Martian 'wobbles' shift climate

Climate clues are locked in the northern cap

By Dr David Whitehouse BBC News Online science editor

Mars undergoes periodic "wobbles" on its axis and variations in its orbit that, like the Earth, may cause it to endure ice ages, say scientists.

The evidence comes from high-resolution images of the planet's northern polar ice cap, a dome of water-ice mixed with dust that is up to 2.5 kilometres (1.5 miles) thick.

Images as well as measurements of the surface profile by the laser altimeter on the Mars Global Surveyor spacecraft reveal a spiral pattern of ice and dust ridges sculpted by fierce winds and evaporation due to sunlight.

The estimated accumulation rate, of 0.05 centimetres (0.02 inches) a year, of these ridges suggest the cap is only about five million years old, and that it was formed at the start of the latest orbital changes that induced the most recent ice age.

Planet-wide ocean

A team led by Jacques Laskar, of France's National Centre for Scientific Research, reports in the journal Nature that Mars experiences dramatic changes in its axis and orbital shape that result in enormous variations in polar "insolation" - the amount of sunlight that falls on the cap.

The same effect happens on the Earth as well, but to a far lesser extent because of the stabilising influence of the Moon's gravity. Mars, however, without a large satellite, can move on its axis up to 47 degrees.

The impact of this can be most easily seen at the north pole.

There, the ice and dust dunes are shaped by the interplay of wind and sunlight. Any variation in solar radiation due to the Sun's changing position in the sky will alter the way the terrain develops.

Dr Laskar's research reveals that the size of the respective layers and the variation in deposition of dust and ice in the past matches Mars' orbital changes.

Drilling missions

The suggestion that the northern cap is only five million years old intrigues astronomers as they calculate that if all the water-ice in the northern cap were melted it would flood the planet with water to a depth of a few metres.

If this happened in the past few million years then it could explain the seemingly freshly cut valleys and erosional features recently identified on the Red Planet.

The answers to such questions will have to wait until man or machine goes to the polar regions.

Commenting on the research, Alan Howard, of the University of Virginia at Charlottesville, US, said: "In the future, missions that land on the planet and drill into the layered deposits may provide definitive interpretations of the climate cycles on Mars."