Partially‐Saturated Brines Within Basal Ice or Sediments Can Explain the Bright Basal Reflections in the South Polar Layered Deposits

Abstract

The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument on Mars Express has detected strong subsurface radar reflections in the region of Ultimi Scopuli (81°S, 193°E), within the South Polar Layered Deposits (SPLD) (Lauro et al., 2021;Orosei et al., 2018). These subsurface reflections are ∼10 dB greater in power than the surrounding reflections and ∼3 dB greater than the reflections from the surface (Orosei et al., 2018). The reflectors are located at the base of the SPLD, approximately 1.5 km below the topographic surface. Because data acquired by MARSIS do not separate the real (ε′) and imaginary (ε″) parts of the complex permittivity of reflectors, the apparent permittivity (ε a ), a single parameter accounting for both ε′ and ε″ (Mattei Abstract Strong radar reflections have been previously mapped at the base of the Martian South Polar Layered Deposits. Here, we analyze laboratory measurements of dry and briny samples to determine the cause of this radar return. We find that liquid vein networks consisting of brines at the grain boundaries of ice crystals can greatly enhance the electrical conductivity, thereby causing strong radar reflections. A brine concentration of 2.7-6.0 vol% in ice is sufficient to match the electrical properties of the basal reflection as observed by Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS). When brine is mixed with sediments, the brine-ice mixture in the pores must be 2-5 times more concentrated in salt, increasing the brine concentration to 6.3-29 vol%. Our best fit of the median observed MARSIS value suggests a salt-bulk sample concentration of ∼6 wt%. Thus, salt enhancement mechanisms on the order of a magnitude greater than the Phoenix landing site are needed. To form brine, the basal reflector must reach a temperature greater than the eutectic temperature of calcium perchlorate of 197.3 ± 0.2 K, which may be possible if more complex thermal modeling is assumed. Colder metastable brines are possible, but stability over millions of years remains unclear. Conversely, gray hematite with a concentration of 33.2-59.0 vol% possess electrical properties that could cause the observed radar returns, but require concentrations 2-3 times larger than anywhere currently detected. We also argue that brines mixed with high-surface-area sediments, or dry red hematite, jarosite, and ilmenite cannot create the observed radar returns at low temperatures.Plain Language Summary Previous research has detected strong radar reflections from the interface between Mars' southern ice cap and their underlying sediments over a region with an area of 20 × 30 km and 1.5 kms beneath the surface. Radar reflections are caused by changes in electrical properties. Here, we analyze electrical property laboratory measurements of materials under Mars-like conditions. We find that a small amount of brine in ice samples could create strong radar reflections similar to those that are observed. A greater concentration of salt is needed in sediment-ice mixtures. We su...

Can Clay Mimic the High Reflectivity of Briny Water Below the Martian SPLD?

“…Smith et al. (2021), however, reported anomalously large complex dielectric permittivity values for a smectite sample, mixed with a large amount of water, at ∼230 K. Because those values contrast significantly with the results reported at MARSIS frequency by other authors (e.g., Stillman et al., 2022), we replicated Smith et al.’s (2021) experiments in an effort to understand the origin of such discrepancy. We used the same type of clay (STx‐1b) and followed the same general approach described by those authors, running experiments over the frequency interval 1–100 MHz and at temperatures between 200 and 290 K. The experimental setup was designed to accurately control the temperature ramping and to monitor the temperature inside and outside the clay sample.…”

“…Recently, Mattei et al (2022) reviewed the literature relevant to the dielectric behavior of clays at very low temperature (around 200 K), highlighting that neither theoretical nor experimental data support the hypothesis that frozen clays have a large complex dielectric permittivity consistent with the subglacial basal reflections detected by MARSIS at Ultimi Scopuli. Smith et al (2021), however, reported anomalously large complex dielectric permittivity values for a smectite sample, mixed with a large amount of water, at ∼230 K. Because those values contrast significantly with the results reported at MARSIS frequency by other authors (e.g., Stillman et al, 2022), we replicated Smith et al's (2021) experiments in an effort to understand the origin of such discrepancy. We used the same type of clay (STx-1b) and followed the same general approach described by those authors, running experiments over the frequency interval 1-100 MHz and at temperatures between 200 and 290 K. The experimental setup was designed to accurately control the temperature ramping and to monitor the temperature inside and outside the clay sample.…”

Revising the Basal Permittivity of the South Polar Layered Deposits of Mars With a Surficial Dust Cover

“…Conversely, ɛ 2′ < ɛ 2 could lower the inverted basal permittivity toward materials more compatible with dry mixture of conductive materials (Bierson et al., 2021; I. B. Smith et al., 2021; Stillman et al., 2022; Stillman & Olhoeft, 2008; Tulaczyk & Foley, 2020). The thickness of the slab is also of importance.…”

“…Indeed, if the slab is assumed to be 5-m thick, as suggested by the excavation depth 160 km away reported by M. E. Landis and Whitten (2022), then the derived basal permittivity for 2 < ε′ 2 < 4.1 could range approximately anywhere between 20 and 60, possibly approaching on its low-end dry mixtures of conductive minerals (Bierson et al, 2021;I. B. Smith et al, 2021;Stillman et al, 2022;Stillman & Olhoeft, 2008;Tulaczyk & Foley, 2020;). This range would change for a different slab thickness, reaching lower basal permittivities for a thicker slab.…”

New Insights Into Composition Variation of Mars South Polar Layered Deposits From SHARAD Radar Sounder

“…Alternative materials such as clays, metal‐bearing minerals, volcanic‐related materials, or saline ice have been suggested as a possible explanation for the bright basal reflections (Bierson et al., 2021; Grima et al., 2022; Smith et al., 2021). However, recent studies have shown that these materials lack the necessary high dielectric permittivity to generate strong radar reflections at the temperatures hypothesized for the base of the SPLD (Mattei et al., 2022; Stillman et al., 2022).…”