ASSESSING SURFACE ROUGHNESS VARIABILITY AND POTENTIAL VOLATILE OCCURRENCE ON MOONS AND SMALL-BODIES USING OPPORTUNISTIC FORWARD-SCATTER BISTATIC RADAR EXPERIMENTS

Characterizing the variability of surface roughness on planetary bodies at centimeter-to-decimeter (cm-dm) scales is key to understanding the primary physical mechanisms that have shaped their surfaces, and to accurately assessing surface trafficability for future landing, anchoring and sampling missions. Airless planetary surfaces are shaped by relatively few mechanisms (impacts, lava flows) and are observed to have a narrow range of surface roughness variability—on the Moon, cm-dm surface slopes range from 2-8°—while planetary bodies with atmospheres or abundant volatiles exhibit a wider range of surface roughness variability due to processes such as aeolian deposition and erosion and potential surface melts and flows.

One technique well-suited to surface roughness characterization is forward-scatter orbital bistatic radar (BSR), which can be conducted opportunistically (at low cost and low risk to mission planning) by using the orbiting spacecraft’s high-gain communications antenna to transmit S/X-band radiowaves and ground stations to receive. During occasional occultation entries and exits of the spacecraft from behind the target, transmitted waves scatter at grazing incidence (89°) from the surface (the ‘echo’). Power spectral analysis of the received echo is then used to measure (1) its strength relative to the direct signal, yielding relative surface radar reflectivity; and (2) frequency broadening due to surface roughness (where cm-dm surface scatterers introduce incremental Doppler shifts), yielding absolute surface roughness. If the surface’s dielectric properties are known, a map of relative surface roughness can also be inferred from radar reflectivity.

Herein, we summarize the results of the opportunistic BSR experiment by the Dawn mission at Vesta, and present minimum requirements to conduct opportunistic orbital BSR at several different moons and small-bodies that are targets of active and planned orbital missions. Objects with D ≲ 200 km require an onboard ultra-stable oscillator to generate sufficient frequency stability to disentangle the direct and echo signals and yield accurate measurements of relative surface radar reflectivity at 89° incidence. Roughness-induced frequency broadening can also be measured in the case of D ≳ 10³ km to yield absolute surface roughness.