Geology of the unusual double-ring Raditladi basin on Mercury

During its first flyby of Mercury in January 2008, MESSENGER viewed a large portion of the surface of the planet not previously seen by spacecraft. From high-resolution images obtained during the flyby we are investigating the geology of the Raditladi basin, a ~~250-km diameter impact feature located west of the Caloris basin and centered at 27°N, 119°E. On the basis of impact crater density, approximately an order of magnitude less than is found on smooth plains to the west of Caloris, Raditladi is thought to be one of the youngest impact basins on Mercury, with an age of 1-2 Ga or less. Raditladi contains an interior peak-ring structure ~~125 km in diameter, and its floor is filled with smooth plains material that clearly embays the central peak ring. The basin walls have undergone modification, with terraces most pronounced within the north and west sides of the rim. The plains are smoother on the eastern and southern side of the basin and appear to have covered more of the underlying material. This difference suggests that the fill may be slightly deeper there than elsewhere in the basin. The plains within the basin floor contain a number of discontinuous, arcuate, flat-floored troughs, interpreted to be graben. The graben are arranged in an approximately circumferential pattern about 70 km in diameter, centered ~~10 km south of the basin center. The graben represent the only major extension identified on Mercury to date outside of the Caloris basin. It is likely that the troughs formed as the result of uplift and extension of the basin floor, but defining the source of uplift at a relatively recent stage in Mercury's history when interior cooling models predict a thick, strong lithosphere presents an interesting challenge. The fill within Raditladi may have a volcanic origin, as appears to be the case for plains in the nearby Caloris basin, but an alternative interpretation is that the fill consists of impact melt. Color analyses are underway to help understand the origin of the plains material, and further studies of the cratering record, topography, and geology will elucidate the history of this enigmatic basin.