Carbon dioxide ice clouds
Mars Express' OMEGA instrument detected carbon dioxide (CO2) ice clouds, a unique feature of the
Red Planet's climate. These clouds are formed from the freezing of carbon dioxide gas, which makes up 96 per cent of the Martian atmosphere.
These clouds were observed at wavelengths of 0.5 and 4.26 microns. This revealed that the abundance of clouds varies from Martian year to year – with one Martian year equating to 687 Earth days. In addition, these carbon dioxide clouds are large and dense enough to form shadows on the planet's surface, which suggests they are 80 kilometres (50 miles) above the surface.