Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p13g..04k&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P13G-04
Astronomy and Astrophysics
Astronomy
[6225] Planetary Sciences: Solar System Objects / Mars, [6297] Planetary Sciences: Solar System Objects / Instruments And Techniques, [6949] Radio Science / Radar Astronomy
Scientific paper
In the search for buried water ice on Mars, radar sounding instruments have unique abilities. Their capacity to resolve glacial structures down to kilometers depth has already provided a huge amount of information related to Martian glaciers. Sounding radars are also relevant tools to determine the composition of the surface, since the radar-waves reflectivity is sensitive to the dielectric properties of the sounding materials. It also has the originality to be representative of the first decameter of the surface (depending on the bandwidth), whereas other observations methods do not exceed few millimeters. The Shallow Radar (SHARAD) is a subsurface sounding instrument aboard the NASA's Mars Reconnaissance Orbiter (MRO) spacecraft [3]. SHARAD is working at a 20 MHz central frequency with a 10 MHz bandwidth. The along-track foot print range is between 0.3 and 1 km, while the typical footprint radius (Fresnel zone) is ~3 km. The radar surface echoes from SHARAD observations are extracted to drawn up a reflectivity map covering almost half of the Martian surface and compared to roughness maps. A 2-steps method, based on a stochastic description of the reflectivity, is proposed in order to (i) separate the coherent/incoherent components of the signal by the mean of PDF (Probability Density Functions) fitting of amplitude distributions, (ii) and to express them with respect to roughness/permittivity values by adapting common backscattering models to the nadir case. We show that scattering is the most important process dominating the reflectivity over the Martian terrains. However some nearly-flat regions clearly exhibit the signature of the surface permittivity. We show that the RMS roughness can be derived at centimeter to decimeter scale without prior calibration of the signal for slightly rough surfaces, which raises SHARAD capabilities in determining surface roughness for landing site selection. Sets of derived dielectric constants are obtained and analyzed regarding to the reference signal used for calibration. Given the very different implications of each set to the composition of the Martian terrains, it is emphasized that a good calibration of the signal should be a major issue for SHARAD and the next interplanetary radar missions.
Grima Cyril
Herique Alain
Kofman Wlodek
Seu Roberto
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