Olympus Mons - Just the facts

Olympus Mons - Just the facts (A article written in 2007)

Olympus Mons a Latin term meaning "Mount Olympus" is the tallest known volcano and mountain in our solar system, located on the planet Mars at approximately 18° N 133° W. Before space probes confirmed its identity as a mountain, Olympus Mons was known to astronomers as the albedo feature, Nix Olympica ("Snows of Olympus"); since the late 19th century, however it had been suspected that it was mountainous.

The central edifice stands 27 kilometres (around 17 miles/88,585 feet) high above the mean surface level of Mars (about three times the height of Mount Everest above sea level and 2.6 times the height of Mauna Kea above its base). It is 550 km (342 miles) in width, flanked by steep cliffs, and has a caldera that is 85 km (53 miles) long, 60 km (37 miles) wide, and up to 3 km (1.8 miles) deep with six overlapping pit craters. Its outer edge is defined by an escarpment up to 6 km (4 miles) tall; unique among the shield volcanoes of Mars.

Both the size of Olympus Mons (roughly the size of the American state of Arizona) and its shallow slope (2.5 degrees central dome surrounded by 5 degree outer region) mean that a person standing on the surface of Mars would be unable to view the upper profile of the volcano even from a distance as the curvature of the planet and the volcano itself would obscure it. The only way to view the mountain properly is from orbit. However, one could view parts of Mons: standing on the highest point of its summit, the slope of the volcano would extend beyond the horizon, a mere 3 kilometres away; from the three kilometre elevated caldera rim one could see 80 kilometres to the caldera's other side; from the southeast scarp highpoint (about 5 km elevation) one could look about 180 km southeast; from the northwest scarp highpoint (about 8 km elevation) one could look upslope possibly 240 km and look northeast possibly 230 km.

An occasional misconception is that the top of Olympus Mons is above the Martian atmosphere. The atmospheric pressure at the top varies between 5 and 8% of the average Martian surface pressure (600 pascals); by comparison the atmospheric pressure at the summit of Mount Everest is about 32% of that at sea level. Even so, airborne Martian dust is still present and high altitude carbon dioxide-ice cloud cover is still possible at the peak of Olympus Mons, though water-ice clouds are not. Although the Martian average atmospheric pressure is less than 1% of that seen on Earth, the much lower gravity on Mars allows its atmosphere to extend much higher.

Two of the craters on Olympus Mons have been provisionally assigned names by the IAU. These are the 15.6 km diameter Karzok crater (18°25'N 131°55'W) and the 10.4 km diameter Pangboche crater (17°10'N 133°35'W).

Olympus Mons is a shield volcano, the result of highly fluid lava flowing out of volcanic vents over a long period of time, and is much wider than it is tall; the average slope of Olympus Mons' flanks is very gradual. In 2004 the Mars Express orbiter imaged old lava flows on the flanks of Olympus Mons. Based on crater size and frequency counts, the surface of this western scarp has been dated from 115 million years in age down to a region that is only 2 million years old. This is very recent in geological terms, suggesting that the mountain may yet have some ongoing volcanic activity.

The Hawaiian Islands are examples of similar shield volcanoes on a smaller scale (see Mauna Kea). The extraordinary size of Olympus Mons is likely due to the fact that Mars does not have tectonic plates. Thus, the crust remained fixed over a hot spot and the volcano continued to discharge lava, bringing it to such a height.

The caldera at the peak of the volcano was formed after volcanism ceased and the roof of the emptied magma chamber collapsed. During the collapse the surface became extended and formed fractures. Later additional caldera collapses were formed due to subsequent lava production. These overlapped the original circular caldera, giving the edge a less symmetrical appearance.

The mountain, as well as a few other of the volcanoes in the Tharsis region, has sufficient height to reach above the frequent sand storms of Mars, and it was visible from Earth already to 19th century observers. The astronomer Patrick Moore points out that during conditions of sand storm, "Schiaparelli had found that his Nodus Gordis and Olympic Snow were almost the only features to be seen. He guessed correctly that they must be high". But only with the Mariner probes could this be confirmed with certainty. After the Mariner 9 probe had photographed it from orbit in 1972, it became clear that the altitude was much greater than that of any mountain found on Earth, and the name was changed to Olympus Mons.

Olympus Mons is located in the Tharsis bulge, a huge swelling in the Martian surface that bears numerous other large volcanic features. Among them are a chain of lesser shield volcanoes including Arsia Mons, Pavonis Mons and Ascraeus Mons, which are small only in comparison to Olympus Mons itself. The land immediately surrounding Olympus Mons is a depression in the bulge 2 km deep.

The volcano is surrounded by a region known as the Olympus Mons Aureole (Latin, "Circle of Light") with gigantic ridges and blocks extending 1000 km (600 miles) from the summit that show evidence of development and resurfacing connected with glacial activity. Both the escarpment and the Aureole are poorly understood. In one theory this basal cliff was formed by landslides and the Aureole consists of material piled up at the bottom of these landslides.

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