3-D Subsurface Imaging and Analysis of Glaciers and Lava Fields with SHARAD Data

We propose to reveal and characterize new details of the internal structure and stratigraphy of Martian glaciers and lava fields by creating and analyzing 3-D volumes of Mars Reconnaissance Orbiter (MRO) Shallow Radar (SHARAD) sounding data. Questions we will address include: (1) Were there multiple phases of Martian glaciation? (2) Do all Martian glacial features have the same origin? and (3) How did the magnitude and style of volcanic eruptions vary over recent Martian History?

Long suspected to contain ice related to a prior climate state, so-called lobate debris aprons were shown to be ice-rich debris-covered glaciers because they allow strong SHARAD reflections at their bases. Elsewhere, young volcanic terrains exhibit some of the most complex subsurface structure detected by SHARAD outside of the polar caps. Multiple, overlapping radar reflectors at depth

have been associated with stacked lava flows and sediments that represent the most recent sequence of eruptions to occur on Mars. However, the ability to fully characterize these features has been hampered by the effects of the associated high topographic relief and complex structures on radar signals. Standard processing of single-pass observations mis-positions sloping interfaces, including the surface itself and geologic contacts at depth, and this effect often leads to interference between off-nadir surface reflections (clutter) and subsurface reflections that obfuscates the latter. Using 3-D imaging routines applied collectively to many observations, clutter can be placed in its true position within the data volume, providing an improved visualization of interfaces, structures, and stratigraphic relationships at depth.

To produce the needed 3-D data volumes for this study, we will leverage previously developed methodologies for existing SHARAD volumes encompassing Planum Boreum and Planum Australe and apply them to four smaller mid-latitude study areas. We collect SHARAD echoes from many orbit passes into a uniform grid, co-registering them geographically and adjusting their delay times to a common datum, taking care to remove residual ionosphere-induced time delays using surface-clutter simulations produced from digital elevation models (DEMs). We then use 3-D processing methods to correct the scattered echoes from varying topography and structure and to enhance the final image. Upon completion of each 3-D radar volume, we will carry out a thorough analysis of the results to identify and map previously obfuscated and finer-scale subsurface structures within the glaciers and lava fields. The site-specific DEMs we will produce from MRO CTX images together with standard imagery from CTX and MRO HiRISE will be used in our analysis to provide context and allow more confidence in inferring the nature of subsurface layering and structures where they may intersect the surface.

Expected outcomes of this study include a better understanding of the emplacement history of glaciers and lava flows, with associated implications for the recent climate history and thermal evolution of Mars. Our proposed work will build on the published results of SHARAD 3D imaging studies developed under prior Mars Data Analysis Program (MDAP) grants.