THICKNESS OF VOLCANIC FILL IN IMPACT BASINS ON MERCURY

In its first Mercury sidereal day in orbit, MESSENGER’s Mercury Dual Imaging System (MDIS) acquired global color and high-resolution monochrome base maps of the innermost planet (at average resolutions of 1 km/pixel and 250 m/pixel, respectively). These data, taken under illumination and observation geometries optimized for observing both color and morphology, allow detailed co-mapping of geologic and spectral features across Mercury’s surface. Impact craters that excavated material spectrally distinct from the surrounding pre-impact surface serve as windows into the subsurface, allowing observations of material at depth that would otherwise remain hidden to remote observations. Such craters provide insight into the stratigraphy of four of Mercury’s younger large basins: Caloris (1550 km in diameter), Rembrandt (720 km), Beethoven (630 km), and Tolstoj (360 km). Spectrally distinct volcanic plains have flooded all of these basins. Post-flooding craters of varying sizes populate the basin interiors and enable an estimation of the thickness of the volcanic fill and the nature of the pre-flooding basin floor. Large craters in all four basins have excavated low-reflectance material (LRM) from beneath the volcanic fill, whereas small craters do not reveal spectrally distinct material. Typical LRM deposits have reflectance values as much as 30% below the global mean and likely constitute a compositional end-member. We use scaling laws and melt volume calculations to bound the depth of origin of the LRM ejecta and central peak structures. Preliminary estimates from MESSENGER flyby data indicated that the surficial volcanic material inside Caloris may be as thick as 2.5 to 4 km in regions and ~2 km near the center of Rembrandt. Higher-resolution orbital images of craters can be used to refine these estimates and map thicknesses across the basin interior. Comparisons with Rembrandt, Beethoven, and Tolstoj help us to interpret and understand the post-impact evolution of Mercury’s younger large basins and to investigate further the extent and role of volcanism. Moreover, knowledge of the stratigraphy and volcanic fill inside these basins constrains models for their subsequent compensation, uplift, and deformation. Topography and gravity data link basin fill with the broader lithospheric evolution of Mercury.