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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. Credit: Jacob Bleacher |
Did flowing water carve the well-known channels on the face of Mars? Or was molten lava perhaps the instigator? This debate has raged for years, and the answer is important, because if there was a lot of surface water, that increases the chances that life may once have existed.
Comparison of images of Martian channels to lava flows in Hawaii indicates that lava, not water, may have been the creative force behind at least some of the channels. So said NASA researcher Jacob Bleacher, speaking before the 41st Lunar and Planetary Science Conference last week.
"To understand if life ? as we know it ? ever existed on Mars, we need to understand where water is or was," Bleacher said.
The notion that water once flowed freely on Mars derives from images showing details resembling fluvial (water-based) erosion. Fine, delicate terrain features usually aren't considered products that lava flows can create.
"The common image [of lava] is of the big, open channels in Hawaii," Bleacher explains.
More detailed view
A single channel on the southwest flank of Mars' Ascraeus Mons volcano, one of three volcanoes collectively known as the Tharsis Montes, formed the basis of Bleacher and his colleagues' research. The team pieced together images covering more than 168 miles (270 kilometers) of this channel utilizing high-resolution pictures from three cameras: the Thermal Emission Imaging System (THEMIS) on board Mars Odyssey spacecraft, the Context Imager (CTX) on Mars Reconnaissance Orbiter and the High/Super Resolution Stereo Color (HRSC) imager on Mars Express, as well as older data from the Mars Orbiter Laser Altimeter (MOLA) on Mars Global Surveyor (MGS).
These data gave a more detailed view of the surface than previously available.
Time has obliterated the fluid that created the Ascraeus Mons channels, but visual clues at the source of the channel in question seem to indicate that water is the culprit. Clues include small islands, secondary channels that branch off and rejoin the main one and eroded bars on the insides of the curves of the channels.
But new close examination of the channel's other end by Bleacher and colleagues revealed 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, he explained.
Water-carved channels don't typically form these types of features, he notes. Bleacher argues that one end of the channel forming by water and the other end by lava is an "exotic" combination. More likely, he thinks, lava formed the entire channel.
To compare the Mars features to those created by lava, Bleacher, along with W. Brent Garry and Jim Zimbelman at the Smithsonian Institution in Washington, examined the 32 mile (51-kilometer) lava flow from the 1859 eruption of Mauna Loa on the Big Island of Hawaii. They focused on a mid-channel island almost a kilometer long. Bleacher says this is much larger than islands typically identified within lava flows.
To survey the island, the team used differential GPS, which provides location information to within about 1.1 to 1.9 inches (3 to 5 centimeters), more accurate than a car's GPS can offer.
"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 said. "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."
The new results make "a strong case that fluid lava can produce channels that look very much like water-generated features," Zimbelman said. "So, we should not jump to a water-related conclusion when we see such channels on other planets, particularly in volcanic terrain such as that around the Tharsis Montes volcanoes."
Lunar evidence, too
Further, researchers discovered more evidence from the moon by studying a detailed image of channels in the Mare Imbrium, a large crater filled with ancient lava rock. In this image, as well, they found channels with terraced walls and branching secondary channels.
The conclusion that lava probably made the channel on Mars "not only has implications for the geological evolution of the Ascraeus Mons but also the whole Tharsis Bulge [volcanic region]," said Andy de Wet, a co-author of the study at Franklin & Marshall College, Lancaster, Penn. "It may also have some implications for the supposed widespread involvement of water in the geological evolution of Mars."
Bleacher notes that the team's conclusions do not preclude the possibility of flowing water on Mars, nor of other channels carved by water.
"But one thing I've learned is not to underestimate the way that liquid rock will flow," he said. "It really can produce a lot of things that we might not think it would."
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