017)  Sp.  Sc. Reviews, 212, 1897-1944. [5] Allen C.C., et al. (1998) Space, 469-476. [6]  Pommerol, A., et al. (2013) JGR, 118, 20452072. [7] Yoldi,  Z.  et  al.  (2015)  GRL, 45, 6205-6212. [8] Pommerol.,  A. et al. (2015a) Planet. and Sp. Sc.,  109-110, 106-122.  [9]  Langevin,  Y.  et  al.  (2007)  JGR,  112.  [10] Roloff, V. et al. (2017) Space Sci Rev, 212, 1871-1896. [11] Pommerol,  A.  et  al  (2013)  Icarus,  225,  911-922.  [12] Appéré, T. et al (2011) JGR, 116. Acknowledgements: The  team  from  the University  of  Bern  is  supported  through  the  Swiss National  Science Foundation and through the NCCR PlanetS. Figure 2: Continuum reflectance versus the depth of H2O 1.5-μm absorption  band  (experimental  and  CRISM  data).  For experimental  data,  different  colors correspond to the substrate and  different  symbols  indicate  the  ice/dust  association.  The colors  of  the  CRISM  points  (stars)  indicate  the  time  the observations were  made, which are detailed in  the legend for both MY 29 and 30. Figure 3: Reflectance measured in the CaSSIS RED filter versus the BLU/RED color ratio. The legend of Fig. 2 applies. 
