Simulating evaporative wet and dry cycles in Gale crater, Mars using
thermochemical modeling techniques.
Abstract
The aim of this work is to understand the formation of primary
evaporites—sulfates, borates, and chlorides—in Gale crater using
thermochemical modeling to determine constraints on their formation. We
test the hypothesis that primary evaporites required multiple wet-dry
cycles to form, akin to how evaporite assemblages form on Earth.
Starting with a basalt-equilibrated Mars fluid, Mars-relevant
concentrations of B and Li were added, and then equilibrated with Gale
lacustrine bedrock. We simulated cycles of evaporation followed by
groundwater recharge/dilution to establish an approximate minimum number
of wet-dry cycles required to form primary evaporites. We determine that
a minimum of 250 wet-dry cycles may be required to start forming primary
evaporites that consist of borates and Ca-sulfates. We estimate that
~14,250 annual cycles (~25.6 k Earth
years) of wet and dry periods may form primary borates and Ca-sulfates
in Gale crater. These primary evaporites could have been remobilized
during secondary diagenesis to form the veins that the Curiosity rover
observes in Gale crater. No LiCl salts form after 14,250 cycles modeled
for the Gale-relevant scenario (approximately 106 cycles would be
needed) which implies Li may be leftover in a groundwater brine after
the time of the lake. No major deposits of borates are observed to date
in Gale crater which also implies that B may be leftover in the
subsequent groundwater brine that formed after evaporites were
remobilized into Ca-sulfate veins.