Lunar Photos Cast Doubt on Recent Mars Water, Astronomers Say
Richard A. Lovett
for National Geographic News
|March 21, 2006|
Six years ago scientists studying images obtained by the Mars Orbital Cameraa probe that has been circling the red planet since 1997saw strange alcoves and gullies that looked like they had been created by liquid water.
The Martian features looked fresh enough that their discoverers, Michael Malin and Kenneth Edgett of Malin Space Science Systems in San Diego, California, concluded that they might have been formed as recently as a million years ago.
But now a Ph.D. student at the University of Arizona's Lunar and Planetary Laboratory has found similar features on the moon, which never had water.
Gwendolyn Bart was reexamining photos taken from lunar orbit in 1969 in an effort to better understand how rocks had been ejected from the moon's craters.
But she kept her eyes open for oddities, and one of the things she found was evidence of gullies that look strikingly similar to the Martian formations.
She spied the gullies in an image of a 10-mile-diameter (17-kilometer-diameter) crater called Dawes.
The lunar gullies' existence throws a potential wet blanket on the theory that Mars supported liquid water in the recent geologic past.
(Read "Mars Water Discovery Spurs Deeper Questions.")
Bart presented her findings on March 16 at a gathering of lunar and planetary scientists in Houston, Texas. She said at the meeting that she believes the lunar gullies were formed by rockslides that were set off when the moon's surface was pelted by meteorites.
"Obviously they weren't formed by water," she told National Geographic News. "There's no evidence for water having been on the moon."
Bart's hunt for lunar rockslides was inspired by the work of Allan Treiman of the Lunar and Planetary Institute in Houston, who has long doubted that water was needed to form the Martian gullies.
Instead, he argues, the same formations could have been caused by flows of sand or dust. Such features aren't common on Earth because our planet is too wet for that much dust to accumulate.
But we do see something similar with avalanches of dry, powdery snow.
"Gwen took my idea and looked in a place where there isn't any water ever," he said.
Both Bart and Treiman point out that the lunar features do not prove that water didn't play a role in the Martian alcoves.
Rather, Bart said, their existence merely shows that there is a way to form such features in waterless environments.
A detailed comparison of lunar and Martian gullies will probably have to await better photos.
The moon images Bart used were taken prior to the first lunar landing in 1969. The shots are the best photos currently available of the moon's surface, but they are not as high-resolution as the orbital photos of Mars. (See a gallery of Mars images.)
Also, Dawes crater is old enough that its contours have been blurred by years of bombardment by dust-sized micrometeorites. That, too, makes it difficult to see details.
Even if details are visible, nobody currently knows what to look for. It's possible, Bart says, that there may be subtle differences between dry landslides and those triggered by water.
Jennifer Heldmann at the SETI Institute and NASA Ames Research Center believes she can see such differences in the existing images.
The Martian gullies, she said via email, look as though they were created by water, because they are sinuous, V-shaped, and cut into the underlying rock.
The gullies on the moon lack these features. Therefore she is "extremely skeptical" that they represent the same thing.
Still, the find is important because there is talk of visiting the Martian gullies in a future landing in the hopes of finding water and possible traces of life.
"In my view, that would be really chancy," Treiman, of the Lunar and Planetary Institute, said. "It might be that all you'd see would be dust."
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