Dielectric signatures of adsorbed and salty liquid water at the Phoenix landing site, Mars

Abstract

The real part of the dielectric permittivity of the Martian regolith was measured by the Thermal and Electrical Conductivity Probe (TECP) on the Phoenix lander. We interpret these data using laboratory measurements of permittivity as a function of H2O and salt content, soil type, and temperature. Due to variability in sensor coupling, we focus on data taken at one locality (“Vestri”) three separate times, spanning multiple sols. A daytime increase in permittivity suggests progressive melting of a heterogeneous, disconnected, salty ice with a eutectic temperature of ∼239 K, which is close to the eutectic temperatures of NaClO4 or MgCl2. We found no evidence for Mg(ClO4)2. NaClO4 and MgCl2 are consistent with precipitation by freezing following a prior epoch of high obliquity. The evaporation of diurnal briny meltwater is inhibited by surface tension in small pores. An increase in permittivity occurred on the night of sol 70 that coincided with surface frost and measurement of a decrease in atmospheric water vapor. The permittivity jump can be matched by an increase in adsorbed H2O from ∼1 monolayer to 3 monolayers in an analog soil with a Viking‐like specific surface area (17 m2/g). However, the amount of adsorbed H2O is an order of magnitude larger than that inferred to have precipitated during the night. We suggest that the electrical signature of adsorbed water on Mars is stronger than we measured in the laboratory, possibly due to radiation damage of the regolith.