Is the Signature Still Wet?
In March 2004, two robotic rovers from NASA delivered clear evidence that liquid waterperhaps large bodies of itexisted on the surface of Mars at some point in the planet's past. But could liquid water still exist on Mars today? The likelihood would seem to be impossibly low: the temperature on the Martian surface averages between -70°C and -100°C (-95°F and -150°F), far below the freezing point of water. In such cold, water should freeze to a depth of several kilometers.
Yet a series of satellite photographs taken by NASA's Mars Global Surveyor, which has been mapping Mars since 1999, raise the exciting possibility that liquid water may have existed there very recently. The photos, examined by scientists Michael Malin and Ken Edgett of Malin Space Science Systems, reveal gullies on Mars seemingly identical to landforms that, on Earth, are created by flowing water. From appearances, water seems to have flowed out and down the sides of Martian craters and canyons. The scientists point to three distinguishing features: a clear starting point, or alcove, where the liquid seems to have come through the wall of a crater or valley; channels like those carved by running water on Earth; and a mound of rubble, or apron, at the bottom that looks like it was dumped by flowing water. (To examine the evidence yourself, click on the accompanying interactive, "Did Water Carve These Canyons?")
"The physical resemblance is stunning," says Nick Schneider, a geoscientist at the University of Colorado. "That's what water on craters looks like." In some photos, several gullies appear in a row. On Earth, these formations, called "weeping layers," form when water flows underground along a porous layer of rock. Wherever these layers end at exposed canyon or valley walls, water seeps out at several points along the layer. Photographs of what appear to be similar weeping layers suggest further evidence of liquid water. Malin and Edgett offered additional evidence in an article published in the journal Science. But Bruce Jakosky, also of the University of Colorado, perhaps summed up their case best: "It comes down to a very technical argument that these are water-carved, and that is: 'Dammit, they look water-carved!'"
One striking feature of the gullies is how young they appear to be. They slice through windswept dunes, which are some of the most rapidly changing features on Mars. They are also remarkably free of dust and impact craters, which cover almost everything else on Mars. According to Malin and Edgett, some of the gullies look to be so recent that they may still be actively forming today. Yet how could that be, if the Martian surface is so cold? Any liquid water should be frozen solid. Scientists are wracking their brains for a logical explanation. Several theories have been proposed, with no clear answer.
One possibility is that the water is very salty. There is plenty of salt on Mars, and salt can significantly lower the freezing point of groundwater. (That's why people sometimes use salt to melt the ice on their sidewalks.) To stay unfrozen on Mars, however, the water would need be so thick with salt that it probably would no longer flow like water. Another possibility is that the water is (or was) spilling from underground quickly enough to avoid freezing. But even then, the water would have to start out warmheated, say, by a nearby volcano or a vein of subsurface magma. (A similar process produces geysers and hot springs at Yellowstone National Park.) So far, however, scientists have seen no sign of active Martian volcanoes or other potential heat sources.
It could also be that the gullies formed in the recent past, perhaps during a temporary warm spell, before Mars's climate cooled down to its present icy state. Steve Clifford, a scientist with the Lunar and Planetary Institute, points out that Mars wobbles on its axis, and that those wobbles could have prompted extreme changes in the planet's climate. If a planet has a tilted axis, then its northern and southern hemispheres will see more daylight during some parts of the year ("summer") and less during others ("winter"); hence the seasons. The greater the tilt, the more extreme the seasons.
Currently Mars sits at a tilt identical to Earth's. But in the past that tilt has varied drastically, from zero degrees (no tilt) to 60 degrees (tipped two-thirds of the way on its side). "That's an amazing variation," Clifford says. When the tilt was greater than 45 degrees, even the Martian poleswhich today are frozencould have received enough sunshine and heat to start melting. Likewise, underground ice elsewhere on the planet could have easily warmed up enough to melt, flow onto the surface, and carve the gullies seen today.
An entirely different theory for the Martian gullies suggests that a mixture of dirt and carbon dioxide gas, not water, was responsible. Michael Carr, a scientist with the U.S. Geological Survey and the author of the book "Water on Mars," notes that under certain conditions, dry particles can become "fluidized" and flow like water. This sometimes occurs on Earthfor example, when particles of ash and gas pour from a volcano, or on the rare occasion when a pocket of carbon dioxide bursts from a hillside and sends debris spilling down a slope. Something similar could have happened on Mars. "When we observe these kind of deposits with channels, which are pretty common on Earth, that's your first reaction, that water was involved," says Ken Tanaka, another USGS scientist. But since the environment on Mars is so different from Earth, he adds, "I think you want to keep your options open."
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Supplement a study of astronomy with a classroom activity drawn from this Science Bulletin essay.
- Working individually or in small groups, have students read the essay (either online or a printed copy).
- Have them write a one-page reaction to the article, explaining in their own words the "recent gully" theory they believe seems most likely or offering one of their own.