PROBING THE POLAR DEPOSITS OF MARS WITH THE MARSIS SOUNDING RADAR

The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) is the first instrument of its kind to visit another planet. Flying onboard the European Space Agency's Mars Express orbiter, MARSIS began sounding the subsurface and ionosphere of Mars in June, 2005. MARSIS has now completed its first global cycle of mapping, sampling most regions of Mars in its subsurface sounding mode. MARSIS is a synthetic-aperture orbital sounding radar. In its subsurface modes, MARSIS operates in two of four frequency bands between 1.3 and 5.5 MHz, with a free-space range resolution of approximately 150 m. Lateral spatial resolution for the cross-track footprint is 10-30 km, and for the along-track footprint, 5-10 km.

The ice-rich polar layered deposits (PLD) are thought to contain a record of climate variations. MARSIS signals easily penetrate the PLD, usually to their basal contact with the substrate. The typically strong echo return from the lower interface of the PLD indicates that only minor attenuation of the radar signal is occurring within the PLD indicating that they consist of ice (most likely water ice) with only a minor component (<10% by mass) of dust. In the south polar region, MARSIS data were used to map the bed topography and the thickness of the PLD, and to calculate their volume. The total volume of the deposits is estimated to be equivalent to a global layer 11 m thick. Preliminary analyses indicate that the North PLD are as transparent to MARSIS signals as are the South PLD. Outside of the PLD, interfaces have been detected in the shallow (<1 km deep) polar subsurface, particularly in the south. A pervasive shallow interface occurs under much of the south polar plains, with a remarkable correlation with the previously mapped Hesperian Dorsa Argentea formation. The polar location, the association with a geologic unit with likely aqueous formation mechanisms, and the relative transparency of this layer lead us to hypothesize that the material is ice-rich. If this proves to be correct, the ice contained in this unit would comprise the second largest known reservoir of H2O on Mars, behind the PLD themselves.