Martian leaks: Hints of present-day water.
Michael C. Malin and Kenneth S. Edgett of Malin Space Science Systems in San Diego announced their results June 22 at a NASA press briefing in Washington, D.C. They also describe their work in the June 30 SCIENCE.
"On Earth, everywhere that you find liquid water below the boiling point, you find life," notes Bruce M. Jakosky of the University of Colorado at Boulder. Although the findings don't directly "tell us anything about whether life does exist at present or ever has in the past on Mars ... the water is much more accessible than we had thought," he says. "We may not have to dig down 2 or 3 or 4 kilometers to find someplace where there is liquid water today."
"This is some of the most exciting data we've had" concerning water on Mars, comments geologist Michael H. Carr of the U.S. Geological Survey in Menlo Park, Calif. "On the one hand, I find the interpretation that these are waterworn [features] very compelling. On the other hand, I'm skeptical--just because of the conditions we know prevail on Mars and knowing how difficult it is to have liquid water close to the surface."
About 250 of the 65,000 images that Surveyor has so far returned to Earth show features that look like gullies gouged by bursts of water that flowed down the sides of valleys and crater walls. At their upper end, the gullies display regions known as alcoves where material has eroded. At the lower end lie deposits, or aprons, of soil and rocks carried downhill. Deep channels, a few hundred meters across, cut through each gully, which runs a few kilometers in length.
"If this had been [found] on Earth, there would absolutely be no question that water was associated with the formation of the features," says Malin.
Gullies had never been seen before on Mars. Not only do they appear to be carved by water, they also seem extremely young. That suggests that the water that created them flowed recently--no more than 1 million years ago and perhaps just a few months ago, says Edgett.
Four lines of evidence point to the youthfulness of the features, he notes. None of the gullies are dimpled with craters, indicating they are not old enough to have been bombarded by space debris.
In one of the images, a gully and channel lie on a field of sand dunes. "Sand dunes tend to be very young features," and the channel that slices through them must be even younger, Edgett says.
Several pictures show that the aprons of debris lie within a mottled region, akin to the surface of a basketball. At Earth's polar regions, such a pattern is well documented, created by repeated thawing and freezing of ice at the surface. Such terrestrial patterns are no more than 10,000 years old and could be as young as a few hundred years, Edgett says.
The scientists don't know whether the same process has produced the mottled appearance on Mars, "but if the aprons of debris are sitting on top of things that are already fairly young, the aprons have to be [even] younger," Edgett notes.
Finally, the images reveal bright and dark surfaces lying adjacent to each other. That's an indication that some process has recently removed dust, which continually settles on the surface of the Red Planet and mutes the contrast between objects. The lack of dust "is telling you something is happening on these slopes right now or in the last year or two," says Edgett.
Malin and Edgett suggest that a supply of water no more than a few hundred meters below ground, stored in the pores of rock, occasionally seeps up through the surface in different locations and creates the landforms. They calculate that the amount of water is relatively small--to create each gully about 2,500 cubic meters of Martian water would have to rush out. That's enough to fill seven Olympic-size swimming pools.
The findings are in stark contrast to the standard view of Mars. Over the past 3 decades, since the Mariner 9 spacecraft found a network of ancient channels resembling dry river beds, most planetary scientists have come to believe that the Red Planet was once a much warmer place with water flowing freely on its surface. Analyses of ancient meteorites believed to have come from Mars also indicate the planet once had a vast ocean.
The bulk of that water evaporated more than 1 billion years ago when the atmosphere became thin, and researchers have thought that if any liquid water remains, it must be buried several kilometers beneath the frigid ground.
"We didn't expect to see anything like this," says Jakosky of the new findings.
The location of the unexpected landforms poses the greatest puzzle. "The features are in the polar regions, the coldest locations on the planet, which is exactly the opposite of what you would have expected for something to [contain] liquid water," says Malin.
Edgett adds, "I was dragged kicking and screaming to the conclusion" that liquid water resides just beneath the surface at the poles.
The gullies lie exclusively in areas where ground temperatures typically don't exceed -70 [degrees] C, which is well below the freezing point of water. Moreover, the images show the gullies and channels primarily on slopes that face away from the sun.
Edgett and Malin suggest that the water freezes as soon as it seeps up onto the surface. The frozen water forms a barrier, or ice dam, that blocks liquid water still percolating beneath the surface. According to their model, the pent-up pressure eventually becomes strong enough for the liquid water to push the dam out of the way, enabling water to gush out and sculpt the gullies before it freezes. Nobody knows whether this process happens once a century or once a week, Edgett says.
Christopher P. McKay of NASA's Ames Research Center in Mountain View, Calif., notes that in Earth's Arctic regions, liquid water manages to penetrate through 600 m of permafrost that has an average temperature of -20 [degrees] C. Although this is not as cold or as deep as the polar ground on Mars, it suggests that cold springs could exist on the Red Planet, he says. McKay and his collaborators, including Dale T. Andersen of NASA Ames, reported their findings in the January 1999 CANADIAN JOURNAL OF EARTH SCIENCES.
Also, the Martian water might be salty, McKay adds, enabling it to remain liquid at low temperatures and not evaporate under the planet's thin atmosphere.
Jakosky suggests that water in the polar regions may leak steadily onto the surface, maintaining a liquid flow the same way that a steady drip keeps household pipes from freezing in the winter.
Carr remains troubled by the depth of the proposed liquid water. "We have a problem," he says. "We would expect the ground to be frozen 3 to 6 kilometers deep, yet we have evidence of [liquid] water close to the surface."
Alternative models could yet emerge, Carr cautions. For example, he proposes that a mixture of frozen carbon dioxide and water, known as a clathrate, might erupt from underground and immediately vaporize. The sudden release of gas bubbles from the frozen brew could create the gullies and channels, he suggests.
"I think the important thing at this stage is to be cautious about the interpretation and get the information out into the science community," he says.
Indirect support for the new findings comes from an analysis of hydrogen isotopes in a meteorite of Martian origin. In the July 15 GEOPHYSICAL RESEARCH LETTERS, Laurie A. Leshin of Arizona State University in Tempe and her colleagues argue that the crust of Mars holds two to three times more water than scientists had previously estimated.
In the thin Martian atmosphere of today, water has a deuterium-to-hydrogen ratio fives times higher than it has on Earth. Because hydrogen is lighter than deuterium, it can more easily escape Mars' lower gravity, leading to the enhanced ratio. Assuming that both Earth and Mars began with the same ratio of deuterium to hydrogen, researchers had calculated that about 90 percent of the water in the Martian atmosphere and upper crust had been lost since the planet formed.
Leshin's team examined tiny, water-bearing crystals locked inside a meteorite called QUE94201. These crystals, they argue, contain hydrogen from the Martian interior that reflects the planet's original composition. They find a ratio of deuterium to hydrogen about double the assumed value, which implies that Mars has lost much less water over the eons.
That water should still exist today within the Martian crust, Leshin says. If confirmed, the study "ties in very well--via a completely independent means--with our recent observation that there may be water at shallow depths beneath the Martian surface today," says Edgett.
Steve Squyres of Cornell University says researchers shouldn't assume that the liquid water comes from a large subsurface aquifer. Instead, he notes, the small amount of water "may just be very local melting of near-surface ground ice," warmed by the planet's internal heat or perhaps by the sun during periods when the north or south pole is tilted closer to the sun.
The polar findings "open up the ball game" of where liquid water may lurk on Mars, says Jakosky. Warmer regions on the planet, nearer the equator, might contain underground deposits that haven't been detected. "It's not your mother's Mars," says Edgett.
Orbiting spacecraft with long-wave-length radar as well as future Mars landers may be required to find out where liquid water lies, he notes. The gullies are too steep for a craft to land on. The aprons are flatter but too small a target for current spacecraft to home in on.
The next lander is scheduled for a 2003 launch and won't have the ability to pin-point a target with the accuracy required, says Edward J. Weiler, NASA's associate administrator for space science in Washington, D.C.
The search for life on Mars had focused on the more temperate, equatorial regions. Scientist will now have to broaden their vision to include the polar areas. They'll also need to design spacecraft to explore near-surface water deposits for signs of past or current life.
Surveyor has at least a year left in its mission to map Mars, and Malin and Edgett are eagerly poring through the images as the craft radios them to Earth.
It's possible but unlikely, they note, that the craft would pass over a gully just as water was gushing out. However, they are also looking for new channels and other modifications that would indicate water had erupted in the past few months.
"If we take pictures of these areas and see changes--Wow!" said Edgett at the press briefing. "We'll be back."
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|Date:||Jul 1, 2000|
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