The team has evaluated high-res pictures from the American space probe "Mars Reconnaissance Orbiter" (MRO) and they show that on the surface of the planet a gully about two metres wide, caused by erosion, has increased in length.
Between November 2006 and May 2009 it lengthened by around 170 metres. “The changes to the gully – especially in its length – are the result of small quantities of water ice melting in spring and the subsequent flow movements of a mixture of water and sand,” is the researchers’ conclusion.
The annual mean temperature on Mars is around minus 60 degrees Celsius, but towards the end of winter it rises and can go above zero. Then, changes on the surface of Mars can be seen. Black areas on the dunes point to carbon dioxide ice which is either thawing or changing directly from a solid to a gaseous state, i.e. undergoing sublimation. In the spring of the first observed year on Mars – a year there lasts 687 days – a small gully caused by erosion in the so-called Russell Crater grew by about 50 metres in length. This was repeated in the spring of the following Mars year. The gully lengthened down the slope by about 120 metres.
How could these gullies develop? Possible explanations are movements of dry masses or the transportation of dry material influenced by liquid carbon dioxide or liquid water. “We can definitely rule out movements of dry masses due to the morphological characteristics of the canals,” says Dennis Reiss. Also, the gullies show one special feature, namely that become thinner and thinner down the slope. This is a general indication of the fact that some liquid seeping into the soil is likely to be responsible for the development. Carbon dioxide becoming liquid for a short time is ruled out by the researchers. “Evaluation of the spectral data shows that in both years all the carbon dioxide ice had already undergone sublimation before the canal arose,” says PhD student Gino Erkeling.
The most likely explanation in the opinion of the researchers is a small quantity of melting water ice which is protected from sublimation by an overlying layer of carbon dioxide ice. The calculations made by the Münster researchers show that the surface temperatures in the Russell Crater at the beginning of spring rise above the freezing point for water. PhD student Karin Bauch is certain that “the carbon dioxide ice – and subsequently the water ice underneath – then begin to melt and there would be a possibility of liquid water on the surface for a short time.” When the water then flows down the slope and collects in gullies, erosion is the result. Moreover, the phases of erosion in both years are almost identical, which leads to the conclusion that it is seasonal effects which are responsible.
Prof. Harald Hiesinger, the Director of the Institute of Planetology at Münster University, is also impressed by the fact that there were changes to the gullies over the past years. “These observations,” he says, “are the clearest evidence so far that today water can still flow on the surface of Mars, and in a quantity that is sufficient to cause erosion.” However, only small gullies are made. “The climate on Mars today only allows very little air humidity which can settle on the surface as frost. The quantities which can melt and lead to liquid water are correspondingly small,” explains Dennis Reiss. “So it’s not enough to make large valleys such as were formed in the early years of Mars.”
Reference: Reiss D. et al. (2010): Evidence for present day gully activity on the Russell crater dune field, Mars. GEOPHYSICAL RESEARCH LETTERS, VOL. 37, doi:10.1029/2009GL042192
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