51st Lunar and Planetary Science Conference (2020) 2419.pdf

MULTI-ANGULAR OBSERVATIONS OF MARTIAN BRIGHT SLOPE STREAKS. A.Valantinas1, N. Thomas1, A. Pommerol1, P. Becerra1, E. Hauber2, L. L. Tornabene3, A. McEwen4, G. Cremonese5 and the CaSSIS Team1. 1Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland ([email protected]), 2Deutsches Zentrum für Luft-und Raumfahrt, Institut für Planetenforschung, Berlin, Germany, 3CPSX, Earth Sci., Western University, London, Canada, 4Lunar and Planetary Lab, University of Arizona, Tucson, USA, 5Osservatorio Astronomico di Padova, INAF, Padova, Italy.

Introduction: Bright slope streaks are enigmatic Discussion: In the past, the contrast reversal of slope surface features of increased albedo found on Martian streaks was attributed to either physical fading [5] or slopes in low thermal inertia regions [1, 2]. These viewing geometry effects [6]. Loose snow avalanches elongated features are thought to result from the more on earth were suggested as an analog for the latter. common dark slope streaks [3, 4], which gradually fade Interestingly, many dark, bright and dark-bright streaks or brighten with time [5]. In fact, in a few rare cases are frequently found on the same slope (Fig. 1 and Fig. slope streaks have been observed to have bright and dark sections [1, 5, 6]. The fading rate of dark slope streaks has been shown to be around 40 years [7] but the contrast reversal rate is unknown. Several hypotheses attempt to explain the origin of dark slope streaks: Dry- based models encompass formation through dust mass wasting, avalanching or granular flows [1, 6, 8]; and aqueous models cite subsurface aquifers as sources, lubricated dust flows and ground staining from saline fluids [9-13]. Various properties of dark slope streak populations were studied in detail to address their origin [14-16]. However, little is known about their photometric parameters, i.e., surface texture, roughness and grain size. We address this issue by acquiring multi- angular observations of bright slope streaks using The Colour and Stereo Surface Imaging System (CaSSIS) [17] onboard the ExoMars Trace Gas Orbiter (TGO). Figure 1. CTX (A) and CaSSIS (B) image of dark and When necessary and available, we obtain high- bright slope streaks in Arabia Terra. Several bright slope resolution views of our measurements from images streaks originating at the northern and southern outcrops taken by the Mars Reconnaissance Orbiter’s (MRO) are visible in (B) image. Notice also the darker dust High Resolution Imaging Science Experiment halos around the outcrops that are only visible in the (HiRISE). Finally, a global Context Camera (CTX) CaSSIS image. The differences in this region possibly indicate dust removal and/or exhumation of the surface mosaic [18] was used to map the locations of all bright over time. (A) was acquired in 2007-05 (P07_003799_ slope streaks in the Arabia Terra region [19]. 1961_XN_ 16N311W) and (B) in 2019-02 Results: A large survey consisting of ~16,000 (MY34_005562_162). hexagons 20 km in diameter provided a base map of all bright slope streak locations in Arabia Terra. We found 2). This suggests that the origin of their differences is that some locations underwent significant surface compositional and independent of viewing geometry. If changes spanning several years (e.g. Fig. 1). However, Fig. 2 is an example of a streak in albedo transition it other locations showed minimal change, which points raises the question of why it follows an apex-to-talus towards heterogeneities within Arabia Terra. brightening pattern. If the streak is affected by Additionally, we have documented ~50 locations of atmospheric dust fallout, then the brightening partially dark-bright slope streaks. One such peculiar mechanism should be homogenous and affect the whole case is seen in Fig. 2. This example features bright streak. Otherwise the dust settling and accumulation is apexes that begin to darken towards the middle, and at not homogenous along different parts of the slope. the distal ends appear indistinguishable from a dark Another hypothesis could be related to gravity-c- slope streak. Some streaks exhibit an impact crater or a ontrolled processes. Particles at the top of the streak dust devil track at their apexes. The latter location and a might be more easily churned by wind and transported few selected others were targeted by CaSSIS, and will downwards as mass wasting. In both cases, a Digital provide the basis for multi-angular observations. Terrain Model (DTM) might hold more clues to the dust Photometric results will be presented at the conference.

51st Lunar and Planetary Science Conference (2020) 2419.pdf

Figure 2. HiRISE image ESP_060672_2050 taken on 2019-06 (A) and zoomed-in view (B). Partially dark-bright slope streaks on a crater wall in Arabia Terra. The same location has been imaged by the Mars Orbiter Camera (MOC) in 2002, and by CTX in 2007. A visual comparison between MOC, CTX and HiRISE observations suggests that slope streaks are brightening from the apex downward. If this is the case, then this location is an example of transitioning slope streaks. Multi-angular observations with CaSSIS will distinguish between viewing geometry or compositional effects. transport hypotheses. Alternatively, a mechanism for (2006) Icarus, 183, 30-45. [7] Bergonio J. R. et al. slope streak brightening might hint at a formation (2013) Icarus, 225, 194-199. [8] Chuang F. C. et al. process unrelated to dust. As the streak forms, the (2007) GRL, 34(L20204). [9] Ferris J. C. et al. (2002), material at the top of the streak will always be older. GRL, 29. [10] Miyamoto H. et al. (2004) JGR, 109, Hence, the talus will remain the least degraded because E06008. [11] Kreslavsky M. A. and Head J. W. (2009) it was formed last. However, this requires a slow Icarus, 201, 517-527. [12] Head J. W. et al. (2007) AGU formation process. Slope streak formation time is poorly Fall Abstracts, (#P22A-08). [13] Bhardwaj A. et al. constrained: numerical modeling suggests streaks can (2019) Rev. Geophys., 57, 48-77. [14] Schorghofer N. form in as little as 1 hour [10], while repeated and King C. M. (2011) Icarus, 216, 159-168. [15] observations show new streaks forming within 5 days Brusnikin E. S. et al. (2016) Icarus, 278, 52-61. [16] [20]. Mushkin A. et al (2010), GRL, 37, L22201. [17] Landscape changes such as those shown in Fig.1 Thomas N. et al. (2017), Space Sci. Rev, 212, 1897- could be related to a dust storm event in 2007. After the 1944. [18] Dickson J. L. et al. (2018), LPSC XLIX, dust settled, bright slope streaks are no longer visible Abstract #2083. [19] Ramsdale J. D. et al. (2017) PSS, because they exhibit a lower albedo difference from the 140, 49-61. [20] Heyer, T. et al. (2019) Icarus, 323, 76- surrounding material than the dark slope streaks (~4% 86. vs. 10%). Later, wind could have exhumed the area and Acknowledgments: The authors thank the remove the thin layer of dust. However, this would spacecraft and instrument engineering teams for the imply that the composition of the material within slope successful completion and operation of CaSSIS. streaks is not the same as that of the atmospheric dust. CaSSIS is a project of the University of Bern funded Future Work: We will employ a Hapke model to through the Swiss Space Office via ESA's PRODEX simulate the photometric parameters of the surface. This programme. The instrument hardware development was will allow us to address the contribution to the albedo of also supported by the Italian Space Agency (ASI) (ASI- the surface texture and composition independently. A INAF agreement no.I/018/12/0), INAF/Astronomical DTM will also be produced from CaSSIS stereo pairs to Observatory of Padova, and the Space Research Center derive local topography and lighting geometry. (CBK) in Warsaw. Support from SGF (Budapest), the References: [1] Sullivan R. et al. (2001) JGR, University of Arizona (Lunar and Planetary Lab.) and 106, 23607-23633. [2] Schorghofer N. et al. (2002) NASA are also gratefully acknowledged. GRL, 29, 2126. [3] Morris, E. C. (1984) JGR, 87, 1164– 1178. [4] Ferguson H. M. & Lucchitta B. K. (1984) NASA Tech. Memo. 86246, 188-190. [5] Schorghofer N. et al. (2007) Icarus, 191, 132-140.[6] Baratoux D. et al.