¹A.M. Lee, ^1,2I.B. Smith, ¹J.A. Isen, ¹M.G. Daly
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2026.117127]
¹Centre for Research in Earth and Space Science, York University, Canada
²Planetary Science Institute, USA
Copyright Elsevier

MARSIS detected bright basal reflections at Ultimi Scopuli, near the south pole of Mars, which could imply the presence of liquid water or water saturated sediments. The dielectric permittivity was originally modelled to have a median value of 33 at 4 MHz, which is lower than expected for water in liquid form. Later investigations estimated values as high as 40 or as low as 4, where basal interface roughness is not considered. Besides roughness and water, alternative materials were suggested, such as brines or smectite clays. Two previous studies measured hydrated smectites at cold temperatures and found an apparent permittivity of 39 and 8.4. The larger number could explain the anomalous reflections, but the lower number would not. To review this discrepancy, we conducted experiments using a well-established smectite simulant at various hydration states at 230 K and 4 MHz, used for comparison to previous measurements. Our results show an apparent permittivity between 7 and 12 at 230 K, however, 180 K is considered a more reasonable estimation of basal South Polar Layered Deposits (SPLD) conditions. Our results at 180 K are close to 4, ruling out the presence of smectites as a potential source of the reflections. This study refines the techniques from a past study at York University, especially related to temperature control and gradients, and demonstrates the increasing capabilities of the MARVIN chamber.