Shadows within shadows: Dark material on the floor of a crater in Mercury's high north
The top left panel shows a view of an unnamed crater in Mercury’s north polar region, with the crater rim outlined in pink and the edge of a low-altitude image in green. The crater has a diameter of 18 kilometers. In the large bottom panel, a different stretch has been applied to the same low-altitude image, revealing details of the shadowed surface inside the crater. In particular, as highlighted with yellow arrows in the top right panel, the image reveals a region inside the crater that has a lower reflectance. The edge of the low-reflectance region has a sharp and well-defined boundary, even as imaged at this highest resolution of 24 m/pixel. The sharp boundary suggests that the low-reflectance material is sufficiently young to have preserved a sharp boundary against lateral mixing by impact craters. The sharp boundary matches the location predicted by temperature models for the stability of a surficial layer of volatile, organic-rich material tens of centimeters thick that overlies a thicker layer of water ice.