Table 1.

Comparison of reservoirs on Earth and Mars at several different stages of planetary evolution. (The ages given are very approximate and are not boundary ages, but the time by which specific characteristics had become established. Thus, Earth's atmosphere did not become oxidizing 2.0 Gyr ago, but by 2.0 Gyr was oxidizing in composition.)

age (Gyr)	Earth	Mars
4.5	Hadean	Noachian
cooling and outgassing; no plate tectonics; strongly reducing atmosphere; no biosphere; no carbon cycle	cooling and outgassing; no plate tectonics; atmosphere similar to early Earth?; no biosphere; no carbon cycle
3.5	Archaean	Hesperian
plate tectonics established; abundant liquid water; reducing atmosphere; biosphere of simple organisms	plate tectonics ceased; hydrosphere/cryosphere of unknown extent; mildly reducing atmosphere, thicker than today (?)
2.0	Proterozoic	Amazonian
abundant liquid water; oxidizing atmosphere; biosphere of simple organisms	hydrosphere/cryosphere of unknown extent; thicker atmosphere (?)
present-day	Phanerozoic
abundant liquid water; oxidizing atmosphere; biosphere of complex organisms	hydrosphere/cryosphere of unknown extent; thin, CO2-rich atmosphere