Stratigraphy of the South Polar Layered Deposits in SHARAD Radar and Images

SHARAD (Shallow Radar) is a synthetic-aperture, orbital sounding radar on the Mars Reconnaissance Orbiter (MRO) operating at 20 MHz, with a 10-MHz bandwidth that provides a free-space range resolution of approximately 15 m after pulse compression. SHARAD observes many tens of internal reflections in the Promethei Lingula (90°-140°E) region of the south polar layered deposits (SPLD). Multiple parallel reflections are traceable over distances of several hundred km in individual radargrams. There are several packets of multiple reflections separated by non-reflective regions with depth. By comparing crossing orbits and using stratigraphic correlations between orbits, a three dimensional picture of the internal structure of the SPLD as observed by SHARAD can be assembled. Reflections extend many hundreds of km throughout the Promethei Lingula region, and generally decrease in elevation towards the margins of the deposit. An angular unconformity between ~ 108°E and 123°E is observed in multiple orbits, and may be related to erosional features observed in the nearby canyon of Australe Sulci. Additional unusual reflections are observed in two orbits near 108°E, 84°S and are currently undergoing closer analysis. Comparisons of SHARAD reflections that intersect with the surface and THEMIS and MOC images indicate that an individual reflection may correlate to multiple (3-7) layers at MOC resolutions (~ 6 m/pxl). The layers in this region are eroding in groups, resulting in a stair-stepped topographic profile; the underlying physical properties of the layers causing this erosional behavior may also be the cause of a radar reflection. By fitting a surface to correlated reflections in multiple orbits using an inverse distance weighting technique and extrapolating that surface to the wall of Chasma Australe, it is possible compare the elevations of the reflectors to the sequence of layers exposed on the wall in THEMIS images. Preliminary results indicate that three major reflectors correlate with changes in layering styles within the SPLD. In particular, a sequence of dark, thin layers near the bottom of the layer stratigraphy in this region may correlate to a single reflection. A shift in layering style between a region where individual layers tend to be subtle and hard to distinguish and a region of thicker (~ 40 m), bright, erosion- resistant layers may correlate to another reflection.