Multi-angular Observations of Martian Bright Slope Streaks
Authors: Valantinas Adomas , Becerra Patricio , Tornabene Livio Leonardo , Pommerol Antoine , Hauber Ernst , Thomas Nicolas , McEwen Alfred , Cremonese Gabriele 
Published: October 2020

Introduction: Bright slope streaks are enigmatic surface features of increased albedo found on Martian slopes in low thermal inertia regions [1, 2]. These elongated features are thought to form as the more common dark slope streaks [3, 4] gradually fade or brighten with time [5]. In fact, in a few rare cases slope streaks have been observed to have bright and dark sections [1, 5, 6], which could be taken as evidence of this transition. 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 the physical parameters that would affect their photometry, e.g., surface texture, roughness and grain size. Here, 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). When necessary and available, we complement these observations with higher-resolution images of our study sites taken by the Mars Reconnaissance Orbiter&#8217;s (MRO) High Resolution Imaging Science Experiment (HiRISE). To obtain regional context for our observations, we used a global Context Camera (CTX) mosaic [18] to grid-map the locations of bright slope streaks in the Arabia Terra region [19].Results: We surveyed the distribution of&#160; bright slope streaks in Arabia Terra using a grid map divided into ~16,000 hexagonal facets, each 20 km in diameter, and each qualitatively surveyed. Additionally, we documented ~30 locations of partially dark-bright slope streaks. We targeted one such peculiar case (first observed by the Mars Orbiter Camera (MOC) [1]) with CaSSIS and HiRISE (Fig. 1). This example features a bright apex that begins to darken towards the middle, and at the distal end appears indistinguishable from a dark slope streak. This same albedo reversal is visible under various lighting geometries (23&#176;-86&#176; incidence). We used these multi angular observations as the basis for photometric measurements of this slope streak, which are seen in Fig 2. We measured I/F values (detected irradiance (I) over the solar irradiance at zero incidence (F), such that I/F = 1 for a normally illuminated, perfectly diffuse re&#64258;ector) of dark and bright slope streak regions-of-interest (ROIs), and ratioed them over the ROIs of the surrounding surface. This method avoids the reflectance dependency on local slope. Fig. 2 indicates that the dark (black squares) and the bright (red circles) part of the slope streak is up to 4.5% darker and 3.5% brighter respectively. It also appears that between years 2009-2014 (three HiRISE observations) the dark slope streak ROI is darker than in the three CaSSIS observations taken in 2020.Discussion: In the past, the contrast reversal of slope streaks was attributed to either physical fading (coverage by atmospheric dust fallout) [5] or viewing geometry effects [6]. Loose snow avalanches on Earth were suggested as a possible analogous process where the latter effect occurs [e.g. 1, 6]. Interestingly, many dark, bright and dark-bright streaks are frequently found on the same slope, which suggests that their albedo is independent of viewing geometry. This is supported by the multiple observations seen in Fig. 1. Reflective behavior appears to be independent of illumination geometry, i.e. the slope streak exhibits the same contrast reversal in all observations. Further, our quantitative photometric measurements (Fig. 2) support this finding. The ratio of the dark ROI and the surroundings within 3 HiRISE observations is almost linear, but the earlier HiRISE image exhibits a higher ratio. The later (2020) 3 CaSSIS images of the dark ROI ratios illustrate a more faded streak. This fading could be a result of atmospheric dust fallout (e.g. dust storm in MY34), but this would not explain the bright ROI behavior. Another hypothesis is that the fading is related to gravity-c&#173;&#173;&#173;ontrolled processes. Particles at the top of the streak might be more easily churned by wind and transported downwards by mass wasting. 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