A survey of ancient Mars – exploring the red planet
Scientists have seen lots of big smooth rocks on the surface of Mars, remnants of catastrophic flooding in the distant past. But where did all that water go? New images from Mars Global Surveyor hint at some answers.
Pathfinder bounded down on Mars last year, 21 years had passed since our previous visit, by two Viking probes, and in that time the Red Planet had gotten a bit remote. That feeling changed, though, as soon as we saw Pathfinder’s panoramic, rock-strewn landscape and the images of the plucky Sojourner rover ambling from rock to rock. Suddenly Man was real again, a place to toddle off to with a picnic lunch and a pair of binoculars. Ares Vallis could be the valley next door.
Even though Pathfinder and Sojourner have now succumbed to the cruel Martian elements, Mars is unlikely to fade from our attention soon. Another probe is picking up where they left off. Mars Global Surveyor is a one-ton package of cameras and sensors, bigger than a Frigidaire but smaller than a Volkswagen Beetle, designed to make us even more familiar with the planet. Surveyor won’t be landing on Mars–not intentionally, at least–but it is sending back stunningly detailed images of the surface from a high elliptical orbit that brings it as close as 75 miles. These early images are but a foretaste of the feast to come, and already they are making researchers take notice. In particular, scientists must reconsider the role of water on this enigmatic planet. swirling around the idea of water on Mars–How much was there? How much is there now? Where did it all go?–have been asked for a long time. A hundred years ago, it was widely thought that water flowed through canals built by an advanced Martian civilization. Astronomers were never comfortable with this hypothesis, but clear evidence that refutes it came only in the early 1960s, when spectroscopic data showed just traces of water in the atmosphere. In 1965, Mariner 4 revealed Mars to be desolate, pockmarked, and bone-dry. But seven years later Mariner 9, the first global surveyor, sent back images that looked like valley networks–more Missouri watershed than lunar landscape. How could a place so desiccated ever have supported rivers, even in its distant geologic past? Pictures from the Viking orbiters in the late 1970s added details such as teardrop-shaped islands behind craters and tributary valley networks, and theorists have been feasting on these data ever since. “Over the last decade or so, the evidence has been much more secure that water has played a major role in the evolution of Mars,” says Michael Carr, a planetary geologist with the U.S. Geological Survey in Menlo Park, California.
Some planetary scientists argue that Mars must have had an “Earth-like” period of hundreds of millions of years to produce the patterns of erosion that the Surveyor photos reveal. This wet period would have been created by a constant peppering of water-laden comets, and according to the crater geology, it would have to have occurred some 4 billion years ago, during the so-called period of heavy bombardment, when the solar system was young and the planets were continually pelted with debris left over from its birth. At that time, however, many scientists argue, the sun was so feeble that no amount of carbon dioxide could have created a greenhouse effect on Mars large enough to warm the planet above freezing. To get liquid water on Mars, proponents of the “warm wet” theory contend that some kind of super greenhouse effect kept the Martian surface warm. Other scientists believe that water acting on the surface of Mars must have been in the form of glaciers cutting across the landscape or in vast, episodic floods spilling over the land after a volcanic eruption or meteor impact melted a block of ice. “The discharges were so great that there’d have been no time to freeze,” Carr says. “Each event didn’t last more than a month, but they happened repeatedly. There are lots and lots of these floodplains on the surface. They left enormous scars.”
The cameras on Surveyor will confirm one view or the other–or maybe send everyone back to the drawing board. Those cameras were built by Michael Malin, a geologist at Malin Space Science Systems in San Diego, and they use a technique like that of a fax machine or photocopier, scanning images one line at a time. Though the high-resolution camera, with one row of light sensors, takes only a ribbonlike picture of a single two-mile-wide swath of the surface, two-dimensional pictures are built up as the orbiter flies from pole to pole over the Martian surface.
Everyone knew the pictures would be impressive–the resolution is some 20 times better than Viking’s best. Still, no one expected anything as powerfully evocative of a once-wet Mars as the image of Nanedi Vallis sent back in January. Winding its way through a featureless Martian plain, Nanedi Vallis looks every bit like a river gorge. The walls slope down to a small channel; dark streaks along the sides suggest layer upon layer of sediments or volcanic ash deposited over the millennia; oxbows hint at the kind of erosion that shapes terrestrial rivers. At first glance, the only tip-off that this is a Martian landform is the sprinkling of impact craters around and within the gorge. “It looks just like the lower Colorado River below Hoover Dam,” Carr says. “There’s a deep meandering valley and you can see the river channel at the bottom of it. This is pretty convincing.”
Other planetary scientists think the look of Nanedi Vallis may be deceiving. Malin believes that some parts of the valley were cut by water flowing on the surface but that other parts were cut from below, as water streaming through an underground aquifer carried off material until the surface layers collapsed to fill the void. “At some point there was a fluid flowing through the valley,” he says. “It does look like a river. But there are immediately adjacent areas that were created by another process.” If so, Nanedi Vallis would be much like the Colorado River, a surface river with many tributaries formed from collapsing underground aquifers. This sapping of the landscape by subsurface water flow has been used to explain many Martian features that, on the face of it, certainly seem like river-cut ravines. And even Carr agrees that this valley is not ironclad evidence of a terribly warm and wet Martian past: small tributaries, which a rain-fed river would have in abundance, are notably absent.
Did Mars ever sustain flows of water? Geologist Robert Craddock of the Smithsonian Institution’s National Air and Space Museum in Washington thinks the soft-looking features in the southern highlands are a key holdover from the early days of the planet. While craters formed in the last 3.5 billion years or so are sharply defined, the very oldest craters are rimless circles, as if they had been stamped into the surface rather than blasted out. “Our analysis shows you need two processes to transform a fresh crater into this degraded shape,” Craddock says. “One is surface runoff and the other is diffusion what you get when you hit an unvegetated surface with raindrops.” To account for the softening of these features, you would need a lot of precipitation–at least occasional heavy rain over thousands of years. That would make these features good evidence of a full hydrologic cycle on ancient Mars.
Craddock hopes images of the older, battered terrain of the southern hemisphere from Mars Global Surveyor, which thus far has been concentrating on the north, will bolster his case for a rainy era. But indirect support may already be available in the pictures of the windblown dunes found all over the planet. “If you have rainfall, you have mechanical weathering, the physical breaking up of material from the raindrops,” Craddock says. “That might explain the soil types we see.” Sweeping across vast stretches of the Martian landscape, the dunes can be quite startling at first glance: they look like oceans with whitecaps set in stone. But while the large quantities of dust suggest aggressive action by water rather than erosion by the thin Martian atmosphere, the dunes can also complicate matters by hiding features researchers most want to see. For example, the channel in Nanedi Vallis is smothered in a blanket of dust, making it difficult to pick out. Geologists looking for signs of ancient shorelines encounter similar problems. all the surface water is so ancient. If water does indeed lie deep underground in permafrost layers, renewed geothermal activity or a meteor impact could shoot superheated water gushing to the surface. “Well, immediately after a meteorite impact, there wouldn’t be a take,” says Nathalie Cabrol, a planetary scientist at NASA’s Ames Research Center. Instead ice at the impact site would be driven away by the tremendous force of the collision. It might take decades for the region to recover. “But as the pressure and temperature go down,” says Cabrol, “the water trapped as ice below the surface surges in. You get an exploding geyser.” Cabrol has identified relatively recent craters that appear to have channel erosion, lake beds, and sediment deposits in or around the sites, and she hopes to explore them with robotic rover missions over the next few decades. Not only are these sites, such as Gusev Crater next to the tropical Ma’adim Vallis, biologically intriguing–the valley’s waters, warmed by local geologic activity, could have been the home of rudimentary life-forms–but such places could also, Cabrol contends, have supported (pen lakes and rivers much more recently than has been generally accepted. “What I’ve seen is so fresh that the deposits must be very young,” Cabrol says. “Maybe only a few million years old.”
Under the conditions now found on the Martian surface, freshwater freezes immediately. Even if it is heated, the ice will evaporate rather than melt in the rarefied atmosphere. But brine, which is water loaded with dissolved salts, has a much lower melting point. Cabrol argues that these mineral-laden waters could have survived on the surface for a long time–maybe thousands of years. In fact, a Surveyor image of an area just upstream of the large Schiaparelli Crater seems to show the dried remains of ancient salt lakes, white patches cracked with spiderwebs of dark lines. Needless to say, Cabrol’s theories are controversial; even the hardiest brines cannot overcome the planet’s present-day wicked cold and withering air. But Cabrol, like every other Mars researcher, hopes to find support in Surveyor images taken this summer.
Of course, Mars Global Surveyor is designed to answer questions other than those concerning water. It will identify minerals on the surface and monitor worldwide weather patterns. And geologists want to find evidence of plate tectonics in the ancient southern highlands–or at least an explanation for why one side of the planet is so high and battered while the other is low-lying and exceedingly flat. But traces of water are hard to escape. Even the new portrait of the so-called Face on Mars sitting on the Cydonia Plain seems to show signs of glacial weathering on the otherwise unnoteworthy rock. Though there’s not a drop to drink on the surface of Mars, we keep seeing water everywhere we look.
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