CRATER STATISTICS FOR THE NORTHERN POLAR REGION OF MERCURY DERIVED FROM MESSENGER ORBITAL DATA

The MESSENGER spacecraft was inserted into orbit about Mercury on 18 March 2011, and soon thereafter the Mercury Dual Imaging System (MDIS) began systematic high-resolution imaging. Previous image coverage of the northern polar region from the Mariner 10 and MESSENGER flybys was limited, and although these data provided an incomplete view, they showed large regions of smooth plains distributed amongst heavily cratered terrain (HCT). MDIS synoptic coverage thus far provides a more complete and higher-resolution view of the north pole than previously possible. Moreover, the ages of the northern polar smooth plains may now be compared, on the basis of impact crater size-frequency distributions, with those of the other large area of smooth plains on Mercury associated with the Caloris basin. We constructed a monochrome mosaic of the north polar region, using MDIS wide-angle camera images at 400 m/pixel, to define boundaries between units and to complete crater measurements. We defined initial measurement areas of ~2 x 10⁶ km² for both the smooth plains and the HCT; these areas will be increased as MDIS coverage increases. Our crater statistics, limited to diameters greater than 10 km, confirm that the smooth plains are a comparatively young geologic unit, similar in age to the Caloris interior plains. Relative crater size-frequency distributions (R-plots) for the northern HCT are consistent with, but deviate slightly from, those for average Mercury HCT. This variation is not unexpected; crater size-frequency distributions on Mercury are highly sensitive to volcanic resurfacing and secondary cratering. The deviation of the northern HCT from average Mercury HCT probably indicates volcanic resurfacing prior to smooth plains emplacement, given that there are embayed and flooded craters interspersed with superposed craters within the HCT. It may be that the evidence for early volcanic resurfacing in the northern HCT has been greatly obscured by subsequent cratering. If documented by further analysis, this earlier activity would provide additional evidence that widespread volcanism occurred through much of Mercury’s early history. Alternatively, smooth plains formation may have been responsible for the embayment relations seen in the northern HCT, an idea testable with additional mapping and MDIS color image analysis.