RESULTS FROM MESSENGER's FIRST SOLAR DAY OF ORBITAL IMAGING OF MERCURY

The imaging investigation of Mercury with the Mercury Dual Imaging System (MDIS) on NASA’s MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft is organized into campaigns by solar day of the mission. Four campaigns being conducted during the first solar day (Apr.-Sep. 2011) include low-Sun monochromatic mapping at an average resolution 0.21 km/pixel, to characterize morphology; high-Sun multispectral mapping at an average 1.0 km/pixel, to characterize spectral variations; repeated imaging of the south polar region at an average 1.5 km/pixel, to determine distribution of permanently shadowed regions and their relation with radar-bright deposits; and targeted, high-resolution imaging of key sites, in monochrome at an average 20 m/pixel and in color at an average 350 m/pixel. Monochrome mapping reveals a variety of landforms and stratigraphic relations that indicate volcanism over much of the planet, and in some locations multiple episodes. Extensional tectonic features are more widespread than recognized previously, with troughs typically arranged in concentric, radial, and more complex polygonal patterns in the medium-sized to larger, volcanically infilled impact basins. Newly recognized extensional features are troughs arranged in polygonal patterns in infilled craters with diameters from tens to a few hundred kilometers. Multispectral and high-resolution images of two types of rimless depressions suggest the importance of volatile phases in shaping Mercury’s surface. Rimless depressions typically 10-40 km in diameter and surrounded by reddish haloes, interpreted as pyroclastic deposits, commonly exhibit evidence for multiple generations of activity. Distinctive smaller, rimless hollows hundreds of meters to a few kilometers in diameter occur on material exposed from depth in the later parts of Mercury’s history; fresher-appearing hollows have a partial to complete halo of bluish deposits. To within uncertainies in map projection, permanently shadowed regions correlate with recognized radar-bright deposits around the south pole. During the second solar day MDIS will acquire a stereo complement to the monochrome map, determine south polar illumination at higher resolution of 0.3 km/pixel, and continue targeted imaging.