TY - JOUR
T1 - Multiband photometry of Martian Recurring Slope Lineae (RSL) and dust-removed features at Horowitz crater, Mars from TGO/CaSSIS color observations
AU - Munaretto, G.
AU - Pajola, M.
AU - Lucchetti, A.
AU - Cremonese, G.
AU - Simioni, E.
AU - Re, C.
AU - Bertoli, S.
AU - Tornabene, L.
AU - McEwen, A. S.
AU - Becerra, P.
AU - Rangarajan, V. G.
AU - Valantinas, A.
AU - Pommerol, A.
AU - Thomas, N.
AU - Portyankina, G.
N1 - Funding Information:
In Fig. 13 we compare the predicted (x-axis) vs observed (y-axis) relative reflectance for the dust (orange) and water (blue) models. The former achieves a χr2≈1 while the latter produces a 5 times higher value, hence again confirming a worse fit. This is consistent with the results of Fig. 11, confirming also that adding or removing the RED and NIR filters does not influence our ability to assess the goodness of fit of each model. The fact that water models return a larger χr2 can be also qualitatively recognized in Fig. 11. Indeed, the modeled reflectance is always “in between” the observed reflectance in the BLU and PAN filters, typically higher in the latter and lower in the former. When considering the observed/modeled ratio, this implies two clusters, one below a 1:1 relationship and one above it. This behavior can be seen in Fig. 13 and implies that errors will be not distributed in a gaussian way. As a consequence, they will be consistently higher than expected by a gaussian error distribution and hence the χr2 will return a larger value. These three checks (SSR distribution in Fig. 11B and C, average absolute residual in Fig. 11D and E and χr2 in Fig. 13) support our hypothesis, suggested by the qualitative comparative analysis in Fig. 9, that RSL, DDTs and DP terrains at Horowitz have the same origin as dust removed features, hence bringing new evidence for a dry origin for RSL. Our estimated dust deposit thicknesses provide a constraint on the process forming RSLs. The similar, and greater than zero, fitted dust deposits of RSL, DDTs and DP materials suggests that the amount of deposited dust on these ROIs is similar. However, the amount of dust deposited outside the RSLs is greater than outside the other features. An explanation of this may be sought in terms of aeolian processes: both dust-devils and winds could first lift some dust from the surface (Reiss et al., 2013). This dust is then redeposited, possibly either over the whole crater or a large, less localized area. This would happen in a more uniform way, hence explaining the same dust content within the DDTs, DP and RSL ROIs. The higher predicted dust content nearby RSLs is consistent with the beiger hue of materials near RSL that can be seen in the CaSSIS NPB composite in Fig. 2, against the relatively bluer tone of regions nearby dust-devil tracks. It is also consistent with the more yellow tone of materials nearby RSLs in the CaSSIS CBRC-ALL product displayed in Fig. 4. The physical process responsible for this dust excess may be explained in different ways, which are difficult to discriminate in this analysis. One possibility could be that the RSL-forming process that may not only lift dust (like in dust devils) but also displaces it laterally, hence enriching in dust the nearby slopes, similarly to the dust movements on the “flat” terrains of Mars (Vincendon et al., 2019). To account for the lack of any observed deposit, its surface density (g/cm2) should be small. This may be accomplished either with a small deposit, or by displacing dust on an larger area than the RSL. Another option, independent from the RSL process, is that dust could then be later lifted by widespread dust deflation processes, similarly to those occurring at Tivat crater (Schaefer et al., 2019), hence explaining the lack of a visible lateral deposit at RSL locations. In this last case, however, the dust lifting mechanism would have a much lower intensity than at Tivat crater to prevent RSL fading. It would redeposit dust at the RSL-hosting slopes, instead of removing it like at Tivat crater (Schaefer et al., 2019). Another possibility is that RSL on Horowitz could form on slopes that are already relatively richer in dust with respect to DDTs and DPs regions, prior to the RSL formation. This pre-existing excess may be due to dust falling from the upslope part of the steep rocky outcrops of Horowitz crater and depositing where it is more dynamically stable, i.e. near RSL locations where the slope angle matches the angle of repose of dry granular flows.We gratefully thank Dr. Mathieu Vincendon and Dr. Ethan I. Schaefer for providing insightful and useful revisions that improved the quality of this paper. CaSSIS is a project of the University of Bern and funded through the Swiss Space Office via ESA's PRODEX programme. The instrument hardware development was also supported by the Italian Space Agency (ASI) (ASI-INAF agreement no. 2020-17-HH.0), INAF/Astronomical Observatory of Padova, and the Space Research Center (CBK) in Warsaw. Support from SGF (Budapest), the University of Arizona (Lunar and Planetary Lab.) and NASA are also gratefully acknowledged. Operations support from the UK Space Agency under grant ST/R003025/1 is also acknowledged.LLT wishes to personally acknowledge funding and support from the Canadian Space Agency (CSA) through their Planetary and Astronomy Missions Co-Investigator programme (19PACOI07) and the Canadian NSERC Discovery Grant programme (RGPIN 2020-06418).
Funding Information:
We gratefully thank Dr. Mathieu Vincendon and Dr. Ethan I. Schaefer for providing insightful and useful revisions that improved the quality of this paper. CaSSIS is a project of the University of Bern and funded through the Swiss Space Office via ESA's PRODEX programme. The instrument hardware development was also supported by the Italian Space Agency ( ASI ) (ASI-INAF agreement no. 2020-17-HH.0 ), INAF /Astronomical Observatory of Padova, and the Space Research Center (CBK) in Warsaw. Support from SGF (Budapest), the University of Arizona (Lunar and Planetary Lab.) and NASA are also gratefully acknowledged. Operations support from the UK Space Agency under grant ST/R003025/1 is also acknowledged.LLT wishes to personally acknowledge funding and support from the Canadian Space Agency (CSA) through their Planetary and Astronomy Missions Co-Investigator programme ( 19PACOI07 ) and the Canadian NSERC Discovery Grant programme ( RGPIN 2020-06418 ).
Publisher Copyright:
© 2022 Elsevier Ltd
PY - 2022/5
Y1 - 2022/5
N2 - Recurring Slope Lineae (RSL) are narrow, dark streaks typically lengthening down Martian steep slopes during warm seasons, fading during the cold ones and regularly recurring every Martian year. Their origin is still debated. Although initially interpreted as possible flows of seeping water, either coming from a subsurface aquifer or through atmospheric processes, recent studies favor a dry granular flow origin. To date, the nature and formation mechanism of RSL represent an open science question about present-day surface processes occurring on Mars. In this study, we analyze color observations of RSL at Horowitz crater, acquired with the Colour and Surface Science Imaging System (CaSSIS) on board ESA's ExoMars Trace Gas Orbiter (TGO) mission. We compare the relative photometry of RSL with respect to nearby terrains with the relative reflectance of dust-removed surfaces, including dust-devil tracks, in the four CaSSIS filters to help assess their properties. Comparing our relative photometry with dust-deposition and soil-wetting models coming from published laboratory experiments, we find that the former results provide a better fit to the observations than the latter, hence supporting a dry origin for Horowitz RSL.
AB - Recurring Slope Lineae (RSL) are narrow, dark streaks typically lengthening down Martian steep slopes during warm seasons, fading during the cold ones and regularly recurring every Martian year. Their origin is still debated. Although initially interpreted as possible flows of seeping water, either coming from a subsurface aquifer or through atmospheric processes, recent studies favor a dry granular flow origin. To date, the nature and formation mechanism of RSL represent an open science question about present-day surface processes occurring on Mars. In this study, we analyze color observations of RSL at Horowitz crater, acquired with the Colour and Surface Science Imaging System (CaSSIS) on board ESA's ExoMars Trace Gas Orbiter (TGO) mission. We compare the relative photometry of RSL with respect to nearby terrains with the relative reflectance of dust-removed surfaces, including dust-devil tracks, in the four CaSSIS filters to help assess their properties. Comparing our relative photometry with dust-deposition and soil-wetting models coming from published laboratory experiments, we find that the former results provide a better fit to the observations than the latter, hence supporting a dry origin for Horowitz RSL.
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U2 - 10.1016/j.pss.2022.105443
DO - 10.1016/j.pss.2022.105443
M3 - Article
AN - SCOPUS:85126301458
VL - 214
JO - Planetary and Space Science
JF - Planetary and Space Science
SN - 0032-0633
M1 - 105443
ER -