Regolith textures on Mercury and the Moon

The surfaces of Mercury and the Moon are covered with a layer of fragmental, highly heterogeneous material known as regolith. Regolith-related processes form short-scale textures seen in the high-resolution images. We carried out a survey of such textures on Mercury and compared them to better-known lunar analogs.

We surveyed the images obtained by MDIS NAC camera onboard the MESSENGER orbiter toward the end of the mission. We select images of the highest resolution and the finest sampling (less than 2.5 m/pix). We selected and screened ~3000 best images of that data set. To compare the typical surface morphology on Mercury to the Moon we used LROC NAC images. To facilitate the comparison we selected a representative set of LROC images that have the same sampling and sunlight incidence angles as the surveyed MDIS images, and degraded their quality.

Primarily, lunar and hermian surfaces as seen at high resolution are similar. The majority of decameter-scale topographic features are smooth and subdued due to the presence of regolith layer and its gardening. The majority of small impact craters are shallow and subdued. On the Moon, regolith-covered slopes, both steep and gentle, often have a specific subtle decameter-scale pattern referred as “elephant hide” or “leathery texture”. Its origin is unknown; however, it is almost certainly related to regolith transport. On Mercury, such a pattern is typically not observed: we identified it in a few occasions only.

Sharp slope breaks, “crisp” morphology and the absence of superposed degraded craters indicate geologically young “fresh” features that are characterized by thin or recently disturbed regolith. We observed fresh morphologies in one large young crater on Mercury; they were similar to their lunar counterparts. Hollows are unique “fresh” hermian features that have no close lunar analogs. They show exceptional sharpness at the highest resolution images, which indicates that their formation is ongoing or extremely recent. We found two more types of fresh morphologies that do not have close lunar analogs. (1) Finely-Textured Slope Patches (FTSP) are patches of finely (meter-scale) textured slopes with sharp outlines. This texture is characterized by a wavy chaotic pattern and occurs amid typical intercrater plains and old impact basins; there are no large young craters or hollows nearby, nor resolvable albedo or color peculiarities close to FTSP locations. They show semblance to some kinds of terrestrial landslides, which might suggest a variant of slide of thick regolith as their formation mechanism. (2) Chevron texture resembles scouring by wind or water in terrestrial environment; however, this cannot suggest a similar formation mechanism hermian conditions. Chevron texture found in one small part of the region with the super resolution images; it is oriented in the same direction. We initially a suggested that it could be related to a ray of a large young crater, but this was not perfectly consistent with observations.

In addition to the expected morphological similarity of regolith textures on the Moon and Mercury, the hermian surface displaces localized traces of geologically recent processes in the regolith having no lunar analogs.