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a b s t r a c tThe Quadrangles Av-11 and Av-12 on Vesta are located at the northern rim of the giant Rheasilvia south polar impact basin. The primary geologic units in Av-11 and Av-12 include material from the Rheasilvia impact basin formation, smooth material and different types of impact crater structures (such as bimodal craters, dark and bright crater ray material and dark ejecta material). Av-11 and Av-12 exhibit almost the full range of mass wasting features observed on Vesta, such as slump blocks, spur-and-gully morphologies and landslides within craters. Processes of collapse, slope instability and seismically triggered events force material to slump down crater walls or scarps and produce landslides or rotational slump blocks. The spur-and-gully morphology that is known to form on Mars is also observed on Vesta; however, on Vesta this morphology formed under dry conditions.

[1] In this study, we analyze Magellan gravity and topography data for Type 1 coronae on Venus to estimate crustal thickness (Z c ), elastic thickness (T e ), and apparent depth of compensation (Z L ). We examine the free-air admittance for all 103 Type 1 coronae (defined as having greater than 50% complete fracture annuli) that are resolved in the gravity data. A spatio-spectral method is used to calculate a localized admittance signature of each corona from a global admittance map. This method extracts spectral information from a region in space whose area is varied as a function of degree to make it large enough to yield robust results at every individual wavelength. Elastic flexure models with either bottom-or top-loading compensation are used to fit the data. The estimated lithospheric flexural parameters span the range obtained for other topographic features on Venus, suggesting that the lithosphere on which coronae form is variable. We find no significant difference in lithospheric properties between Type 1 and 2 coronae. Fifty-four percent of all coronae are consistent with local isostasy, which may indicate that they are no longer active. Very few coronae with dome or plateau morphologies have a bottomloading signature or the small T e and large Z L expected if a mantle plume were present. Approximately 71% have Z L estimates less than 70 km, indicating that compensation probably occurs within the crustal layer. We find no systematic relationship between T e or Z c and corona diameter, as would be expected for the spreading-drop model of formation.

In 1966, while examining some of the earliest images of the lunar surface, researchers identified several dark features, which were postulated to be cave entrances (Heacock et al., 1966). Later that year, Halliday (1966) further mused over the existence of lunar caves and briefly discussed their potential importance for future human

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