The image of John Carter's Mars, crisscrossed with canals, has been relegated to pulp sci-fi curiosity, but there is much evidence that liquid water existed on the planet at some time. Spurred by this and the tantalizing, if inconclusive, suggestion of microfossils in the martian meteorite ALH 84001, scientists have sought to determine the best site to prospect for proof of ancient life. A new study that determines where liquid water might still form on Mars may help them do so. Christopher P. McKay of NASA's Ames Research Center, Space Science Div., and his colleagues calculated where liquid water could be stable based on atmospheric pressure and temperature data from the Viking and Mars Global Surveyor missions. The laser altimetry data from the latter enabled them to extend the average pressure measurements collected at the two Viking lander sites and to estimate the pressure at all locations on the planet. The team repeated the calculations to generate pressure and temperature maps throughout the planet's year. The researchers, who published their results in the Feb. 27 issue of Proceedings of the National Academy of Sciences, found that conditions are right for liquid water in large swaths of the martian lowlands and plains for at least part of the year. They caution that it does not follow that liquid water does exist at these locations. 'Nor any drop to drink' Because the planet's atmosphere is so dry, any liquid water would form only in thin films that would rapidly evaporate, but it nevertheless could be important. "From the point of view of biology, liquid water is essential," McKay said. "No organisms are known that can grow or reproduce without liquid water. From a geochemical point of view, even transient liquid water could be a very important weathering agent." The researchers doubt that thin films of moisture could support life, but the sites where water could form today overlap with ancient fluvial channels. The team suggests that if any liquid water ever did form on Mars, it was probably dependent on atmospheric conditions, not underground sources, and that the locations that are still able to support liquid water most likely were more favorable in the past. They would therefore be the best places to search for signs of ancient life. "Hopefully, this study and similar studies might be used to help determine where on Mars to obtain samples for return to Earth," McKay said.