Washington, Jan 19: Contrary to popular perception the arid planet Mars' orange sky also possesses dense clouds that can cast shadow on the surface.
Until now, Mars has generally been regarded as a desert world, where a visiting astronaut would be surprised to see clouds scudding across the orange sky. Mars is not entirely a heaven for Sun worshippers. Clouds of water ice particles do occur, for example on the flanks of the giant Martian volcanoes. There have also been hints of much higher, wispy clouds made up of carbon dioxide (CO2) ice crystals.
This is not too surprising, since the thin Martian atmosphere is mostly made of carbon dioxide, and temperatures on the fourth planet from the Sun often plunge well below the 'freezing point' of carbon dioxide, Science Daily reported.
Now, a team of French scientists has shown that such clouds of dry ice do, indeed, exist. Furthermore, they are sometimes so large and dense that they throw quite dark shadows on the dusty surface.
''This is the first time that carbon dioxide ice clouds on Mars have been imaged and identified from above,'' said Franck Montmessin, lead author of the paper in the Journal of Geophysical Research.
''This is important because the images tell us not only about their shape, but also their size and density,'' he said.
Even more surprising is the fact that the CO2 ice clouds are made of quite large particles - more than a micron (one thousandth of a millimetre) across and they are sufficiently dense to noticeably dim the Sun. Normally, particles of this size would not be expected to form in the upper atmosphere or to stay aloft for very long before falling back towards the surface.
Since the CO2 clouds are mostly seen in equatorial regions, the research team believes that the unexpected shape of the clouds and large size of their ice crystals can be explained by the extreme variations in daily temperature that occur near the equator.