Mars Global Surveyor
Mars Orbiter Camera

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.

Reference Material:

• The meteorological story of the Arsia Mons spiral cloud was described by Scot Rafkin and colleagues in 2002:
  Rafkin, S. C. R., M. R. V. Sta. Maria, and T. I. Michaels (2002), "Simulation of the atmospheric thermal circulation of a martian volcano using a mesoscale numerical model," Nature, 419, 697-699, doi:10.1038/nature01114
  
  
• The year-to-year repeatability of martian weather phenomena was initially discussed by Bruce Cantor and colleagues of Malin Space Science Systems, also in 2002:
  Cantor, B., M. Malin, and K. S. Edgett (2002), "Multiyear Mars Orbiter Camera observations of repeated Martian weather phenomena during the northern summer season," Journal of Geophysical Research, 107(E3), 5014, doi:10.1029/2001JE001588 (click HERE for a no-cost link to the paper)
  
  
• Another view of the 2001 spiral cloud was presented in a MGS MOC release on 11 February 2002: Arsia Mons Spiral Cloud