Radio study finds drier Martian atmosphere.

There's dry and then there's dry.

Although researchers have known that the atmosphere of Mars contains very little water vapor, a newly reported study shows that in December 1990 the Martian atmosphere contained the smallest concentration of water vapor ever recorded for the Red Planet. Indeed, if all the vapor then present in the atmosphere had condensed on the planet's surface, it would have formed an ocean only 3 micrometers deep -- too shallow to cover even the thickness of a human hair.

The study marks the first time researchers have used a ground-based instrument -- in this case, the Very Large Array radiotelescope near Socorro, N.M. -- to measure the thermal radio emissions of water in a planetary atmosphere other than Earth's. Previous surveys, both in space and on the ground, have relied on an entirely different technique to measure the concentration of Martian water vapor.

Past surveys, notes study coauthor R. Todd Clancy of the University of Colorado at Boulder, employed near-infrared detectors that record how much sunlight the vapor absorbs. In particular, infrared instruments aboard the Viking spacecraft in the mid-1970s found twice as much water vapor during the same Martian season, early northern spring, as the 1990 study; a 1988 infrared study from the ground revealed four times as much water vapor. Clancy and his colleagues, Arie W. Grossman of the University of Maryland in College Park and Duane O. Muhleman of the California Institute of Technology in Pasadena, report their work in the November Icarus.

Bruce M. Jakosky, also at the University of Colorado, cautions that scientists have not yet rigorously compared the infrared absorption and radio-emission methods. But if the apparent variation in water vapor proves accurate, he says, it suggests that the concentration of water in the atmosphere varies as much from year to year on Mars as it does from season to season.

Water may have played a key role in etching the rugged face of the Red Planet, and it remains an influence on climate. But planetary scientists, says Jakosky, have begun to realize that they "don't have a sense of what the typical behavior of water vapor is in the present epoch on Mars and how much variation there can be. The extremes of water vapor that have been measured -- if they're real -- are telling us that our previous understanding is not the whole story, that here's a year that has less water than we thought was possible."

Clancy adds that the views of Mars gathered by the two Viking craft, and earlier by Mariner 9, may not reflect the general status of water vapor on the planet. Each of these missions made observations during and soon after Martian dust storms, which probably warmed the planet. Such warming could melt some of the ice on the surface, temporarily boosting the amount of water vapor in the atmosphere. Jakosky notes that water stored in the planet's frozen polar caps, as well as ice mixed in with surface soil, could influence the amount of water in the atmosphere.

Clancy says his team's radio study, though restricted to detecting vapor above the limb, or edge, of the planet, has an advantage over infrared measurements: It recorded the density of water vapor at different altitudes above the Martian surface. The team found that the density of water vapor was nearly constant up to 50 kilometers above the surface.

This finding suggests that Mars' windy atmosphere may transport water between middle and low latitudes, he says. Jakosky adds that he looks forward to further observations that can indicate whether water density remains uniform from year to year. A greater density at lower depths, he notes, could force more water vapor into the soil.

Clancy's team plans to examine water vapor concurrently with other studies using a detector aboard the Mars Observer, which is expected to reach Mars in 1994 (SN: 9/19/92, p. 181).