 |  |
Figure 1:
19 June 2001, Ls 180° | Figure 2:
24 April 2003, Ls 173° |
Three wide angle views taken by the Mars Orbiter Camera on NASA's Mars
Global Surveyor at intervals approximately one Mars year apart show
similar spiral dust clouds over a volcano named Arsia Mons. The upper-left
image (figure 1) was taken on June 19, 2001, the first day of southern
winter on Mars. The upper-right image (figure 2) was taken on April 24,
2003, in late southern autumn on Mars. The lower image was taken on Feb.
25, 2005, slightly earlier in late southern autumn on Mars.
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
Arsia Mons, which is located near 9 degrees south latitude, 121 degrees
west longitude. 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.
The spiral dust cloud over Arsia Mons can tower 15 to 30 kilometers (9 to
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 kilometers (6 miles)
above its surrounding plains.
The Mars Orbiter Camera was built and is operated by Malin Space Science
Systems, San Diego, Calif. Mars Global Surveyor left Earth on Nov. 7,
1996, and began orbiting Mars on Sept. 12, 1997. JPL, a division of the
California Institute of Technology, Pasadena, manages Mars Global Surveyor
for NASA's Science Mission Directorate, Washington.
