Using MARSIS signal attenuation to assess the presence of South Polar Layered Deposit subglacial brines

Knowledge of the physical and thermal properties of the South Polar Layer Deposits (SPLD) is key to constrain the source of bright basal reflections at Ultimi Scopuli detected by the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) radar sounder. Here we present a detailed analysis of attenuation, based on data acquired by MARSIS at 3, 4, and 5 MHz. We show that attenuation is frequency dependent, and that its behavior is consistent throughout the entire region. This suggests that the SPLD are compositionally homogeneous at Ultimi Scopuli, and our results are consistent with dust contents of 5 to 12%. Using these values as input, and plausible estimates of surface temperature and heat flux, we inferred basal temperatures around 200 K: these are consistent with perchlorate brines within liquid vein networks as the source of the reflections. Furthermore, extrapolation of the attenuation to higher frequencies explains why SHARAD (Shallow Radar) has thus far not detected basal reflections within the SPLD at Ultimi Scopuli. MARSIS attenuation and thermal data confirm that liquid brines are the most plausible source for the bright reflections at the base of the South Polar Layered Deposits. Such results also justify why SHARAD does not penetrate to the base of the ice.

Automated summary

This study provides a detailed analysis of the physical and thermal properties of the South Polar Layer Deposits (SPLD) using data acquired by MARSIS. The frequency-dependent attenuation suggests that the SPLD are compositionally homogeneous at Ultimi Scopuli, and the results support the presence of perchlorate brines within liquid vein networks as the source of the bright basal reflections.