Science results from sixteen years of MRO SHARAD operations

In operation for >16 years to date, the Mars Reconnaissance Orbiter (MRO) Shallow Radar (SHARAD) sounderhas acquired data at its nominal 300–450 m along-track and 3-km cross-track resolution covering >55% of theMartian surface, with nearly 100% overlap in coverage at that scale in the polar regions and in a number ofsmaller mid-latitude areas. While SHARAD data have opened a new window into understanding the interiorstructures 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. Herewe 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 thatcomprise up to several dozen reflecting interfaces that extend to depths of 3 km, which inform the evolution ofMartian climate in the late Amazonian period. SHARAD observations of mid-latitude lobate debris aprons andother glacier-like landforms detect strong basal reflections and low dielectric loss, confirming that they are icerichdebris-covered glaciers. In other mid-latitude terrains, SHARAD data demonstrate the presence of widespreadground ices, likely at lower concentrations. SHARAD signals also probe non-icy materials, mapping outstacked lava flows, probing low-density materials thought to be ash-fall deposits, and occasionally penetratingsedimentary deposits, all of which reveal the structures and interior properties diagnostic of emplacementprocesses. SHARAD signals are impacted by their passage through the Martian ionosphere, revealing variationsin time and space of the total electron content linked with the remanent magnetic field. Advanced techniquesdeveloped over the course of the mission, which include subband and super-resolution processing, coherent andincoherent summing, and three-dimensional (3D) radar imaging, are enabling new discoveries and extending theutility of the data. For 3D imaging, a cross-track spacing at the nominal 3-km resolution is more than sufficient toachieve good results, but finer spacing of 0.5 km or less significantly improves the spatially interpolated radarimages. Recent electromagnetic modeling and a flight test show that SHARAD’s signal-to-noise ratio can begreatly improved with a large (~120◦) roll of the spacecraft to reduce interference with the spacecraft body. BothMRO and SHARAD are in remarkably fine working order, and the teams look forward to many more years inwhich to pursue improvements in coverage density, temporal variability in the ionosphere, and data quality thatpromise exciting new discoveries at Mars.