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

Strong radar reflections have been previously mapped at the base of the Martian South Polar Layered Deposits (SPLD).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, therebycausingstrong radar reflections. A brine concentration of 2.7–6.0vol% in ice issufficientto matchthe electrical properties of the basal reflection as observed by MARSIS. When brine is mixed with sediments, the brine-ice mixture in the pores must be 2–5times 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 ~6wt%.Thus, salt enhancement mechanisms on the order of a magnitude greater than the Phoenix landing site are needed. To form brine, thebasal reflectormust reach a temperature greater than the eutectic temperature of calcium perchlorate of 197.30.2K, 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, grey hematite with a concentration of 33.259.0 vol% possess electrical properties that could cause the observedradar returns, but require concentrations 23 times larger than anywhere currently detected.We also argue thatbrines mixed with high-surface-area sediments, or dry red hematite, jarosite, and ilmenite cannot create the observedradar returns at low temperatures