Studies of impact crater densities on the Martian surface have delineated three broad periods in the planet’s geologic history. The periods were named after places on Mars that have large-scale surface features, such as large craters or widespread lava flows, that date back to these time periods. The absolute ages given here are only approximate. From oldest to youngest, the time periods are:
- Pre-Noachian Represents the interval from the accretion and differentiation of the planet about 4.5 billion years ago (Gya) to the formation of the Hellas impact basin, between 4.1 and 3.8 Gya. Most of the geologic record of this interval has been erased by subsequent erosion and high impact rates. The crustal dichotomy is thought to have formed during this time, along with the Argyre and Isidis basins.
- Noachian Period (named after Noachis Terra): Formation of the oldest extant surfaces of Mars between 4.1 and about 3.7 billion years ago (Gya). Noachian-aged surfaces are scarred by many large impact craters. The Tharsis bulge is thought to have formed during the Noachian, along with extensive erosion by liquid water producing river valley networks. Large lakes or oceans may have been present.
- Hesperian Period (named after Hesperia Planum): 3.7 to approximately 3.0 Gya. Marked by the formation of extensive lava plains. The formation of Olympus Mons probably began during this period. Catastrophic releases of water carved extensive outflow channels around Chryse Planitia and elsewhere. Ephemeral lakes or seas formed in the northern lowlands.
- Amazonian Period (named after Amazonis Planitia): 3.0 Gya to present. Amazonian regions have few meteorite impact craters but are otherwise quite varied. Lava flows, glacial/periglacial activity, and minor releases of liquid water continued during this period.
Gya and Ga, are symbols for units of time, specifically gigayears ago and gigaannum (or giga-annum), respectively. They represent 1,000,000,000 years.
- Caplinger, Mike. “Determining the age of surfaces on Mars”. Archived from the original on February 19, 2007. Retrieved 2007-03-02.
- Carr, M.H.; Head, J.W. (2010). “Geologic History of Mars”. Earth Planet. Sci. Lett. 294 (3–4): 185–203. Bibcode:2010E&PSL.294..185C. doi:10.1016/j.epsl.2009.06.042