Tharsis (, ) is a vast volcanic plateau centered near the equator in the western hemisphere of Mars. The region is home to the largest volcanoes in the Solar System, including the three enormous shield volcanoes Arsia Mons, Pavonis Mons, and Ascraeus Mons, which are collectively known as the Tharsis Montes. Current consensus is that Tharsis overlays a hot spot, similar to those thought to underlay volcanic island chains on Earth.
The tallest volcano on the planet, Olympus Mons, is often associated with the Tharsis region but is actually located off the western edge of the plateau. The name Tharsis is the Greco-Latin transliteration of the biblical Tarshish, the land at the western extremity of the known world.
Tharsis can have many meanings depending on historical and scientific context. The name is commonly used in a broad sense to represent a continent-sized region of anomalously elevated terrain centered just south of the equator around longitude 265°E.Carr, M.H. (2006). The Surface of Mars; Cambridge University Press: Cambridge, UK, p. 46. . Called the Tharsis bulge or Tharsis rise, this broad, elevated region dominates the western hemisphere of Mars and is the largest topographic feature on the planet, after the global dichotomy.Boyce, J.M. (2008). The Smithsonian Book of Mars; Konecky & Konecky: Old Saybrook, CT, p. 101. .
Location
Tharsis has no formally defined boundaries,Morton, O. (2002). Mapping Mars: Science, Imagination, and the Birth of a World; Picador: New York, p. 98, . so precise dimensions for the region are difficult to give. In general, the bulge is about across and up to high (excluding the volcanoes, which have much higher elevations). It roughly extends from Amazonis Planitia (215°E) in the west to Chryse Planitia (300°E) in the east. The bulge is slightly elongated in the north–south direction, running from the northern flanks of Alba Mons (about 55°N) to the southern base of the Thaumasia highlands (about 43°S). Depending on how the region is defined, Tharsis covers , or up to 25% of Mars's surface area.Tanaka, K.L.; Scott, D.H.; Greeley, R. (1992). Global Stratigraphy in Mars, H.H. Kieffer et al., Eds; University of Arizona Press: Tucson, AZ, p. 369. .
Extant
The greater Tharsis region consists of several geologically distinct subprovinces with different ages and volcano-tectonic histories. The subdivisions given here are informal and may rise all or parts of other formally named physiographic features and regions.
North and south
Tharsis is divided into two broad rises: a northern and a larger southern rise.Boyce, J.M. (2008). The Smithsonian Book of Mars; Konecky & Konecky: Old Saybrook, CT, p. 103. . The northern rise partially overlies sparsely cratered, lowland plains north of the dichotomy boundary. This region is dominated by Alba Mons and its extensive volcanic flows. Alba Mons is a vast, low-lying volcanic construct that is unique to Mars. Alba Mons is so large and topographically distinct that it can almost be treated as an entire volcanic province unto itself. The oldest part of the northern rise consists of a broad topographic ridge that corresponds to the highly fractured terrain of Ceraunius Fossae. The ridge is oriented north–south and forms part of the Noachian-aged basement on which Alba Mons sits. Also located in the northern rise are lava flows of the Ceraunius Fossae Formation, which are somewhat older than the Amazonian-aged flows that make up much of the central Tharsis region to the south.Scott, D.H.; Tanaka, K.L. (1986). Geologic Map of the Western Equatorial Region of Mars. USGS; I-1802-A.
Plana The larger southern portion of Tharsis (pictured right) lies on old cratered highland terrain. Its western boundary is roughly defined by the high lava plains of Daedalia Planum, which slope gently to the southwest into the Memnonia and Terra Sirenum regions. To the east, the southern Tharsis bulge consists of the Thaumasia Plateau, an extensive stretch of volcanic plains about 3,000 km wide.Carr, M.H. (2006). The Surface of Mars; Cambridge University Press: Cambridge, UK, p. 92-93. . The Thaumasia Plateau is bounded to the west by a highly elevated zone of fractures (Claritas Fossae) and mountains (the Thaumasia Highlands) that curves south then east to northeast in a wide arc that has been compared to the shape of a scorpion's tail. The plateau province is bounded to the north by Noctis Labyrinthus and the western three-quarters of Valles Marineris. It is bounded to the east by a north–south oriented ridge called the Coprates rise. These boundaries enclose a broad high plateau and shallow interior basin that include Syria, Sinai, and Solis Plana (see list of plains on Mars). The highest plateau elevations on the Tharsis bulge occur in northern Syria Planum, western Noctis Labyrinthus, and the plains east of Arsia Mons.
Montes
Between the northern and southern portions of the Tharsis bulge lies a relatively narrow, northeast-trending region defined by the three massive Tharsis Montes volcanoes: Arsia Mons, Pavonis Mons, and Ascraeus Mons. There are a number of smaller volcanic edifices, and adjacent plains consisting of young (mid to late Amazonian) lava flows. The lava plains slope gently to the east where they overlap and embay the older (Hesperian-aged) terrain of Echus Chasma and western Tempe Terra. To the west, the lava plains slope toward a system of immense northwest-oriented valleys up to wide. These northwestern slope valleys (NSVs) - which debouch into Amazonis Planitia - are separated by a parallel set of gigantic "keel-shaped" promontories. The NSVs may be relics from catastrophic floods of water, similar to the huge outflow channels that empty into Chryse Planitia, east of Tharsis. Central Tharsis is approximately long and includes most of the region covered by the Tharsis quadrangle and the northwestern portion of the adjoining Phoenicis Lacus quadrangle to the south.
Olympus Mons and its associated lava flows and aureole deposits form another distinct subprovince of the Tharsis region. This subregion is about across. It lies off the main topographic bulge, but is related to the volcanic processes that formed Tharsis. Olympus Mons is the youngest of the large Tharsis volcanoes.
Geology Composition
Tharsis is commonly called a volcano-tectonic province, meaning that it is the product of volcanism and associated tectonic processes that have caused extensive crustal deformation. According to the standard view, Tharsis overlies a hot spot, similar to the one thought to underlie the island of Hawaii. The hot spot is caused by one or more massive columns of hot, low-density material (a superplumeDohm, J.M. et al. (2007). Traits and Evolution of the Tharsis Superplume, Mars in Superplumes: Beyond Plate Tectonics, D.A. Yuen et al., Eds.; Springer, pp. 523–536, .) rising through the mantle. The hot spot produces voluminous quantities of magma in the lower crust that is released to the surface as highly fluid, basaltic lava. Because Mars lacks plate tectonics, the lava is able to build up in one region for billions of years to produce enormous volcanic constructs.
On Earth (and presumably Mars as well), not all of the magma produced in a large igneous province erupts at the surface as lava. Much of it stalls in the crust where it slowly cools and solidifies to produce large intrusive complexes (plutons). If the magma migrates through vertical fractures it produces swarms of dikes that may be expressed at the surface as long, linear cracks (fossae) and crater chains (catenae). Magma may also intrude the crust horizontally as large tabular bodies, such as sills and laccoliths, that can cause a general doming and fracturing of the overlying crust. Thus, the bulk of Tharsis is probably made of these intrusive complexes in addition to lava flows at the surface.
One key question about the nature of Tharsis has been whether the bulge is mainly the product of active crustal uplifting from buoyancy provided by the underlying mantle plume or whether it is merely a large, static mass of igneous material supported by the underlying lithosphere. Theoretical analysis of gravity data and the pattern of faults surrounding Tharsis suggest the latter is more likely.Boyce, J.M. (2008). The Smithsonian Book of Mars; Konecky & Konecky: Old Saybrook, CT, p. 107. . The enormous sagging weight of Tharsis has generated tremendous stresses in the crust, producing a broad trough around the region and an array of radial fractures emanating from the center of the bulge that stretches halfway across 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
Age
Geologic evidence, such as the flow direction of ancient valley networks around Tharsis, indicates that the bulge was largely in place by the end of the Noachian Period, some 3.7 billion years ago. Although the bulge itself is ancient, volcanic eruptions in the region continued throughout Martian history and probably played a significant role in the production of the planet's atmosphere and the weathering of rocks on the planet's 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 thick. Martian magmas also likely contain significant amounts of sulfur and chlorine. These elements combine with water to produce acids that can break down primary rocks and minerals. Exhalations from Tharsis and other volcanic centers on the planet are likely responsible for an early period of Martian time (the Theiikian) when sulfuric acid weathering produced abundant hydrated sulfate minerals such as kieserite and gypsum.
Two European Space Agency (ESA) probes have discovered water frost deposits on Tharsis during the morning. Previously, it was thought that water frost on the equator Mars was impossible. Such findings are significant to understanding the present day water cycle on Mars and its habitability.
True polar wander on Mars
The total mass of the Tharsis bulge is approximately 1021 kg,A volume of 3 × 108 km3 (Phillips et al., 2001) multiplied by average density of the igneous material (3.1 × 103 kg/m3) times conversion factor of 1 × 109 m3/km3 gives a mass of 9.3 × 1020 (or ~1021) kg. Average density is from Nimmo and Tanaka (2005), p. 138. about the same as the dwarf planet Ceres. Tharsis is so large and massive that it has likely affected the planet's moment of inertia, possibly causing a change in the orientation of the planet's crust with respect to its rotational axis over time. According to one recent study, Tharsis originally formed at about 50°N latitude and migrated toward the equator between 4.2 and 3.9 billion years ago. Such shifts, known as true polar wander, would have caused dramatic climate changes over vast areas of the planet. A more recent study reported in Nature agreed with the polar wander, but the authors thought the eruptions at Tharsis happened at a slightly different time.
Volcanism