SHARAD: NO FLEXURE BENEATH THE NORTHERN CAP; IMPLICATIONS FOR THE THERMAL STATE OF MARS

SHARAD (SHAllow RADar) is a sounding radar on the Mars Reconnaissance Orbiter mission and was provided by the Italian Space Agency. It has a 20-MHz center frequency and 10-MHz bandwidth, providing a vertical resolution of 15 m (free space) and a horizontal resolution of several km. The radar has been particularly successful in sounding the polar deposits of Mars. SHARAD is complemented by the lower frequency, narrower bandwidth, capability of the MARSIS sounding radar aboard the Mars Express orbiter. MARSIS has a coarser vertical resolution of 150 m, but in many places can map the interface at the base of the polar deposits whereas SHARAD cannot. The northern deposits can be divided into two lobes, in part separated by Chasma Boreale, centered roughly on 0° and 180° longitudes. In the 0° lobe, SHARAD sounds to the base of the deposits and finds essentially no flexural/membrane downwarping due to the depositional load at the cap. The uncertainty in this measurement is about 50 m, and conservatively we state that there has been no more than 100 m of flexural/membrane response to the load. Thin elastic spherical shell loading models show that to get less than 100 m of deflection, the elastic thickness must be at least 300 km. By Earth analogy, the base of the elastic lithosphere is in the temperature range 500-600 °C. This yields a lithospheric temperature gradient of approximately 2 K/km, a significantly sub-Chondritic value and likely an unrealistic result. The alternative interpretation is that the lithosphere has not yet reached elastic equilibrium with the load. The controlling factor is the viscosity of the mantle, and a bound on the age of the load provides a bound on mantle viscosity. For example, for a more reasonable elastic thickness of 100 km, a load age of 100 Kyr corresponds to a mantle viscosity of about 10²³ Pa-s. The flexural deflection limit, using MARSIS, under the South Polar deposits is about 300 m due largely to the rugged nature of the heavily cratered substrate. But the age of the load is greater than 10⁷ years, implying a mantle viscosity greater than roughly 10²⁴ Pa-s. In all, relatively high viscosities and relatively low temperatures are implied for the martian mantle.