Mars water cycle: an 11 Mars year climatology of water vapour column abundances by SPICAM on Mars Express

Water vapour on Mars has long been an important target for exploration, as its detection revealed that Mars was home to an active water cycle fuelled by exchanges between ice on the surface and the atmosphere. From its first spectroscopic identification in 1963 to the most recent studies carried out by the many spacecrafts that have orbited Mars, our understanding of the water cycle on Mars has made considerable progress. Here we present a climatology of water vapour column abundances over 11 Martian years (MY), observed by the “Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars” (SPICAM) instrument on the European Space Agency's Mars Express mission. Despite uneven spatial coverage due to the orbital configuration of Mars Express, SPICAM succeeded in monitoring the abundance of water vapour in daylight at almost all latitudes and seasons. As its water vapour measurements are based on sunlight reflected by Mars in the near infrared, SPICAM has not been able to observe during the polar night, where water vapour is predicted to be anyway present in almost undetectable quantities.

The 11MY-climatology encompasses two years with a Global Dust Event (GDE), allowing us to perform an initial exploration of the differences between years with and years without a GDE. We have also compared our measurements with those of past and present missions, a topic that has long resisted reconciliation attempts. Furthermore, we attempted to fill observation gaps with the well-known kriging (a Gaussian process regression) technique to allow better appraisal of the year-to-year variations. Finally, we propose a reference water vapor annual cycle based on averaging all the years of observations.