MESSENGER VIEWS OF UNUSUAL BRIGHT-HALOED DEPRESSIONS ON MERCURY: POSSIBLE RECENT VOLATILE-RELATED ACTIVITY

The MESSENGER spacecraft entered orbit about Mercury on 18 March 2011, enabling the Mercury Dual Imaging System (MDIS) to acquire the highest-spatial-resolution images of this enigmatic planet ever obtained. MDIS targeted observations have resolutions up to 10 m/pixel for monochrome and 80 m/pixel for multispectral images. Targeted MDIS observations complement the global mapping campaigns by providing resolutions that can be more than a factor of ten higher at latitudes north of 20° N. These high-resolution morphology and color images reveal an unusual landform on Mercury, characterized by small (tens of meters to a few kilometers), irregularly shaped, shallow, rimless depressions, often occurring in clusters and in association with high-reflectance materials. The features ("hollows") are commonly found on the central peaks, floors, walls, and rims of impact craters or basins, implying a link to material brought near the surface from depth during crater formation. The most likely formation mechanisms for the hollows involve recent loss of volatiles through some combination of sublimation, sputtering, pyroclastic volcanism, or outgassing. The involvement of volatiles in formation mechanisms for the hollows fits with growing evidence that Mercury's interior contains higher abundances of volatile materials than predicted by most scenarios for the formation of the Solar System's innermost planet.