General Description
The volcano is located in the southeast-central portion of the Tharsis quadrangle at 11.8°N, 255.5°E in Mars' western hemisphere. A group of three smaller volcanoes (the Ceraunius-Uranius group) lies about 700 km to the northeast, and Pavonis Mons (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 caldera (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 albedo (reflectivity) and low thermal inertia, indicating that the volcano and surrounding areas are covered with large amounts of fine dust. (See the Martian Surface.) 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 HiRISE camera on the Mars Reconnaissance Orbiter (MRO) in November 2010 (pictured in gallery below). The pits resemble those imaged around Arsia Mons 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 dike or lava tube. They are analogous to volcanic pit craters 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 caves.
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