Radar hints possible surprise: an 'oasis' of water on Mars

New studies of old radar data indicate Mars may have "an oasis" of water in an area known as Solis Lacus -- Lake of the Sun. This means that any future mission to look for life on the Red Planet now has a prime target for its search.

The imagery of "lake" and "oasis" is deceptive. It evokes visions of waving palm trees and sparkling waves. But this is not at all what scientists studying the Solis Lacus region have in mind.

Apart from a little atmospheric water vapor, some thin winter frost deposits, and ice at the poles, Mars presents a very dry appearance. However, scientists suspect there may be substantial deposits of ice or water underground. And Solis Lacus now appears to be a likely place for such a reservoir.

Robert L. Huguenin of the University of Massachusetts has called it "an oasis" -- "the wettest spot on the planet." A year ago, he reported signs of underground water in Solis Lacus and another area called Noachis-Hellespontus. Both areas are a little south of the Martian equator in the warmest part of the planet's surface.

Indeed, Solis Lacus was no named because it is on the sunward side when Mars is closest to the sun. Thus it is quite possible that subsurface water in such an area would be at least partially in liquid form of at least some part of the Martian year.

The evidence for water Dr. Huguenin used is indirect. Dust storms in that region are associated with much more water and ice haze than are dust storms elsewhere. The regions cool more slowly after sunset and stay warmer at night than do comparable areas, which is consistent with a "heat reservoir" effect of water not too far underground. As long ago as 1973, Dr. Huguenin had noted that these areas brighten as Mars comes closest to the sun, suggesting water released during the day produces frost overnight. Measurements by the orbiting Vicking spacecraft later found considerably more water vaport over these sites than over another place observed on Mars.

Now Peter Mouginis-Mark of Brown University and Stanley H. Zisk of the Haystack Observatory in Westford, Mass., have found radar evidence to back up Dr. Huguenin's conclusion. During two Martian oppositions in 1971 and 1973, radar operated at Goldstone, Calif., by the Jet Propulsion Laboratory (JPL) of the National Aeronautics and Space Administration scanned the entire circumference of Mars between latitudes 14 and 23 degrees south, including the northern part of the Solis Lacus. Intrigued by Dr. Hugueninhs findings last year, Drs. Mouginis-Marck and Zisk made a new study of the radar data.

"What we discovered was a little surprising," Dr. Zisk says, "You don't expect to find your evidence so clearly on a first pass through the data." What they found was an area 300 to 400 kilometers across with high radar reflectivity and a dielectric constant (a measure of properties) indicating high smoothness. But pictures of the area do not look any smoother than the standard rockstrewn Martian landscape. This suggests radar echoes from water. Seasonal changes in the radar returns support this conclusion. They vary as expected if an underground water body were warming and cooling, melting and freezing.

While none of this proves there is water under Solis Lacus or implies life exists there, it does strongly suggest that Mars has at least one location where organic life would have needed water. "If we go to Mars with another biological mission, this would be a good target," Dr. Zisk says.