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

Article


Stillman, D. E., Pettinelli, E., Lauro, S. E., Mattei, E., Caprarelli, G., Cosciotti, B., Primm, K. M. and Orosei, R.. 2022. "Partially-Saturated Brines Within Basal Ice or Sediments Can Explain the Bright Basal Reflections in the South Polar Layered Deposits." Journal of Geophysical Research: Planets. 127 (10). https://doi.org/10.1029/2022JE007398
Article Title

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

ERA Journal ID210883
Article CategoryArticle
AuthorsStillman, D. E., Pettinelli, E., Lauro, S. E., Mattei, E., Caprarelli, G., Cosciotti, B., Primm, K. M. and Orosei, R.
Journal TitleJournal of Geophysical Research: Planets
Journal Citation127 (10)
Article Numbere2022JE007398
Number of Pages16
Year2022
PublisherJohn Wiley & Sons
Place of PublicationUnited States
ISSN2169-9097
2169-9100
Digital Object Identifier (DOI)https://doi.org/10.1029/2022JE007398
Web Address (URL)https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022JE007398
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.

KeywordsBrines ; radar reflections
Byline AffiliationsSouthwest Research Institute, United States
Third University of Rome, Italy
Centre for Astrophysics
Planetary Science Institute, United States
National Institute for Astrophysics, Italy
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