Hubbry Logo
NoachianNoachianMain
Open search
Noachian
Community hub
Noachian
logo
8 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Noachian
Noachian
Noachian
View on Wikipedia
from Wikipedia
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
MOLA colorized relief map of Noachis Terra, the type area for the Noachian System. Note the superficial resemblance to the 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 bodyMars
Time scale(s) usedMartian Geologic Timescale
Definition
Chronological unitPeriod
Stratigraphic unitSystem
Type sectionNoachis Terra

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.[1] The absolute age of the Noachian period is uncertain but probably corresponds to the lunar Pre-Nectarian to 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 spacecraft landing sites to search for fossil evidence of life.[5][6][7] 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.[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] 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.[3] Weathering of surface rocks produced a diversity of clay minerals (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

[edit]

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 40°S 340°W / 40°S 340°W / -40; -340.[2] 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.[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-NoachianNoachianHesperianAmazonian (Mars)
Martian time periods (millions of years ago)

Epochs:

Martian Time Periods (Millions of Years Ago)

Noachian chronology and stratigraphy

[edit]
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 geological mapping of surface units from spacecraft 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 a stratigraphic approach pioneered in the early 1960s for photogeologic studies of the Moon.[32][33] 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.[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 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.[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.

System vs. Period

[edit]
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 column based on the physical rock record of a type area (type section) correlated with rocks sections from many different locations planetwide.[38] 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.[38] 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.[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]

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

[edit]

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.[25] 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.[41]

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 (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 Noachian 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)[36]

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.[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]

Mars during the Noachian Period

[edit]
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

[edit]

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 the crust to depths of several kilometers[47] 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.[48] High impact rates probably played a role in removing much of Mars's early atmosphere through impact erosion.[49]

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.[50]

By analogy with the Moon, frequent impacts produced a zone of fractured bedrock and breccias in the upper crust called the megaregolith.[51] The high porosity and permeability of the megaregolith permitted the deep infiltration of groundwater. Impact-generated heat reacting with the groundwater produced long-lived hydrothermal systems that could have been exploited by thermophilic microorganisms, 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]

Erosion and valley networks

[edit]

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 to precipitation and surface runoff.[8][56] Many (but not all) Noachian-aged terrains on Mars are densely dissected by 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.[57]

Lakes and oceans

[edit]
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 Lake Baikal or the Caspian 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 or fans are commonly present where a valley enters the crater floor. Particularly striking examples occur in 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.[59] This dichotomy 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 the geomorphic 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 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.[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]

Volcanism

[edit]

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.[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 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]

Four outcroppings of Lower Noachian rocks showing spectral signatures of mineral alteration by water. (CRISM and HiRISE images from the Mars Reconnaissance Orbiter)

Extensive volcanism also occurred in the cratered highlands outside of the Tharsis region, but little geomorphologic evidence remains because surfaces have been intensely reworked by impact.[3] Spectral evidence from orbit indicates that highland rocks are primarily basaltic in composition, consisting of the minerals pyroxene, plagioclase feldspar, and olivine.[68] Rocks examined in the Columbia Hills by the Mars Exploration Rover (MER) Spirit may be typical of Noachian-aged highland rocks across the planet.[69] The rocks are mainly degraded basalts with a variety of textures indicating severe fracturing and brecciation from impact and alteration by hydrothermal fluids. Some of the Columbia Hills rocks may have formed from pyroclastic flows.[3]

Weathering products

[edit]

The abundance of olivine in Noachian-aged rocks is significant because olivine rapidly weathers to clay 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 Noachian outcroppings from orbit indicate that olivine is mostly restricted to rocks of the Upper (Late) Noachian Series.[3] In many areas of the planet (most notably Nili Fossae and Mawrth 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 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

[edit]

Notes

[edit]

References

[edit]

Further reading

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Noachian

View on Grokipedia
from Grokipedia
The Noachian is the earliest geological period in Mars' history, spanning from approximately 4.1 billion years ago to about 3.7 billion years ago and representing the planet's formation and initial surface evolution under intense extraterrestrial bombardment. Named after , a vast, ancient highland region in the southern hemisphere situated between the Argyre and Hellas impact basins, this epoch is defined by its high density of impact craters, exceeding 400 craters larger than 5 km per million square kilometers on mid-Noachian surfaces. It marks a time of heavy and asteroid impacts that shaped much of the Martian crust, including the formation of massive basins like Hellas, Argyre, and Isidis, while also witnessing the onset of significant volcanic activity and the development of the bulge. Key characteristics of the Noachian include widespread evidence of liquid water activity, such as dendritic valley networks indicative of and runoff, as well as the alteration of rocks by non-acidic to form clay minerals like phyllosilicates. These features suggest a dynamic hydrological system, potentially including lakes, episodic rivers, and even a stable northern that could have remained liquid despite globally cold conditions, supported by a thicker atmosphere rich in CO₂ and . The Noachian holds critical significance for understanding Mars' potential habitability, as the late phase (roughly 4.1–3.5 billion years ago) featured conditions conducive to life, including near-equatorial rainfall and glacial features in the southern highlands. Geologic evidence from craters and sediments points to a reducing atmosphere and episodic warmth, contrasting with the planet's later oxidized, arid state, and recent models extend the window for microbial life into the early Hesperian by about 500 million years. Recent explorations by the Perseverance rover in Jezero Crater (as of 2025) have identified potential biosignatures and redox-driven minerals in Noachian-age rocks, while new studies reveal over 15,000 km of ancient river systems in Noachis Terra and atmospheric oxidation processes driving the transition to aridity. This epoch's terrains, preserved in the heavily cratered southern highlands, provide the primary record of Mars' primordial environment and its divergence from Earth's early history. The period's end is associated with a decline in impact rates and the gradual loss of Mars' global as the planet's core cooled, transitioning into the Hesperian epoch around 3.7–3.5 billion years ago.

Etymology and Definition

Name Origin

The Noachian epoch derives its name from , a vast, heavily cratered highland region in Mars's that spans over 2,000 km and resembles a continental landmass amid the planet's ancient terrain. This region, located between the Argyre and Hellas impact basins, exemplifies the densely cratered southern highlands formed during Mars's early history. The designation "Noachis Terra," meaning "Land of Noah," was assigned by Italian astronomer in 1877 during his mapping of Mars's surface features. The Noachian epoch itself was formalized in the as part of the Martian , with David H. Scott and Michael H. Carr defining it in their 1978 U.S. Geological Survey global geologic map based on imagery, identifying it as the oldest system characterized by high crater densities. The (IAU) oversees the official nomenclature for Martian features, including , ensuring consistency in planetary mapping efforts initiated in that .

Geological Epoch Overview

The Noachian epoch represents the oldest named unit in the Martian geologic time scale, encompassing the initial phase of the planet's surface evolution following its accretion. It is characterized by profound modifications to the primordial crust through widespread geological activity, establishing the foundational that dominates much of Mars today. This epoch marks a critical transition in the planet's history, from the chaotic processes of formation to the development of a more stable lithospheric structure. Unlike terrestrial geologic epochs, which are often delineated by fossil records or of rock layers, the Noachian is primarily defined through methods reliant on the density of impact s preserved on the surface. Higher densities indicate older terrains exposed for longer durations to meteoritic , providing a proxy for chronological ordering without direct absolute age constraints. This approach is essential for Mars, where direct sampling remains limited, allowing to the relative antiquity of surface units across the . The hallmark of Noachian terrains includes the heavily cratered southern highlands, which constitute the majority of the planet's ancient crust formed during this epoch. These regions reflect the culmination of early and the solidification of a rigid crust capable of retaining impact features, signifying the shift from a molten, accreting body to one with enduring geological stability. The preservation of these densely cratered surfaces underscores the epoch's role in shaping Mars' hemispheric dichotomy and long-term surface integrity.

Chronostratigraphy

Period Versus System

In planetary geology, the term "Noachian" serves dual purposes within the framework of and lithostratigraphy. The Noachian Period designates a temporal interval in Mars' history, spanning approximately 4.1 to 3.7 billion years ago, during which key geological events such as intense impact occurred. In contrast, the Noachian System refers to the corresponding stratigraphic rock units—primarily the heavily cratered highlands and ancient terrains—that were formed or modified during this timeframe. This distinction aligns with standard geological nomenclature, where periods define spans of geologic time and systems encompass the material record preserved in the planet's crust. On Mars, the Noachian is characterized by its predominance in the southern highlands, including rugged, crater-saturated surfaces that represent the planet's earliest preserved . Unlike , where stratigraphic correlations often rely on involving records, Martian depends entirely on and size-frequency distributions to establish relative and absolute ages, calibrated against lunar impact chronologies. This approach is analogous to 's hierarchy, such as the eon (a geochronologic unit) versus its corresponding (a chronostratigraphic unit), but adapted to a non-biological record. The separation between period and system is crucial for Martian studies, as it allows researchers to correlate temporal events with physical rock layers without assuming direct equivalence, especially given the challenges of and limited sample return.

Boundaries and Subdivisions

The Noachian period on Mars is bounded below at approximately 4.1 billion years ago (Ga), marking the onset of the preserved crustal record after the planet's primary accretion and early giant impacts, near the beginning of the period. This lower limit is supported by of Martian meteorites, such as ALH 84001, which crystallized around 4.09 Ga from a parent melt shortly after the cessation of major accretionary impacts. The upper boundary occurs at about 3.7 Ga, defined by a sharp decline in global impact cratering rates and the emergence of extensive volcanic activity that characterizes the onset of the period. This transition reflects a shift from a bombardment-dominated to one of prolonged geological stability and resurfacing. Within the Noachian, three main subdivisions are recognized based on stratigraphic and cratering evidence: the Early Noachian (~4.1–3.9 Ga), Middle Noachian (~3.9–3.8 Ga), and Late Noachian (~3.8–3.7 Ga). The Early Noachian encompasses the peak of late-stage , during which much of the heavily cratered southern highlands formed amid intense impacts that excavated deep basins like Hellas and Argyre. The Middle Noachian saw gradual crustal stabilization, with reduced but still elevated impact rates allowing for the development of intercrater plains through localized and deposition. The Late Noachian featured a further decline in bombardment and marked an increase in aqueous alteration processes, setting the stage for the period's environmental transitions, though specific features of this activity are detailed elsewhere. These subdivisions align with the chronostratigraphic framework where the Noachian System corresponds to the period's rock record, distinct from the time-rock units of earlier discussions. Dating of these boundaries and subdivisions relies primarily on crater size-frequency distributions (CSFDs), which model surface exposure ages by counting craters above a reference , such as N(20)—the number of craters greater than 20 km in per 10^6 km². Noachian terrains typically exhibit N(20) values exceeding 100, with Early Noachian surfaces approaching saturation (N(20) > 200 in some highlands), Middle Noachian around 100–150, and Late Noachian closer to 50–100, calibrated against lunar impact chronologies like those of and Neukum. Absolute ages are anchored by radiometric data from Martian meteorites, including ALH 84001's crystallization age of ~4.1 Ga, which provides a key tie-point for the lower Noachian, and cross-validated with in situ measurements from orbiters and landers. These methods yield uncertainties of about 0.1–0.2 Ga due to variations in impact flux models and resurfacing effects.

Geological Processes

Impact Bombardment

The Noachian epoch on Mars was marked by an intense period of meteoritic that profoundly influenced the planet's early geological evolution, with impact fluxes extending from the (LHB) phase observed in the inner Solar System. This , peaking around 4.1–4.0 billion years ago (Ga), is evidenced by the formation of numerous large impact basins and a high density of craters across the southern highlands, reflecting collision rates substantially higher than those of the based on lunar chronologies scaled to Mars. Prominent among these events were the basin-forming impacts that sculpted the southern highlands, including the Hellas and Argyre basins, both dated to approximately 4.0 Ga during the Early Noachian. These massive collisions, involving projectiles hundreds of kilometers in , excavated deep depressions and ejected vast amounts of crustal material, contributing to the thick, ancient highland crust that dominates Mars' southern hemisphere today. The Hellas basin, the largest unambiguous impact structure on Mars with a exceeding 2,000 km, exemplifies this era's cataclysmic scale, while Argyre's formation similarly disrupted and reshaped regional . Noachian craters exhibit distinctive morphologies, often appearing degraded and partially filled due to subsequent processes, in contrast to the sharper, younger craters of later s. Crater density metrics, such as N(1)—the number of craters with diameters greater than 1 km per million square kilometers—reveal saturation levels in highland terrains, typically exceeding 1,000 for Early Noachian surfaces, indicating minimal resurfacing and a record of the bombardment's intensity. These degraded features, including buried rims and infilled interiors, provide key stratigraphic markers for the . Recent geophysical analyses from 2023–2025, incorporating seismic data from the mission, suggest that this bombardment drove progressive crustal thickening during the Noachian, with models indicating an initial elastic and crust as thin as 20–30 km that gradually increased to around 40 km through repeated impacts. These elastic simulations demonstrate how large impacts induced isostatic and magmatic underplating, enhancing crustal stability without invoking solely internal heat sources. Such findings underscore the bombardment's role in establishing Mars' bimodal crustal .

Volcanism and Tectonics

During the Noachian period, early shield contributed to the formation of proto-plateaus in the and regions, with activity spanning approximately 3.9 to 3.7 Ga. These precursors involved widespread effusive eruptions that built the foundational structures of these major volcanic provinces, contrasting with the more localized activity seen in later epochs. Evidence from crater counting and stratigraphic analysis indicates that this was part of a broader phase of crustal construction, potentially linked to mantle upwelling beneath the developing bulge. Tectonic processes in the Noachian highlands are evidenced by extensive fault systems and grabens, which record deformation associated with internal planetary stresses rather than dominant impact-related features. These structures, including radial and circumferential fractures around proto-Tharsis, suggest episodic rifting and compression driven by volcanic loading and . Debates persist regarding the possibility of early , with some geophysical models proposing limited or lithospheric recycling to explain the crustal 's formation around 4.0–3.8 Ga, though stagnant-lid remains the prevailing paradigm. Recent 2024 models favor an exogenic giant impact origin for the dichotomy, followed by endogenic modifications through Noachian . Magma compositions during Noachian volcanism were predominantly basaltic, as inferred from spectroscopic data of highland terrains and comparisons to preserved flows in and . This basaltic nature facilitated efficient of volatiles such as H2O and CO2, contributing to the early Martian atmosphere, though the exact volatile budget remains constrained by meteorite analyses.

Erosion and Sedimentation

During the early Noachian epoch, erosion on Mars was primarily driven by impact bombardment and processes, which mechanically broke down the nascent crust and highlands into fragmented materials. Impact events generated widespread and breccias, while aeolian activity sculpted surfaces through and abrasion, contributing to the degradation of primary impact craters. These processes dominated the modification before the late Noachian, when began transitioning to include aqueous mechanisms, though and impact influences persisted. Sedimentation during this period resulted in the accumulation of depositional records, including interbedded layers of impact breccias and within craters and basin margins. Volcanic materials from contemporaneous eruptions served as key sources for these sediments, intermixing with impact-derived debris to form stratified sequences. In regions precursor to , such layered terrains emerged as stacked deposits in ancient basins, preserving a record of episodic burial and . These sedimentary units highlight the interplay between destructive and constructive geological forces in shaping Noachian . Erosion rates in the early Noachian were relatively high, as inferred from crater degradation and surface models, but they declined sharply toward the late Noachian and into subsequent epochs. This decline is evident in etched terrains, where differential erosion has exhumed underlying units, revealing yardang-like features and subdued indicative of prolonged aeolian modification. Recent geophysical analyses from 2025, including ejecta studies near Hellas basin, provide evidence of significant sediment infill during the middle Noachian, with burial depths exceeding hundreds of meters in select s, supporting models of basin-scale deposition.

Hydrology and Paleoenvironments

Fluvial Features and Valley Networks

Dendritic valley networks, resembling terrestrial river systems with branching tributaries, are prominent geomorphic features in the southern highlands of Mars, primarily formed during the Late Noachian epoch approximately 3.8 to 3.7 billion years ago (Ga). These networks, often incised into ancient cratered terrain, exhibit characteristics indicative of sustained , including high drainage densities and integrated channel patterns that suggest prolonged episodes of liquid water flow rather than isolated events. Their distribution is concentrated in regions like Terra Sabaea and , where intercrater plains provided gentle slopes conducive to runoff accumulation and channel incision. Key features of these Noachian valley networks include precursors to later outflow channels and instances of inverted relief, where resistant sedimentary fills now form elevated ridges amid surrounding erosion. For example, the Naktong-Scamander-Mamers Valles system, spanning over 4,700 km, shows late-stage overflow and entrenchment that may have contributed to the development of massive outflow channels in Arabia Terra. Inverted channels, such as those in Eberswalde crater, preserve sinuous ridges up to 130 meters wide, formed by differential erosion of less resistant surrounding materials. Discharge estimates for these networks, derived from Viking Orbiter imagery and (MGS) topographic data, range from approximately 10³ to 10⁵ cubic meters per second (m³/s), implying basin-wide runoff rates of 0.3 to 2.3 cm per day over areas of 5,000 to 23,000 km². These values support episodic but voluminous flows capable of transporting sediments and eroding V-shaped profiles up to several hundred meters deep. A significant recent advancement came in 2025, when mapping efforts using high-resolution images from the Mars Reconnaissance Orbiter's Context Camera (CTX), , and MGS's (MOLA) revealed over 15,000 km of previously unidentified "lost rivers" in , manifested as fluvial sinuous ridges. These features, often isolated segments but forming extensive networks in places, date to the Noachian-Hesperian boundary around 3.7 Ga and indicate stable conditions during this transitional period, with implications for regionally persistent hydrological activity. The discovery, led by researchers at the and presented at the Royal Astronomical Society's National Astronomy Meeting, highlights how inverted channels can preserve evidence of ancient fluvial systems obscured by later erosion. The formation mechanisms of Noachian valley networks remain debated, with two primary hypotheses: precipitation-driven versus . Proponents of argue that the dendritic patterns, high drainage densities, and correlations with gradients require widespread rainfall and overland flow, as supported by MGS-derived morphometric analyses showing similarities to terrestrial humid- networks. In contrast, the model posits that valleys formed through seepage from subsurface aquifers, potentially recharged by impacts or geothermal sources, explaining the limited development and alcove-headed channels without necessitating a globally warm . Hybrid scenarios suggest was modulated by -recharged , reconciling observations from both Viking-era and modern orbital data. This ongoing debate underscores the challenges in reconstructing early Martian from preserved landforms.

Lakes, Rivers, and Ocean Hypotheses

Evidence for standing bodies of water during the Noachian epoch includes filled impact craters that hosted long-lived lakes, as indicated by sedimentary deposits and deltas preserved within them. In Jezero Crater, rover observations reveal an ancient delta-lake system formed by rivers depositing sediments into a standing body of water, with inclined strata and boulder conglomerates suggesting episodic flooding into the lake basin during the late Noachian. Similarly, precursors to the lake systems explored by the Curiosity rover in Gale Crater show evidence of persistent water bodies with deltaic features, supporting the presence of crater lakes that accumulated sediments over extended periods. These lakes were likely fed by valley networks from surrounding highlands, indicating regional hydrological connectivity. Hypotheses for large-scale focus on the northern lowlands, where topographic and mineralogical features suggest episodic bodies of standing around 3.7 billion years ago, potentially covering 20–50% of Mars' surface during peak extents. Proposed shorelines, such as the Arabia and Deuteronilus contacts, align with levels and are associated with phyllosilicate-rich mounds, implying water-rock interactions from inundation by a northern ocean. Clay minerals like Fe/Mg smectites and saponites in these lowlands, detected via orbital , further support aqueous alteration linked to standing , though the exact duration and stability of such remain debated due to the lack of widespread evaporites or carbonates. A 2025 study of Barnard Crater in southern highlights glaciofluvial infill dated to approximately 3.81 billion years ago, revealing Middle-to-Late Noachian lakes with ice-rich deposits and sinuous ridges indicative of meltwater channels, alongside evidence of ice-rafted debris transported into the basin. These features suggest cold-based glacial activity contributing to lake formation and in high-latitude craters. Estimates of the total volume involved in Noachian hydrological systems range from 0.9 × 10^{20} to 4.1 × 10^{20} kg, equivalent to a global layer of 0.6–2.8 km depth, primarily sourced from subsurface reservoirs released by or impacts, with possible contributions from cometary delivery. This volume could have sustained episodic lakes and oceans, though much was likely cycled through transient surface expressions before loss to space or the subsurface.

Mineralogical Evidence for Water

Mineralogical evidence for past during the Noachian period primarily comes from the detection of hydrated silicates, particularly phyllosilicates such as smectites, which indicate low-temperature aqueous alteration of basaltic crust. These minerals form through water-rock interactions under neutral to alkaline conditions, preserving chemical signatures of prolonged surface or near-surface hydration. Orbital spectrometers like the Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA) on and the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on have identified these signatures in Noachian-aged terrains, revealing widespread phyllosilicate deposits. Phyllosilicates, including Fe/Mg-rich smectites like nontronite and saponite, are abundant in layered outcrops within regions such as Mawrth Vallis and Nili Fossae, where spectral analyses show absorption features at 1.9 μm and 2.3 μm indicative of hydroxyl groups bound to iron and magnesium. These deposits formed primarily between approximately 3.9 and 3.7 billion years ago, during the early to middle Noachian, through pedogenic processes involving the of or melted snow in a , leading to stratified alteration profiles with Fe/Mg-smectites in lower layers and Al-rich smectites like or in upper zones. Hydrothermal alteration, potentially linked to impact cratering or magmatic activity, represents a secondary mechanism, confined to localized sites and characterized by different assemblages, though it is less dominant than surface . Sulfates, such as and other Mg/Fe/Ca varieties, provide additional evidence of episodic acidic aqueous environments, formed through H₂SO₄-HCl of basalts that generated sulfate-rich solutions. These minerals coexist with phyllosilicates in Noachian stratigraphies, particularly in Nili Fossae, suggesting initial formation in acidic lakes or systems followed by later neutralization and of sulfates alongside clays in deeper, alkaline zones. The presence of sulfates indicates transient, acidic conditions interspersed with more neutral hydrological episodes, reflecting variable water chemistry during the Noachian. A 2025 study analyzing CRISM data revealed thick clay layers exceeding 100 meters in depth within Noachian craters and lowlands, formed near ancient standing bodies of and exhibiting minimal , which could have provided stable conditions potentially conducive to microbial over extended periods. These extensive deposits underscore the duration and intensity of aqueous alteration, with phyllosilicates comprising significant portions of the stratigraphic succession from 4.1 to 3.7 Ga.

Climate Evolution

Early Atmospheric Conditions

During the Early Noachian epoch, Mars is inferred to have possessed a thick atmosphere dominated by (CO₂), with partial pressures estimated between 0.1 and 1 bar, primarily sourced from extensive volcanic associated with the planet's early magmatic activity. This CO₂-rich envelope was accompanied by nitrogen (N₂) as a secondary component and (H₂O), contributing to a total sufficient to support transient stability. The atmosphere exhibited reducing conditions, characterized by the presence of gases such as (H₂) and (CH₄), produced through volcanic emissions, serpentinization of ultramafic rocks, and meteoritic impacts. Greenhouse warming from this dense CO₂ atmosphere played a critical role in mitigating the effects of the faint young Sun, which delivered approximately 75% of its present-day to Mars around 4.1 billion years ago. Climate models indicate that the enhanced raised surface temperatures above freezing thresholds, enabling episodic warm and wet conditions despite the reduced stellar insolation. These models suggest that without such a thick atmosphere, Mars would have experienced a global freeze, inconsistent with geological evidence for surface modification by liquid . Evidence for atmospheric pressures exceeding 0.3 bar during the Early Noachian derives from the stability of magnesium carbonate minerals preserved in Martian meteorites like ALH 84001, which require such conditions to form without rapid decomposition. A 2024 study further reconstructs the atmospheric evolution, showing that oxidation processes began in the Middle Noachian, transitioning from a reducing state (dominated by H₂ and CH₄) to a more oxidizing one with gradual buildup of oxygen (O₂), driven by loss of reducing gases and surface . This shift marked the onset of climatic changes but preserved the early epoch's warmer baseline established by volcanic outgassing.

Transition to Arid Climate

The cessation of Mars' global magnetic dynamo around 3.9–4.0 billion years ago (Ga) marked a pivotal point in the late Noachian climate transition, as the loss of this protective field exposed the atmosphere to intensified solar wind erosion. Without the magnetic shield, charged particles from the Sun began stripping away atmospheric gases, accelerating the planet's cooling by reducing the greenhouse effect and overall pressure. This process contributed to a dramatic decline in surface atmospheric pressure to below 0.1 bar by the close of the Noachian epoch, transforming the once relatively warmer environment into a progressively colder and thinner one. Amid this overarching cooling, episodic intervals of warmth punctuated the late Noachian and early periods, likely driven by transient geological activity such as large impacts or volcanic eruptions that temporarily thickened the atmosphere or released heat-trapping gases. These events provided brief windows of , allowing for localized hydrological activity before the dominant resumed. Building on evidence from valley networks and elevation distributions, 2025 analyses suggest that in the Martian highlands during the Noachian included rather than solely from ice, implying mean annual temperatures () greater than 0°C in the early to middle epochs to support such processes. As atmospheric loss progressed, however, declined toward subzero values, leading to widespread formation by the epoch's end and curtailing liquid water stability. Sedimentary strata in Arabia Terra preserve a record of this humid-to-arid evolution across the Noachian-Hesperian boundary, with layered deposits indicating initial aqueous deposition under wetter conditions followed by dust-dominated and reduced . These archives highlight how the interplay of declining atmospheric and solar forcing drove the irreversible shift to the planet's current cold, dry state.

References

  1. https://sci.esa.int/web/mars-express/-/55481-the-ages-of-mars
  2. https://pubs.usgs.gov/imap/i2694/i2694pamphlet.pdf
  3. https://www.merriam-webster.com/dictionary/Noachian
  4. https://planetpailly.com/2019/04/16/sciency-words-a-to-z-noachian/
  5. https://cseligman.com/text/planets/1877map.htm
  6. https://pubs.usgs.gov/publication/i1083
  7. https://pubs.usgs.gov/publication/ofr76573
  8. https://link.springer.com/article/10.1023/A:1011945222010
  9. https://nap.nationalacademies.org/read/10715/chapter/6
  10. https://pubs.usgs.gov/sim/3292/pdf/sim3292_map.pdf
  11. https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/JB091iB13p0E139
  12. https://www.sciencedirect.com/science/article/pii/S0019103523003470
  13. https://www.lpl.arizona.edu/hamilton/sites/lpl.arizona.edu.hamilton/files/courses/ptys551/Mars_Global_Geology_USGS_Map_sim3292_Pamphlet.pdf
  14. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/jgre.20053
  15. http://www2.ess.ucla.edu/~nimmo/ess250/hartmann.pdf
  16. https://adsabs.harvard.edu/full/1986LPSC...17..139T
  17. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2008GL033515
  18. https://www.sciencedirect.com/science/article/abs/pii/S001910351500069X
  19. https://www.hou.usra.edu/meetings/lpsc2019/pdf/2612.pdf
  20. https://www.researchgate.net/publication/397150749_Geophysical_evidence_of_progressive_Noachian_crustal_thickening_on_Mars_revealed_by_meteorite_impacts
  21. https://www.sciencedirect.com/science/article/abs/pii/S0012821X24005879
  22. https://www.lyellcollection.org/doi/10.1144/SP401.22
  23. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2007JE002924
  24. https://www.sciencedirect.com/science/article/pii/S001910352200272X
  25. https://www.mdpi.com/2072-4292/14/22/5664
  26. https://www.sciencedirect.com/science/article/pii/S0019103524001970
  27. https://www.hou.usra.edu/meetings/tenthmars2024/pdf/3501.pdf
  28. https://www.pnas.org/doi/10.1073/pnas.2404260121
  29. https://www.sciencedirect.com/science/article/pii/S001910352300204X
  30. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2006JE002875
  31. https://pubs.usgs.gov/imap/1802a/plate-1.pdf
  32. https://www.lpi.usra.edu/meetings/lpsc2002/pdf/2058.pdf
  33. https://iopscience.iop.org/article/10.3847/PSJ/ae031d
  34. https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2009JE003548
  35. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2005JE002460
  36. https://pubs.geoscienceworld.org/gsa/geology/article/33/6/489/103797/Interior-channels-in-Martian-valley-networks
  37. https://www.sciencedaily.com/releases/2025/07/250711082924.htm
  38. https://conference.astro.dur.ac.uk/event/7/contributions/607/
  39. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2010JE003709
  40. https://ntrs.nasa.gov/api/citations/20000092094/downloads/20000092094.pdf
  41. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2008JE003301
  42. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2011JE003983
  43. https://www.nature.com/articles/s41561-024-01634-8
  44. https://geosci.uchicago.edu/~kite/doc/Wordsworth_2016.pdf
  45. https://www.nature.com/articles/s41467-024-47326-0
  46. https://ntrs.nasa.gov/api/citations/20150021504/downloads/20150021504.pdf
  47. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016GL071766
  48. https://www.hou.usra.edu/meetings/lpsc2016/pdf/1220.pdf
  49. https://www.science.org/doi/10.1126/sciadv.ade9071
  50. https://ntrs.nasa.gov/citations/20230001221
  51. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2024JE008637
  52. https://ui.adsabs.harvard.edu/abs/2025epsc.conf..746P/abstract
Add your contribution
Related Hubs
User Avatar
No comments yet.