Mars mapping with delay-Doppler radar

Mars radar imaging results from Arecibo 12.6-cm observations are presented. The images were derived from delay-Doppler mapping using a coded-long-puls e technique to mitigate the effects of echo overspreading. Images of the de polarized echo are used to identify regions of high decimeter-scale roughne ss. Some of the strongest echo features are located on the major shield vol canoes or on relatively young off-shield flows such as the Olympus and Pavo nis lava aprons. The shields themselves have highly irregular radar signatu res suggesting complex volcanic histories. Some Mars radar features have tw ice the depolarized brightness of the roughest terrestrial lava flows, appa rently due to higher levels of multiple scattering from surfaces of spectac ular roughness or from volume scattering. Low-brightness (smooth) areas are associated with older surfaces such as fractured and highland terra, as we ll as with terrain interpreted to be debris lobes, ash flows, and aureoles; in particular, a close connection was found between the 12.6-cm counterpar t of the "Stealth" feature and the Medusae Fossae Formation (postulated to be deep ignimbrite deposits). Marte Vallis is anomalous in being the only o utflow channel showing strongly enhanced echoes, which supports the idea th at this channel and the Elysium Basin that it drained are filled with lava flows. A weak radar feature was found for the south polar residual ice cap. Comparisons with Goldstone 3.5-cm data show that the south polar enhanceme nt is much weaker at 12.6 cm than at 3.5 cm, indicating that the southern i ce cap becomes optically thin at the longer wavelength. A north polar enhan cement has also been found, which is comparable in strength to the 12.6-cm south polar feature.