Even in the clearest, bluest sky on Earth, there is still water vapour in our atmosphere. If you could condense all the water vapour out of the atmosphere above you, it would form a layer of water two centimetres deep. On Mars today, there is also water vapour in the atmosphere but it would create a layer just 10 micrometres thick.

With Robert Haberle, NASA Ames Research Center and Oleg Korablev, Space Research Institute, Moscow, Russia.

With Bill Feldman, Planetary Science Institute, Arizona, USA and Bernard Schmitt, Laboratoire de Planetologie de Grenoble, France.