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Science results from sixteen years of MRO SHARAD operations
In operation for >16 years to date, the Mars Reconnaissance Orbiter (MRO) Shallow Radar (SHARAD) sounder
has acquired data at its nominal 300–450 m along-track and 3-km cross-track resolution covering >55% of the
Martian surface, with nearly 100% overlap in coverage at that scale in the polar regions and in a number of
smaller mid-latitude areas. While SHARAD data have opened a new window into understanding the interior
structures and properties of Martian ices, volcanics, and sedimentary deposits up to a few kilometers in depth,
they have also led to new revelations about the deeper interior and the behavior of the planet’s ionosphere. Here
we summarize the data collected by SHARAD over this time period, the methods used in the analysis of that data,
and the resulting scientific findings. The polar data are especially rich, revealing complex structures that
comprise up to several dozen reflecting interfaces that extend to depths of 3 km, which inform the evolution of
Martian climate in the late Amazonian period. SHARAD observations of mid-latitude lobate debris aprons and
other glacier-like landforms detect strong basal reflections and low dielectric loss, confirming that they are icerich
debris-covered glaciers. In other mid-latitude terrains, SHARAD data demonstrate the presence of widespread
ground ices, likely at lower concentrations. SHARAD signals also probe non-icy materials, mapping out
stacked lava flows, probing low-density materials thought to be ash-fall deposits, and occasionally penetrating
sedimentary deposits, all of which reveal the structures and interior properties diagnostic of emplacement
processes. SHARAD signals are impacted by their passage through the Martian ionosphere, revealing variations
in time and space of the total electron content linked with the remanent magnetic field. Advanced techniques
developed over the course of the mission, which include subband and super-resolution processing, coherent and
incoherent summing, and three-dimensional (3D) radar imaging, are enabling new discoveries and extending the
utility of the data. For 3D imaging, a cross-track spacing at the nominal 3-km resolution is more than sufficient to
achieve good results, but finer spacing of 0.5 km or less significantly improves the spatially interpolated radar
images. Recent electromagnetic modeling and a flight test show that SHARAD’s signal-to-noise ratio can be
greatly improved with a large (~120â—¦) roll of the spacecraft to reduce interference with the spacecraft body. Both
MRO and SHARAD are in remarkably fine working order, and the teams look forward to many more years in
which to pursue improvements in coverage density, temporal variability in the ionosphere, and data quality that
promise exciting new discoveries at Mars