Layered basalt on the wall of Marius A crater partially covered by debris. The crater rim is to the right of the image, and the floor of the crater is to the left. LROC NAC M137848463R, image width is 460 m, incidence angle is 33° [NASA/GSFC/Arizona State University].
Marius A is an ~15 km diameter crater (12.58°N, 46.05°W) located within Mare Insularum. The Featured Image shows basalt layering partially covered by streaks of granular material that slid down from higher up on the wall. Craters with visible basalt layers like Marius A are windows to the history of basalt deposition. Each thin layer seen in the wall of Marius A is probably a single flow or flow lobe, each spreading out across the lunar surface due to the low viscosity of mare basalt (basalt has a viscosity similar to that of ketchup). How much time passed between each layer is still an unanswered question. By studying many craters with visible basalt flows, however, scientists may be able to piece together a more detailed, local history for the various mare on the lunar surface. Not all craters in the mare have visible mare basalt layering, though. Additionally, over time post-impact processes like the debris in today's Featured Image and slumping of the crater walls reduce the visibility of basalt layers.
LROC WAC context image of Marius A crater. The image is 59 km across. The red box shows the area of the Featured Image NAC frame [NASA/GSFC/Arizona State University].
There are many craters with visible basalt layers and LROC has given us our first look at these incredible details of the lunar surface. Check out the NAC frame of Marius A and then explore the rest of the lunar maria!