ESA Science & Technology - Transport of water vapour in the Martian atmosphere (annotated)

New analysis of data sent back by the SPICAM spectrometer on board ESA's Mars Express spacecraft has revealed for the first time that the planet's atmosphere is supersaturated with water vapour. This surprising discovery, reported in Science by Maltagliati et al (2011), has major implications for understanding the Martian water cycle and the historical evolution of the atmosphere.

As illustrated in this schematic diagram the water cycle in the atmosphere of Mars can be described as follows:

• When the polar caps - the main reservoir of water vapour on Mars – are illuminated by the Sun during the spring and summer seasons, their water vapour (H₂O) content progressively sublimates and is released into the atmosphere.
• These water vapour molecules are transported by winds to higher altitudes where, in the presence of dust aerosols, they condense to clouds. When there are too few dust aerosols, condensation is impeded, leaving substantial amounts of water vapour, i.e. the atmosphere is supersaturated.
• Supersaturated water vapour may be transported by winds to the southern hemisphere or may be carried high in the atmosphere.
• In the upper atmosphere the supersaturated water vapour is affected by photodissociation; solar radiation splits the water vapour molecules into the constituent hydrogen and oxygen atoms, which can then escape into space.