Science Special Section: Images from Mars Suggest Evidence of Water Hard to Find

The residual ice cap of Mars' south pole (in white) tops the smooth layered deposits that overlie the cratered southern highlands. This image is a shaded relief mosaic combined with color, using data from the Mars Orbiter Laser Altimeter on the Mars Global Surveyor spacecraft.

[Image courtesy of NASA/MOLA Science Team]

A series of articles in a Science special section suggest that finding liquid water on the Red Planet may be more difficult than previously thought.

The Mars Reconnaissance Orbiter (MRO), launched in 2005 by NASA to study the hydrological history of Mars, has imaged geological features including craters, gullies, and river beds that scientists believe could have been formed by flowing water.

But there are other explanations, wrote Alfred S. McEwen, a researcher at the California Institute of Technology in Pasadena and the first author of the lead article in the special section.

In their article, McEwen and his coauthors imaged six gullies with very recent geologic activity indicated by the presence of bright sediments at their base.

While the sediment could have been eroded by flowing water, the researchers said they cannot rule out that the sediment piles were formed by small rocks falling off the steep gully walls during a dry landslide.

"We cannot confirm the reality of ancient oceans or water in active gullies" on Mars, the team wrote, but they add that they do see geological features that could have been formed by runoff from melting icecaps, for example.

In a second article, Windy L. Jaeger, a researcher at the U.S. Geological Survey in Flagstaff, Ariz., and colleagues write that the Athabasca Valles, a channel system thought to be produced by catastrophic water flooding as recently as a few million years ago, now appears to be a lava flow instead.

The MRO images reveal that the thin layer of lava, which probably erupted from a fissure, inundated the Athabasca Valles channels and drained downstream. Although erosion by water may also have been involved, lava flows may be a better explanation than flooding for this and other channel systems like it, the authors write.

Previous missions to Mars have located frozen water dusting the ground, contained within hydrous minerals, lining volcano walls, and locked up in the polar ice caps.

In a third paper, Kenneth E. Herkenhoff, also of the U.S. Geological Survey in Flagstaff, and his coauthors found that Mars' north polar ice cap contains many thin layers that reveal ancient climate oscillations.

By studying the series of repeating layers of ice, along with the dust and sand that has become trapped in the ice, researchers are beginning to "create a record of global climate history within the layers" of the Martian North Pole, Herkenoff and his coauthors wrote.

In a fourth paper, Roberto Seu, a researcher from the Università di Roma in Rome, Italy, and colleagues used radar data to map out layers within the south polar plateau, Planum Australe, that suggest episodes of erosion occurred in between periods of ice buildup.

While the North Pole is composed of sediments and ice, Mars' south polar ice cap, which is the largest reservoir water on the planet's surface, consists of relatively clean water with approximately 15 percent dust, Maria Zuber and colleagues report in the fifth paper.

Using data from the MRO as well as previous Mars missions, Zuber's team calculated the North Pole's ice density, leading the team to hypothesize that it is composed of frozen water with significantly less dust than the Martian North Pole.

"These deposits represent the largest known surface reservoir of water on Mars today and the largest in the inner solar system outside of Earth," wrote Zuber and her coauthors.

Kathy Wren and Benjamin Somers

21 September 2007