Posted: September 30, 2008
The Martian weather system and a giant impact crater could explain why the residual ice cap at the south pole of Mars is offset by several degrees.
Mars has frozen polar caps just like the Earth, but the Martian caps are made of carbon dioxide ice as well as water ice. During the southern hemisphere's summer much of the ice cap sublimates, that is, turns straight from solid ice to gas, leaving behind what is known as the residual polar cap. But until recently, scientists had been baffled as to why this residual cap is offset from the south pole by 3-4 degrees, when the northern cap lies symmetrically across the north pole.
Thanks to ESA's Mars Express, new information is now available to explain things in more detail. The Planetary Fourier Spectrometer (PFS) instrument was used to measure the temperature of the Martian atmosphere from the ground up to an altitude of 50 kilometres in the southern hemisphere for over half a Martian year.
This is a mosaic of images taken by the Mars Express’s Visible and Infrared Mineralogical Mapping Spectrometer, OMEGA. It shows the residual south polar cap at the end of northern winter on the Red Planet. The cap appears clearly asymmetric, its centre being displaced by 3° from the geographic pole. Image: ESA/ Image Courtesy of F. Altieri (IFSI-INAF) and the OMEGA team.
"It is not a straightforward process,” says Marco Giuranna of the Istituto di Fisica dello Spazio Interplanetario CNR (IFSI), Italy. ”We found that two regional weather systems developed from mid-fall through the winter."
These weather systems are derived from strong eastward winds that characterise the Martian atmospheric circulation at mid-latitudes. The winds blow straight into the Hellas Basin, a 2,300 kilometre wide and 7 kilometre deep impact crater, which deflects the winds, rerouting them towards the south pole. Such winds are known as Rossby waves on the Earth, and are essentially large scale meanders in high-altitude winds, such as the jet stream. On Mars, this action creates a strong low-pressure system in the western hemisphere and a high-pressure system in the eastern hemisphere, both near the south pole.
The Mars Express scientists found that the temperature of the low-pressure system permits carbon dioxide gas to condense and fall as snow, which builds up on the ground. In the high-pressure system, the conditions are too warm for snow, so only ground frost forms, resulting in two different mechanisms to build the south polar ice cap. Because the regions with extensive snow cover reflect more light than the larger and rougher frost grains, it is the frosty zones that sublimate more rapidly in the summer months. This gives rise to the eastern area of frost disappearing completely, explaining why the residual cap is not symmetrically placed around the south pole.
"This has been a Martian curiosity for many years," says Giuranna. A curiosity that has, thanks to Mars Express, now been solved.