You could hardly call the weather on Mars pleasant, and presently it is far from predictable. As well as having an average surface temperature of -63 C, and a thin, inhospitable atmosphere of mainly carbon dioxide, last year, a springtime dust storm smothered the entire planet for months.

There are also dust devils, which are swirling columns of dust that can reach several kilometres in height that frequently race across the barren surface. Such severe and dusty weather conditions can interfere with the manoeuvres made by spacecraft as they enter an orbit around the planet. For a craft landing on the surface, scientific investigations would have to be put on hold until the weather got better.

Like all travellers, ESA scientists want to make sure that the weather is good when future missions arrive on Mars. They have developed a global atmospheric circulation model for Mars, which uses the same principles as those developed for Earth's erratic weather patterns.

Here on Earth, it is water that is the driving force behind our weather. On Mars, where there is only a tiny amount of water vapour from its north pole in summer, dust takes over the role, heating the atmosphere by absorbing sunlight. Since the atmosphere on Mars is so thin and there are no oceans on the surface to store the heat from the Sun, temperatures rise and fall more quickly and dramatically because of surface changes and atmospheric heating by the Sun.

Similarly to Earth's weather, hot air rising at the equator is replaced by cooler air, giving rise to trade winds which stir up the Martian dust, provoking the small storms that can merge to form the monster tempests.

The climate models formulated by ESA scientists show us the pattern of the storms throughout the Martian year, but the mystery remains why some years are dustier than others. Mars Express, the ESA mission to the Red Planet that is due to arrive in December 2003, will be sending back data that could help us solve this mystery and perfect our weather maps of Mars.

The mission has been timed to arrive at Mars at the end of the Martian winter in the northern hemisphere, making it highly unlikely that there is a dust storm at the time of arrival.

A super-resolution camera will take more accurate and detailed images of the Martian atmosphere than ever before, imaging the entire planet in full colour, 3D, and at high resolution. Two other instruments will take vital measurements of the dust, atmospheric gases, and surface composition.

Over its four-year lifetime, Mars Express will be closely watching the clouds, fog, dust devils, and storms, looking for clues to explain the climate changes on Mars, now and in the past. The data it returns will help to check and refine the new Martian global climate models so that future missions make sure they arrive there during the high season.

Related Links
The seasons on Mars
The Martian atmosphere
More about Mars Express
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