The geology of Mars is the scientific study of the surface, crust, and interior of the planet Mars. It emphasizes the composition, structure, history, and physical processes that shape the planet. It is analogous to the field of terrestrial geology. In planetary science, the term geology is used in its broadest sense to mean the study of the solid parts of planets and moons. The term incorporates aspects of geophysics, geochemistry, mineralogy, geodesy, and cartography. A neologism, areology, from the Greek word Arēs (Mars), sometimes appears as a synonym for Mars's geology in the popular media and works of science fiction (e.g. Kim Stanley Robinson's Mars trilogy). The term areology is also used by the Areological Society.
Geological map of Mars (2014)
The 2014 map was compiled as a digital geographic information system dataset rather than solely as a printed map. Its principal base layers were the Mars Orbiter Laser Altimeter global digital elevation model and the daytime infrared mosaic from the Thermal Emission Imaging System on 2001 Mars Odyssey. Higher-resolution images from the Context Camera on the Mars Reconnaissance Orbiter and other instruments were used to refine contacts and identify smaller landforms where the global base data were insufficient.
Digital mapping allows units and structures to be compared directly with topography, mineral spectra and subsurface observations. Mars datasets are commonly processed with planetary image software and analyzed in GIS, while calibrated mission products and their metadata are preserved by archives such as NASA's Planetary Data System.
Composition of Mars
Mars is a terrestrial planet, which has undergone the process of planetary differentiation.
The InSight lander mission is designed to study the deep interior of Mars. The mission landed on 26 November 2018. and deployed a sensitive seismometer to enable 3D structure mapping of the deep interior.InSight lander completes seismometer deployment on Mars. Stephen Clark, Space Flight Now. 4 February 2019. On 25 October 2023, scientists, helped by information from InSight, reported that the planet Mars has a radioactive magma ocean under its crust.
Global physiography Mars has a number of distinct, large-scale surface features that indicate the types of geological processes that have operated on the planet over time. This section introduces several of the larger physiographic regions of Mars. Together, these regions illustrate how geologic processes involving volcanism, tectonism, water, ice, and impacts have shaped the planet on a global scale.
Hemispheric dichotomy
The northern and southern hemispheres of Mars are strikingly different from each other in topography and physiography. This dichotomy is a fundamental global geologic feature of the planet. The northern part is an enormous topographic depression. About one-third of the surface (mostly in the northern hemisphere) lies 3–6 km lower in elevation than the southern two-thirds. This is a first-order relief feature on par with the elevation difference between Earth's continents and ocean basins. The dichotomy is also expressed in two other ways: as a difference in impact crater density and crustal thickness between the two hemispheres. The hemisphere south of the dichotomy boundary (often called the southern highlands or uplands) is very heavily cratered and ancient, characterized by rugged surfaces that date back to the period of Late Heavy Bombardment. In contrast, the lowlands north of the dichotomy boundary have few large craters, are very smooth and flat, and have other features indicating that extensive resurfacing has occurred since the southern highlands formed. The third distinction between the two hemispheres is in crustal thickness. Topographic and geophysical gravity data indicate that the crust in the southern highlands has a maximum thickness of about , whereas the crust in the northern lowlands "peaks" at around in thickness. The location of the dichotomy boundary varies in latitude across Mars and depends on which of the three physical expressions of the dichotomy is being considered.
The origin and age of the hemispheric dichotomy are still debated. Hypotheses of origin generally fall into two categories: one, the dichotomy was produced by a mega-impact event or several large impacts early in the planet's history (exogenic theories) or two, the dichotomy was produced by crustal thinning in the northern hemisphere by mantle convection, overturning, or other chemical and thermal processes in the planet's interior (endogenic theories). One endogenic model proposes an early episode of plate tectonics producing a thinner crust in the north, similar to what is occurring at spreading plate boundaries on Earth. Whatever its origin, the Martian dichotomy appears to be extremely old. A new theory based on the Southern Polar Giant Impact and validated by the discovery of twelve hemispherical alignments shows that exogenic theories appear to be stronger than endogenic theories and that Mars never had plate tectonics that could modify the dichotomy. Laser altimeters and radar-sounding data from orbiting spacecraft have identified a large number of basin-sized structures previously hidden in visual images. Called quasi-circular depressions (QCDs), these features likely represent derelict impact craters from the period of heavy bombardment that are now covered by a veneer of younger deposits. Crater counting studies of QCDs suggest that the underlying surface in the northern hemisphere is at least as old as the oldest exposed crust in the southern highlands. The ancient age of the dichotomy places a significant constraint on theories of its origin.
Tharsis and Elysium volcanic provinces
Straddling the dichotomy boundary in Mars's western hemisphere is a massive volcano-tectonic province known as the Tharsis region or the Tharsis bulge. This immense, elevated structure is thousands of kilometers in diameter and covers up to 25% of the planet's surface. Averaging 7–10 km above datum (Martian "sea" level), Tharsis contains the highest elevations on the planet and the largest known volcanoes in the Solar System. Three enormous volcanoes, Ascraeus Mons, Pavonis Mons, and Arsia Mons (collectively known as the Tharsis Montes), sit aligned NE-SW along the crest of the bulge. The vast Alba Mons (formerly Alba Patera) occupies the northern part of the region. The huge shield volcano Olympus Mons lies off the main bulge, at the western edge of the province. The extreme massiveness of Tharsis has placed tremendous stress on the planet's lithosphere. As a result, immense extensional fractures (grabens and rift valleys) radiate outward from Tharsis, extending halfway around the planet.Carr, M.H (2007). Mars: Surface and Interior in Encyclopedia of the Solar System, 2nd ed., McFadden, L.-A. et al. Eds. Elsevier: San Diego, CA, p.319
A smaller volcanic center lies several thousand kilometers west of Tharsis in Elysium. The Elysium volcanic complex is about 2,000 kilometers in diameter and consists of three main volcanoes, Elysium Mons, Hecates Tholus, and Albor Tholus. The Elysium group of volcanoes is thought to be somewhat different from the Tharsis Montes, in that development of the former involved both lavas and pyroclastics.
Large impact basins Several enormous, circular impact basins are present on Mars. The largest one that is readily visible is the Hellas basin located in the southern hemisphere. It is the second largest confirmed impact structure on the planet, centered at about 64°E longitude and 40°S latitude. The central part of the basin (Hellas Planitia) is 1,800 km in diameterBoyce, J.M. (2008) The Smithsonian Book of Mars; Konecky&Konecky: Old Saybrook, CT, p. 13. and surrounded by a broad, heavily eroded annular rim structure characterized by closely spaced rugged irregular mountains (massifs), which probably represent uplifted, jostled blocks of old pre-basin crust.Carr, M.H.; Saunders, R.S.; Strom R.G. (1984). Geology of the Terrestrial Planets; NASA Scientific and Technical Information Branch: Washington DC, 1984, p. 223. http://www.lpi.usra.edu/publications/books/geologyTerraPlanets/ (See Anseris Mons, for example.) Ancient, low-relief volcanic constructs (highland paterae) are located on the northeastern and southwestern parts of the rim. The basin floor contains thick, structurally complex sedimentary deposits that have a long geologic history of deposition, erosion, and internal deformation. The lowest elevations on the planet are located within the Hellas basin, with some areas of the basin floor lying over 8 km below datum.
The two other large impact structures on the planet are the Argyre and Isidis basins. Like Hellas, Argyre (800 km in diameter) is located in the southern highlands and is surrounded by a broad ring of mountains. The mountains in the southern portion of the rim, Charitum Montes, may have been eroded by valley glaciers and ice sheets at some point in Mars's history. The Isidis basin (roughly 1,000 km in diameter) lies on the dichotomy boundary at about 87°E longitude. The northeastern part of the basin rim has been eroded and is now buried by northern plains deposits, giving the basin a semicircular outline. The northwestern rim of the basin is characterized by arcuate grabens (Nili Fossae) that are circumferential to the basin. One additional large basin, Utopia, is completely buried by northern plains deposits. Its outline is clearly discernable only from altimetry data. All of the large basins on Mars are extremely old, dating to the late heavy bombardment. They are thought to be comparable in age to the Imbrium and Orientale basins on the Moon.
Equatorial canyon system
Near the equator in the western hemisphere lies an immense system of deep, interconnected canyons and troughs collectively known as the Valles Marineris. The canyon system extends eastward from Tharsis for a length of over 4,000 km, nearly a quarter of the planet's circumference. If placed on Earth, Valles Marineris would span the width of North America.Kargel, J.S. (2004) Mars: A Warmer Wetter Planet; Springer-Praxis: London, p. 52. In places, the canyons are up to 300 km wide and 10 km deep. Often compared to Earth's Grand Canyon, the Valles Marineris has a very different origin than its tinier, so-called counterpart on Earth. The Grand Canyon is largely a product of water erosion. The Martian equatorial canyons are of tectonic origin, i.e. they were formed mostly by faulting. They could be similar to the East African Rift valleys. The canyons represent the surface expression of a powerful extensional strain in the Martian crust, probably due to loading from the Tharsis bulge.
Chaotic terrain and outflow channels The terrain at the eastern end of the Valles Marineris grades into dense jumbles of low rounded hills that seem to have formed by the collapse of upland surfaces to form broad, rubble-filled hollows. Called chaotic terrain, these areas mark the heads of huge outflow channels that emerge full size from the chaotic terrain and empty (debouch) northward into Chryse Planitia. The presence of streamlined islands and other geomorphic features indicate that the channels were most likely formed by catastrophic releases of water from aquifers or the melting of subsurface ice. However, these features could also be formed by abundant volcanic lava flows coming from Tharsis. The channels, which include Ares, Shalbatana, Simud, and Tiu Valles, are enormous by terrestrial standards, and the flows that formed them correspondingly immense. For example, the peak discharge required to carve the 28-km-wide Ares Vallis is estimated to have been 14 million cubic metres (500 million cu ft) per second, over ten thousand times the average discharge of the Mississippi River.
Ice caps