TY - JOUR
T1 - Subsurface Cl-bearing salts as potential contributors to recurring slope lineae (RSL) on Mars
AU - Wang, Alian
AU - Ling, Zongcheng
AU - Yan, Yuanchao
AU - McEwen, Alfred S.
AU - Mellon, Michael T.
AU - Smith, Michael D.
AU - Jolliff, Bradley L.
AU - Head, James
N1 - Funding Information:
AW and YCY thank the support by NASA MoO project (06-Scout06-0027- #49137-NRA 1295053) for participating RLS investigations for the ESA-ExoMars rover mission that enabled the accomplishment of this extensive laboratory investigation, which began in 2012. AW thanks the McDonnell Center for the Space Sciences at Washington University in St Louis for specific financial support to continue a collaboration with Shandong University in China. ZCL thanks the National Natural Science Foundation of China (41473065, JQ201511, 41373068 and 41490634) that supported his visit to WUSTL in 2015–2016. We thank Professor Raymond Arvidson for providing a critical suggestion on the mass-balance of H 2 O budget relevant to the H 2 O recharging process. We thank Dr. J. Wei and Ms. Y. L. Lu for their assistances in the laboratory experiments of earlier years. We would express our appreciation to two anonymous reviewers whose critics, comments, and suggestions have helped the improvement of this manuscript.
Publisher Copyright:
© 2019 Elsevier Inc.
PY - 2019/11/15
Y1 - 2019/11/15
N2 - We report laboratory experimental results that support a brine-related hypothesis for the recurring slope lineae (RSL) on Mars in which the subsurface Cl-salts, i.e., hydrous chlorides and oxychlorine salts (HyCOS) are the potential source materials. Our experiments revealed that within the observed RSL temperature window TRSL (250–300 K), the deliquescence of HyCOS could occur in relative humidity ranges (RH ≥ 22%–46%) much lower than those for hydrous (Mg, Fe)-sulfates (RH ≥ 75%–96%). In addition, we demonstrated that the RH values kept by common HyCOS and hydrous sulfates in enclosures have a general trend as RHsulfates > RHperchlorates > RHchlorides (with same type of cation) in wide T range. It means that the required RH range for a Cl-bearing salt to deliquescence can be satisfied by a co-existing salt of different type, e.g., in the subsurface layers of mixed salts on Mars. Furthermore, we found a strong temperature dependence of the deliquescence rates for all tested HyCOS, e.g., a duration of 1–5 sols for all HyCOS at the high end (300 K) of TRSL, and of 20–70 sols for all tested HyCOS (except NaClO4·H2O) at the low end (250 K) of TRSL, which is consistent with the observed seasonal behavior of RSL on Mars. From a mass-balance point of view, the currently observed evidences on Mars do not support a fully-brine-wetted track model, thus we suggest a brine-triggered granular-flow model for the most RSL. Considering the recurrence of RSL in consecutive martian years, our experimental results support the rehydration of remnant HyCOS layers during a martian cold season through H2O vapor-to-salt direct interaction. We found that the evidences of HyCOS rehydration under Mars relevant P-T-RH conditions are detectable in a few minutes by in situ Raman spectroscopy. This rehydration would facilitate the recharge of H2O back into subsurface HyCOS, which could serve as the source material to trigger RSL in a subsequent warm season. The major limiting factor for this rehydration is the H2O supply, i.e., the H2O vapor density carried by current Mars atmospheric circulation and the diffusion rate of H2O vapor into the salt-rich subsurface in a cold season. In a worst-case scenario, these H2O supplies can support a maximum increase of hydration degrees of two for totally dehydrated HyCOS, whereas the full rehydration of subsurface HyCOS layers can be easily reached during a >30° obliquity period that has H2O vapor density 10× to 20× times the value of current obliquity. Overall, our results imply the existence of a large amount of Cl-bearing salts in the subsurface at RSL sites.
AB - We report laboratory experimental results that support a brine-related hypothesis for the recurring slope lineae (RSL) on Mars in which the subsurface Cl-salts, i.e., hydrous chlorides and oxychlorine salts (HyCOS) are the potential source materials. Our experiments revealed that within the observed RSL temperature window TRSL (250–300 K), the deliquescence of HyCOS could occur in relative humidity ranges (RH ≥ 22%–46%) much lower than those for hydrous (Mg, Fe)-sulfates (RH ≥ 75%–96%). In addition, we demonstrated that the RH values kept by common HyCOS and hydrous sulfates in enclosures have a general trend as RHsulfates > RHperchlorates > RHchlorides (with same type of cation) in wide T range. It means that the required RH range for a Cl-bearing salt to deliquescence can be satisfied by a co-existing salt of different type, e.g., in the subsurface layers of mixed salts on Mars. Furthermore, we found a strong temperature dependence of the deliquescence rates for all tested HyCOS, e.g., a duration of 1–5 sols for all HyCOS at the high end (300 K) of TRSL, and of 20–70 sols for all tested HyCOS (except NaClO4·H2O) at the low end (250 K) of TRSL, which is consistent with the observed seasonal behavior of RSL on Mars. From a mass-balance point of view, the currently observed evidences on Mars do not support a fully-brine-wetted track model, thus we suggest a brine-triggered granular-flow model for the most RSL. Considering the recurrence of RSL in consecutive martian years, our experimental results support the rehydration of remnant HyCOS layers during a martian cold season through H2O vapor-to-salt direct interaction. We found that the evidences of HyCOS rehydration under Mars relevant P-T-RH conditions are detectable in a few minutes by in situ Raman spectroscopy. This rehydration would facilitate the recharge of H2O back into subsurface HyCOS, which could serve as the source material to trigger RSL in a subsequent warm season. The major limiting factor for this rehydration is the H2O supply, i.e., the H2O vapor density carried by current Mars atmospheric circulation and the diffusion rate of H2O vapor into the salt-rich subsurface in a cold season. In a worst-case scenario, these H2O supplies can support a maximum increase of hydration degrees of two for totally dehydrated HyCOS, whereas the full rehydration of subsurface HyCOS layers can be easily reached during a >30° obliquity period that has H2O vapor density 10× to 20× times the value of current obliquity. Overall, our results imply the existence of a large amount of Cl-bearing salts in the subsurface at RSL sites.
KW - Deliquescence
KW - Hydrous Cl-bearing salts
KW - Mars
KW - Raman spectroscopy
KW - Recurring slope lineae
KW - Rehydration
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U2 - 10.1016/j.icarus.2019.06.024
DO - 10.1016/j.icarus.2019.06.024
M3 - Article
AN - SCOPUS:85068920160
SN - 0019-1035
VL - 333
SP - 464
EP - 480
JO - Icarus
JF - Icarus
ER -