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

Abstract

It has recently been suggested that clay minerals, which are widespread on the Martian surface, could be the possible source of the basal bright reflections detected by MARSIS at Ultimi Scopuli, instead of briny water. This hypothesis is based on dielectric measurements on a wet Ca‐Montorillonite (STx‐1b) sample conducted at 230 K, which reported permittivity values (apparent permittivity of 39 at 4 MHz) compatible with the median value of 33 retrieved by MARSIS 4 MHz data inversion in the high reflectivity area. These experimental results are, however, incompatible with well‐established dielectric theory and with laboratory measurements on clays, at MARSIS frequency and Martian temperatures, reported in the literature. Here, we replicate the experiment using a setup to precisely control the rate of cooling/warming and the temperature inside and outside the clay sample. We found that the rate of cooling, the position of the temperature sensor and, consequently, the thermal equilibrium between the sample and the sensor play a fundamental role in the reliability of the measurements. Our results indicate that even for a large water content in the clay sample, at 230 K and 4 MHz, the apparent permittivity is only 8.4, dropping to 4.1 at 200 K, ruling out clays as a possible source of the bright reflections detected by MARSIS at the base of the SPLD.

Radar Observations of Liquid Water in the South Polar Region of Mars: Indications from Astrobiology Perspectives

“…They found that even in clay samples with high moisture content, the apparent permittivity exhibited was only 8.4 at 4 MHz and 230 K, and it was only 4.1 at 200 K. These values were significantly lower than the actual inversed values. This research has conclusively demonstrated that materials such as clays cannot produce the strong reflections observed by MARSIS [33].…”

“…In response to these doubts, researchers have conducted laboratory experiments to measure the dielectric properties of relevant materials under corresponding conditions. They argue that materials such as clay cannot cause such bright reflections and they continue to support the view that the cause of the bright reflections is brines with high chloride content [32,33]. Currently, the cause of these bright reflections remains inconclusive.…”

“…Therefore, they argued that, compared to other substances like minerals or clay, high-concentration brines are better suited to explaining the bright reflection at the the SPLDs base [32]. On the other hand, Cosciotti et al [33] replicated experiments measuring the dielectric properties of clay samples. They found that even in clay samples with high moisture content, the apparent permittivity exhibited was only 8.4 at 4 MHz and 230 K, and it was only 4.1 at 200 K. These values were significantly lower than the actual inversed values.…”

Water Ice Resources on the Shallow Subsurface of Mars: Indications to Rover-Mounted Radar Observation

“…Layered interference within the SPLDs and high-density igneous material with metallic inclusions are also thought to cause (Lalich et al [117], Grima et al [118]). Laboratory experiments have been conducted to measure the dielectric properties of relevant materials under similar conditions, and materials like clay cannot produce such bright reflections (Mattei et al [119], Cosciotti et al [120]).…”

Planetary Radar—State-of-the-Art Review

“…Because of its significance in the study of the biologic potential of Mars, the identification of liquid water has been closely scrutinized by the scientific community at large, and several counterarguments and alternative interpretations of the MARSIS measurements have been proposed over the last four years. Although the debate has yet to reach a conclusion, geologic evidence, all experimental work, and most electromagnetic modeling favor the presence of brines, possibly as interstitial fluid in sediments [160][161][162][163][164][165][166][167]. Dense measurement coverage in the polar regions has enabled 3-dimensional processing of the sounding data to improve resolution and better map continuous structures in the ice caps [168,169].…”