Reconsidering Martian Winds

Thanks to the Mars Global Surveyor (MGS) spacecraft, scientists are now armed with a wealth of new data about the Red Planet¿including dramatic images of its furrowed surface in the South and flat plains in the North. The pictures have revealed gaping channels, spidery valley networks, steep gullies and, most recently, ancient layers of what appears to be sedimentary rock. Many researchers seeking to explain these peculiar scenes have, like travelers lost in the desert, thought almost exclusively of one thing: water. Indeed, a mounting collection of papers suggests that vast oceans and gushing streams once helped wash, erode and flood away surface rocks to shape the planet¿s current face.

This vision of Mars as a wet and warm world is naturally inviting¿in no small part because it increases the likelihood that life may have flourished there at one time. "We all love water," says Conway Leovy, a meteorologist and Mars researcher at the University of Washington in Seattle. "And it¿s the most important question on Mars." But forces other than water¿among them tectonic shifts, volcanism, dry debris flows, ice and wind¿certainly contributed to the formation of Mars¿ remarkable surface features. And Leovy believes that the power of wind in particular has been grossly underestimated.

On supporting science journalism

If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.

At the annual meeting of the American Astronomical Society¿s Division for Planetary Sciences in late October, Leovy outlined his case for aeolian influences on Martian geology when he delivered the 2000 Kuiper Prize Lecture. "Looking at the high resolution images, it¿s clear that wind erosion and deposition have relatively recently modified the surface in many areas," he says. And if surface pressure on the planet were only slightly greater in the past, wind would have been a far more potent force. "Going from a surface pressure of 7 to 50 millibars, you get a factor of 100 increase in the rate of dust transport," he says.

Just as you lift up more dust with sandpaper if you press down harder on a surface, Martian winds, carrying coarse sand-like grains, could have scraped away at the planet¿s bumps and ridges, and widened existing channels and grooves, such as the outflow channels and valley networks. "Wind erosion isn¿t responsible for a lot of these features," Leovy explains. "There are many factors, but modification by wind of many of these features must be recognized as more important than it has been."

What is particularly compelling about giving wind greater credit in creating the outflow channels and valley networks on Mars, Leovy notes, is that it absolves researchers from accounting for the massive amounts of flowing water these processes would have otherwise required. Catastrophic flooding is generally held responsible for creating the outflow channels¿some of which measure hundreds of kilometers wide¿during the Hesperian to early Amazonian periods on Mars, for instance. But how such volumes of water could have survived the planet¿s earlier stages is a mystery. "We¿re talking about gulf stream-sized amounts of water in some cases," Leovy says. "And it¿s hard to account for where that water came from, where it went or how you had a warmer climate."

So too, the valley networks, which appear to snake through the Martian terrain like dried river beds, are difficult to justify in terms of water, Leovy notes. Based on the appearance of these networks, most scientists have assumed that repeated flows of running water or groundwater sapping formed them during the Noachian period. And yet for Mars to have had either regular flows of liquid water or continually replenished groundwater, it had to be a much warmer place at the time¿which is unlikely, Leovy says. Warmth from the sun during the late Noachian to early Hesperian was probably 25 percent less than it is today. And researchers have yet to come up with a mechanism by which Mars could have sustained a sufficiently warming greenhouse effect to compensate.

"There are many kinds [of valley networks] in the Southern latitudes," Leovy notes. "They look like debris flows and there is a lot of talk about ice or ice motion." Once scratched out in the surface by debris and ice, these grooves, like the outflow channels, could have been selectively widened and deepened by wind erosion and dust transport, he argues. The eroded material would settle in depressions: "In crater bottoms and valley bottoms, you find dunes," he says of contemporary pictures. And kicked up by more wind, this loose material would in turn further scour away at the roughened walls of a surface depression¿creating a positive feedback loop. Such reinforcement, Leovy suggests, could even help explain why, in contrast to the South, Mars' Northern plains are so flat.

He emphasizes that water has certainly played its part in shaping Mars' appearance. The planet now bears water in the form of ice and vapor--and likely had some liquid water in the past. "The gullies look the most like water," Leovy says, "and I think they were [formed by] water." Michael Malin, principal investigator for photography on the MGS spacecraft, and Kenneth Edgett--both from Malin Space Systems--described these gullies in a paper in the June 30th issue of Science. Most scientists feel that these chasms, which arose more recently than did either the outflow channels or valley networks, were created by short floods of water, erupting from ice buried beneath the planet's surface.

And additional signs of liquid water on Mars--at least in its very ancient times--were described last week. In a paper in the December 8th issue of Science, Malin and Edgett revealed images of what look like sedimentary rock. "We see distinct, thick layers of rock within craters and other depressions for which a number of lines of evidence indicate that they may have been formed in lakes or shallow seas," Malin stated in a press briefing. "We have never before had this type of irrefutable evidence that sedimentary rocks are widespread on Mars."

What is particularly striking about these layers is their uniform thickness. "These layers are magnificent stuff," Leovy says, "and maybe they're produced by water." To date, he has not considered the influence of wind on features older than the outflow channels and valley networks: "My thinking only goes back that far," he says. Still, he believes an aspect of his theory might fit because wind probably would have been more significant in ancient Martian times too. For now, a final accounting of the wind and water on Mars may have to wait for more pictures or future missions, in which scientists hope to collect rock samples from the Red Planet.