Paris, September 23 : Data from ESA's (European Space Agency's) Mars Express spacecraft has enabled scientists to better explain why Mars's residual southern ice cap is misplaced, attributing the cause of the mystery to the Martian weather system.
Like Earth, Mars has frozen polar caps, but unlike Earth, these caps are made of carbon dioxide ice as well as water ice.
During the southern hemisphere's summer, much of the ice cap sublimates, a process in which the ice turns straight back into gas, leaving behind what is known as the residual polar cap.
The problem is that while the winter cap is symmetrical about the South Pole, the residual cap is offset by some three to four degrees.
This misplacement, which has puzzled planetary scientists for years, was solved by scientists in 2005, but now, thanks to ESA's Mars Express, new information is available to explain the misplacement.
Marco Giuranna of the Istituto di Fisica dello Spazio Interplanetario CNR (IFSI), Rome, Italy, and colleagues have used the Planetary Fourier Spectrometer (PFS) onboard Mars Express to measure the temperature of the Martian atmosphere from the ground up to an altitude of 50 km above the south polar region.
The team used the profiles to chart the way the atmosphere changes in temperature and other characteristics over more than half a Martian year.
They monitored the way carbon dioxide builds into the southern ice cap as the martian autumn, or fall, turns into the Martian winter.
"It is not a straightforward process. We found that two regional weather systems developed from mid-fall through the winter," said Giuranna.
These weather systems are derived from strong eastward winds that characterise the Martian atmospheric circulation at mid-latitudes. They blow straight into the Hellas Basin, the largest impact structure on Mars with a diameter of 2300 km and a depth of 7 km.
The crater's depth and the steep rise of the walls deflect the winds and create what are called Rossby waves on Earth.
These waves reroute the high altitude winds on Mars and force the weather system towards the south pole.
In the western hemisphere of Mars, this creates a strong low-pressure system near the south pole, and a high-pressure system in the eastern hemisphere, again near the south pole.
Giuranna found that the temperature of the low-pressure system is often below the condensation point for carbon dioxide, so the gas condenses and falls from the sky as snow and builds up on the ground as frost.
In the high-pressure system, the conditions are never appropriate for snow, so only ground frost occurs.
Thus, the south polar cap is built by two different mechanisms.