Astronomers have found the first direct evidence that the structure of the polar caps on Mars is linked to climate changes driven by fluctuations in the planet’s motion. A team led by Jacques Laskar of the CNRS Institute of Celestial Mechanics in France established the link by inspecting new high-resolution images. The researchers believe that the polar caps could reveal as much about the history of Mars as the terrestrial ice caps have told us about the history of the Earth (J Laskar et al 2002 Nature 419 375).
Taken over 30 years ago, the first pictures of the martian polar caps showed that they consisted of layers of material, each of which is thought to have been deposited during different climatic periods. Astronomers believe that these horizontal layers consist of frozen water and dust, with a seasonal coating of frozen carbon dioxide. But the poor resolution of these images made it impossible to study the caps in detail.
Now the Mars Orbiter camera has taken pictures that reveal details on the polar caps as small as tens of centimetres. Laskar and colleagues focused on an escarpment in the northern polar cap in which many layers were visible. The team calculated the thickness of these successive layers by measuring their reflectivity, and then modelled the motion of Mars over the last ten million years.
Over such long periods, both the orbital eccentricity and the rotational obliquity of Mars – the angle its axis makes with the plane of its orbit – fluctuate greatly. These irregularities combine to make the long-term motion of Mars highly complex. Since this motion determines how much sunlight reaches the martian poles, it also has a strong influence on the climate in those regions.
When Laskar and colleagues compared their measurements with the past motion of Mars, they discovered that the thickness of the layers in the escarpment was strongly linked to the amount of sunlight that had reached the north pole over a given period.
Laskar and colleagues also estimated that the material in the northern polar cap was deposited at a rate of about 0.05 cm per year, which would suggest that it is just five million years old. This period coincides with a spell of high rotational obliquity – and therefore sunny conditions at the north pole that might aid deposition – but Laskar and colleagues stress that this is an open question.
Now that Laskar and co-workers have discovered the link between the structure of the polar caps and climate, they hope that further studies will reveal the processes underpinning it. This could help astronomers to understand how water was distributed on Mars in the past, and how apparently young features elsewhere on Mars formed.
