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Thursday, 21 March, 2002, 00:40 GMT
Two-faced Mars explained
Mars is a two-sided planet
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By Dr David Whitehouse
BBC News Online science editor
Researchers may have explained why the north and south poles of Mars are so different.

It is because the planet's atmospheric circulation is affected by the higher terrain in the southern hemisphere. The north polar cap is made mainly of water-ice, while the southern cap appears to be mainly frozen carbon dioxide or "dry ice".

Using a sophisticated computer model of the planet's atmosphere, researchers report that the thin Martian air (which is mostly carbon dioxide) rises and falls more vigorously in the southern than in the northern hemisphere.

They say this creates an overall south-to-north flow of water vapour, which could explain the observed difference in the compositions of the poles.

Circulation problems

Mars is a lopsided world. Its northern hemisphere is relatively smooth and free of craters, and has a permanent ice cap composed of water-ice.

How the gas goes round on Mars
The southern hemisphere is more heavily cratered, higher and has a polar cap composed of carbon dioxide that almost vanishes in the Martian summer.

Even the seasons of Mars are not equal. Southern hemisphere summers are much warmer than northern ones because the south side of the planet faces the Sun when it is at its closest.

It had been thought that the differences between the two poles would average out when viewed over a long timescale. For example, over tens of thousands of years, there are changes in the planet's orbit around the Sun that should have a significant influence on behaviour in the two regions.

But scientists say their new analysis suggests the geography of Mars plays a far more important role than was previously realised.

Lumpy planet

Mars' southern hemisphere is on average about five kilometres (three miles) higher in elevation than the north. The difference appears to have a dramatic impact.

Using a computer simulation of global atmospheric circulation on Mars, the researchers show that the atmosphere flows northward at high altitudes above the equator and southward over the equator at lower levels.

This results in different conditions at either pole.

According to Dr Peter Gierasch of Cornell University, Ithaca, New York, US, this work "should help us understand the planet much better". The research is published in the journal Nature.

See also:

04 Mar 02 | Sci/Tech
Mars probe finds evidence of water
21 Feb 02 | Sci/Tech
Digital data puts Mars on map
10 Dec 01 | Sci/Tech
Mars may be changing
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