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Mars topography (MOLA dataset) HiRes (1).jpg
MC-09 Tharsis 0–30° N 90–135° W Ascraeus Mons Quadrangles Atlas

Tharsis is a huge volcanic plateau in the western hemisphere near the equator of Mars. It covers almost 25 % of the surface of the planet.[1] Tharsis is sometimes called the Tharsis Bulge, Tharsis Rise, or the Tharsis Dome. [2] Officially, "Tharsis" is an albedo feature.[3] The region contains the largest volcanoes in the Solar System, which includes three enormous shield volcanoes: Arsia Mons, Pavonis Mons, and Ascraeus Mons. They are known as the Tharsis Montes. Although these three look small as compared to Olympus Mons, one, Ascraeus Mons is about twice the height of Pikes Peak and almost as high as Mount Everest.[4] Olympus Mons, the tallest volcano on the planet, is often associated with the Tharsis region, however, it is actually located off the western edge of the plateau. Noctis Labyrinthus and the western part of Valles Marineris cut through the middle of Tharsis. Alba Patera is old and low in elevation, but it has the most area of any Martian volcano. Pavonis Mons can serve as a marker for location, as it is very prominent and sits almost directly on the equator. Find Pavonis and you have found the equator.

The name Tharsis is the Greco-Latin of the biblical Tarshish, the land at the western extremity of the known world. [5] [6]

Tharsis features, as seen by Viking 1
Tharsis features with clouds, as seen by Mars Orbiter Camera (MOC) onboard the Mars Global Surveyor (MGS) spacecraft.

Nature of Tharsis Volcanoes

Volcanoes in the Tharsis region are believed to be constructed of the volcanic rock basalt.[7] Basalt is rich in iron and magnesium (mafic) minerals and typically dark in color. It is a very fluid lava so its flows travel long distances and do not make steep volcanoes. Volcanoes of basaltic composition look like shields sitting on the ground. Volcanoes on Mars are able to grow many times larger than those on the Earth because Mars has experienced very little, if any, plate tectonics. Lava from a stationary hot spot is able to accumulate at one location for a billion years or longer. If magma and lava behaves on Mars the way it does on Earth much of the magma (liquid rock under the surface) does not erupt at the surface as lava, rather it never reaches the surface before cooling; hence, it forms large intrusive rock complexes, such as sills and laccoliths, that can cause a general doming and fracturing of the overlying crust. In other words, most of the once liquid rock of Tharsis probably never made it to the surface.[8]

Implications of large volcanoes

The great mass of Tharsis volcanoes has stretched the crust and caused it to crack in places to form long troughs, called fossae. Some extend half way across the planet.[9] Sometimes lava, and maybe even water, can flow from these fossae. When lava erupts, it throws out liquid rock, but it also releases enormous amounts of gas like water and carbon dioxide. So when Tharsis volcanoes were erupting, they may have caused the atmosphere to become thicker and warmer. One study, calculated that the Tharsis bulge contains around 300 million cubic km of igneous rock. If this magma contained carbon dioxide and water vapor in percentages similar to that observed in Hawaiian basaltic lava, then the total amount of gases released could have produced a 1.5-bar carbon dioxide atmosphere and a 120 meter thick layer of water that covered the whole planet. A 1.5 bar atmosphere has one and a half times the pressure of the Earth’s.[10] Gases given off in the eruptions also probably contained large amounts of sulfur and chlorine. When these elements combine with water they produce acids that can break down rocks and minerals. Gases from Tharsis together with other Martian volcanoes are likely responsible for an early period of Martian time (the Theiikian) when sulfuric acid weathering created hydrated sulfate minerals such as kieserite and gypsum that have been detected on the planet by orbiting spectroscopes.[11] Many of the gases, like carbon dioxide, from volcanoes cause global warming. Although the Martian atmosphere is now cold, thin, and dry, in the past it was probably much thicker and warmer since there is widespread evidence of rivers, streams, floods, lakes, and maybe oceans. Much of the planets’ early water has either become frozen into the ground or has escaped into space. [12] [13] [14] [15] [16] Research, reported in 2018, about when Tharsis developed, gave support to the existence of oceans. The team of scientists proposed that Martian oceans appeared very early, before or along with the growth of Tharsis. This proposal increases the chance of oceans because oceans would only need to be half as deep as had been thought. The full weight of Tharsis would have created deep basins, but if the ocean occurred before the mass of Tharsis had formed deep basins, much less water would be required. In addition, the later growth of Tharsis would have caused irregular shorelines, which is what is observed.[17] [18] Some researchers suggested that the great mass of lava in the Tharsis region caused the rotation of the planet to change, but later research cast doubt on this idea.[19] [20] [21]

Many volcanoes of various sizes are found here or closely nearby. Towards the north is Alba Mons the volcano with the largest area on the planet. Ceraunius Tholus, Uranius Tholus, Jovis Tholus, Tharsis Tholus, and Biblis Tholus may only be the tops of old volcanoes that have been mostly buried by younger lava flows; perhaps by as much as 4 km thick lava flows.[22] Other volcanoes are classified as patera. Patera display craters with scalloped edges. Patera is a Latin term meaning a shallow drinking bowl. Ulysses Patera is one such volcano in Tharsis.[23]

Tharsis quadrangle

The Tharsis quadrangle is famous for its many large volcanoes. Tharsis contains Olympus Mons, Ascraeus Mons and Pavonis Mons, three of the four largest shield volcanoes on Mars. Another major geologic feature here is Ceraunius Fossae—a group of troughs. This article will show the most significant landscapes of this region and what finding them means.

Name and location

The name Tharsis refers to a land mentioned in the Bible. It may be at the location of the old town of Tartessus at the mouth of Guadalquivir.[24] The quadrangle covers the area from 0° to 30° north latitude and 90° to 135° west longitude (270--225 E) and contains most of the Tharsis Rise, a large plateau. The plateau is about as high as Earth's Mount Everest and about as big in area as all of Europe. Tharsis contains a group of large volcanoes. Olympus Mons is the tallest.[25]


Topography of Ascraeus Mons. Ascraeus Mons is about twice the height of Pikes Peak and almost as high as Mount Everest.[26]

Tharsis is a land of great volcanoes. Olympus Mons is the tallest known volcano in the Solar System; it is 100 times larger than any volcano on Earth. Ascraeus Mons and Pavonis Mons are at least 200 miles across and are over six miles above Tharsis Rise, the plateau that they sit on. And, get this, the plateau is three to four miles above the zero altitude of Mars.[27]

As of 2001, based on measurements of the Mars Orbiter Laser zero elevation on Mars (standard topographic dataum) was defined as the average radius of the planet at the equator.[28] This type of agreement was necessary since Mars does not have a sea level like the Earth. Before this data came in from the Mars Orbiter, zero altitude was defined as a specific atmospheric pressure of 610.5 Pascals, about six millibars--it is a special number, it's where water can exist as gas, liquid or solid, called "the triple point of water."[29]

Pavonis Mons, the middle in a line of three volcanoes, sits at just about dead center on the equator. So, this middle volcano can serve as a good marker for the position of the Martian equator. Mons is a term used for a large raised feature on Mars. Tholus is about the same, but smaller. A patera is flatter and resembles a volcano with a super large opening. A patera is formed when the top of a volcano collapses because its magma chamber is empty. Crater Lake Oregon was formed in that manner, as well. Several volcanoes form a straight line in the Tharsis Uplift. Two major ones lie in the Tharsus quadrangle: Ascraeus Mons and Pavonis Mons. It has been proposed that these are the result of plate motion which on Earth makes volcanic arc islands.[30][31] [32] [33] [34] Even though Mars presents many volcanoes here and other places, there has been no evidence of recent volcanic activity, even at a very low level. Research, published in 2017, found no active release of volcanic gases during two successive Martian years. The team looked for the outgassing of sulfur-bearing chemicals with spectrometers.[35]

THEMIS mosaic of Uranius Tholus (upper volcano) and Ceraunius Tholus (lower volcano). The latter is about as high as Earth's Mount Everest.
Ulysses Tholus, with its location in relation to other volcanoes shown (photo by THEMIS).

How volcanoes changed climate

Some scientists maintain that Tharsis has shaped the climate of Mars. Volcanoes give off large amounts of gas when they erupt. Most of the gases are water vapor and carbon dioxide. Some estimates put the amount of gas released to the atmosphere as enough to make the atmosphere thicker than the Earth's. In addition, the water discharged by the volcanoes could have been enough to cover all of Mars to a depth of 120 meters. The greenhouse effect of carbon dioxide raises the temperature of a planet by trapping heat. Through this process, volcanic eruptions on Tharsis could have made Mars more Earth-like in the past. This reasoning provides a theoretical basis for how Mars may have once aquired a much thicker and warmer atmosphere. [36] Oceans and/or lakes may have been present. [37] [38] [39] [40] [41] Over the decades of observations with orbiting instruments, we have found much evidence for great amounts of water in the past[42] [43] [44] [45] [46] [47]


Line of pits turning into a trough, as seen by HiRISE under HiWish program

The Tharsis quadrangle is also home to large troughs (long narrow depressions) called fossae in the geographical language used for Mars. Fossae in this area are: Ulysses Fossae, Olympica Fossae, Ceraunius Fossae, and Tractus Fossae. These troughs form when the crust is stretched until it breaks. The stretching on Mars is from the mass of volcanoes. Studies show that the volcanoes of Tharsis caused most of the major fossae on Mars. The stress that caused the fossae and other tectonic features is centered in Noctis Labyrinthus, at 4 S and 253 E. But the center has moved somewhat over time.[48][49] Fossae/pit craters are common near volcanoes both in the Tharsis and Elysium system of volcanoes.[50] Many troughs on Mars are classified as “graben.” They have two breaks with a middle section moving down, leaving steep cliffs along the sides.[51] Grabens often are closely connected to pits (also called pit craters). These may be due to the formation process. Faults may have voids under them. Studies have found that on Mars a fault may be as deep as 5 km, and may widen at depth. It is this widening that causes a void to form with a relatively high volume. When material slides into the void, a pit crater or a pit crater chain forms. Pit craters do not have rims or ejecta around them, like impact craters do. On Mars, individual pit craters can join to form chains or even to form troughs that are sometimes scalloped.[52] A different idea for the formation of these features is the effects of dikes. Dikes present as thin, rock walls. They form when hot magma underground moves along a fault or break in the rock. As the magma moved up it may melt ice in the ground. The resulting action would cause a crack to form at the surface. Knowledge of the locations and formation mechanisms of pit craters and fossae is important for the future colonization of Mars because they may be reservoirs of water.[53]

Troughs and channels Troughs are straight and channels are curved.


There are many signs of glaciers on many of the volcanoes in Tharsis, including Olympus Mons, Ascraeus Mons, and Pavonis Mons.[54] [55] Ceraunius Tholus may have even had its glaciers melt to form some temporary lakes in the past.[56] [57] [58] [59] [60] [61]

Dark slope streaks

Dark slope streaks are widspread on Mars. They occur on steep slopes of craters, troughs, and valleys. The streaks are dark at first. They get lighter with age.[62] Sometimes they start in a tiny spot, then spread out and go for hundreds of meters. They have been seen to travel around obstacles, like boulders.[63] Today, they are generally accepted to be avalanches of bright dust that expose a darker underlying layer. Streaks appear in areas covered with dust. Much of the Martian surface is covered with dust. Fine dust settles out of the atmosphere covering everything.

Streaks, as seen by HiRISE under HiWish program Bright dust moves down and exposes underlying dark material.

Research, published in January 2012 in Icarus, found that at least one group of dark streaks were set off by airblasts from meteorites traveling at supersonic speeds. The team of scientists was led by Kaylan Burleigh, an undergraduate at the University of Arizona. After counting some 65,000 dark streaks around the impact site of a group of 5 new craters, the team found patterns. The number of streaks was greatest closer to the impact site. Somehow the impact probably caused the streaks. The largest crater in the cluster is about 22 m in diameter with close to the area of a basketball court. As the meteorite traveled through the Martian atmosphere it probably broke up; hence a tight group of impact craters resulted.[64] [65]

Lava flows

All of the volcanoes have covered the surface with lava flows. Below are pictures of some of the lava flows. Often there are many separate flows that come from a volcano. they just pile up on one another. Lava flows on Mars are usually of the basalt variety. Such flows can spread over wide areas because they are so fluid. Scientists would say that they have low viscosity.

Lava flows with older and younger flows labeled
Lava flow in Tharsis quadrangle, as seen by HiRISE under HiWish program
Close-up of lava flow with labels Note: this is an enlargement of the previous image of lava flows.

Close view of lava moving over cliff around Olympus Mons, as seen by HiRISE under HiWish program


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