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Mars Global Surveyor
Mars Orbiter Camera

8 Years at Mars #5: Repeated Weather -- Arsia Mons Spiral Cloud

MGS MOC Release No. MOC2-1224, 20 September 2005

    MOC2-1224a: 19 June 2001, Ls 180°   --   MOC2-1224b: 24 April 2003, Ls 173°   --  MOC2-1224c: 25 February 2005, Ls 166°
Arsia Mons spiral cloud in 2001 at Ls 180 Arsia Mons spiral cloud in 2003 at Ls 173 Arsia Mons spiral cloud in 2005 at sLs 166
NASA/JPL/Malin Space Science Systems

Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) images have shown that some parts of Mars experience weather phenomena that repeat each year at about the same time. In some regions, the repeated event may be a dust storm that appears every year, like clockwork, in such a way that we can only wish the weather were so predictable on Earth.

One of the repeated weather phenomena occurs each year near the start of southern winter over the martian volcano, Arsia Mons. The volcano is located near 9°S, 121°W. Each year, just before southern winter begins, sunlight warms the air on the slopes of the volcano. This air rises, bringing small amounts of dust with it. Eventually, the rising air converges over the volcano's caldera, the large, circular depression at its summit. The fine sediment blown up from the volcano's slopes coalesces into a spiraling cloud of dust that is thick enough to actually observe from orbit.

The spiral dust cloud over Arsia Mons repeats each year, but observations and computer calculations indicate it can only form during a short period of time each year. Similar spiral clouds have not been seen over the other large Tharsis volcanoes, but other types of clouds have been seen.

Shown here are MOC wide angle views taken during three of the four Mars years that MGS has been observing the planet. The first image was taken on the first day of southern winter in 2001. The second image is from late southern autumn in 2003, and the third image shows that the phenomenon appeared a little bit earlier in late southern autumn in 2005.

The spiral dust cloud over Arsia Mons can tower 15-30 kilometers (9-19 miles) above the volcano. The white and bluish areas in the images are thin clouds of water ice. In the 2005 case, more water ice was present than in the previous years at the time the pictures were obtained. For scale, the caldera of Arsia Mons is about 110 kilometers (~68 miles) across, and the summit of the volcano stands about 10 km (~6 miles) above its surrounding plains.

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Malin Space Science Systems and the California Institute of Technology built the MOC using spare hardware from the Mars Observer mission. MSSS operates the camera from its facilities in San Diego, California. The Jet Propulsion Laboratory's Mars Surveyor Operations Project operates the Mars Global Surveyor spacecraft with its industrial partner, Lockheed Martin Astronautics, from facilities in Pasadena, California and Denver, Colorado.