By Paul Rincon
BBC News science reporter, Cambridge
An artist's impression of Mars Express, Nasa
Mars Express entered orbit at the end of 2003
Europe's Mars Express probe may have found evidence for a band of ice that once spanned the Martian equator.
A frozen sea and patterns of glacial activity on the planet may be a relic of this ancient belt of ice, says a top scientist.
The ice may have formed just before five million years ago due to a change in the tilt of Mars.
This change caused moisture from the poles to be deposited as snow at the equator.
The idea is based on work by a team of scientists led by astronomer Jacques Laskar of the Paris Observatory, France.
Laskar's team has shown that the tilt of Mars on its axis can vary between 15 degrees and 40 degrees, largely because of its lack of a significant moon. By contrast, the Earth varies little from its tilt of 23.5 degrees.
Groups of modellers in Paris, Oxford and at the Nasa Ames Research Center in the US have also modelled the climatic effects of these changes in obliquity.
They found that when Mars' tilt changed to an obliquity of about 35 degrees around five million years ago, moisture trapped at the North and South Poles may have been re-deposited in equatorial regions as snow.
Mars Express picture of hourglass-shaped crater at Promethei Terra, European Space Agency
These Martian craters show signs of glacial flow
It is also possible that water trapped in the Martian tropics since ancient Noachian times was mobilised around five million years ago.
Eventually, the poles may have got smaller and a thick belt of ice formed around the tropics.
Now, Dr Bernard Foing, the European Space Agency's (Esa) chief scientist, said Mars Express could have found multiple lines of evidence for this, including a pattern of glacial activity in the so-called tropics.
"When we look at some of the Mars Express data we find evidence of glacial deposits or even flows on the flanks of some of the equatorial mountains and volcanoes," Dr Foing told the BBC News website.
He added it was possible the ice could have got as thick as several hundred metres at high altitudes.
Evidence for recent and recurring glacial activity at tropical and mid-latitude regions on Mars has also been found by James Head of Brown University, US.
Another line of evidence from Mars Express is the possible discovery of a frozen sea in the Elysium region, near the Martian equator. The finding was announced by Dr John Murray of the Open University in Milton Keynes and colleagues earlier this year.
The sea, which would have been about the size of the North Sea and about 45m deep, froze as pack ice and was covered over as a layer of dust, the researchers said.
Intriguingly, the best estimate of its date comes out at about five million years, exactly when the icy belt may have existed around the tropics.
"This could be a future site for exploration, to search for possible life on Mars," said Dr Foing.
Details were announced at the American Astronomical Society's Division of Planetary Sciences meeting in Cambridge, UK.