Venus is so much like the Earth in its size and composition that it is sometimes called our sister planet, but it differs in at least one relatively dramatic way: it has very little water. Scientists suspect this lack of water might help to explain why Venus has such a dense cloud cover of carbon dioxide, and why its surface is so hot (about 750 degrees kelvin), among other things. Although water is scare on Venus, it is not entirely absent. There are about 50 parts per million of water vapor in the Venusian atmosphere; this amount can be compared to about 2500 parts per million of water vapor in the Earth's atmosphere (about 0.25% of the Earth's atmospheric gases). A better understanding of the atmosphere of Venus will lead as well to a better understanding of processes in our own atmosphere.
A team of SAO astronomers -- Mark Gurwell, Gary Melnick, Volker Tolls, and Brian Patten -- together with a colleague who was previously at SAO, has used the Submillimeter Wave Astronomy Satellite (SWAS) to study water vapor in the Venusian atmosphere. (SWAS is a NASA Small Explorer mission that was proposed, designed, and operated by an SAO team led by Gary Melnick.) Writing in this week's journal Icarus, the scientists report on the first detection of an important water line from the atmosphere of Venus. The water emission arises primarily from an altitude of 70 - 100 kilometers on Venus, its mesosphere, a transition region between the lower cloud decks with their massive sulfuric acid clouds and the upper atmosphere with its strong, solar-heat-driven winds.
The SWAS data, taken between 2002 and 2004, find dramatic variability in the water vapor abundance in the atmosphere, over a factor of one hundred. Just as interesting was the timescale of these variations -- as rapid as a factor of about fifty in only two days, with longer term variations also observed.
The scientists note that the total water in the atmosphere is thought to be relatively constant -- only the amount of water vapor is changing. The reasons for these rapid changes are still uncertain. However, it appears likely that temperature changes of perhaps 10 - 15 degrees kelvin in the mesosphere can prompt chemical reactions with the sulfuric acid aerosols that can account for the water-vapor abundance changes. The results indicate that the atmosphere of Venus is extremely dynamic, with relatively large changes in temperature and global circulation occurring on short time scales. The results will help scientists unravel the behavior of the weather on Venus, our sister planet, and perhaps of the evolution of the Earth's atmosphere.