Geological Chronicle
Situated on Oceanus Procellarum, Marius is a lunar impact crater measuring 40.09 kilometers in diameter with a low, generally circular rim. Impact rays extending from the crater Kepler to the east-southeast reach its rim, while Reiner lies to the southwest.
This impact dates to the Imbrian epoch of the lunar geologic timescale. Basaltic lava flooded the floor of Marius, leaving a relatively smooth, flat surface with no central rise, punctuated only by the small craterlet Marius G in the northeast part of the floor.
The surface to the west and north contains the Marius Hills, an area over a hundred kilometers in diameter with a large number of lunar domes. These domes may be of volcanic origin, formed by magma that is fairly more viscous than the material that formed the basaltic lunar maria.
In 2009, observations by the Japanese probe SELENE identified a probable cave skylight of a lava cave in a nearby rille, measuring about 90 meters deep beneath a 25-meter-thick roof. The crater area was proposed for an Apollo mission before cancellation, and Luna 7 landed about 50 kilometres to the southeast.