D. Biccari

Radar Soundings of the Subsurface of Mars

The martian subsurface has been probed to kilometer depths by the Mars Advanced Radar for Subsurface and Ionospheric Sounding instrument aboard the Mars Express orbiter. Signals penetrate the polar layered deposits, probably imaging the base of the deposits. Data from the northern lowlands of Chryse Planitia have revealed a shallowly buried quasi-circular structure about 250 kilometers in diameter that is interpreted to be an impact basin. In addition, a planar reflector associated with the basin structure may indicate the presence of a low-loss deposit that is more than 1 kilometer thick.

Radar Sounding Evidence for Buried Glaciers in the Southern Mid-Latitudes of Mars

Lobate features abutting massifs and escarpments in the middle latitudes of Mars have been recognized in images for decades, but their true nature has been controversial, with hypotheses of origin such as ice-lubricated debris flows or glaciers covered by a layer of surface debris. These models imply an ice content ranging from minor and interstitial to massive and relatively pure. Soundings of these deposits in the eastern Hellas region by the Shallow Radar on the Mars Reconnaissance Orbiter reveal radar properties entirely consistent with massive water ice, supporting the debris-covered glacier hypothesis. The results imply that these glaciers formed in a previous climate conducive to glaciation at middle latitudes. Such features may collectively represent the most extensive nonpolar ice yet recognized on Mars.

An incoherent simulator for the SHARAD experiment

SHARAD (shallow radar) is a sounder and altimeter with synthetic aperture radar (SAR) capability provided by the Italian Space Agency (ASI) as a Facility Instrument to NASApsilas 2005 Mars Reconnaissance Orbiter. Primary objective of this nadir-looking sounder is to map Martian surface and subsurface down to several hundred meters depth with vertical resolution of 15 m and horizontal resolution of a few hundred meters (300 m-1 Km). In this paper we are going to present an incoherent simulator for the surface echoes received from SHARAD that utilizes only geometrical projections and the operating principle of the synthetic aperture radar. Making use of a convenient model of the Martian surface, our simulator is an important instrument for discriminating subsurface echoes from clutter artifacts.

Mars surface models and subsurface detection performance in MARSIS

The MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument is a multi-spectral, low. frequency, nadir looking pulse limited radar sounder and altimeter with ground penetration capability. Moreover the detection of a subsurface interface will be possible only if the following conditions are met:-the level of the subsurface reflection is higher than the noise floor-the surface/subsurface dynamic is included in the system dynamic range-the subsurface reflection is higher than the corresponding surface clutter reflection For MARSIS the noise floor has been evaluated to be about 60 dB below the fully coherent surface echo, so that an overall 60 dB dynamic range will be allowed if sidelobes and nonlinearities are controlled and reduced down to the noise level with a proper design. In this case we can assess that the penetration depth can be defined as that depth where the subsurface power is equal to the surface clutter power. In this paper the subsurface to surface clutter ratio will be evaluated, taking also into account the results related to the new fractal models of the structure of the planets surface and in particular the new MARS surface models obtained from the MGS/MOLA data.

Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS): subsurface performances evaluation

According to the Mars Express mission, the MARSIS primary scientific objectives are to map the distribution of water, both liquid and solid, in the upper pot-lions of the crust of Mars. Three secondary objectives are also defined subsurface geologic probing, surface characterization, and ionosphere sounding. In order to obtain the primary objectives the Radar Sounder design was based on the Ice/water interface and Dry/ice interface scenario: defining the material composition of the first layers and porosity and the pore filling materials. Concerning the surface, we have characterized the geometric structure in terms of a large-scale morphology, on which a small-scale geometric structure, due to rocks, is superimposed, taking into account also that recently the structure of the planets surface was described by means of fractals and in particular the new MARS surface models obtained by processing of the MOLA data. According to these models, this paper provides a description of the operational planning approach and expected performances of MARSIS.

Mars high resolution Shallow Radar (SHARAD) for the MRO 2005 mission

The search for water is a primary focus of Mars exploration. At the surface of Mars, water is present as ice in the polar ice caps and in trace quantities in the atmosphere. To detect liquid water on Mars today probably requires searching below the surface. As noted in the MRO SDT Report, the unambiguous detection of liquid water in the upper crust of Mars and the profiling of ice in the subsurface, particularly within one kilometer of the surface, would be major discoveries in the exploration of Mars. To that end, the MRO SDT recommended that flight of a subsurface sounding radar be considered for MRO, if the radar could confidently detect liquid water and profile ice in the topmost 1 km of subsurface with approximately 10 in vertical resolution. In response to this recommendation a team of italian researchers, sponsored by the Italian Space Agency (ASI), has proposed a Shallow Radar sounder (SHARAD). In this paper the main features of the SHARAD (to be complementary to MARSIS) in terms of the expected penetration performance according to proposed models of the martian crust composition and surface scattering are described.

MARSIS data inversion approach: Preliminary results

An approach to the inversion of the data available from the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument on Mars Express is described. The data inversion gives an estimation of the materials composing the different detected interfaces, including the impurity (inclusion) of the first layer, if any, and its percentage, by the evaluation of the values of the permittivity that would generate the observed radio echoes. 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 constraints, due to the known geological history of the surface, the local temperature and the thermal condition of the observed zones and the results of other instruments on Mars Express and other missions to Mars, have to be considered to improve the validity of the utilized models and the obtained results that are given in parametric way.

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).

Adaptive compensation of Mars ionosphere dispersion in the MARSIS experiment

MARSIS (Mars Advanced Radar for Subsurface Ionosphere Sounding), selected as scientific payload of the ESA mission Mars Express, is a low-frequency nadir-looking pulse limited radar sounder and altimeter with ground penetration capabilities, which uses synthetic aperture techniques and a secondary receiving antenna to isolate subsurface reflections. In order to maximize the penetration capabilities of the transmitted pulse MARSIS must operate at a frequency as low as possible (few MHz). Moreover the requirement for fine range resolution calls for a relatively large transmitted bandwidth (1 MHz) so that MARSIS will operate with a very high fractional bandwidth and very dose to the expected Martian ionosphere plasm frequency. This will result in a generally large phase distortion across the spectrum of the received pulses (due to either the antenna frequency response or the propagation through the ionosphere) which will cause severe degradation of the matched filter performance in term of SNR, pulse spreading and sidelobe level. This paper deals with the definition and implementation of an adaptive range compression algorithm, which makes use of the contrast maximization technique to estimate the phase dispersion spectrum, in order to perform a matched filtering of radar sounder echoes in the presence of phase distortions across the signal bandwidth.

SHARAD, a shallow radar sounder to investigate the red planet

SHARAD is a sounder provided by ASI, which is participating as a facility instrument to 2005 NASApsilas Mars Reconnaissance Orbiter mission. SHARAD is, together with MARSIS, the only sounding instrument that orbits around Mars. It has a higher vertical resolution than MARSIS, however the latter has a greater penetration capability. Goal of this nadir-looking Altimeter with synthetic aperture capabilities is to investigate surface and subsurface of Mars and subsequently provide radar data that supply unique information concerning dielectric interfaces. The sounderpsilas low-frequency and wideband features, meaning respectively a carrier band of 20 MHz and a bandwidth of 10 MHz, give the opportunity to achieve a theoretical vertical resolution of 15 meters in free space, maintaining an acceptable penetration capability of approximately 1500 meters. The instrument is composed of two main subsystems: the antenna (a 10 meter foldable dipole) and the SHARAD Electronic Box. In order to maximize the signalpsilas received power and increase the quality of the data acquired, a certain number of calibration measurements on-ground and in-flight have been executed. The calibration outcome is part and parcel of the on ground processing of the radar. The processing method through which Planetary Data System compliant products are generated at the SHARAD operational center (SHOC) includes both range compression for the vertical resolution and synthetic aperture processing to achieve along-track resolution. To discern surface and subsurface echoes from clutter and thus support the scientific analysis of the data, the SHOC team developed an incoherent simulator for surface echoes.