| Noachian | |
|---|---|
 MOLA colorized relief map ofNoachis Terra, thetype area for the Noachian System. Note the superficial resemblance to thelunar highlands. Colors indicate elevation, with red highest and blue-violet lowest. The blue feature at bottom right is the northwestern portion of the giantHellasimpact basin. | |
| Chronology | |
| Subdivisions | Early Noachian Middle Noachian |
| Usage information | |
| Celestial body | Mars |
| Time scale(s) used | Martian Geologic Timescale |
| Definition | |
| Chronological unit | Period |
| Stratigraphic unit | System |
| Type section | Noachis Terra |
TheNoachian is ageologic system and earlytime period on the planetMars characterized by high rates ofmeteorite andasteroidimpacts and the possible presence of abundantsurface water.[1] Theabsolute age of the Noachian period is uncertain but probably corresponds to the lunarPre-Nectarian toEarly Imbrian periods[2] of 4100 to 3700 million years ago, during the interval known as theLate Heavy Bombardment.[3] Many of the large impact basins on theMoon and Mars formed at this time. The Noachian Period is roughly equivalent to the Earth'sHadean and earlyArchean eons when Earth's first life forms likely arose.[4]
Noachian-aged terrains on Mars are primespacecraft landing sites to search forfossil evidence oflife.[5][6][7] During the Noachian, theatmosphere of Mars was denser than it is today, and the climate possibly warm enough (at least episodically) to allow rainfall.[8] Large lakes and rivers were present in the southern hemisphere,[9][10] and an ocean may have covered the low-lying northern plains.[11][12] Extensivevolcanism occurred in theTharsis region, building up enormous masses of volcanic material (theTharsis bulge) and releasing large quantities of gases into the atmosphere.[3]Weathering of surface rocks produced a diversity ofclay minerals (phyllosilicates) that formed under chemical conditions conducive tomicrobial 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 (SeeWater 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 consequentgreenhouse 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, ororbital 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 earlyHesperian.[22][23][24]
TheNoachian System and Period is named afterNoachis Terra (lit. "Land ofNoah"), a heavily cratered highland region west of theHellas basin. Thetype area of the Noachian System is in theNoachis quadrangle (MC-27) around40°S340°W / 40°S 340°W /-40; -340.[2] At a large scale (>100 m), Noachian surfaces are very hilly and rugged, superficially resembling thelunar highlands. Noachian terrains consist of overlapping and interbeddedejecta blankets of many old craters. Mountainous rim materials and upliftedbasement rock from large impact basins are also common.[25] (SeeAnseris 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 inTempe andXanthe Terrae,Acheron Fossae, and around the Isidis basin (Libya Montes).[28][29]
Epochs:
Martian time periods are based ongeological mapping of surface units fromspacecraft images.[25][30] 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.[31] Mappers use astratigraphic approach pioneered in the early 1960s for photogeologic studies of theMoon.[32][33] Although based on surface characteristics, a surface unit is not the surface itself or group oflandforms. It is aninferredgeologic unit (e.g.,formation) representing a sheetlike, wedgelike, or tabular body of rock that underlies the surface.[34][35] A surface unit may be a crater ejecta deposit, lava flow, or any surface that can be represented in three dimensions as a discretestratum bound above or below by adjacent units (illustrated right). Using principles such assuperpositioning (illustrated left),cross-cutting relationships, and the relationship ofimpact crater density to age, geologists can place the units into arelative age sequence from oldest to youngest. Units of similar age are grouped globally into larger, time-stratigraphic (chronostratigraphic) units, calledsystems. For Mars, four systems are defined: the Pre-Noachian, Noachian,Hesperian, and Amazonian. Geologic units lying below (older than) the Noachian are informally designatedPre-Noachian.[36] 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.
| e h | ||
| Segments of rock (strata) inchronostratigraphy | Periods of time ingeochronology | 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 andPeriod are not interchangeable terms in formal stratigraphic nomenclature, although they are frequently confused in popular literature. A system is an idealized stratigraphiccolumn based on the physical rock record of atype area (type section) correlated with rocks sections from many different locations planetwide.[38] A system is bound above and below bystrata with distinctly different characteristics (on Earth, usuallyindex fossils) that indicate dramatic (often abrupt) changes in the dominant fauna or environmental conditions. (SeeCretaceous–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 theCretaceous 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 thestrata of a system were deposited, including any unknown amounts of time present in gaps.[38] Periods are measured in years, determined byradioactive dating. On Mars, radiometric ages are not available except fromMartian meteorites whoseprovenance and stratigraphic context are unknown. Instead,absolute ages on Mars are determined by impact crater density, which is heavily dependent uponmodels of crater formation over time.[39] 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.[36][40]
Across many areas of the planet, the top of the Noachian System is overlain by more sparsely cratered, ridged plains materials interpreted to be vastflood basalts similar in makeup to thelunar 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.[25] However, work by Herbert Frey and colleagues at NASA'sGoddard Spaceflight Center usingMars 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.[41]
The Noachian System is subdivided into three chronostratigraphicseries: Lower Noachian, Middle Noachian, and Upper Noachian. The series are based onreferents 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 theArgyre basin. The plains overlie (are younger than) the more rugged cratered terrain of the Middle Noachian and underlie (are older than) the less cratered, ridged plains of the Lower Hesperian Series.[2][42] The corresponding geologic time (geochronological) units of the three Noachian series are the Early Noachian, Mid Noachian, and Late NoachianEpochs. Note that an epoch is a subdivision of a period; the two terms are not synonymous in formal stratigraphy.
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 toCincinnati, Ohio and visit a rockoutcrop in the UpperOrdovicianSeries of the OrdovicianSystem. You can even collect a fossiltrilobite there. However, you cannot visit the Late OrdovicianEpoch in the OrdovicianPeriod 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.[43] (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.[44]
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 abundantphyllosilicates (clay minerals). These processes imply a wetter global climate with at least episodic warm conditions.[3]
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.[45] 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 thecrust to depths of several kilometers[47] and left thickejecta 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.[48] High impact rates probably played a role in removing much of Mars's early atmosphere through impact erosion.[49]
By analogy with the Moon, frequent impacts produced a zone of fracturedbedrock andbreccias in the upper crust called themegaregolith.[51] The highporosity andpermeability of the megaregolith permitted the deep infiltration ofgroundwater. Impact-generated heat reacting with the groundwater produced long-livedhydrothermal systems that could have been exploited bythermophilicmicroorganisms, if any existed.[52] 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.[53]
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[54]) 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.[55]
The high erosion rates during the Noachian may have been due toprecipitation andsurface runoff.[8][56] Many (but not all) Noachian-aged terrains on Mars are densely dissected byvalley networks.[3] Valley networks are branching systems of valleys that superficially resemble terrestrial riverdrainage 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 resembledrainage 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 bygroundwater 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.[57]
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 asLake Baikal or theCaspian Sea on Earth.[58] Many Noachian craters show channels entering on one side and exiting on the other. This indicates that large lakes had to be present inside the crater at least temporarily for the water to reach a high enough level to breach the opposing crater rim.Deltas orfans are commonly present where a valley enters the crater floor. Particularly striking examples occur inEberswalde Crater,Holden Crater, and inNili 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.[59] Thisdichotomy has existed since the Pre-Noachian.[60] Water draining from the southern highlands during the Noachian would be expected to pool in the northern hemisphere, forming an ocean (Oceanus Borealis[61]). Unfortunately, the existence and nature of a Noachian ocean remains uncertain because subsequent geologic activity has erased much of thegeomorphic evidence.[3] The traces of several possible Noachian- and Hesperian-aged shorelines have been identified along the dichotomy boundary,[62][63] but this evidence has been challenged.[64][65]Paleoshorelines mapped withinHellas 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.[66] 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]
The Noachian was also a time of intense volcanic activity, most of it centered in theTharsis region.[3] The bulk of the Tharsis bulge is thought to have accumulated by the end of the Noachian Period.[67] 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 containedcarbon dioxide (CO2) and water vapor in percentages comparable to that observed in Hawaiianbasalticlava, then the total amount of gases released from Tharsismagmas could have produced a 1.5-bar CO2 atmosphere and a global layer of water 120 m deep.[3]
Extensivevolcanism also occurred in the cratered highlands outside of the Tharsis region, but littlegeomorphologic evidence remains because surfaces have been intensely reworked by impact.[3]Spectral evidence from orbit indicates that highland rocks are primarilybasaltic in composition, consisting of themineralspyroxene,plagioclase feldspar, andolivine.[68] Rocks examined in theColumbia Hills by theMars Exploration Rover (MER)Spirit may be typical of Noachian-aged highland rocks across the planet.[69] The rocks are mainly degradedbasalts with a variety of textures indicating severe fracturing andbrecciation from impact and alteration by hydrothermal fluids. Some of the Columbia Hills rocks may have formed frompyroclastic flows.[3]
The abundance of olivine in Noachian-aged rocks is significant because olivine rapidly weathers toclay minerals (phyllosilicates) when exposed to water. Therefore, the presence of olivine suggests that prolonged water erosion did not occur globally on early Mars. However, spectral and stratigraphic studies of Noachianoutcroppings from orbit indicate that olivine is mostly restricted to rocks of the Upper (Late) Noachian Series.[3] In many areas of the planet (most notablyNili Fossae andMawrth Vallis), subsequent erosion or impacts have exposed older Pre-Noachian and Lower Noachian units that are rich in phyllosilicates.[70][71] Phyllosilicates require a water-rich,alkaline environment to form. In 2006, researchers using the OMEGA instrument on theMars Express spacecraft proposed a new Martian era called the Phyllocian, corresponding to the Pre-Noachian/Early Noachian in which surface water andaqueous 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.
