Analysis of data collected by ESA’s Mars Express spacecraft leaves no room for doubt: the Martian atmosphere of contains water vapor in a supersaturated state. This surprising finding will enable scientists to better understand the water cycle on Mars, as well as the evolution of its atmosphere. The research was led by a team from the Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS, CNRS / UPMC / UVSQ), in collaboration with Russian and French colleagues(1), and received support from CNES. It is published in the 30 September 2011 issue of the journal Science.
On Earth, water vapor tends to condense, i.e. turn into a liquid, when the temperature falls below dew point. The atmosphere is said to be ‘saturated’ since it cannot hold any more moisture at that temperature and pressure. The excess water vapor then condenses around suspended particles and dust, forming precipitation. However, condensation may sometimes be much slower, especially when particles and dust are scarce. Unable to condense, the excess water vapor therefore remains in the gaseous state: this is known as supersaturation. Until now, it was assumed that this phenomenon could not occur in the Martian atmosphere, although this had never been proved.
While several spacecraft have visited Mars since the 1970s, most of their instruments were focused on surface data. Because of this, they only observed the horizontal component of the Martian atmosphere. The way in which water content on Mars varies with height remained largely unexplored. The survey carried out by the SPICAM(2) spectrometer on board the Mars Express spacecraft has now made it possible to fill this gap. SPICAM can establish vertical profiles of the atmosphere using solar occultation, i.e. by observing light from the Sun as it travels through the Martian atmosphere at sunrise and sunset.
Contrary to previous belief, the researchers discovered that water vapor supersaturation is a frequent phenomenon on Mars. They even observed very high levels of supersaturation in the Martian atmosphere, up to ten times greater than those found on Earth. “This ability of water vapor to exist in a highly supersaturated state would, for example, allow to supply the southern hemisphere of Mars with water, far more efficiently than models currently predict,” points out Franck Montmessin, CNRS researcher at LATMOS and SPICAM(3) project leader. Moreover, a far greater quantity of water vapor than thought may be transported high enough in the atmosphere to be destroyed by photodissociation(4). If confirmed, this phenomenon would have consequences for the whole issue of Martian water, a significant fraction of which is known to have continually escaped to space for billions of years, which partly explains today’s low abundance of water on the planet(5).
The vertical distribution of water vapor is key to the study of the hydrological cycle on Mars. The hypothesis according to which the amount of water in the Martian atmosphere is limited by the saturation process therefore needs revising. This finding has major implications for the current understanding of both the climate and water transport on Mars.