By HENRY FOUNTAIN
Published: November 18, 2003
A Dry Run on the Moon
Water is to the Moon as oil is to Earth. Just the prospect that it exists in large quantities is enough to excite certain people, like the planetary scientists, who could learn by studying it, and the interplanetary dreamers, who view it as a source of nourishment and ultimately of fuel for future explorers.
Now, though, scientists from the Smithsonian Institution, Cornell and Arecibo Observatory in Puerto Rico have rained on the parade. Radar imaging of the lunar poles, they report in the journal Nature, suggests that there is relatively little water there.
The prospect of lunar water was first raised in the mid-1990's, when satellite data gave hints that significant amounts of ice might exist in craters at the poles. The water, billions of tons of it, would have come from the bombardment of comets over the several billion years of the Moon's existence (thus the planetary scientists' interest in studying it).
Elsewhere on the Moon the water would have long since boiled away, but at the polar craters there are areas permanently in shadow, and thus well below freezing. So ice -- large reservoirs of it slightly below the surface -- might exist there. Among other things, it's easy to produce oxygen and hydrogen fuel from water (thus the interplanetary dreamers' interest in it).
But the radar data showed none of the kind of strong echoes that would be a sign of ice reservoirs and that have been seen in similar studies of polar craters on Mercury. The researchers suggest that if there is any ice at the poles, it is most likely in the form of distributed grains or thin layers. That may be enough for scientists to study someday, but the dreamers may have to look elsewhere.
Signs of a Martian Delta
Mars is a different story. While spacecraft have returned data hinting that there is water ice just beneath the surface near the planet's poles, there are also increasing signs that a liquid (water, most scientists say) may have flowed on the surface in the past.
The latest evidence comes in the form of new pictures from the Mars Global Surveyor, which has taken more than 150,000 high-resolution images in the six years it has been orbiting the planet.
The new images, analyzed by Dr. Michael C. Malin and Dr. Kenneth S. Edgett of Malin Space Science Systems, the maker and operator of the spacecraft's camera, are of a fanlike landform in the Mare Erythraeum region, in the southern hemisphere. The researchers, writing in the journal Science, suggest that the landform is the remnant of a delta, formed by the deposition of sediment from a slow-moving river.
The delta must have been created long ago because Mars is now bone-dry and the sediment is now weathered rock. The landform consists of a complex of rock ridges separated by smooth areas. The researchers say the ridges are evidence of the kind of meandering flows that occur in a delta: a wide area of sediment is laid down, the water cuts channels in it, and the number and direction of channels changes over time.
On Earth, deltas form where a river meets a larger body of water. So if the landform is a delta, the researchers suggest, it is a strong sign that Mars once had lakes and other long-lasting water features, rather than brief intense floods as some scientists think.
The Beagle Is Landing
Further evidence of water on Mars may come sometime after Christmas Day. That's when a British-built lander, the Beagle 2, is expected to touch down.
The European Space Agency confirmed last week that the Mars Express, the spacecraft carrying the Beagle 2, was on schedule to arrive at Mars on Dec. 25. That same day, if all goes as planned, the lander will reach the surface at Isidis Planitia, a sedimentary basin near the equator.
Instruments on the lander will collect and analyze soil and rock samples and otherwise search for water, organic molecules and other signs of past life on Mars.
Drawing (Drawing by Nurit Karlin)