Morphometry of Small Flat Floored Craters on Mercury: Implications for Regolith Thickness

Abstract:

The surfaces of bodies without atmospheres, including Mercury, are covered with a layer of regolith, fragmental, heterogeneous material formed due to meteoritic impacts on more coherent substrate material. Among the regolith-related characteristic morphologies observed on the Moon are distinctive small fresh impact craters with 1) flat floors, 2) single knobs on their floors, 3) double-rings. These morphologies are interpreted to indicate the penetration down to more coherent material below the fragmental regolith, for example lava flows in the lunar maria, and can be used to estimate the thickness of the regolith (Quaide & Oberbeck 1968 JGR 73, 5247). We found about 150 small craters of similar morphologies as the result of a survey of more than 1600 high-resolution images obtained by MESSENGER’s MDIS NAC camera in the northern high latitudes, and we inferred the regolith thickness using the method of Bart (2014 Icarus 235, 130). These estimates indicate that the regolith on Mercury is significantly thicker than on the Moon. Our analysis showed that the small craters with flat floors are mostly confined to Mercury’s smooth plains, which have been interpreted as features of volcanic origin (Denevi et al. 2013 JGR 118, 891). Smooth plains are likely to be characterized by a relatively sharp boundary between the regolith and underlying bedrock; association with smooth plains is consistent with a regolith-related origin for the specific crater morphologies. Using stereo pairs of MDIS NAC images we created DEMs with a resolution of 30 m/pix using PHOTOMOD software for several sites on Mercury’s surface. The depth of several flat-floored craters measured using these DEMs also indicates a thick regolith layer on Mercury. This technique can be used to estimate regolith thickness, and detect plains of volcanic origin, during the ESA BepiColombo mission. Acknowledgement: This work was performed at MIIGAiK and supported by Russian Science Foundation, project 14-22-00197.