[3] Smooth plains are widespread flat areas resembling the lunar maria of the Moon, which fill depressions of various sizes.
[3] Inter-crater plains are the oldest visible surface on Mercury,[3] predating the heavily cratered terrain.
This combination of surface features has been called "inter-crater plains" by the Mariner 10 Imaging Science Team.
The higher resolution of the MESSENGER datasets compared with those of Mariner 10 enables the most ancient plains units on Mercury to be better characterized.
The inter-crater plains units are densely cratered at diameters under ten km, producing a highly textured surface that yields ancient pre-Tolstojan and Tolstojan ages over 3.9 Ga (billion years).
[13] There is no clear correlation with topography; inter-crater plains cover high-standing plateaus and continue into topographic depressions.
Inter-crater plains and heavily cratered terrain typical of much of Mercury outside the area affected by the formation of the
Caloris Basin
. Abundant shallow elongate craters and crater chains are present on the plains. This image, taken during the first mission of
Mariner 10
, shows a large tract of inter-crater plains centered at 3° N, 20° W. The scarp running down the middle of the image,
Santa Maria Rupes
cuts through both the plains and large craters. The scene is 200 km across; north is the top.
Close up view of the Mercury crater named
Rudaki
. Image taken from the
MESSENGER
mission. On the floor of Rudaki and the region surrounding the crater to the west, or left, are areas flooded with lava, leaving only the rims of these craters. This terrain is known as smooth plains, formed by volcanic flows on the surface of Mercury. To the east, or right, of this crater are the inter-crater plains which can be present at many different elevations due to previous uplift after formation.
Caloris Basin
—Mercury's largest impact crater (left side of image), is surrounded by a ring of mountains with chaotic terrain following this and eventually leading to smooth and inter-crater plains.
An example of inter-crater plains on Mercury. Yellow shows the inter-crater plains, whereas green shows younger impact craters. White is the surrounding areas of these features. Black is other craters of the area.
