Topographically, the Moon is very different from Earth. The Moon's surface is characterized by highlands and lowlands, mountains, and most notably, craters (bowl-shaped cavities of meteoric origin). These craters are often marked by secondary craters and by rays from ejecta — ejected matter from the meteor's impact.
The Moon's dark regions, call maria, are lava-filled basins up to 1,000 kilometers in diameter. Maria are sites of immense meteoric strikes early in lunar history that later were filled by molten lava seeping up from the interior.
These maria are also the sites of gravity anomalies, or mascons, which are caused by the concentration of very dense material beneath the surface of the Moon. Mascons are found only on the near side of the Moon (the side that faces Earth), suggesting that the influence of Earth's gravitation altered the trajectories of the impacting objects that produced these features.
