Mare Surface
This is an Apollo photo of the surface in southern Mare Imbrium. It shows
some young, fairly pristine mare lava flows. These lavas are probably 1 to
2 billion years old. Still, individual flow lobes can be clearly seen at
the top of the image. Similarly, the gully-like features in the lower left
do not mark any kind of erosion. Rather, they mark shallow lava channels
(sinuous rilles) which formed at the lava flow surfaces. The image also
shows many small circular impact craters. While meteorite impacts on the
Earth and Moon are rare, such craters are quite common within the lunar
mare. The mare are so old that a large number of meteorite impacts have
occurred. Indeed, the number of impact craters within a mare provides a
method for guessing its age. Because older surfaces are more likely to
have been hit by meteorites, older mare should contain both more craters
and larger craters than younger mare. Note -- Younger mare lavas can bury
craters formed on older lavas. This image shows one such example near the
crater in the bottom center. The rough ejecta unit surrounding this crater
is cut and partially buried by younger lava flows. (Mosaic of Apollo
photographs A17 M-2295 and A15 M-1701)