Crater Floor Slope as a Measure of Long-wavelength Changes in Topography on Mercury

During the course of three flybys and an orbital mission phase that began on 18 March 2011, the MESSENGER spacecraft has been performing a detailed survey of Mercury in order to characterize the planet's origin and evolution. Precise topographic information about the surface of Mercury is being collected by the Mercury Laser Altimeter (MLA), largely over the northern hemisphere where the spacecraft slant range from the surface is less than 1500 km. Complementary knowledge of surface relief is gained through stereographic imaging by the Mercury Dual Imaging System (MDIS). Analysis of stereographic images returned during MESSENGER's first flyby of Mercury revealed, and orbital MLA profiles have confirmed, the presence of unexpected long-wavelength topography within and adjacent to the Caloris impact basin. In particular, basin topography is far from radially symmetric, and portions of the northern basin floor lie at higher elevation than the nearby basin rim. The anomalously high areas of basin floor appear to be part of a larger-scale topographic variation that extends outside the basin. To assess the nature and development time of this long-wavelength topography we examine surface features that may have been tilted during its formation. In particular, we investigate the idea that the slopes of the floors of nominally flat-floored impact craters within and near the Caloris basin may, depending on their age, reflect changes in long-wavelength slopes associated with the large-scale topography. Whereas floor slopes for individual craters may be the result of any of several volcanic, tectonic, or impact processes, a large-scale organization of slope direction and magnitude can be an indicator of a common origin. Results from the measurement of crater floor slopes from MLA profiles across the northern Caloris region of Mercury reveal that a majority of flat crater floors profiled by MLA have along-track slopes between ~0.25 and 1.5°. Moreover, the magnitudes and along-track slope directions of these crater floors are generally spatially correlated with the long-wavelength slope of the Caloris floor topography. Ongoing collection of topographic profiles by MLA will serve to extend the statistical sample of craters that may have been influenced by the development of this large-scale feature as well as permit estimation of cross-track slopes for some craters, both crucial for understanding its development. Results to date also suggest that measurement of post-impact tilting of crater floors may provide a means more generally to assess the existence and development of comparable late-stage long-wavelength surface deformation across the planet.