Flowing lava probably carved at least one of the river-like channels on the surface of Mars, according to results of a study presented at the Lunar and Planetary Science Conference at NASA's Goddard Space Flight Center. Whether channels on Mars were formed by water or by lava has been debated for years, and the outcome is thought to influence the likelihood of finding life there.

Geologists carried out a careful study of a single channel on the southwest flank of Mars' Ascraeus Mons, one of the three Tharsis Montes volcanoes. The researchers relied on detailed images from three cameras: the Thermal Emission Imaging System (THEMIS), the Context Imager (CTX) and the High/Super Resolution Stereo Color (HRSC) imager, as well as earlier data from the Mars Orbiter Laser Altimeter (MOLA). From these images, the team pieced together more than 270 kilometers (~168 miles) of the channel.

At the source of the channel, the visual clues seem to point to water. But at the channel's other end, an area not clearly seen before, researchers found a ridge that appears to have lava flows coming out of it. In some areas, "the channel is actually roofed over, as if it were a lava tube, and lined up along this, we see several rootless vents," or openings where lava is forced out of the tube and creates small structures, researcher Jacob Bleacher explains. These types of features don't form in water-carved channels, he notes.

These are details from the Ascraeus channel (red), meandering across the surface of Mars. The insets in the black boxes show close-ups of some of the structures that lava can form: (left) branched channels, (middle) a snaking channel and (right) rootless vents; the rootless vents are also marked by yellow spots on the main image.

(Photo Credit: Jacob Bleacher)

Evidence that lava can produce finely detailed features came from a survey of the 51-kilometer (32 mile) lava flow from the 1859 eruption of Mauna Loa on Hawaii. The main focus was an island nearly a kilometer long in the middle of the channel.

"We found terraced walls on the insides of these channels, channels that go out and just disappear, channels that cut back into the main one, and vertical walls 9 meters (~29 feet) high," Bleacher says. "So, right here, in something that we know was formed only by flowing lava, we found most of the features that were considered to be diagnostic of water-carved channels on Mars."

Further evidence came from the examination of a detailed image of the Mare Imbrium, a dark patch on the moon that is actually a large crater filled with ancient lava rock. Here, too, the researchers found channels with terraced walls and branching secondary channels.

The team's conclusions do not rule out the possibility of flowing water on Mars, nor of the existence of other channels carved by water. Even so the findings have implications for the geological evolution of this volcanic region of Mars and could ultimately change our ideas about water's role in the geological evolution of Mars.