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.Anderson, R.C.; Dohm, J.M.; Buczkowski, D.; Wyrick, D.Y. (2022). Early Noachian Terrains: Vestiges of the Early Evolution of Mars. Icarus, 387, 115170. 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.
Although there is abundant geologic evidence for surface water early in Mars history, the nature and timing of the climate conditions under which that water occurred is a subject of vigorous scientific debate.Wordsworth, R. Ehlmann, B.; Forget, F.; Haberle, R.; Head, J.; Kerber, L. (2018). Healthy Debate on Early Mars (Letter to the Editor). Nature Geoscience, 11, 888. Today Mars is a cold, hyperarid desert with an average atmospheric pressure less than 1% that of Earth. Liquid water is unstable and will either freeze or evaporate depending on season and location (See Water on Mars). Reconciling the geologic evidence of river valleys and lakes with computer climate models of Noachian Mars has been a major challenge.Kite, E.S. (2019). Geologic Constraints on Early Mars Climate. Space Sci. Rev. 215(10), https://doi.org/10.1007/s11214-018-0575-5. Models that posit a thick carbon dioxide atmosphere and consequent greenhouse effect have difficulty reproducing the higher mean temperatures necessary for abundant liquid water. This is partly because Mars receives less than half the solar radiation that Earth does and because the sun during the Noachian was only about 75% as bright as it is today.Wordsworth, R. et al. (2013). Global Modelling of the Early Martian Climate under a Denser CO2 Atmosphere: Water Cycle and Ice Evolution. Icarus, 222, 1–19.Gough, D. O. (1981). Solar interior structure and luminosity variations. Solar Physics, 74(1), 21–34. https://doi.org/10.1007/BF00151270. As a consequence, some researchers now favor an overall Noachian climate that was "cold and icy" punctuated by brief (hundreds to thousands of years) climate excursions warm enough to melt surface ice and produce the fluvial features seen today.Fastook, J. L.; Head, J. W. (2015). Glaciation in the Late Noachian Icy Highlands: Ice Accumulation, Distribution, Flow Rates, Basal Melting, and Top-Down Melting Rates and Patterns. Planetary and Space Science, 106, 82–98. https://doi.org/10.1016/j.pss.2014.11.028 Other researchers argue for a semiarid early Mars with at least transient periods of rainfall warmed by a carbon dioxide-hydrogen atmosphere.Ramirez, R.M.; Craddock, R.A. (2018). The Geological and Climatological Case for a Warmer and Wetter Early Mars. Nature Geoscience, 11, 230–237. Causes of the warming periods remain unclear but may be due to large impacts, volcanic eruptions, or orbital forcing. In any case it seems probable that the climate throughout the Noachian was not uniformly warm and wet.Wordsworth, R. (2016). The Climate of Early Mars. Annu. Rev. Earth Planet. Sci., 44, 381–408. In particular, much of the river- and lake-forming activity appears to have occurred over a relatively short interval at the end of the Noachian and extending into the early Hesperian. Howard, A.D.; Moore, J.M.; Irwin, R.P. (2005). An Intense Terminal Epoch of Widespread Fluvial Activity on Early Mars: 1. Valley Network Incision and Associated Deposits. J. Geophys. Res., 110, E12S14, doi:10.1029/2005JE002459.Fassett, C.I.; Head, J.W. (2008a). The Timing of Martian Valley Network Activity: Constraints from Buffered Crater Counting. Icarus, 195, 61–89.Fassett, C.I.; Head, J.W. (2008b). Valley Network-Fed, Open-Basin Lakes on Mars: Distribution and Implications for Noachian Surface and Subsurface Hydrology. Icarus, 198, 37–56.
Description and name origin
The Noachian System and Period is named after Noachis Terra (lit. "Land of Noah"), a heavily cratered highland region west of the Hellas basin. The type area of the Noachian System is in the Noachis quadrangle (MC-27) around . At a large scale (>100 m), Noachian surfaces are very hilly and rugged, superficially resembling the lunar highlands. Noachian terrains consist of overlapping and interbedded ejecta blankets of many old craters. Mountainous rim materials and uplifted basement rock from large impact basins are also common. (See Anseris Mons, for example.) The number-density of large impact craters is very high, with about 200 craters greater than 16 km in diameter per million km2.Werner, S.C.; Tanaka, K.L. (2011). Redefinition of the Crater-Density and Absolute-Age Boundaries for the Chronostratigraphic System of Mars. Icarus, 215, 603–607. Noachian-aged units cover 45% of the Martian surface;Tanaka, K.L. et al. (2014). Geologic Map of Mars. U.S. Geological Survey Scientific Investigations Map 3292, pamphlet they occur mainly in the southern highlands of the planet, but are also present over large areas in the north, such as in Tempe and Xanthe Terrae, Acheron Fossae, and around the Isidis basin (Libya Montes).Scott, D.H.; Tanaka, K.L. (1986). Geologic Map of the Western Equatorial Region of Mars. U.S. Geological Survey Miscellaneous Investigations Series Map I–1802–A.Greeley, R.; Guest, J.E. (1987). Geologic Map of the Eastern Equatorial Region of Mars. U.S. Geological Survey Miscellaneous Investigations Series Map I–1802–B.
Noachian chronology and stratigraphy
Martian time periods are based on geological mapping of surface units from spacecraft images.Scott, D.H.; Carr, M.H. (1978). Geologic Map of Mars. U.S. Geological Survey Miscellaneous Investigations Series Map I-1083. A surface unit is a terrain with a distinct texture, color, albedo, spectral property, or set of landforms that distinguish it from other surface units and is large enough to be shown on a map.Greeley, R. (1994) Planetary Landscapes, 2nd ed.; Chapman & Hall: New York, p. 8 and Fig. 1.6. Mappers use a stratigraphic approach pioneered in the early 1960s for photogeologic studies of the Moon. Although based on surface characteristics, a surface unit is not the surface itself or group of landforms. It is an inferred geologic unit (e.g., formation) representing a sheetlike, wedgelike, or tabular body of rock that underlies the surface.Wilhelms, D.E. (1990). Geologic Mapping in Planetary Mapping, R. Greeley, R.M. Batson, Eds.; Cambridge University Press: Cambridge UK, p. 214.Tanaka, K.L.; Scott, D.H.; Greeley, R. (1992). Global Stratigraphy in Mars, H.H. Kieffer et al., Eds.; University of Arizona Press: Tucson, AZ, pp. 345–382. A surface unit may be a crater ejecta deposit, lava flow, or any surface that can be represented in three dimensions as a discrete stratum bound above or below by adjacent units (illustrated right). Using principles such as superpositioning (illustrated left), cross-cutting relationships, and the relationship of impact crater density to age, geologists can place the units into a relative age sequence from oldest to youngest. Units of similar age are grouped globally into larger, time-stratigraphic (chronostratigraphic) units, called systems. For Mars, four systems are defined: the Pre-Noachian, Noachian, Hesperian, and Amazonian. Geologic units lying below (older than) the Noachian are informally designated Pre-Noachian. The geologic time (geochronologic) equivalent of the Noachian System is the Noachian Period. Rock or surface units of the Noachian System were formed or deposited during the Noachian Period.
System vs. Period
System and Period are not interchangeable terms in formal stratigraphic nomenclature, although they are frequently confused in popular literature. A system is an idealized stratigraphic column based on the physical rock record of a type area (type section) correlated with rocks sections from many different locations planetwide.Eicher, D.L.; McAlester, A.L. (1980). History of the Earth; Prentice-Hall: Englewood Cliffs, NJ, pp 143–146, . A system is bound above and below by strata with distinctly different characteristics (on Earth, usually index fossils) that indicate dramatic (often abrupt) changes in the dominant fauna or environmental conditions. (See Cretaceous–Paleogene boundary as example.)
At any location, rock sections in a given system are apt to contain gaps (unconformities) analogous to missing pages from a book. In some places, rocks from the system are absent entirely due to nondeposition or later erosion. For example, rocks of the Cretaceous System are absent throughout much of the eastern central interior of the United States. However, the time interval of the Cretaceous (Cretaceous Period) still occurred there. Thus, a geologic period represents the time interval over which the strata of a system were deposited, including any unknown amounts of time present in gaps. Periods are measured in years, determined by radioactive dating. On Mars, radiometric ages are not available except from Martian meteorites whose provenance and stratigraphic context are unknown. Instead, absolute ages on Mars are determined by impact crater density, which is heavily dependent upon models of crater formation over time. Accordingly, the beginning and end dates for Martian periods are uncertain, especially for the Hesperian/Amazonian boundary, which may be in error by a factor of 2 or 3.Hartmann, W.K.; Neukum, G. (2001). Cratering Chronology and Evolution of Mars. In Chronology and Evolution of Mars, Kallenbach, R. et al. Eds., Space Science Reviews, 96: 105–164.
Boundaries and subdivisions Across many areas of the planet, the top of the Noachian System is overlain by more sparsely cratered, ridged plains materials interpreted to be vast flood basalts similar in makeup to the lunar maria. These ridged plains form the base of the younger Hesperian System (pictured right). The lower stratigraphic boundary of the Noachian System is not formally defined. The system was conceived originally to encompass rock units dating back to the formation of the crust 4500 million years ago. However, work by Herbert Frey and colleagues at NASA's Goddard Spaceflight Center using Mars Orbital Laser Altimeter (MOLA) data indicates that the southern highlands of Mars contain numerous buried impact basins (called quasi-circular depressions, or QCDs) that are older than the visible Noachian-aged surfaces and that pre-date the Hellas impact. He suggests that the Hellas impact should mark the base of the Noachian System. If Frey is correct, then much of the bedrock in the Martian highlands is pre-Noachian in age, dating back to over 4100 million years ago.Frey, H.V. (2003). Buried Impact Basins and the Earliest History of Mars. Sixth International Conference on Mars, Abstract #3104. http://www.lpi.usra.edu/meetings/sixthmars2003/pdf/3104.pdf.