Quieter now.
Currently spring-fed, Box Canyon, Idaho, may have been carved by a megaflood or two.

M. Lamb et al., Science

Martian Canyons by a Trickle or a Gush?

The Red Planet has never been a tropical paradise, but after investigating how a canyon in south-central Idaho formed, a group of terrestrial geologists is now questioning whether early Mars had even the minimal conditions for life. Here on Earth, the slow seepage of groundwater did not scratch out 3-kilometer-long Box Canyon, the group says. Instead, one or more catastrophic megafloods gouged it out in a matter of days or weeks, the geologists say. If similar-looking martian canyons also formed catastrophically, early Mars may have been only episodically warm and wet, if that.

Box Canyon seemed like the ideal place to test the seepage, or "sapping," mechanism of canyon formation, says geomorphologist Michael Lamb of the University of California, Berkeley, who, with Berkeley colleagues, reports the results in this week’s issue of Science. It has the classic signs of sapping: the stubby, rounded shape and steep walls at its upstream head that give the impression of a natural amphitheater; little evidence of water having carved channels draining into the amphitheater; and a substantial spring in the amphitheater. By conventional thinking, the spring had ever so slowly weakened the head-wall rock, which fell as boulders and debris that the seepage eventually eroded away.

But when Lamb and his colleagues looked closer, the sapping hypothesis fell apart. Their calculations indicate that flowing spring water is too feeble by a factor of 22 to move the existing bouldery rubble downstream and make room for more. Moreover, spring water is chemically incapable of eroding the rubble by dissolution. And rock dating showed that erosion of the canyon's head wall ceased 45,000 years ago--further evidence that the present water flow isn’t up to the job of cutting a canyon.

A better explanation is a megaflood, the group says. Pools carved into solid rock remain where torrents of water may have plunged off the head wall. And a slight notch on the top of the head wall is finely scoured as if once crossed by high-speed, rock-laden waters. From the scale of that scouring, the group calculates a flow that could have easily moved rubble downstream. Indeed, the flow could have resulted from a partial diversion of known catastrophic floods unleashed upstream when natural lake dams broke tens of thousands of years ago.

Catastrophic floods may have carved similar-looking canyons on Mars, the group writes. "We're not saying canyons on Mars couldn't have been carved by springs," says Lamb. But "just because a canyon head is amphitheater-shaped, it shouldn't be assumed to have been carved by springs," he says. If martian canyons were gouged out only by rare floods rather than many millennia of slow seepage, Mars may have lacked the continually warm and wet climate needed for the origin and evolution of life.

Box Canyon may well be the product of floods, says Mars geologist Michael Carr of the U.S. Geological Survey in Menlo Park, California, but "I'm not throwing out the sapping hypothesis" for Mars. Solid rock may not be liable to spring erosion, he says, but "who knows what's underneath" the lava-strewn surface of Mars? It could be easily erodable sands. Finding out sounds like a job for a nimble-footed rover.