Water Vapor in the Martian Atmosphere Higher Than Expected

Sep 30, 2011
John Austin

global image of Mars taken from the wide field camera of the Hubble Space Telescope. - NASA Goddard Space Flight Center

New observations show that water vapor in the Martian atmosphere is supersaturated, leading to concerns of future destabilization of Mars' atmosphere.

Mars Compared with Earth

The atmospheres of the planets have evolved over the thousands of millions of years since the birth of the solar system. Mars is, of course, further from the sun and is smaller than Earth, characteristics that have influenced its atmosphere over time.

In particular, the atmosphere of Mars is considerably thinner than that of Earth by a factor of over 100. Its atmosphere contains mainly carbon dioxide (95%), and nitrogen (3%) with trace amounts of oxygen and water vapor.

Water is a strong greenhouse gas, but because of its very low concentration in the Martian atmosphere, it causes a negligible warming effect on the surface. Nonetheless, the presence of water ice clouds in the martian atmosphere has an important influence on its climate.

The Significance of Water on Mars

The presence of life on Earth has been tied to the occurrence of water in its three phases (gas, liquid, solid). If life can be shown to exist on Mars, even primitive life, it would appear to increase the probability of complex life forms elsewhere in the galaxy. Many of the probes to the planet have therefore been searching for surface water.

Measurements of Atmospheric Water

A study published in Science magazine on 30 September 2011 has revealed considerably larger amounts of atmospheric water than expected. The Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) measurements reveal that water vapor is supersatuated by ratios approaching 10 in some locations. Although this doesn't affect the total amount of water in the atmosphere by a substantial amount, water is a dynamical quantity and its vertical distribution is important for the hydrological cycle.

Simulating the Climate of Mars

Similar to Earth climate models, computer models of the atmosphere of Mars have been built. Such models are essentially a set of physical equations which describe the known physical processes including heating from the sun and the role of water ice clouds. The models usually assume that water is condensed as soon as saturation is reached. However, the latest measurements from SPICAM contradict that.

Condensation of water usually requires the presence of "condensation nuclei" such as dust particles. If these are unavailable, water remains in the vapor phase, and supersaturation occurs. Also, the low pressures in the Martian atmosphere slow down the condensation process even in the presence of dust.

The Importance of Supersaturation

In their report, Maltagliati et al. point out the profound consequences of supersaturation for the Martian atmosphere. The formation of the equatorial cloud belt, which is one of the major climatic features on Mars, acts like a valve to reduce cross-hemispheric movement of water when Mars is farthest from the sun.

The new results show that there is more water in the vapor phase for transport across hemispheres which need now to be taken into account in climate models. Also, supersaturation will affect the escape of water from the atmosphere. This process occurs first by photodissociation of water into its component atoms that then have the energy to exceed the escape velocity of Mars. This could reduce the stability of the atmosphere over time.

The Importance of Clouds for the Climate of Mars

The observations reinforce the conclusions reached by other scientists that clouds need to be properly included in climate models. John Wilson (NOAA Geophysical Fluid Dynamics Laboratory) in commenting on these results indicated that he has been aware also that clouds provide an important source of heating in the Mars atmosphere and other groups such as NASA Ames, and the Lab. for Meteorological Dynamics (France) have come to agree with this. Wilson also commented that the clouds simulated in models likely underestimate the observed cloud depths. Models which allow for supersaturation are now being developed to overcome the discrepancies with observations.

Terrestrial Implications

Supersaturation of water vapor is infrequently observed in the Earth's atmosphere because of the presence, usually, of copious amounts of aerosols which can act as condensation nuclei. Nonetheless, in the Earth's lower stratosphere over the poles, clouds can form. These are known as polar stratospheric clouds (PSCs) and are implicated in the formation of the Antarctic ozone hole. Hence, further careful analysis and simulation of Martian clouds might provide useful scientific insights into PSCs and the destruction of stratospheric ozone.