New images obtained by a sharp-eyed Martian satellite reveal that some Red Planet features once thought to have been carved by flowing water were in fact created by other processes.
The images were taken during the first 100 days of the Mars Reconnaissance Orbiter (MRO) mission and are detailed in a special section of the Sept. 21 issue of the journal Science.
While the results don't confirm or deny the existence of liquid water on Mars' surface, they are no less fascinating, say the scientists involved.
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For instance, one team found no evidence that flowing water caused bright deposits on the planet. Instead, the scientists proposed dry landslides caused the deposits.
"All findings are good findings," said one team leader Alfred McEwen, a planetary geologist at the University of Arizona.
Philip Christensen of Arizona State University said the MRO results reiterate that "Mars has been fairly dry for the recent past and we need to be careful and not overestimate how much water may have been present, or may have shaped the surface" in ancient times.
"I have been a 'dry Mars guy' for a long time," Christensen said. "These findings are basically saying you look at very high resolution and you do see some evidence for water, there's no disputing that. But you don't see an overwhelming amount of evidence for water."
The bus-sized MRO orbiting spacecraft, launched in 2005, is equipped with six instruments, including the High-Resolution Imaging Science Experiment camera, or HiRISE, which provides 10 times the resolution of any past Mars imagers.
While the MRO images are in some cases inconclusive on the question of Martian water, they are painting a picture of the Martian surface for scientists in unprecedented detail.
In some cases, the images refute past speculation that some of the features were created by flowing water.
A team led by Windy Jaeger of the U.S. Geological Survey in Arizona analyzed HiRISE images of the Athabasca Valles, a young outflow channel system speculated to have been carved out by past catastrophic floods.
"That entire surface is coated with a thin layer of solidified lava, very hard rock that's almost preserved the channel system," Jaeger said, adding: "Catastrophic water floods probably did carve the channel system, but lava flowed through it more recently."
The findings suggest that rather than flooding, steam explosions left behind trails of cone-shaped features found on the floor of Athabasca Valles.
"When water and lava interact it causes a steam explosion," Jaeger told SPACE.com. "And so the lava-covered ground had ground ice in it. And as that water was heated it exploded in steam explosions through the lava."
McEwen led another research team, which studied a variety of landforms also thought to be associated with past water on Mars.
They examined images of gully deposits that had been detected last year by the Mars Global Surveyor. The gully deposits were not present in 1999 images but appeared by 2004.
The before-and-after images raised hopes that modern flows of liquid water created the deposits. However, observations from MRO suggest a dry origin, McEwen said.
Both chemical analyses and images of one of the fresh deposits showed no signs of frost or ice and no evidence for even hydrated minerals, all of which could have given the deposits a "bright" appearance.
"We think dry landsliding could've created the bright deposits," McEwen said.
The slopes above this deposit and five other locations are steep enough for sand or loose, dry dust to flow down the gullies, the scientists say. Material uphill could be the source.
In science, discrediting a theory can be just as important as supporting one.
"Some science reporters are acting as if we should be disappointed these new bright deposits weren't deposited by water," McEwen said. "We're excited by any advance in understanding Mars no matter what it is."
The researchers also ruled out a hypothesis for an ancient ocean on Mars.
The Vastitas Borealis Formation, which covers low-lying northern plains of Mars, was thought to be the result of fine-grained deposits left by an ancient ocean.
The new HiRISE images reveal the area, which appeared as flat and featureless in prior missions, is peppered with large boulders.
The mixed-bag of findings intrigue scientists involved.
While Mars is dry now, there remains a lot of water locked up as ice at the poles and beneath the surface away from the poles.
"Ninety-nine percent of Mars is pretty dry and pretty average and not all that exciting," Christensen said. "But the one percent is extremely interesting. So imagine stumbling across an oasis or hot spring out in the middle of a desert. It's a barren desert but gosh that little oasis sure looks attractive."
As an astrobiologist, Christensen says Mars holds plenty of hideouts for life, "I think there are still plenty of places to look for life on Mars."
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