Noachian
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 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 and releasing large quantities of gases into the atmosphere. Weathering of surface rocks produced a diversity of clay minerals 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. 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. Reconciling the geologic evidence of river valleys and lakes with computer climate models of Noachian Mars has been a major challenge. 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. As a consequence, some researchers now favor an overall Noachian climate that was “cold and icy” punctuated by brief climate excursions warm enough to melt surface ice and produce the fluvial features seen today. Other researchers argue for a semiarid early Mars with at least transient periods of rainfall warmed by a carbon dioxide-hydrogen atmosphere. 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. 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.
Description and name origin
The Noachian System and Period is named after Noachis Terra, a heavily cratered highland region west of the Hellas basin. The type area of the Noachian System is in the Noachis quadrangle around. At a large scale, 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. The number-density of large impact craters is very high, with about 200 craters greater than 16 km in diameter per million km2. Noachian-aged units cover 45% of the Martian surface; 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.Noachian chronology and stratigraphy
Martian time periods are based on geological mapping of surface units from spacecraft images. 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. 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 representing a sheetlike, wedgelike, or tabular body of rock that underlies the surface. 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. Using principles such as superpositioning, 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 units, called systems. For Mars, four systems are defined: the Pre-Noachian, Noachian, Hesperian, and Amazonian. Geologic units lying below the Noachian are informally designated Pre-Noachian. The geologic time 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 correlated with rocks sections from many different locations planetwide. A system is bound above and below by strata with distinctly different characteristics that indicate dramatic changes in the dominant fauna or environmental conditions.At any location, rock sections in a given system are apt to contain gaps 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 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.
Image:Ridged Plains Overlying Noachian.jpg|right|thumb|Geologic contact of Noachian and Hesperian Systems. Hesperian ridged plains embay and overlie older Noachian cratered plains. Note that the ridged plains partially bury many of the old Noachian-aged craters. Image is THEMIS IR mosaic, based on similar Viking photo shown in Tanaka et al., Fig. 1a, p. 352.
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. 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 data indicates that the southern highlands of Mars contain numerous buried impact basins 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.The Noachian System is subdivided into three chronostratigraphic series: Lower Noachian, Middle Noachian, and Upper Noachian. The series are based on referents or locations on the planet where surface units indicate a distinctive geological episode, recognizable in time by cratering age and stratigraphic position. For example, the referent for the Upper Noachian is an area of smooth intercrater plains east of the Argyre basin. The plains overlie the more rugged cratered terrain of the Middle Noachian and underlie the less cratered, ridged plains of the Lower Hesperian Series. The corresponding geologic time units of the three Noachian series are the Early Noachian, Mid Noachian, and Late Noachian Epochs. Note that an epoch is a subdivision of a period; the two terms are not synonymous in formal stratigraphy.
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Stratigraphic terms are often confusing to geologists and non-geologists alike. One way to sort through the difficulty is by the following example: You can easily go to Cincinnati, Ohio and visit a rock outcrop in the Upper Ordovician Series of the Ordovician System. You can even collect a fossil trilobite there. However, you cannot visit the Late Ordovician Epoch in the Ordovician Period and collect an actual trilobite.
The Earth-based scheme of formal stratigraphic nomenclature has been successfully applied to Mars for several decades now but has numerous flaws. The scheme will no doubt become refined or replaced as more and better data become available. Obtaining radiometric ages on samples from identified surface units is clearly necessary for a more complete understanding of Martian history and chronology.