A radar-derived 3-D basal map of the south polar layered deposits on Mars

Abstract:

The martian south pole hosts a large reservoir of water ice mixed with dust and lithic fragments that collectively form the south polar layered deposits (SPLD). The SPLD are hypothesized to have formed on the order of 100 thousand to 100 million years ago from ice deposition and erosion rates driven by the planet’s varying eccentricity and obliquity. Therefore, the SPLD provides a useful tool for studying the recent climatic history of Mars.

Orbital penetrating radar has been used for climate studies to peer into the interior of the SPLD to variable depths. Here, Shallow Radar (SHARAD) and Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) are used to understand the radar behaviors of the SPLD. Echoes from the SHARAD instrument onboard the Mars Reconnaissance Orbiter quickly attenuate across most of the SPLD and exhibit a fog-like return that is likely associated with volume scattering of the signal. In most cases, longer wavelength signals from MARSIS penetrate to the base of the SPLD, while even in the thinnest regions SHARAD cannot.

In part of a broader effort to better understand the “fog” scattering phenomenon and the overall paucity of SHARAD basal reflectors, we use MARSIS data in conjunction with SHARAD to produce a map of the basal interface. Noting where radar sounder echoes traverse the entire SPLD is a first step in characterizing the cause of the SHARAD fog-behavior. 1584 separate basal detections from 435 MARSIS radargrams were identified. These reflectors are located in the SPLD interior where the ice is relatively thick, with two notable gaps in the inner parts of Australe and Promethei Lingulua, as well as another around 280­­^oE. Several hundred SHARAD detections from ~1000 radargrams were mapped predominantly around the perimeter of the SPLD. Overlapping detections between both instruments tend to occur in regions that appear smooth on the surface, most noticeably apparent in the southwest region of the SPLD.

The elevation of these basal reflectors is calculated, assuming a real dielectric constant of water ice (3.15). The power of these reflectors is also recorded. The beginning and end points of each detection are mapped by hand, followed by a semi-automated process that fills in the profile of the reflector. The detection profiles are interpolated together to form a 3D map of the basal interface.