Dry Debris, Not Water, Caused Recent Flows on Mars

Richard A. Lovett
for National Geographic News
March 3, 2008
A dry landslide—not liquid water—is the most likely explanation for at least one of the bright streaks seen recently on Mars, a new study concludes.

The streaks first made waves in 2006, when scientists reported them in pictures of Martian gullies taken in 2005.

The features had not been in images of the same region from 1999, indicating that something had flowed down those gullies recently.

At the time the researchers figured the most likely candidate was a flash flood—which would have offered proof of liquid water on modern Mars.

The new paper, published this month in the journal Geology, concludes that the shape of at least one of the bright deposits doesn't match the way water would flow.

This doesn't mean that Mars never had water, but it does cast doubt on more recent liquid flowing on the red planet.

The study team, led by Jon Pelletier of the University of Arizona, examined topographic maps made using high-resolution 3-D imagery from the HiRISE Mars-orbiting camera. (See some of the first color images of Mars released last year by the HiRISE team.)

The new maps not only provided a more detailed look at the features, they allowed the team to model the flows based on the steepness of the slopes.

"If we know where the flow originated and know the slope angle, we can mathematically model how [material] ran down the slope," Pelletier said.

"We tried to get both the extent of the runout and also match the shape of the flow," he said. "We were actually trying to strengthen the case for [water]."

Telltale Fingers

But the team instead found that the shape of the deposit just didn't match with a liquid flash flood.

"At the bottom it [spreads] out like the fingers on a hand," Pelletier said.

"There's something about the mechanics of dry, granular flows that produce this fingering in a way that a water flood doesn't."

This doesn't mean that water couldn't have been involved, he added.

Wet ground might have triggered the landslide. And it's possible that the flow was muddy slurry containing about 50 to 60 percent sediment.

"If you have a really sediment-rich [flow], like a very soupy mudflow, the mechanics are very similar to dry, granular flows."

Still, he said, "the simplest model is dry, granular flow."

Rainy Crater

Pelletier noted that his finding involves only the recently created bright streaks.

"This says nothing about water in the Martian past," he said.

In fact, another paper in the same issue of Geology examined the water-related history of Holden Crater, one of six landing sites being considered for the Mars Science Laboratory scheduled to be launched next year.

(Related: "Mars, Planet Ice" in National Geographic magazine [January 2004].)

This study—led by John Grant of Smithsonian's National Air and Space Museum—found that the crater once contained a lake that probably existed for thousands of years.

Also using images from HiRISE, Grant's team found signs of water-related erosion on the crater walls, suggesting that rain must once have fallen there.

That's important for understanding early Mars as a whole, Grant said, because "it's very difficult to envision how you can make it rain and be persistently wet in one place and … not someplace else."

But when it comes to more recent evidence for water on Mars, Grant said, "it looks like the story behind these gullies may be more complicated than we've thought."

Grant and Pelletier are both members of the HiRISE team, although they were not directly involved in each other's research.

The original discoverer of the bright streaks, Michael Malin of Malin Space Science Center, did not reply to National Geographic News's request for comment by press time.

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