Water vapour in the atmosphere of Mars

Abstract

Water vapour on Mars has always been in the focus of Martian research. A set of fundamental questions concerning the past and present of the planet is closely related to the peculiarities of its water cycle. Spacecraft and ground-based observations revealed complex behavior of the water vapour. This gas appears to be the most variable minor constituent in the atmosphere of Mars. Its seasonal and latitudinal variations reach a factor of 10. They are driven by condensation and sublimation of the polar caps, exchange with regolith and surface frost, and advective transport by the general circulation. Both seasonal changes and observed spatial and diurnal H₂O variability imply that the regolith is one of the main reservoirs of atmospheric water on Mars. The vertical distribution of water vapour is affected by the vertical transport, condensation and regolith-atmosphere exchange and can be strongly inhomogeneous. Current numerical models succeed to reproduce the water cycle in general. However some important details remain unclear. This paper reviews our knowledge about the Martian water cycle and discusses the open questions and future studies.