Paleohydrology on Mars constrained by mass balance and mineralogy of pre-Amazonian sodium chloride lakes

TL;DR

This study combines modeling, spectroscopy, and terrain data to constrain the conditions of chloride-bearing lakes on Mars, revealing they were deep, long-lasting, and formed under cold, arid conditions without deep groundwater involvement.

Contribution

It provides new constraints on the hydrologic boundary conditions and source mechanisms for chloride deposits on Mars, integrating multiple data sources and modeling approaches.

Findings

Lakes were >100 m deep and persisted for decades or more.

Chloride likely originated from volcanic degassing or igneous sources, requiring specific conditions.

Mars's hydrosphere was probably cold and horizontally segregated during chloride formation.

Abstract

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 degrees 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/m^2, 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.