The Noachian is a geologic system and early time period on the planet Mars characterized by high rates of meteorite and asteroid impacts and the possible presence of abundant surface water. The absolute age of the Noachian period is uncertain but probably corresponds to the lunar Pre-Nectarian to Early Imbrian periods of 4100 to 3700 million years ago, during the interval known as the Late Heavy Bombardment. Many of the large impact basins on the Moon and Mars formed at this time. The Noachian Period is roughly equivalent to the Earth's Hadean and early Archean eons when Earth's first life forms likely arose.
Noachian-aged terrains on Mars are prime spacecraft landing sites to search for fossil evidence of life. During the Noachian, the atmosphere of Mars was denser than it is today, and the climate possibly warm enough (at least episodically) to allow rainfall. Large lakes and rivers were present in the southern hemisphere, and an ocean may have covered the low-lying northern plains. Extensive volcanism occurred in the Tharsis region, building up enormous masses of volcanic material (the Tharsis bulge) and releasing large quantities of gases into the atmosphere. Weathering of surface rocks produced a diversity of clay minerals (phyllosilicates) that formed under chemical conditions conducive to microbial life.
