"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."
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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