If you look at the surface of Mars, the first thing that may strike you is that Mars looks more similar to the Moon than to the Earth. There are no oceans or forests- just deserts whose brightness, or albedo, varies according to the amount of dust there is on the surface. Like the Moon, areas that are rocky and relatively free of dust appear dark, whilst areas with more of a covering of dust are brighter.
There are of course two major apparent differences between Mars and the Moon: the dust of Mars is reddish in colour (because of iron compounds in the soil), and Mars has visibly bright polar caps and sometimes clouds. This gives away a major difference between Mars and the Moon: Mars actually has an atmosphere and weather.
Mars has an surface pressure of 6.1 hPa and an average surface temperature of 220K, or -50C. It is a rapidly rotating planet with a day length of just over 24 hours, meaning that the approximations used for examining large-scale flow on Earth, such as hydrostatic balance and geostrophy, also work for understanding flow on large scales on Mars.
Like Earth, Mars has seasons (a year is about 690 days), and has a strongly seasonal cross-equatorial flow associated with a vigorous Hadley circulation. Like Earth, Mars has midlatitude westerly winds and baroclinic weather systems during autumn, spring and winter. Unlike Earth, the lack of oceans causes the near-surface meridional or north-south thermal gradient to reverse so that the summer midlatitudes are warmer than the summer tropics (why do you think this happens?). This means that the summer midlatitudes have prevalent easterly winds, causing midlatitude weather systems to mostly disappear during this time (why do you think happens?).
The weather on Mars has a very large bearing on what happens to the dust that lies on its surface. At most times Mars has some dust suspended in its atmosphere; this haze is relatively thin, having an optical depth of around 0.05-0.2. However, on some occasions- usually when Mars is closest to the Sun, and surface winds are strongest- significant amounts of dust are raised into the atmosphere. Below are two images comparing a dust plume near the martian North polar region (taken using the Mars Global surveyor camera), and a dust plume off the northwest coast of Africa. Note the similarities.
Image courtesy of the
Mars Orbiter Camera website
The particles in Martian dust are also much smaller than the particles in dust storms on Earth. Heavier particles tend to drop out of the thin atmosphere much more quickly than on Earth.
On rare occasions these dust storms can grow into planet-encircling events. In global dust storms the optical depth of dust of large parts of the atmosphere exceeds 1, meaning that significant parts of the surface can be completely obscured from the view of Earthbound telescopes, or even satellite instruments orbiting Mars itself. These storms tend to die down after 50-100 days. The result of these storms is to transport large amounts of dust around the planet. The effect of one of these global storms is shown below:
Above are two images of Mars taken using the Hubble Space telescope: the image on the left in June 2001 shows quiescent conditions; the image on the right shows a global dust storm: note the differences
Although we understand a lot about Martian dust storms, a lot of mystery still surrounds them: for instance why do they occur in some years and not others? Why do small mesoscale storms coalesce into planet-encircling storms and some not? How is dust transported from the bright regions back to the darker “scoured” regions?
These questions are being answered by researchers around the world using methods similar to those employed in terrestrial weather and climate research: a combination of data from satellites, computer modelling, and even soil data from Mars’ surface. In addition, understanding phenomena such as Martian storms gives us more confidence that our theories and models correctly predict weather and climate on Earth too, given the similarities and differences between the climates of Earth and Mars.
Right now (Early May 2011), Mars is just past its southern summer solstice period: this is the the season when global dust storms are most common, so perhaps one of these storms may actually happen in the next few days to weeks.
The Mars Today website, showing Mars in its orbit, a simulated image, and prediction of winds in a (simplified) global circulation model