Noachian

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This article is about the Martian geologic system and period. For the Biblical patriarch of whom "Noachian" is the derived adjective, see Noah.
Noachian
4100 – 3700 Ma
lunar highlands. Colors indicate elevation, with red highest and blue-violet lowest. The blue feature at bottom right is the northwestern portion of the giant Hellas impact basin
.
Chronology
SubdivisionsEarly Noachian

Middle Noachian

Late Noachian
Usage information
Celestial body
Period
Stratigraphic unitSystem
Type sectionNoachis Terra

The Noachian is a

Early Imbrian periods[2] of 4100 to 3700 million years ago, during the interval known as the Late Heavy Bombardment.[3] 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.[4]

Noachian-aged terrains on Mars are prime

phyllosilicates) that formed under chemical conditions conducive to microbial life.[13][14]

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.[15] 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.[16] 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.[17][18] 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.[19] Other researchers argue for a semiarid early Mars with at least transient periods of rainfall warmed by a carbon dioxide-hydrogen atmosphere.[20] 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.[21] 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.[22][23][24]

Description and name origin

The Noachian System and Period is named after

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.[25] (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.[26] Noachian-aged units cover 45% of the Martian surface;[27] 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).[28][29]

Pre-NoachianHesperianAmazonian (Mars)
Martian Time Periods (Millions of Years Ago)

Noachian chronology and stratigraphy

Schematic cross section of image at left. Surface units are interpreted as a sequence of layers (strata), with the youngest at top and oldest at bottom in accordance with the law of superposition.
HiRISE image illustrating superpositioning, a principle that lets geologists determine the relative ages of surface units. The dark-toned lava flow overlies (is younger than) the light-toned, more heavily cratered terrain (older lava flow?) at right. The ejecta of the crater at center overlies both units, indicating that the crater is the youngest feature in the image. (See schematic cross section, right.)

Martian time periods are based on

Hesperian, and Amazonian. Geologic units lying below (older than) the Noachian are informally designated Pre-Noachian.[35] 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

e  h
Segments of rock (strata) in chronostratigraphy Periods of time in geochronology Notes (Mars)
Eonothem
Eon
 not used for Mars
Erathem
Era
 not used for Mars
System
Period
 3 total; 108 to 109 years in length
Series
Epoch
 8 total; 107 to 108 years in length
Stage
Age
 not used for Mars
Chronozone
Chron
 smaller than an age/stage; not used by the ICS timescale

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

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 (

absolute ages on Mars are determined by impact crater density, which is heavily dependent upon models of crater formation over time.[38] 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.[35][39]

Geologic contact of Noachian and Hesperian Systems. Hesperian ridged plains (Hr) embay and overlie older Noachian cratered plains (Npl). 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. (1992), 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

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.[40]

The Noachian System is subdivided into three chronostratigraphic

Epochs
. Note that an epoch is a subdivision of a period; the two terms are not synonymous in formal stratigraphy.

Noachian Epochs (Millions of Years Ago)[35]

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.[42] (See mineralogical timeline below as example of alternative.) Obtaining radiometric ages on samples from identified surface units is clearly necessary for a more complete understanding of Martian history and chronology.[43]

Mars during the Noachian Period

Artist's impression of an early wet Mars. Late Hesperian features (outflow channels) are shown, so this does not present an accurate picture of Noachian Mars, but the overall appearance of the planet from space may have been similar. In particular, note the presence of a large ocean in the northern hemisphere (upper left) and a sea covering Hellas Planitia (lower right).

The Noachian Period is distinguished from later periods by high rates of impacts, erosion, valley formation, volcanic activity, and weathering of surface rocks to produce abundant

phyllosilicates (clay minerals). These processes imply a wetter global climate with at least episodic warm conditions.[3]

Impact cratering

The lunar cratering record suggests that the rate of impacts in the Inner Solar System 4000 million years ago was 500 times higher than today.[44] During the Noachian, about one 100-km diameter crater formed on Mars every million years,[3] with the rate of smaller impacts exponentially higher.[a] Such high impact rates would have fractured the crust to depths of several kilometers[46] and left thick ejecta deposits across the planet's surface. Large impacts would have profoundly affected the climate by releasing huge quantities of hot ejecta that heated the atmosphere and surface to high temperatures.[47] High impact rates probably played a role in removing much of Mars’ early atmosphere through impact erosion.[48]

Branched valley network of Warrego Valles (Thaumasia quadrangle), as seen by Viking Orbiter. Valley networks like this provide some of the strongest evidence that surface runoff occurred on early Mars.[49]

By analogy with the Moon, frequent impacts produced a zone of fractured

microorganisms, if any existed.[51] Computer models of heat and fluid transport in the ancient Martian crust suggest that the lifetime of an impact-generated hydrothermal system could be hundreds of thousands to millions of years after impact.[52]

Erosion and valley networks

Most large Noachian craters have a worn appearance, with highly eroded rims and sediment-filled interiors. The degraded state of Noachian craters, compared with the nearly pristine appearance of Hesperian craters only a few hundred million years younger, indicates that erosion rates were higher (approximately 1000 to 100,000 times[53]) in the Noachian than in subsequent periods.[3] The presence of partially eroded (etched) terrain in the southern highlands indicates that up to 1 km of material was eroded during the Noachian Period. These high erosion rates, though still lower than average terrestrial rates, are thought to reflect wetter and perhaps warmer environmental conditions.[54]

The high erosion rates during the Noachian may have been due to

valley networks.[3] Valley networks are branching systems of valleys that superficially resemble terrestrial river drainage basins. Although their principal origin (rainfall erosion, groundwater sapping
, or snow melt) is still debated, valley networks are rare in subsequent Martian time periods, indicating unique climatic conditions in Noachian times.

At least two separate phases of valley network formation have been identified in the southern highlands. Valleys that formed in the Early to Mid Noachian show a dense, well-integrated pattern of tributaries that closely resemble drainage patterns formed by rainfall in desert regions of Earth. Younger valleys from the Late Noachian to Early Hesperian commonly have only a few stubby tributaries with interfluvial regions (upland areas between tributaries) that are broad and undissected. These characteristics suggest that the younger valleys were formed mainly by groundwater sapping. If this trend of changing valley morphologies with time is real, it would indicate a change in climate from a relatively wet and warm Mars, where rainfall was occasionally possible, to a colder and more arid world where rainfall was rare or absent.[56]

Lakes and oceans

Further information: Water on Mars
Delta in Eberswalde Crater, seen by Mars Global Surveyor.
Layers of phyllosilicates and sulfates exposed in sediment mound within Gale Crater (HiRISE).

Water draining through the valley networks ponded in the low-lying interiors of craters and in the regional hollows between craters to form large lakes. Over 200 Noachian lake beds have been identified in the southern highlands, some as large as

Eberswalde Crater, Holden Crater, and in Nili Fossae region (Jezero Crater). Other large craters (e.g., Gale Crater) show finely layered, interior deposits or mounds that probably formed from sediments deposited on lake bottoms.[3]

Much of the northern hemisphere of Mars lies about 5 km lower in elevation than the southern highlands.

Paleoshorelines mapped within Hellas Planitia, along with other geomorphic evidence, suggest that large, ice-covered lakes or a sea covered the interior of the Hellas basin during the Noachian period.[65] In 2010, researchers used the global distribution of deltas and valley networks to argue for the existence of a Noachian shoreline in the northern hemisphere.[12] Despite the paucity of geomorphic evidence, if Noachian Mars had a large inventory of water and warm conditions, as suggested by other lines of evidence, then large bodies of water would have almost certainly accumulated in regional lows such as the northern lowland basin and Hellas.[3]

Volcanism

The Noachian was also a time of intense volcanic activity, most of it centered in the Tharsis region.[3] The bulk of the Tharsis bulge is thought to have accumulated by the end of the Noachian Period.[66] The growth of Tharsis probably played a significant role in producing the planet's atmosphere and the weathering of rocks on the surface. By one estimate, the Tharsis bulge contains around 300 million km3 of igneous material. Assuming the magma that formed Tharsis contained carbon dioxide (CO2) and water vapor in percentages comparable to that observed in Hawaiian basaltic lava, then the total amount of gases released from Tharsis magmas could have produced a 1.5-bar CO2 atmosphere and a global layer of water 120 m deep.[3]

CRISM and HiRISE images from the Mars Reconnaissance Orbiter
)

Extensive

pyroclastic flows.[3]

Weathering products

The abundance of olivine in Noachian-aged rocks is significant because olivine rapidly weathers to

alkaline environment to form. In 2006, researchers using the OMEGA instrument on the Mars Express spacecraft proposed a new Martian era called the Phyllocian, corresponding to the Pre-Noachian/Early Noachian in which surface water and aqueous weathering was common. Two subsequent eras, the Theiikian and Siderikian, were also proposed.[13]
The Phyllocian era correlates with the age of early valley network formation on Mars. It is thought that deposits from this era are the best candidates in which to search for evidence of past life on the planet.

See also

Notes

  1. ^ The size-distribution of Earth-crossing asteroids greater than 100 m in diameter follows an inverse power-law curve of form N = kD−2.5, where N is the number of asteroids larger than diameter D.[45] Asteroids with smaller diameters are present in much greater numbers than asteroids with large diameters.

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Bibliography

Further reading
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