Distribution and intensity of water ice signature in South Xanadu and Tui Regio
Authors: Coutelier Maélie , Cordier Daniel , Rannou Pascal , Seignovert Benoît 
Published: October 2020

Titan is a complexe moon of Saturn, with dense atmosphere, hydrocarbon cycle, and water-ice crust. Titan was targeted by two major space missions: Voyager and Cassini, and has been the subject of many studies. Lakes and seas of liquid hydrocarbons were discoveredby Cassini's RADAR in Titan's polar regions [15, 14]. This instrument also detected on equatorial regions geomorphological structures related to the presence of liquid, like fluvial valleys incised in the bedrock, and alluvial fans [8, 3]. The existence of evaporitic terrains where also suggested [1, 4, 9], often in place of paleo-sea [12]. Water ice signal is not present everywhere on Titan, contrary to other icy moon of Saturn and Jupiter. It was detected with Cassini's VIMS instrument in Titan's dark region, often at the transition between a dark and bright unit, and mixed with a darkening material [11, 13]. Ref. [6] also highlighted an equatorial corridor of exposed water-ice using a principal component analysis (PCA) with VIMS, showing on a large scale terrains with low and high water ice content.We accentuate our study on Titan's river bed, where the erosion due to liquid hydrocarbon can reveals the bedrock, and where we can have clues on the sediment transport. Here we propose a study of the water ice signal distribution in Xanadu and Tui regio, where we have channels [8] and paleo-seas of hydrocarbures [12].We focused on 2 radar bright channels [8], and the surrounding areas. The channels are located at the south of Xanadu, directly exiting the mountains. They are at the north of the evaporitic region TuiRegio [10], and form dry dendritic fluvial valleys [7], with a width up to 8~km. We apply a radiative transfer analysis to retrieve the surface albedo of 2 VIMS cubes located in our area of study : 1590648776_1, and 1809727868_1.&#160;We establish a criteria &#948; representing the intensity of the water-ice signature, based on wavelength ratios and means of the surface albedo significant for the identification of the water-ice spectrum. They are detailed in Tab. 1. Those ratio and mean are normalized with respect to themselves. Then, they are added if the expected value indicating a high water ice signal is high or >1, and subtracted otherwise. We then have a range of values going from &#948; = -4 for no water ice signature, and &#948; = 1 for a high water ice signal. The results of this multicriteria analysis are in Fig. 1 and 2. An example of the different spectra for a low and high &#948; are given is Fig.3.
Wavelength ratioand mean

Expected value ifwater-ice signature
1.58/1.28
&#160;< 1
2.03/1.28
&#160;< 1
2.03/1.58
&#160;< 1
2.7/2.8
&#160;high
4.85 - 5.1&#181;m
&#160;low
The radiative transfer analysis highlighted areas of high water-ice signal on Xanadu mountains, and also at the end of the dry channels, corresponding to potential deltas or alluvial fans, supporting [2] and [7] hypothesis of sediment transport from the mountains of Xanadu to downstream of the channels. The ice signal is lower inside the channels, perhaps due to their sizes, smaller than the spatial resolution of a pixel. The Fig. 3 shows that water ice lower the surface albedo.This combined analysis also shows a gradient of low water-ice signal perfectly superposed to Tui Regio in the RADAR map. This gradient could correspond to the decreasing thickness from the center to the border of the evaporite layer of Tui Regio, as suggested by [5].Detailed analysis of the retrieved albedo could bring further information on the composition of those terrains if the error bars are low enough. A map of the inversion of the surface signal allows us to have a finer vision than just an analysis of the I/F. With the parameter &#948;, we have better contrasts, and a direct appreciation of the strength of the water-ice signature.&#160;AcknowledgementsThe present research was supported by the Programme National de Plan&#233;tologie (PNP) of CNRS-INSU co-funded by CNES, and also partially supported by the French HPC Center ROMEO, the R&#233;gion Grand-Est and the University of Reims Champagne-Ardenne.&#160;References&#160;[1] J. W. Barnes et al., Organic sedimentary deposits in Titan&#8217;s dry lakebeds: Probable evaporite. Icarus, 216:136&#8211;140, November 2011.[2] Jason W. Barnes et al., Near-infrared spectral mapping of Titan&#8217;s mountains and channels. Journal of Geophysical Research (Planets), 112(E11):E11006, Nov 2007.[3] S.P.D. Birch et al., Alluvial fan morphology, distribution and formation on titan. Icarus, 270:238 &#8211; 247, 2016. Titan&#8217;s Surface and Atmosphere.[4] D. Cordier et al., On the chemical composition of Titan&#8217;s dry lakebed evaporites. Icarus, 226(2):1431&#8211;1437, 2013.[5] D. Cordier et al., Structure of titan&#8217;s evaporites. Icarus, 270:41&#8211;56, may 2016.[6] Caitlin A. Griffith et al., A corridor of exposed ice-rich bedrock across Titan&#8217;s tropical region. Nat. Astron., page 343, Apr 2019.[7] M. H. Langhans et al., Titan&#8217;s fluvial valleys: Morphology, distribution, and spectral properties. Planet. Space Sci., 60(1):34&#8211;51, Jan 2012.[8] A. Le Gall et al., Radar-bright channels on Titan. Icarus, 207:948&#8211;958, June 2010.[9] S. M. MacKenzie et al., Evidence of Titan&#8217;s climate history from evaporite distribution. Icarus, 243:191&#8211;207, November 2014.[10] Shannon MacKenzie and Jason Barnes. Compositional Similarities and Distinctions between Titan&#8217;s Evaporitic Terrains. The Astrophysical Journal, 821(1):0, 2016.[11] T. B. McCord et al., Composition of Titan&#8217;s surface from Cassini VIMS. Planet. Space Sci., 54(15):1524&#8211;1539, Dec 2006.[12] Jeffrey M. Moore and Alan D. Howard. Are the basins of titan&#8217;s hotei regio and tui regio sites of former low latitude seas ? Geophysical Research Letters, 37(22), 2010.[13] Laurence A. Soderblom et al., Correlations between cassini vims spectra and radar sar images: Implications for titan&#8217;s surface composition and the character of the huygens probe landing site. Planetary and Space Science, 55(13):2025 &#8211; 2036, 2007. Titan as seen from Huygens.[14] K. Stephan et al., Specular reflection on Titan: Liquids in Kraken Mare. Geophysical Research Letters, 37:L07104, April 2010.[15] E. R. Stofan et al.,The lakes of Titan. Nature, 445:61&#8211;64, January 2007.