Paleohydrology on Mars constrained by mass balance and mineralogy of pre-Amazonian sodium chloride lakes: Deep groundwater not required

1M. Melwani Daswani,1E. S. Kite
Journal of Geophysical Research Planets (in Press) Link to Article [DOI: 10.1002/2017JE005319]
1Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois, USA
Published by arrangement wit John Wiley & Sons

Chloride-bearing deposits on Mars record high-elevation lakes during the waning stages of Mars’ wet era (mid-Noachian to late Hesperian). The water source pathways, seasonality, salinity, depth, lifetime, and paleoclimatic drivers of these widespread lakes are all unknown. Here we combine reaction-transport modeling, orbital spectroscopy, and new volume estimates from high-resolution digital terrain models, in order to constrain the hydrologic boundary conditions for forming the chlorides. Considering a T = 0 °C system, we find: (1) individual lakes were >100 m deep and lasted decades or longer; (2) if volcanic degassing was the source of chlorine, then the water-to-rock ratio or the total water volume were probably low, consistent with brief excursions above the melting point and/or arid climate; (3) if the chlorine source was igneous chlorapatite, then Cl-leaching events would require a (cumulative) time of >10 yr at the melting point; (4) Cl masses, divided by catchment area, give column densities 0.1 – 50 kg Cl/m2, and these column densities bracket the expected chlorapatite-Cl content for a seasonally-warm active layer. Deep groundwater was not required. Taken together, our results are consistent with Mars having a usually cold, horizontally segregated hydrosphere by the time chlorides formed.

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