MARSIS and SHARAD radar reflections within Promethei Lingula, South Polar Layered Deposits, Mars

Recently, two sounding radar instruments have begun collecting information about the subsurface of the south polar layered deposits (SPLD) of Mars, which are made up of layers of primarily water ice mixed with small amounts of dust. Both MARSIS, the radar onboard Mars Express (operating at 1.8-5 MHz) and SHARAD, the radar onboard Mars Reconnaissance Orbiter (operating at 20 MHz), detect subsurface reflections at multiple depths within the SPLD in several locations, in particular the Promethei Lingula (90˚-140˚E) region of the SPLD. MARSIS observes up to three strong internal reflections, plus a strong basal reflection at approximately 1.2 - 1.5 km depth (assuming pure ice, consistent with the strength of the basal reflection). Reflections where orbits cross are at the same elevations in each radargram (within 100 m, or the resolution of the data) and the reflections in the remaining orbits are at similar elevations; we therefore conclude that the same reflectors are being observed in each orbit. SHARAD detects many tens of reflections with several packets of multiple reflections separated by non-reflective regions with depth; the lowest reflection is observed intermittently at approximately 1 km depth. Comparisons of neighboring and crossing orbits indicate that MARSIS reflections correlate to the boundaries of packets of reflections in the SHARAD data; whatever change in composition of the SPLD that causes the SHARAD reflections to occur in packets may also be the source of the MARSIS reflections. The deepest observable reflection in each SHARAD radargram is at a shallower depth than the deepest reflection in many MARSIS radargrams. The lack of any deeper reflectors in SHARAD may be due to attenuation of the radar signal; however, the contact between the SPLD and the underlying cratered plains as observed in images and topography data at the margins occurs at a similar elevation to the SHARAD basal reflection rather than the MARSIS basal reflection. Thus, SHARAD may be penetrating to the interface between the base of the SPLD and the underlying plains, and MARSIS may be detecting an ice-rich deposit underlying the SPLD in this region. Such a deposit may be related to the nearby volatile-rich Dorsa Argentea Formation.