Ascraeus Mons is a large
shield volcanoA shield volcano is a type of volcano usually built almost entirely of fluid lava flows. They are named for their large size and low profile, resembling a warrior's shield. This is caused by the highly fluid lava they erupt, which travels farther than lava erupted from more explosive volcanoes...
located in the
TharsisThe Tharsis region on Mars is a vast volcanic plateau centered near the equator in Mars’ western hemisphere. 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...
region of the
planetA planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
MarsMars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
. It is the northernmost and tallest of three shield volcanoes collectively known as the
Tharsis MontesThe Tharsis Montes are three large shield volcanoes in the Tharsis region of the planet Mars. From north to south, the volcanoes are Ascraeus Mons, Pavonis Mons and Arsia Mons. Mons is the Latin word for mountain...
. The volcano's location corresponds to the
classical albedo featureThe classical albedo features of Mars are the light and dark features that can be seen on the planet Mars through an Earth-based telescope. Before the age of space probes, several astronomers created maps of Mars on which they gave names to the features they could see. The most popular system of...
Ascraeus Lacus.
Ascraeus Mons was discovered by the
Mariner 9Mariner 9 was a NASA space orbiter that helped in the exploration of Mars and was part of the Mariner program. Mariner 9 was launched toward Mars on May 30, 1971 from Cape Canaveral Air Force Station and reached the planet on November 13 of the same year, becoming the first spacecraft to orbit...
spacecraft in 1971. The volcano was originally called North Spot because it was the northernmost of only four spots visible on the surface due to a global dust storm that was then enshrouding the planet. As the dust cleared, the spots were revealed to be extremely tall volcanoes whose summits had projected above the dust-laden, lower atmosphere. The volcano's name officially became Ascraeus Mons in 1973.
General Description
The volcano is located in the southeast-central portion of the
Tharsis quadrangleThe Tharsis quadrangle is one of a series of 30 quadrangle maps of Mars used by the United States Geological Survey Astrogeology Research Program. The Tharsis quadrangle is also referred to as MC-9 ....
at 11.8°N, 255.5°E in Mars' western hemisphere. A group of three smaller volcanoes (the
Ceraunius-Uranius groupThe Uranius group of volcanoes is located on planet Mars in the northeast part of Tharsis and include Uranius Patera, Ceraunius Tholus, and Uranius Tholus. They are dated to the Late Hesperian Epoch and are part of earliest phases of volcanism in the Tharsis province. All three volcanoes are...
) lies about 700 km to the northeast, and
Pavonis MonsPavonis Mons is a large shield volcano located in the Tharsis region of the planet Mars. It is the middle member of a chain of three volcanic mountains that straddle the Martian equator between longitudes 235°E and 259°E. The volcano was discovered by the Mariner 9 spacecraft in 1971 and was...
(the middle volcano of the Tharsis Montes) lies 500 km to the southwest. The 70-km diameter crater Poynting is located 300 km to the west-southwest.
Ascraeus Mons is roughly 480 km in diameter and is the second highest mountain on Mars, with a summit elevation of 18.1 km. The volcano has a very low profile with an average flank slope of 7°. Slopes are steepest in the middle portion of the flanks, flattening out toward the base and near the top where a broad summit plateau and
calderaA caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption, such as the one at Yellowstone National Park in the US. They are sometimes confused with volcanic craters...
(collapse crater) complex are located.
Volcanic vents, located on the northeastern and southwestern edges of the volcano, are sources for broad lava aprons, or fans, that bury nearby portions of the volcano and extend over 100 km out into the surrounding plains. The southwest-northeast orientation of the aprons matches the orientation of the Tharsis Montes, suggesting that a major fissure or rift in the Martian crust is responsible for the orientation of both the aprons and the Tharsis Montes chain. The presence of the lava aprons causes some disagreement in the actual dimensions of the volcano. If the aprons are included as part of the edifice then Ascraeus Mons has dimensions closer to 375 × 870 km.
Like most of the Tharsis region, Ascraeus Mons has a high
albedoAlbedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
(reflectivity) and low thermal inertia, indicating that the volcano and surrounding areas are covered with large amounts of fine dust. (See the
Martian SurfaceThe study of surface characteristics is a broad category of Mars science that examines the nature of the materials making up the Martian surface. The study evolved from telescopic and remote-sensing techniques developed by astronomers to study planetary surfaces...
.) The dust forms a mantle over the surface that obscures or mutes much of the fine-scale topography and geology of the region. Tharsis is probably dusty because of its high elevations. The atmospheric density is too low to mobilize and remove dust once it is deposited. The atmospheric pressure at the summit of Ascraeus Mons averages 100 pascal (1.0 mbar); this is only 17% of the average surface pressure of 600 pascal.
Ascraeus Mons is surrounded by lava flow plains that are mid to late Amazonian in age. The elevation of the plains averages about 3 km above datum (Martian "sea" level), giving the volcano an average vertical relief of 15 km. However, the elevation of the plains varies considerably. The plains northwest of the volcano are less than 2 km in elevation. The plains are highest (>3 km) southeast of the volcano.
The lava plains northwest of Ascraeus Mons are notable for having two dark collapse pits photographed by the
HiRISEHigh Resolution Imaging Science Experiment is a camera on board the Mars Reconnaissance Orbiter. The 65 kg , $40 million instrument was built under the direction of the University of Arizona's Lunar and Planetary Laboratory by Ball Aerospace & Technologies Corp....
camera on the
Mars Reconnaissance OrbiterMars Reconnaissance Orbiter is a NASA multipurpose spacecraft designed to conduct reconnaissance and Exploration of Mars from orbit...
(MRO) in November 2010 (pictured in gallery below). The pits resemble those imaged around
Arsia MonsArsia Mons is the southernmost of three volcanos on the Tharsis bulge near the equator of the planet Mars. To its north is Pavonis Mons, and north of that is Ascraeus Mons. The tallest mountain in the solar system, Olympus Mons, is to its northwest...
by the Mars Odyssey spacecraft. The two pits measure about 180 and 310 m wide, and the larger pit is approximately 180 meters deep. The eastern walls of the pits consist of steep, overhanging ledges. The bottoms of both pits contain sediments and large boulders. These rimless pit craters are believed to form by collapse of surface material into a subsurface void created either by a
dikeA dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
or lava tube. They are analogous to volcanic
pit craterA pit crater is a depression formed by a sinking of the ground surface lying above a void or empty chamber, rather than by the eruption of a volcano or lava vent. It is often found in chains or troughs. Several craters may merge into a linear alignment...
s on Earth, such as the Devil's Throat crater on the upper east rift zone of Kilauea Volcano, Hawaii. In some cases, they may mark skylights/entrances to subsurface
lava caveA lava cave is any cave formed in volcanic rock, although in typical usage it is reserved for caves formed by volcanic processes, which are more properly termed volcanic caves...
s.
Geology
Ascraeus Mons was built by many thousands of fluid
basaltBasalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
ic lava flows. Except for its great size, it resembles terrestrial shield volcanoes like those that form the
Hawaiian IslandsThe Hawaiian Islands are an archipelago of eight major islands, several atolls, numerous smaller islets, and undersea seamounts in the North Pacific Ocean, extending some 1,500 miles from the island of Hawaii in the south to northernmost Kure Atoll...
. The flanks of Ascraeus Mons are covered with narrow, lobate lava flows and lava channels. Many of the lava flows have levees along their margins. 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.
By examining the morphologies of lava flow structures on Ascraeus Mons, geologists are able to calculate the
rheologicalRheology is the study of the flow of matter, primarily in the liquid state, but also as 'soft solids' or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force....
properties of the lava and estimate the rate at which it poured out during eruption (effusion rate). Results show that the lava was highly fluid (low
viscosityViscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
) with low
yield strengthThe yield strength or yield point of a material is defined in engineering and materials science as the stress at which a material begins to deform plastically. Prior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removed...
, resembling Hawaiian and
IcelandIceland , described as the Republic of Iceland, is a Nordic and European island country in the North Atlantic Ocean, on the Mid-Atlantic Ridge. Iceland also refers to the main island of the country, which contains almost all the population and almost all the land area. The country has a population...
ic basaltic lavas. Average effusion rates are about 185 m
3/sec. These rates are comparable to those seen in Hawaii and Iceland. Earth-based radar studies show that Ascraeus Mons has a higher radar echo strength than other volcanic structures on the planet. This could indicate that the lava flows on the flanks of Ascraeus Mons consist of rough Aā-type flows, a conclusion supported by photogeologic analysis of lava flow morhpologies.
The flanks of Ascraeus Mons have a rumpled appearance caused by numerous low, rounded terrace-like structures arranged concentrically around the summit of the volcano. The terraces are spaced 30 to 50 km apart, have lengths up to 100 km, radial widths of 30 km, and heights of about 3 km. Individual terraces are not continuous around the volcano, but instead consist of arcuate segments that overlap with each other, forming an
imbricateIn sedimentology imbrication refers to a primary depositional fabric consisting of a preferred orientation of clasts such that they overlap one another in a consistent fashion, rather like a run of toppled dominoes...
pattern. They are interpreted to be the surface expression of
thrust faultA thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger...
s that formed due to compression along the volcano's flanks. Flank terraces are also common on
Olympus MonsOlympus Mons is a large volcanic mountain on the planet Mars. At a height of almost , it is one of the tallest mountains in the Solar System, three times as tall as Mount Everest and more than twice the height of Mauna Kea the tallest mountain on Earth. Olympus Mons is the youngest of the large...
and the other Tharsis shield volcanoes. The source of the compressive stresses is still debated. The flank terraces may be due to compressional failure of the volcano, flexing of the underlying
lithosphereThe lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
due to the volcano's massive weight, cycles of
magma chamberA magma chamber is a large underground pool of molten rock found beneath the surface of the Earth. The molten rock in such a chamber is under great pressure, and given enough time, that pressure can gradually fracture the rock around it creating outlets for the magma...
inflation and deflation, or shallow gravitational slumping.
Fissures, or flank vents, at the southwestern and northeastern edges of the volcano are the sources of the lava aprons that spread out across the surrounding plains. The fissures seem to have formed by the merger of numerous, narrow
rilleRille is typically used to describe any of the long, narrow depressions in the lunar surface that resemble channels. Typically a rille can be up to several kilometers wide and hundreds of kilometers in length...
-like depressions. In places, the depressions form sinuous channels with islands and other features suggestive of erosion by a fluid. Whether the channels were formed predominately by water or lava is still a topic of debate.
The caldera complex consists of a central caldera surrounded by four coalesced calderas. The central caldera measures about 24 km across and 3.4 km deep and is the youngest of the collapse structures. Crater counting indicates that the central caldera is about 100 million years (Myr) old. The surrounding calderas have ages of about 200, 400, and 800 Myr old, or earlier. A small, partly preserved depression southeast of the main caldera may be as old as 3.8 billion years (Gyr). If the dates are valid, then Ascraeus Mons may have been active through most of Mars' history.
An area of peculiar, fan-shaped deposits (FSD) lies on the volcano's western flank. The FSD consists of a zone of knobby terrain outlined by a semicircular zone of concentric ridges. Similar deposits are also found at the northwestern edges of Pavonis Mons and Arsia Mons, the other two of the Tharsis Montes. The FSD at Ascraeus Mons is the smallest of the three, covering an area of 14,000 km
2 and extending outward from the volcano's base for about 100 km. The origin of these deposits has been debated for decades. However, recent geologic evidence suggests that FSDs are deposits left by glaciers, which covered portions of the volcanoes during a recent period of high obliquity. During periods of high obliquity (axial tilt) the polar regions receive higher levels of sunlight. More water from the poles enters the atmosphere and condenses as ice or snowfall in the cooler equatorial regions. Mars changes its obliquity from about 15° to 35° in cycles of 120,000 years.
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