LARGE RAYED CRATERS ON MERCURY

Ray systems are generally associated with morphologically fresh (and hence young) craters, so studies of rayed craters yield information about the relatively recent history of planetary surfaces. MESSENGER's first two Mercury flybys have provided a global view of the innermost planet for the first time. Only about 45% of Mercury's surface had been imaged by Mariner 10. With the addition of images returned from MESSENGER's first two Mercury flybys, about 90% of Mercury's surface has been imaged by spacecraft at a resolution of 1 km/pixel or better. MESSENGER's third Mercury flyby, scheduled for 29 September 2009, will image much of the remaining unseen terrain at low and mid latitudes. These new data enable a global look at rayed craters on Mercury.

We have identified large (≥ 20 km diameter) rayed craters on Mercury using a global mosaic created from both MESSENGER and Mariner 10 data at a resolution of 1 km/pixel. Lighting conditions have a significant effect on the ability to discern crater rays; at high incident Sun angles, topography is well defined but reflectance differences due to relatively brighter or immature rays are often not detectable. Consequently, the search area was limited to latitudes less than 60°, consistent with the practice adopted by the lunar studies, and to longitudes more than 25° from the terminators in the Mercury flyby datasets. The resulting search area was thus about 75% of the planet between latitudes of 60°N and 60°S.

Comparison of our new results for Mercury with previously published ones for the Moon indicates that the surface density of large rayed craters on Mercury is about 50-60% of that of the lunar surface density. Thus, while some large craters on Mercury have spectacular ray systems, overall there are fewer large rayed craters per unit area preserved on Mercury than on the Moon. Due to a greater flux and higher impact velocities, it is predicted that 20 times more vapor is produced on Mercury than on the Moon during micrometeorite impacts, resulting in a considerably higher rate of regolith maturation on Mercury. Our comparison of large rayed craters appears to be consistent with this prediction of a faster space weathering rate on Mercury than on the Moon, and we are examining if more quantitative conclusions can be reached about the rates and processes of space weathering on these two bodies.