Presentation #316.05 in the session Moon & Mercury (iPosters).
The Surveyor 7 spacecraft landed on the northern rim of Tycho crater (86 km diameter, age ~110 Ma) on January 10, 1968. It is the only mission that has obtained surface-level observations of the ejecta blanket of a large, young impact crater on the Moon, and thus is a critical site for interpretation of impact processes. Its main scientific instruments were an alpha scatterer experiment, a soil mechanics and surface sampling robotic arm, and a television camera system. Together, these confirmed the expected youthfulness and distinct composition of the lunar highlands around Tycho compared to the maria, as well as the material properties and morphology of a young, impact-melt-rich ejecta deposit. Later analysis with the Lunar Reconnaissance Orbiter (LRO)’s Narrow-Angle Camera showed that Surveyor 7 likely landed on a thin impact melt veneer associated with the “patterned flow” unit defined by Shoemaker et al. (1969), rather than particulate ejecta or a coherent impact melt pond/flow (Meyer et al., 2017).
To corroborate this finding and better understand the subsurface geology of the landing area, we analyze data from LRO’s Miniature Radio Frequency (Mini-RF) S-Band radar, LRO’s Diviner thermal infrared radiometer, and Chandrayaan-2’s Dual Radio-Frequency Synthetic Aperture Radar (DFSAR). Measurements of the total backscattered power and circular polarization ratio (CPR) from both Mini-RF and DFSAR confirm that Surveyor 7 is on the edge of a unit that is radar-distinct from both nearby particulate ejecta and lobate impact melt flows. This unit has an elevated CPR of 0.98±0.35 at L-band (wavelength=24 cm) and 1.13±0.65 at S-band (wavelength=12.6 cm), compared to values of 0.67±0.25 and 0.94±0.54 for the nearby “patterned debris” (particulate ejecta) unit, and 0.66±0.23 and 0.89±0.50 for the “smooth flow” (impact melt flow) unit, at the same respective wavelengths. Thermal models with variable subsurface density profiles fit to nighttime surface temperature data from Diviner confirm that this radar-distinct unit contains high-thermal inertia material buried under a thin layer of regolith (less than the thermal skin depth, ~4-20 cm). This supports observations by Surveyor 7’s surface sampler instrument, which recorded maximum trench depths of 2.5-15 cm in its working area before hitting immovable buried material (Scott and Roberson, 1969). Thus, we reaffirm that Surveyor 7 observations likely provide the first ground-truth of a young lunar impact melt veneer on the Moon, and will present re-interpretations of its television images and other instrument data in light of this hypothesis.