Multiple subglacial water bodies below the south pole of Mars unveiled by new MARSIS data

The detection of liquid water by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) at the base of the south polar layered deposits in Ultimi Scopuli has reinvigorated the debate about the origin and stability of liquid water under present-day Martian conditions. To establish the extent of subglacial water in this region, we acquired new data, achieving extended radar coverage over the study area. Here, we present and discuss the results obtained by a new method of analysis of the complete MARSIS dataset, based on signal processing procedures usually applied to terrestrial polar ice sheets. Our results strengthen the claim of the detection of a liquid water body at Ultimi Scopuli and indicate the presence of other wet areas nearby. We suggest that the waters are hypersaline perchlorate brines, known to form at Martian polar regions and thought to survive for an extended period of time on a geological scale at below-eutectic temperatures

Liquid Water Detection under the South Polar Layered Deposits of Mars—a Probabilistic Inversion Approach

Liquid water was present on the surface of Mars in the distant past; part of that water is now in the ground in the form of permafrost and heat from the molten interior of the planet could cause it to melt at depth. MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) has surveyed the Martian subsurface for more than fifteen years in search for evidence of such water buried at depth. Radar detection of liquid water can be stated as an inverse electromagnetic scattering problem, starting from the echo intensity collected by the antenna. In principle, the electromagnetic problem can be modelled as a normal plane wave that propagates through a three-layered medium made of air, ice and basal material, with the final goal of determining the dielectric permittivity of the basal material. In practice, however, two fundamental aspects make the inversion procedure of this apparent simple model rather challenging: (i) the impossibility to use the absolute value of the echo intensity in the inversion procedure; (ii) the impossibility to use a deterministic approach to retrieve the basal permittivity. In this paper, these issues are faced by assuming a priori information on the ice electromagnetic properties and adopting an inversion probabilistic approach. All the aspects that can affect the estimation of the basal permittivity below the Martian South polar cap are discussed and how detection of the presence of basal liquid water was done is described

Assessing the role of clay and salts on the origin of MARSIS basal bright reflections

Anomalously bright basal reflections detected by MARSIS at Ultimi Scopuli have been interpreted to indicate the presence of water-saturated materials or ponded liquid water at the base of the South Polar Layered Deposits (SPLD). Because conventional models assume basal temperatures (≤200 K) much lower than the melting point of water, this interpretation has been questioned and other explanations for the source of the bright basal reflections have been proposed, involving clay, hydrated salts, and saline ices. Combining previous published data, simulations, and new laboratory measurements, we demonstrate that the dielectric properties of these materials do not generate strong basal reflections at MARSIS frequencies and Martian temperatures. Plausible candidates remain perchlorates and chlorides brines that exhibit a strong dielectric response at much lower temperatures than other materials. This explanation might require that metastability could be maintained for a long period of time on a geological scale

Reply to: Explaining bright radar reflections below the south pole of Mars without liquid water

In their Matter Arising Lalich et al.1 simulate MARSIS echoes at the base of the South Polar Layered Deposits (SPLD) assuming three different layering scenarios (Fig. 1 in ref.1): (a) dusty water ice overlaying bedrock; (b) one CO2 ice layer between dusty water ice and bedrock; and, (c) two basal CO2 ice layers interbedded with one layer of dusty water ice. A surficial layer of CO2 ice ranging from 0 m (no layer) to 2 m in thickness is also considered. The first layer in each simulation is a semi-infinite half space assigned the permittivity of free space, and the bedrock is a semi-infinite half space with pure basaltic rock permittivity. These authors argue that constructive interference generated by some layered configurations produce waveforms (Fig. 2 in ref.1) with local maxima corresponding to the bright basal reflections observed by MARSIS at Ultimi Scopuli 2,3. They conclude that this explanation is more plausible than liquid brines being the source of the bright reflections, as posited instead by Orosei et al.2 and Lauro et al.3. In an earlier paper, however, Orosei et al.4 explored the same model and mathematics covering the entire range of possible parameters for two and three basal CO2 ice layers. Through the quantitative analysis of 3.45 x 108 simulation results, these authors demonstrated that local maxima at one of the MARSIS operating frequencies are not matched by local maxima at the other operating frequencies: that is, a layer stack producing constructive interference at one frequency, does not produce the same effect at the other frequencies, which is inconsistent with MARSIS real data. Thus, constructive interference by basal layers is not a viable mechanism to explain the bright basal reflections at Ultimi Scopuli. Because most of the points in Lalich et al.1 are superseded by Orosei et al.’s4 work, we refer interested readers to that earlier paper for a full discussion of the models and results. Here, we focus on three critical aspects: electromagnetic model; dielectric values used in the simulations; and materials and geology

Dielectric characterization of ice/MgSO4.11H2O mixtures as Jovian icy moon crust analogues

One of the main objectives of proposed missions to the icy Jovian moons is to prove the existence of the postulated subsurface ocean on Europa using radar sounders. The success of these missions will rely on the ability of the radar signals to penetrate ten kilometers of icy material that could potentially contain various types of impurities. In this work we quantify the impact of magnesium sulfate hydrates on the electrical properties of water ice by performing a series of dielectric measurements on different ice/MgSO4.11H2O mixtures as a function of frequency and at temperatures comparable with those expected on the icy satellite surfaces. Our results indicate that the salt only affects the real part of permittivity of the mixtures, whereas the imaginary part, hence the attenuation, does not significantly differ from that of pure ice. This means that in some regions signal penetration may be better than previously thought