MORPHOLOGY OF PIT CRATERS AND HOLLOWS ON MERCURY (Invited Presentation)

MESSENGER mission data reveal invaluable new information about the cratering on Mercury. The Mercury Dual Imaging System (MDIS) has acquired over 125,000 images, providing nearly global, high-resolution coverage of Mercury since the spacecraft began orbiting in March 2011. These data reveal a treasure trove of information with which to study crater morphology. MDIS data has led to the discovery of two features found only in craters on Mercury: hollows and pit craters. Hollows are a class of impact crater that contain unusual high-reflectance patches that have relatively flat reflectance spectra at visible to near-infrared wavelengths. These areas consist of shallow irregular rounded depressions with flat floors and bright interiors and halos. The formation of the hollows likely involves loss of a volatile phase(s) that is unstable when exposed to Mercury surface conditions. One hypothesis for creating the source of volatile layer is that massive sulfide or chloride deposits formed by impact melt differentiation. Space weathering slowly volatilzes the sulfied/chloride to create small, shallow depressions that can coalesce. The second crater connected feature found to exist only within Mercury craters are pit craters. Pit craters are rimless depressions, with no observed ejecta, are irregularly and often arcuately shaped, often steep sided, range in size from 10 km to almost 80 km, which is comparable to sizes of large calderas on the Earth, Venus, and Mars. In a majority of occurrences pits and hollows are found in close proximity. The overwhelming majority of these pits are found on the floors of impact craters, suggesting some connection. However, these pit craters do not share characteristics of pit craters on Mars or Ganymede, which are thought to occur as a result of the impact process. No correlation exists between impact crater diameter and size of pit crater, and the pit craters do not occur in the center of the host impact crater, as often observed for Mars. The shape, depth, size, and geologic setting suggests that endogenic modification, similar to caldera formation, of a pre-existing impact crater is responsible for the formation of these pit craters. These two distinctive products, hollows and pit craters, are evidence of the unique interaction between endogenic and cratering processes found only on Mercury.