Last week Malin and his collaborator, Kenneth Edgett, unveiled photos they believe offer the strongest evidence yet that water flowed billions of years ago on Mars. The photos show layered rock outcroppings scattered around the planet’s equator in canyons and craters. Some of the outcroppings, separated by 30 miles or more, are made of identical sequences of thin layers–like slices from the same cake. The evidence hints at a long history of standing water on the planet’s surface. “We think it’s likely that these very thin layers formed underwater,” says Malin. “It’s hard to come up with an alternative.” A bonus: the rock could contain fossils.
Scientists have long thought dry riverbeds and flood channels are signs that liquid water used to gush across Mars’s dusty surface. But they have had no idea how long the torrents lasted. The new photos of layered rocks, snapped by a camera onboard the Mars Global Surveyor, suggest that water could have flowed for millions of years on early Mars. Malin believes the rocks formed when sand and dust settled into lakes, where water cemented the dust into rock. The lakes then evaporated, leaving behind bands of rock. When the lakes filled with water again, new loads of dust trickled in, creating new bands of rock on top of the old ones. Over many millenniums, this process could have created huge outcroppings made up of hundreds of layers. Wind and other forces then cut into Mars’s surface to expose the banded hills captured by the Global Surveyor’s camera, which was built by Malin’s company, Malin Space Science Systems. Malin and Edgett reported their theory in last week’s issue of the journal Science. “The Global Surveyor images are much better than anything we had before,” says Michael Carr of the U.S. Geological Survey. “These pictures further strengthen the argument that early Mars was warm and wet.”
For decades, scientists have tried to figure out what Mars was like 4i billion years ago, when it formed along with the rest of our solar system. The young planet probably had an atmosphere, and the surface bears witness to ancient, massive volcanoes. But at some point our neighboring planet lost its atmosphere and became an icy desert. Or so most scientists thought until this summer, when Malin and Edgett reported that the Global Surveyor had spotted gullies that might have been cut by running water. That discovery sparked debate about whether water flows under–or on–Mars’s surface today. If not, life probably had its best shot on Mars long ago, when the planet was racked by volcanoes and floods and protected by an atmosphere.
If Malin and Edgett are right, a network of lakes circling the planet’s equator might have increased the chances that life evolved on the Red Planet. “The longer water stands around on the surface, the more you think life could have started, and maybe even established itself well enough that it still exists,” says Michael Meyer, who runs NASA’s astrobiology program. “These pictures suggest water has been around for a substantial period of time.” Probably not long enough to support little green men–if life exists on Mars, it’s most likely microscopic. Bacteria can thrive in extreme heat or cold, as long as they have access to moving water. To Bruce Jakosky, an interdisciplinary scientist on the Global Surveyor mission, that suggests that life may be a matter of a simple chemical equation. “It could be that if your environment meets a straightforward set of conditions, life can evolve quickly. The evidence is plausible that life could have started on Mars or been transported there. But we’ll only find out by going there,” Jakosky says.
Humans won’t head for Mars any time soon, but scientists will send unmanned missions there every 26 months through 2018. Next April NASA will launch an orbiter to make more detailed maps of the minerals and features near Mars’s surface. In June 2003, two Mars Exploration Rovers will take their turn. When they arrive on Mars, the rovers will examine rocks in a one-mile search area for evidence of water–and perhaps life. Space scientists are already talking about where they’ll send the rovers; should they head for long-dead volcanoes, since we suspect life on Earth evolved at hot spots? Or should we send them to check out the gullies Malin and Edgett spotted this summer? The layered outcroppings might also be a solid choice. They’re huge and flat, which leaves lots of wiggle room for a good landing. The layers may also yield evidence of ancient water. And they could be the best place to look for fossils, if they exist. Of course, scientists would love to get their hands on a piece of Mars, but it will take until at least 2011 to plan a sample-return mission.
In the meantime, researchers will keep analyzing the data coming back from Global Surveyor. Since its launch four years ago, Surveyor has taken 85,000 photos, 500 million laser measurements, hundreds of millions of infrared profiles and gigabytes of other new data about Mars. Malin and Edgett will comb this stockpile of information for answers to some unresolved questions about their theory. For instance, they’re not sure how the loads of dirt that eventually turned into rock arrived in the crater lakes. The craters aren’t surrounded by evidence of rivers; if water didn’t carry the dirt in, what did? Malin and Edgett offer one possibility: perhaps climate changes caused waves of dust to fall out of the sky into layers on Mars’s surface. Future missions might help answer questions like these. No doubt, they’ll also uncover a few new mysteries.
