High-Resolution Radar Images of Mercury from the 2019 and 2020 Inferior Conjunctions

Previous ground-based radar imaging campaigns of Mercury revealed radar-bright features consistent with volume backscatter from a low-loss volatile, such as clean water ice, located largely within polar impact craters. The MESSENGER spacecraft later confirmed that polar radar-bright features are associated with locations permanently in shadow that are on average hydrogen-rich. Additionally, early Arecibo radar data from the mid-latitudes of Mercury revealed the variety of fresh impact craters and their ray systems. This included the discovery of several rayed impact craters, such as Hokusai. Here we revisit Mercury with radar observations to resolve its surface features by incorporating insights gained during the MESSENGER mission. We conducted radar observations using the Arecibo Observatory S-band (12.6 cm, 2380 MHz) planetary radar system during the 2019 and 2020 inferior conjunctions. The subradar latitude during these observations allowed for radar imaging of only the north pole. We used the long-code delay-Doppler radar imaging method with a baud of 5 μs (i.e., 750 m/pixel resolution in the delay dimension) to eliminate Doppler aliasing of the slightly overspread target. Echoes in both the opposite circular and same circular polarization as transmitted were recorded, permitting radar scattering studies. Delay-Doppler radar images were calibrated and transformed to a simple cylindrical equidistant projection with a resolution of 0.25° for the mid-latitudes and a stereographic projection for the polar terrain. The final maps were georegistered with MESSENGER data to adjust to hermiocentric coordinates. We used radar imagery of features over a broad range of incidence angles to constrain their radar scattering properties. Particularly, here we will present results for four distinct features: Fonteyn, Hokusai, and Rachmaninoff craters, as well as the north polar region. For the mid-latitude craters, we constrained the dielectric properties of the terrain and RMS slopes of the topography. We compared the RMS slopes to those derived from MESSENGER optical imagery. Additionally, we studied the radar return from the polar craters and compared with MESSENGER data to resolve peculiarities in these craters.