Lucia Marinangeli

MARSIS Data Inversion Approach

In this paper we describe an inversion approach in order to analyze data from the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument on Mars Express. The inversion process allows the dielectric constant of the subsurface material to be estimated provided the dielectric constant of the surface is known. In addition, if impurity are present, it is possible to estimate the dielectric constant of any inclusions as well as the percentage amount of material in the inclusions relative to the host material provided knowledge of the host material up to the depth where the interface has been detected is available. The data inversion method is based on the analysis of the surface to subsurface power ratio and the relative time delay as measured by MARSIS. The data inversion has been performed at several frequencies in order to estimate the frequency dependent parameters affecting the behavior of the radar echoes. It is necessary that the surface and subsurface interfaces have the same roughness in order to estimate the Subsurface Fresnel reflectivity. As a preliminary approach, only flat surface have been selected. MOLA (Mars Orbiter Laser Altimeter) has already provided detailed data on the visible Martian surface and a simulator, with a facet model, has been utilized to use MOLA data in order to verify the correct selection of the frames that will be used for the data inversion (absence of clutter echoes).

“Unconformity-Bounded” Stratigraphic Units in the South Polar Layered Deposits (Promethei Lingula, Mars)

We conducted a stratigraphic analysis of the South Polar Layered Deposits (SPLDs) in Promethei Lingula (PL, Mars) based on the identification of regional unconformities at visible and radar wavelengths. According to the terrestrial classification, this approach constrains the stratigraphy of the region and remedies the ambiguous interpretation of stratigraphy through marker layers, bypassing the problem related to the morphologic and radiometric appearance of the layers. Thus, the approach does not exclude diverse classifications, but complements them, whereas other discriminant elements are doubtful or difficult/impossible to be defined. Using this approach, we defined two stratigraphic units (or synthems: PL1 and PL2) in PL, which are morphologically different and divided by a regional unconformity (AuR1). This stratigraphic architecture implies that the geological history of PL has been conditioned by periodic changes in climate, which in turn are related to orbital variations of Mars.

Application of Sequence-Stratigraphic Concepts to Mars: Eberswalde Crater

Sedimentary deposits within Eberswalde Crater are generally interpreted as having formed in a fluviolacustrine depositional environment. The Eberswalde fan delta consists of five lobes (four deltaic), the relative stratigraphy of which can be unravelled through simple cross-cutting relationships, allowing inferences to be made of the approximate water level from the transition between delta plain and delta front. Switching between different lobes is inferred to have been controlled by allogenic control. We distinguish three water-level fluctuations, possibly associated with system tracts, on which a higher order regressive trend is superimposed. The three lower-order cycles may partly reflect control by transient and/or localized processes, but the cyclic behaviour suggests the presence of climatic control.

Electromagnetic parameters measurements of clay soils for Mars radar sounding

The martian shallow crust has been studied through two subsurface sounding radars, MARSIS and SHARAD which operate at 3-5 and 20 MHz, respectively. The capability of radar to resolve the subsurface structures and the stratigraphy relates both on the radar features and the electromagnetic parameters of the shallow crust. For small grain size sediments, like clay minerals, the dielectric properties strongly affect the penetration depth of the radar signal. In this study, we measured several clayey samples, collected from various geological settings, with different mineralogy and water content using a Vector Network Analyzer (VNA) in the frequency range 1MHz-6GHz. The electrical properties were estimated as a function of temperature (200-298K) via the Nicolson-Ross-Weir algorithm using a coaxial-cage probe connected to the VNA. These measurements are used to interpret radar data in terms of depth penetration and surface echoes strength which could be affected by the temperature diurnal variation of martian surface.

Radar subsurface sounding over the putative frozen sea in Cerberus Palus, Mars

The area of Mars known as Cerberus Palus, suspected of harboring a frozen body of water, has been observed by the two subsurface sounding radar MARSIS and SHARAD. SHARAD data reveal subsurface interfaces at depths ranging from ~50 m to ~150 m which could be interpreted as either the bottom of an ice sheet lying over bedrock, or an interface between two lava flows. Echoes have been analyzed to estimate the dielectric properties of the surface layer, and results favor the interpretation that no ice is present in the area.