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Localized
tropical dust storms occur on Mars during all seasons. A typical storm
is about 1 million km2 size and comprises microscopic particles which
move with speeds of 15-30 m/s (33--66 mph) before dissipating after a
few days. Dust devils, about 2 km width and a few kilometers high, have
also been observed in the tropics by the Viking orbiters. However, the
most dramatic aspect of the Martian climate is when a dust storm
expands to encompass nearly one or both hemispheres. Indeed, sometimes
these great dust storms can become completely global.
The
observational record in the 1970s by the Viking and Mariner 9
spacecraft suggests that the occurrence of great dust storms is highly
variable from year-to-year. For some years, no great dust storm occurs;
for other years one or even two great storms occur. For most Martian
years this century there are no records of great dust storms but this
does not mean they did not occur: the telescopic observations are too
sparse to quantify the true frequency of occurrence.
So
what causes a dust storm? Wind tunnel studies show that winds in the
free atmosphere above the surface (a few kilometers altitude) must
reach a threshold of about 45 m/s (100 mph) to lift typical dust grains
at the surface, depending on the surface roughness. All dust storms
require high surface winds to start and to be sustained. Also great
dust storms, in particular, always occur close to southern summer on
Mars which is the season when Mars is nearest to the sun and there is
maximum solar heating.
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Several
hypotheses have been advanced to explain the onset of regional and
planet-wide dust storms but the sheer lack of data means that we cannot
single out one theory in particular. Proposed theories include
1.
a dust hurricane which pulls in dust at its base - although there is
little evidence of spiral structures in pictures of storms
2. winds resulting from regional and planetary-scale effects which add
together to give speeds great enough to raise dust
3. a sudden latitudinal expansion of the tropical circulation when
tropical heating exceeds a critical value - this circulation consists
of large-scale overturning air, i.e. warm air that rises in the
tropics, travels at altitude to the opposite hemisphere, cools, sinks,
and returns to its origin as near-surface flow
4. a global resonance of the atmosphere (i.e. when the atmosphere
oscillates at its natural frequency) which produces surface winds
strong enough to initiate dust storms.
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Dust
devil in Sinus Sabaeus (MOC2-546)
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At some stage, most
proposed theories (e.g. 2 and 3 above) invoke an amplification
mechanism caused by the absorption of sunlight by airborne dust during
the day and cooling of the dust at night. This increases the day-night
temperature changes in the atmosphere, which, in turn, raises the winds
resulting from the daily expansion and contraction of the atmosphere
(called "thermal tides"). Higher winds lift more dust causing further
amplification of the storm in positive feedback.
For a dust storm to
extinguish itself and the dust to fall back to the surface, the winds
must drop. Computer simulations show that as the atmospheric dust load
increases, the vertical mixing of the dust decreases but winds near the
surface continue to increase. This makes dust storm extinction
difficult to explain. But maybe dust gets transported to regions where
settling out is possible or perhaps the dust is scavenged by
condensation of water vapor or carbon dioxide in the polar regions. Or
if dust storms are caused by global resonance of the atmosphere then
they would naturally extinguish themselves because a dusty atmosphere
would no longer be conducive to resonance.
In conclusion, the issue
of how and why dust storms occur will remain unresolved until we get
better systematic global observations from the surface and orbit of
Mars.
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