Formed between 3.8 and 3.2 billion years ago, after a violent period of asteroid and cometary impacts known as the Late Heavy Bombardment, the four lunar “seas” (or mare) pictured are now defunct lava basins of basaltic rock.
Mantle magma is thought to have risen through the shattered crust to fill these giant resevoirs. Lava channels known as rilles wind their way throughout each of the basins, and are better viewed in the picture’s larger version.
Early in the evolution of the Earth-Moon system, the Moon gravitationally locked into synchronous rotation with our planet, which is why the same half of the lunar sphere faces Earth at all times. The Moon’s far side is nearly devoid of mare basins, so it’s rather fortuitous that we see the more topographically varied portion of its surface.
Along the top of the night/day line, or terminator, sharp shadows are cast out tens of kilometers from the western edge of the Caucasus Mountains.
This was captured with my Nikon D90 adapted to LSU’s Landolt Telescope. The exposure was 1/60 second at ISO 400.