September 24, 2007

Volume 85, Number 39

p. 18

Planetary science

Mars Is Drier Than Expected

New images show lava, landslides instead of watery residues

Elizabeth Wilson

PUTTING A DAMPER on some of the recent effusiveness about possible evidence for past and present water on Mars, new images of the red planet show that many features once attributed to gushing floods and rivers may instead be landslides or lava flows.

NASA's Mars Reconnaissance Orbiter (MRO) spacecraft, which sent back to Earth the striking new images, like the gullies shown here (right image), has a suite of instruments with much greater resolving power than those in previous craft. With its ability to pick out 1-meter-sized boulders and narrow crevasses, it was expected to answer a lot of questions about Mars's geologic history.

Just last year, NASA's Mars Global Surveyor spotted a new bright patch in a gully, which was believed to be evidence of a recent gush of water on Mars (C&EN Online, Latest News, Dec. 11, 2006).

But international teams of planetary scientists now report in a series of papers (Science 2007, 317, 1706, 1709, 1711, and 1715) that signs of water flow have been harder to find than expected in the new images. Some gullies do resemble those formed by water flow on Earth, but the new bright patches appear likely to be the result of dry landslides. MRO also reveals that a large field of channels once thought to be formed by water is covered by lava. Other regions of the planet are littered with boulders, rather than the snow or ancient ocean sediments once postulated.

But that doesn't mean that Mars is and always has been dry as a bone. Some geologic features, such as the fan-shaped swath of channels shown (left image) in an impact crater, appear to have been created by fluid flow.

MRO will continue mapping the planet for another year, after which it will serve as a communications relay for future Mars missions.

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