Water found (and lost) on Mars
Posted: Thursday, September 24, 2009 2:00 PM by Alan Boyle
NASA / JPL-Caltech / Univ. of Ariz. |
Water ice surrounds a 26-foot-wide meteorite impact crater on Mars in this HiRISE picture, taken in November 2008.
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Researchers have caught Martian water ice in the midst of a triply amazing disappearing act. Why triply amazing? The ice was spotted amazingly close to the Red Planet's surface, and amazingly far away from the north pole. The third amazing thing about the observations, made using NASA's Mars Reconnaissance Orbiter and reported in Friday's issue of the journal Science, is that the researchers knew it was 99 percent pure water ice because of how slowly it disappeared.
The findings should cheer up astrobiologists, who have said even a little trickle of liquid water might sustain life beneath Mars' forbidding surface. It should also cheer up would-be space explorers, who are already over the moon because of this week's reports that lunar ice deposits are more prevalent than previously thought.
"We knew there was ice below the surface at high latitudes of Mars, but we find that it extends far closer to the equator than you would think, based on Mars' climate today," the University of Arizona's Shane Byrne, a member of the research team for the orbiter's High Resolution Imaging Science Experiment (HiRISE), said in a news release issued today.
The ice was identified using three of the scientific instruments on Mars Reconnaissance Orbiter.
First, scientists sifted through images captured by the spacecraft's wide-angle Context Camera in August 2008. They found dark patches of debris, apparently thrown up from impact craters when meteorites crashed into the Martian surface.
That gave the scientists a list of targets to look at more closely starting a month later, using the higher-resolution HiRISE camera. They were elated to find that, in some cases, bright blue-white material had been thrown up from the craters along with the dark stuff. They became even more elated when they saw that the material slowly faded away over the months that followed.
That's just the kind of signature researchers saw last year when NASA's Phoenix Mars Lander came upon subsurface water ice during its diggings. That ice slowly vaporized once it was exposed to Mars' thin, dry, cold atmosphere. (On Mars, atmospheric conditions are such that water usually passes right from the solid to the gaseous state, as carbon dioxide "dry ice" does on Earth.)
The icy patches spotted by HiRISE did the same thing. In fact, the scientists worked out a mathematical model to determine just how much the ice and soil were mixed together, based on how quickly the dark material shrank away. Based on a conservative reading of the model, Byrne and his colleagues estimated that the water ice was 99 percent pure.
The model provided one line of evidence that the bright material was really water ice. If it were, say, frozen carbon dioxide, the frozen stuff would have disappeared more quickly. But just to make sure, the scientists checked the readings made with a third instrument, the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). The right readings were available for one of the craters, and they matched the spectral "fingerprint" of water ice.
"Everyone thought it was water ice, but it was important to get the spectrum for confirmation," Kim Seelos of Johns Hopkins University's Applied Physics Laboratory, a CRISM team member and co-author of the Science paper, said in NASA's news release.
Malin Space Science Systems' Megan Kennedy, another co-author who is on the Context Camera team, said that "we now know we can use new impact sites as probes to look for ice in the shallow subsurface."
So much care was taken to confirm the detection of water ice in part because the location of the ice was unexpected.
Substantial water ice deposits had been previously detected above 60 degrees latitude on Mars, roughly equivalent to the locations of Anchorage, Alaska, and Helsinki, Finland, on Earth. Indirect signs of buried ice have been seen at lower latitudes. But the latest research reveals visible ice deposits at northern latitudes of 55 degrees and even 45 degrees, which is about where Seattle and Paris are on Earth.
By rights, that ice shouldn't be there, Byrne told me.
"Probably what happened was, in the recent past, Mars had a more humid atmosphere," he said. "That allows water ice to be more stable."
Byrne explained that Mars appears to be going through a climate cycle lasting thousands or tens of thousands of years. During the more humid times, water vapor would be more easily transported through the atmosphere to lower latitudes, where it would freeze out and get mixed up with Martian soil. The ice deposits seen by the HiRISE camera are likely leftovers from that phase of the climate cycle.
Based on the depths of the craters, the scientists figured out that a layer of almost pure ice, perhaps measuring 20 inches (half a meter) or so, must lie no more than a couple of yards (meters) beneath the surface at the sites that were studied.
The crater sites are all roughly in the same region where the Viking 2 lander touched down in 1976, and NASA said the latest results suggest Viking might have struck water ice back then if only it had dug 4 inches (10 centimeters) deeper than it did.
How could a layer of ice that pure have formed so close to the surface?
"One way to do that is to use this mechanism called 'frost heave,'" Byrne said. On Earth, thin films of liquid water can form around grains of ice, even at temperatures below freezing. That liquid water can migrate through the soil to form a pure frozen "lens" on top of the ice table.
Frost heave is a big nuisance on Earth, because the movement of the ice can create cracks in pavements or building foundations. But if this is what's happening on Mars, that opens up a couple of intriguing possibilities.
First, it could have provided that precious trickle of liquid water for, say, subsurface microbes of the kind found in Earth's polar regions. Astrobiologists have theorized that microbial life just might still exist beneath Mars' surface if there were a source of liquid water. "They'll definitely be excited about this," Byrne said.
Second, those ice lenses could someday provide a valuable resource for colonists from Earth, at latitudes that would be more hospitable for exploration and settlement than the polar regions. "It would probably be a little bit easier to get drinking water from a block of pure ice instead of a block of 50-50 material," Byrne observed.
NASA / JPL-Caltech / Univ. of Ariz. |
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This series of pictures shows how the ice detected at two Martian impact craters
gradually faded and became completely covered over with dust.
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But once the ice is exposed to the thin Martian air, it doesn't hang around all that long. Almost all of the ice that was spotted last year is now gone. "We were able to image one crater where there's just the slightest trace of ice still left, and that won't last long," Byrne said.
Which means it was a good thing that the scientists in charge of Mars Reconnaissance Orbiter's various instruments worked together to monitor the disappearing act as it unfolded. "If we had taken HiRISE images just a few months later, we wouldn't have noticed anything unusual," Byrne said. "This discovery would have just passed us by."
Update for 6:35 p.m. ET Sept 24: Several commenters asked how the researchers knew that the ice seen in the craters was not actually ice that came down with the object that created the impact crater. I had to admit that's a question I didn't think to ask, so I went back and asked Byrne about that in a follow-up e-mail. Here's his answer:
"The objects that created the craters are quite small. Their volume is probably only a fraction of a percent of the volume of the crater. So the amount of material they could have contributed is pretty minor. Also, icy objects this size probably wouldn't make it to the surface but rather break up in the atmosphere. The impactors are much more likely to be fragments of rocky or iron-rich asteroids, which are stronger."
This report was last updated at 5:52 p.m. ET Sept. 24.
In addition to Byrne, Kim Seelos and Kennedy, the authors of the Science paper include Colin Dundas, Alfred McEwen and Ingrid Daubar of the University of Arizona's Lunar and Planetary Laboratory; David Shean, Bruce Cantor, Kenneth Edgett, Tanya Harrison and Liliya Posilova of Malin Space Science Systems; Michael Mellon of the University of Colorado's Laboratory for Atmospheric and Space Physics; Selby Cull of Washington University at St. Louis; Scott Murchie and Frank Seelos of Johns Hopkins University's Applied Physics Laboratory; and Andreas Reufer and Nicolas Thomas of the University of Bern.
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