Abstract
Flowing water and brine have been proposed to cause seasonally reappearing dark streaks called recurring slope lineae (RSL) on steep warm slopes on Mars, along with other formation mechanisms that do not involve water. This study aims to examine whether the evaporation of water vapor from the RSL, whether from fresh water or brine, is detectable by observing water vapor and/or clouds. In this study, we summarize the possible rate and duration of water-vapor emission from RSL in different scenarios, simulate how the emitted water vapor behaves in a global climate model, and discuss the detectability of water vapor in nadir observations during existing and future explorations. We found that, in typical cases, rapid horizontal dissipation within the planetary boundary layer (PBL) following the release of water vapor prohibits cloud formation and the excess water vapor from being distinguished from the background with existing observations. Thus, we conclude that the lack of correlation between the RSL activities and the overlying water-vapor column density does not necessarily rule out the wet origin of RSL. Nevertheless, we also found that water vapor tends to accumulate in basins and valleys in some cases due to the combined effects of topography and low PBL; we suggest the locations of such configuration as targets for future atmospheric studies of Mars dedicated to quantifying water-vapor release (associated with RSL) to elucidate the formation mechanism(s) of the RSL on the planet.
Highlights
Water on the surface of present-day Mars has attracted great attention for a long time as a record of the history of the surface environment, target for astrobiological investigations, and a possible resource for future human exploration missions
∆α listed here were smaller than the “background,” the water-vapor column density observed by Mars Global Surveyor (MGS)-Thermal Emission Spectrometer (TES) (Section 2.1; Smith 2008) by 2–3 orders of magnitude due to the horizontal grid size being larger than that of a recurring slope lineae (RSL) site, the actual values at the emission spots can be much higher when investigated on a smaller scale
We note that we consider the loss of water vapor to the surface neither by regolith adsorption nor by deliquescence of salts, and water vapor emitted in our Mars Global Climate Model (MGCM) simulations should be considered as an upper limit
Summary
Water on the surface of present-day Mars has attracted great attention for a long time as a record of the history of the surface environment, target for astrobiological investigations, and a possible resource for future human exploration missions. RSL are lengthening streaks on Mars that reappear each year during the warm seasons and fade during the cold seasons; they are possibly linked to temperature and/or solar irradiation on the surface of the planet. Liquid water and brine have been proposed as the flowing material on RSL itself (McEwen et al 2011, 2014; Grimm et al 2014; Stillman et al 2014, 2016, 2017) or as a trigger of the granular flow (e.g., melting and boiling) (Massé et al 2016; Raack et al 2017). Several studies have proposed that RSL are dry granular flows triggered by liquid-free mechanisms, such as solar irradiation-induced Knudsen pump, and that the albedo change is caused by the sorting of grains and removal/deposition of dust (Dundas et al 2017; Dundas 2020; Schmidt et al 2017; Schaefer et al 2019)
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