Olympus Mons

File:Olympus Mons alt.jpg

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This article is about the volcano on Mars

Olympus Mons (Latin for Mount Olympus) is a very large shield volcano on the planet Mars. By one measure, it has a height of nearly 25km (16mi).[3] Olympus Mons stands almost three times as tall as Mount Everest's height above sea level. It is the youngest of the large volcanoes on Mars, having formed during Mars's Amazonian Period. It is currently the biggest discovered Volcano in the Solar System, and had been known to astronomers since the late 19th century as the albedo feature Nix Olympica (Latin for "Olympic Snow"). Its mountainous nature was suspected well before space probes confirmed its identity as a mountain.[4] The volcano is located in Mars's western hemisphere at approximately 18.65°N 226.2°E,[1] just off the northwestern edge of the Tharsis bulge. The western portion of the volcano lies in the Amazonis quadrangle (MC-8) and the central and eastern portions in the adjoining Tharsis quadrangle (MC-9). Two impact craters on Olympus Mons have been assigned provisional names by the International Astronomical Union. They are the 15.6km (9.7mi)-diameter Karzok crater (18°25′N 131°55′W) and the 10.4km (6.5mi)-diameter Pangboche crater (17°10′N 133°35′W).[5] The craters are notable for being two of several suspected source areas for shergottites, the most abundant class of Martian meteorites.[6]
As a shield volcano, Olympus Mons resembles in its morphology the large volcanoes making up the Hawaiian Islands. The edifice is about 600km (370mi) wide.[7] Because the mountain is so large, with complex structure at its edges, allocating a height to the structure is difficult. It stands 21km (13mi) above the Mars global datum, and its local relief, from the foot of the cliffs which form its margin to the northwest to its peak, is nearly 22km (14mi)[3] (a little over twice the height of Mauna Kea as measured from its base on the ocean floor). The total elevation change from the plains of Amazonis Planitia, over 1,000km (620mi) to the northwest, to the summit approaches 26km (16mi).[2] The summit of the mountain has six nested calderas (collapse craters) forming an irregular depression 60km (37mi) × 80km (50mi) across[8] and up to 3.2km (2.0mi) deep.[9] The volcano's outer edge consists of an escarpment, or cliff, up to 8km (5.0mi) tall, a feature unique among the shield volcanoes of Mars. Olympus Mons covers an area approximately the size of Arizona, or about 295,254km2 (113,998sqmi).[10]

Being a shield volcano, Olympus Mons has a very low profile. The average slope on the volcano's flanks is only 5°.[9] Slopes are highest near the middle part of the flanks and grow shallower toward the base, giving the flanks a concave upward profile. The shape of Olympus Mons is distinctly unsymmetrical. Its flanks are shallower and extend out further from the summit in the northwestern direction than they do to the southeast. The volcano's shape and profile have been likened to a "circus tent" held up by a single pole that is shifted off center.[11]Because of the size of Olympus Mons and its shallow slopes, an observer standing on the Martian surface would be unable to view the entire profile of the volcano, even from a great distance. The curvature of the planet and the volcano itself would obscure such a synoptic view.[12]Similarly, an observer near the summit would be unaware of standing on a high mountain, as the slope of the volcano would extend beyond the horizon, a mere 3 kilometers away.[13]
However, the basal escarpment seen from the plains around the volcano would likely present an astonishing view. The sight of an immense wall of rock shooting up 5miles into the air is sure to impress any future explorers.
Olympous Mons - LEGO Mars Mission Wiki
Olympous Mons - LEGO Mars Mission Wiki
Mars Global Surveyor image of clouds over Tharsis. Olympus Mons is in the upper left.
The typical atmospheric pressure at the top of Olympus Mons is 72 pascal, about 12% of the average Martian surface pressure of 600 pascal.[14][15]Both are exceedingly low by terrestrial standards. By comparison, the atmospheric pressure at the summit of Mount Everest is 32,000 pascals, or about 32% of Earth's sea level pressure.[16]Even so, high-altitude orographic clouds are frequently observed over the Olympus Mons summit, and airborne Martian dust is still present.[17]Although the average Martian surface atmospheric pressure is less than one percent of Earth's, the much lower gravity on Mars increases the atmosphere's scale height; in other words, Mars' atmosphere is puffy and doesn't drop off in density with height as sharply as Earth's.Olympus Mons is an unlikely landing location for automated space probes in the near future. The high elevations preclude parachute-assisted landings because of insufficient atmospheric thickness to slow the spacecraft down. Moreover, Olympus Mons is located in one of the dustiest regions of Mars. A mantle of fine dust covers much of the terrain, obscuring the underlying bedrock (rock samples might be hard to come by). The dust layer would also likely cause severe maneuvering problems for rovers.


Olympous Mons - LEGO Mars Mission Wiki
Olympous Mons - LEGO Mars Mission Wiki
Mars Global Surveyorimage showing lava flows of different ages at the base of Olympus Mons. The flat plain is the younger flow. The older flow has lava channels withleveesalong the edges. Levees are quite common on many lava flows on Mars.
Olympus Mons is the result of many thousands of highly fluid,basalticlavaflows that poured from volcanic vents over a long period of time. (TheHawaiian Islandsexemplify similar shield volcanoes on a smaller scale - seeMauna Kea.) The extraordinary size of Olympus Mons is likely because Mars lacks mobiletectonic plates. Unlike on Earth, the crust of Mars remains fixed over a stationaryhotspot, and a volcano can continue to discharge lava until it reaches an enormous height.[18]The flanks of Olympus Mons are made up of innumerable lava flows and lava channels. Many of the flows have levees along their margins (pictured). Levees are parallel ridges formed at the edges of lava flows. The cooler, outer margins of the flow solidify, leaving a central trough of molten, flowing lava. Partially collapsed lava tubes are visible as chains of pit craters, and broad lava fans formed by lava emerging from intact, subsurface tubes are also common.
[19]In places along the volcano's base, lava flows can be seen spilling out into the surrounding plains, forming broad aprons, and burying the basal escarpment. (Note: Lava flows refer to both actively flowing lava and the solidified landforms they produce. The meaning here is the latter, since Mars has no active flows at the present time.) Crater counts from high resolution images taken by theMars Expressorbiter in 2004 indicate that lava flows on the northwestern flank of Olympus Mons range in age from 115 million years old (Mya) to only 2 Mya.[20]These ages are very recent in geological terms, suggesting that the mountain may still be volcanically active, though in a very quiescent and episodic fashion.[21]
Olympous Mons - LEGO Mars Mission Wiki
Olympous Mons - LEGO Mars Mission Wiki
Calderason Olympus Mons summit. The youngest calderas form circular collapse craters. Older calderas appear as semicircular segments because they are transected by the younger calderas.
The caldera complex at the peak of the volcano is made of at least six overlapping calderas and caldera segments (pictured).[22]Each caldera was formed by roof collapse following depletion and withdrawal of the subsurfacemagma chamberafter an eruption. Each caldera thus represents a separate pulse of volcanic activity on the mountain.[23]The largest and oldest caldera segment appears to have formed as a single, large lava lake.[24]The size of a caldera is a reflection of the size of the underlying magma chamber. Using geometric relationships of caldera dimensions from laboratory models, scientists have estimated that the magma chamber associated with the largest caldera on Olympus Mons lies at a depth of about 32km below the caldera floor.
.[25]Crater size-frequency distributions on the caldera floors indicate the calderas range in age from 350Mya to about 150Mya. All probably formed within 100 million years of each other.[26][27]Olympus Mons is asymmetricalstructurallyas well astopographically. The longer, more shallow northwestern flank displays extensional features, such as large slumps andnormal faults. In contrast, the volcano's steeper southeastern side has features indicating compression. They include step-like terraces in the volcano's mid-flank region (interpreted asthrust faults[28]) and a number ofwrinkle ridgeslocated at the basal escarpment. Why opposite sides of the mountain should show different styles of deformation is puzzling. The answer may lie in understanding how large shield volcanoes grow laterally and on how variations within the substrate of the volcano affect the final shape of the mountain.
Olympous Mons - LEGO Mars Mission Wiki
Olympous Mons - LEGO Mars Mission Wiki
MOLA-generated oblique view of Olympus Mons, showing the asymmetry of the volcano. View is from northeast. The wider, gently sloping northern flank is to the right. The more narrow and steeply sloping southern flank (left) displays low, rounded terraces resembling rumpled carpet. These features are interpreted asthrust faults. The volcano's basal escarpment is prominent. The vertical exaggeration in this image is high (10x), so the relief is more striking than it would appear in reality.
Large shield volcanoes grow not only by adding material to their flanks as erupted lava, but also by spreading laterally at their bases. As a volcano grows in size, thestressfield underneath the volcano changes from compressional to extensional. A subterranean rift may develop at the base of the volcano, causing the underlying crust to spread apart.[29]If the volcano rests on sediments containing mechanically weak layers (e.g., beds of water-saturated clay), detachment zones (decollements) may develop in the weak layers. The extensional stresses in the detachment zones can produce giant landslides and normal faults on the volcano's flanks, leading to the formation of a basal escarpment.[30]Further from the volcano, these detachment zones can express themselves as a succession of overlapping, gravity driven thrust faults. This mechanism has long been cited as an explanation of the Olympus Mons aureole deposits (discussed below).[31]Olympus Mons lies at the edge of the Tharsis bulge, a vast volcanic plateau that is very ancient. The formation of Tharsis was likely complete by the end of theNoachian Period. At the time Olympus Mons began to form inHesperiantimes, the volcano was located on a shallow slope that descended from the high in Tharsis into the northern lowland basins. Over time, these basins would have received large volumes of sediment eroded from Tharsis and the southern highlands. The sediments likely contained abundant Noachian-agedphyllosilicates(clays) formed during a early period on Mars when surface water was abundant.[32]The sediments would be thickest in the northwest where basin depth was greatest. As the volcano grew through lateral spreading, low-friction detachment zones preferentially developed in the thicker sediment layers to the northeast, creating the basal escarpment and widespread lobes of aureole material (Lycus Sulci). Spreading also occurred to the southeast; however, it was more constrained in that direction by the Tharsis rise, which presented a higher-friction zone at the volcano's base. Friction was higher in that direction because the sediments were thinner and probably consisted of coarser grained material resistant to sliding. The competent and rugged basement rocks of Tharsis acted as an additional source of friction. Thus, basal spreading of Mons Olympus was inhibited in the southeast direction, accounting for the structural and topographic asymmetry of the mountain. Numerical models of particle dynamics involving lateral differences in friction along the base of Olympus Mons have been shown to reproduce the volcano's present shape and asymmetry fairly well.[33]The detachment along the weak layers was likely aided by the presence of high-pressure water in the sediment pore spaces. This possibility has interesting astrobiological implications. If water-saturated zones still exist in sediments under the volcano, they would likely have been kept warm by a high geothermal gradient and residual heat from the volcano's magma chamber. Potential springs or seeps around the volcano would offer exciting possibilities for detecting microbial life.[34]

Early observations and naming

Olympus Mons and a few other volcanoes in theTharsis regionare high enough to reach above the frequentMartian dust stormsrecorded by telescopic observers as early as the 19th century. The astronomerPatrick Moorepoints out thatSchiaparelli(1835-1910) "had found that hisNodus GordisandOlympic Snow[Nix Olympica] were almost the only features to be seen" during dust storms, and "guessed correctly that they must be high".[35]TheMariner 9spacecraft arrived in orbit around Mars in 1971 during a global dust-storm. The first objects to become visible as the dust began to settle were the tops of the Tharsis volcanoes, demonstrating that the altitude of these features greatly exceeded that of any mountain found on Earth. Afterward, astronomers adopted the nameOlympus Monsfor the albedo feature known as Nix Olympica.

Regional setting and surrounding features

Olympus Mons is located at the northwestern edge of theTharsisregion, about 1200 km from the other three large Martian shield volcanoes, collectively called theTharsis Montes(Arsia Mons,Pavonis Mons, andAscraeus Mons). The Tharsis Montes are slightly smaller than Olympus Mons.A wide,annulardepression ormoatabout 2km (1mi) deep surrounds the base of Olympus Mons and is thought to be due to the volcano's immense weight pressing down on the Martian crust. The depth of this depression is greater on the northwest side of the mountain than on the southeast side.Olympus Mons is partially surrounded by a region of distinctive grooved or corrugated terrain known as the Olympus Monsaureole. The aureole consists of several large lobes. Northwest of the volcano, the aureole extends a distance of up to 750km and is known asLycus Sulci(24.6° N, 219° E). East of Olympus Mons, the aureole is partially covered by lava flows, but where it is exposed it goes by different names (Gigas Sulci, for example). The origin of the aureole remains debated, but it was likely formed by huge landslides or gravity-driventhrust sheetsthat sloughed off the edges of the Olympus Mons shield.[36]