BURIED BASINS IN THE NORTHERN LOWLANDS OF MARS REVEALED BY THE MARSIS RADAR SOUNDER

The relatively featureless lowland plains of the northern hemisphere and the heavily cratered highlands of the southern hemisphere are the surface expression of the crustal dichotomy on Mars. The young appearance of the northern plains lead to the interpretation that the northern lowlands crust could also be relatively young. This view changed when subdued quasi-circular depressions (QCDs) in the northern lowlands were revealed in Mars Orbiter Laser Altimeter (MOLA) data. Interpreted to be buried impact craters, the QCDs suggest that the crater age of the northern lowlands crust is ancient. The initial operations of the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument on Mars Express in June-July, 2005 were over the northern lowlands. Data obtained over this period revealed evidence of 11 buried impact basins ranging in diameter from about 130 km to 470 km. Four of these basins coincide with previously mapped QCDs. The buried basins detected by MARSIS were used to estimate an independent areal crater density N(200), a measure of the cumulative number of basins with diameters >200 km per 10⁶ km², for the lowland crust. The area of the northern lowlands surveyed in the data obtained in 2005 by MARSIS is estimated to be about 14% of the lowlands. This corresponds to an N(200) crater density for the lowland crust of ~1.9. The N(200) crater density and the cumulative frequency plot of the MARSIS basins suggest an Early Noachian age for the northern lowland crust. MARSIS data collected over northern lowlands from about April 2006 to April 2007 are now being analyzed for evidence of other buried basins. Preliminary results suggest the presence of many previously unidentified buried basins. This work, in combination with ongoing efforts to refine the number of QCDs, should greatly improve our understanding of the age of the northern lowlands crust and ultimately the origin of the crustal dichotomy.