 |
 |
||
 |
|
||
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
||||||||
 |
Planetary News: Mars (2006)Mars Global Surveyor Discovers Current Liquid Water Activity on MarsMOC Images Also Reveal New Impact CratersBy Emily LakdawallaDecember 6, 2006
Newly released images from Mars
Global Surveyor contain telltale deposits
left behind by liquid water flowing on the surface within the few years that
the spacecraft surveyed Mars. Scientists had previously announced the
discovery of features that must have been carved by water within the last
several million years, but this is the first evidence that water has flowed
on Mars' surface while humans have been studying it. "Ten years
ago, Mars scientists were talking about water billions of years ago. Five
years ago, [Mike Malin and Ken Edgett] were talking about water millions of
years ago. I think now we can honestly talk about liquid water on the
surface of Mars today. And that revolution in our thinking truly has
changed how we view Mars and how we should think about exploring Mars," said
scientist Phil Christensen at a press conference held today at NASA Headquarters. The discoveries were made as part of a campaign to monitor curious light-toned deposits that had been observed on the walls of craters in the mid-latitudes of Mars (from 30 to 60 degrees north and south latitude, as differentiated from the equatorial and polar regions). "With the multiple extensions of Mars Global Surveyor's mission," remarked Malin Space Science Systems scientist Ken Edgett, "we have been able to image gullies repeatedly, the same ones over and over again. In doing that we found something that Mike [Malin] and I didn't expect, but we were looking for just in case it might happen." Edgett and Malin suspected that the light-toned deposits had formed geologically recently, and that therefore, if they kept watching long enough, they might find that the appearance of the deposits evolved over time. It would seem they were also hoping, but not expecting, to spot a new deposit after it formed. Their hope became reality last year, when they spotted two newly brightened gully floors on the walls of craters in the southern hemisphere. Their discoveries were published today in the journal Science. "This is material that flowed down a slope," Edgett said. "As it encountered little obstacles, in some cases it flowed around those, and in other cases it appears that part of that flow branched off and diverted away from the main body of the material. Where that channel met the crater floor, it broke up into five little fingers."
The MOC images clearly demonstrate that these features formed in the last few years, while Mars Global Surveyor has been in orbit at Mars. But how do they demonstrate that liquid water was involved? Edgett stated three lines of evidence: their geological context, their morphology, and their brightness with respect to their surroundings. "The context is, these are in gullies. People have been talking for six and a half years about what could form gullies and what could flow through gullies, and, by and large, the consensus is liquid water. It could be acidic water, it could be briny water, it could be water carrying sediment, it could be slushy, but water is involved." This is in contrast to the consensus opinion for the formation mechanism of another currently forming feature on Mars, the so-called slope streaks. Slope streaks are interpreted to be scars left on slopes by an essentially dry process of dust avalanching. "These things are very far away from regions where dry dust avalanches occur -- they occur in a region where those things are not found," Edgett said. Additionally, Edgett continued, the light-toned gullies have the shape or morphology one would expect from features that formed as a result of the flow of a liquid. "They flow around obstacles, they break off at the end into little digits, little fingers as they're flowing down the slope. Those things indicate a very fluid material." But most compelling, Edgett said, was the brightness of the features. "A light-toned flow-like feature is something that is very hard to make on Mars. When we disrupt dust on the surface of Mars, the surface usually darkens. When a rover drives across the surface of Mars and you look at it with an orbiter, you see a dark track that the rover left behind. When Viking dug a trench, the trench was darker. When an avalanche of dust comes down a slope, the new [avalanche scars] are always dark. They fade or brighten over time, but the new ones are always dark. And, when you get a new impact crater, those are also dark. The only thing we know from the 240,000 Mars Global Surveyor images that brightens, other than these, in a short period of time, is seasonal frost, which comes and goes. These are not coming and going; they came, and they are still there." It's not clear, however, what the light-toned material is that has been deposited on the floor of these two gullies, as Edgett explained. "This could be frost from the original water that came down the slope. But these features have persisted for several years now; they're still bright. So we suspect it's not likely to be frost." Frost on Mars is a seasonal feature; the warmer temperatures of the Martian summer would likely cause a frost to sublimate. However, it is also possible that a frost could be continually replenished from moisture seeping out of the interior of the deposit. "An alternative is that it's minerals that were precipitated from the water -- in other words, salts. The other option is that you're actually seeing fine grains. If this thing was flowing down a slope and the coarser sediments settled to the bottom of the flow and the finer sediments rose to the top, then that would also make these appear bright." "This is a remarkable set of new observations that show, once again, how dynamic and active the surface of Mars is," says Phil Christensen. "Clearly, something unusual and exciting goes on in the midlatitudes. And this activity, particularly the fact that it's going on today, to me says that the midlatitudes of Mars are one of the more remarkable places in the solar system. We've seen evidence for ancient water at the equator; but that region of Mars is quite dry today. We see lots of evidence for ice and frozen water in the poles, and the Phoenix mission will explore that. But those regions are cold today. The midlatitudes, with their snowpacks and their aquifers that release water into these gullies, I think are really tremendous places where we really ought to think about focusing some exploration." "If Malin and Edgett are right -- if there really is liquid water close to and even (for a short time) on the surface of Mars today -- then we have to figure out a way to get to these places," states Jim Bell, lead scientist on the Pancam instruments on the Mars Exploration Rovers, and a member of The Planetary Society's Board of Directors. "As hard as it is to believe, the search for life on Mars just became even more compelling. These Martian oases could be the best places on the planet to look for evidence of life on Mars today." Spend Long Enough Looking, and You're Bound to See Something ChangeAt the same press conference, Mike Malin, lead scientist for Mars Global Surveyor's camera systems, unveiled new sets of "before" and "after" shots of newly formed small impact craters. "We had not anticipated, though we probably should have, that we could actually see craters forming," Malin said. "Given the extended duration of the Mars Global Surveyor mission, up to nine years, we could have hypothesized that we might be able to see them, but we didn't. So this was a completely serendipitous discovery." Malin stated that as a result of a systematic survey, they had found a total of 20 new craters in images covering the 30 percent of Mars that they were able to photograph twice. Two of the areas were even imaged twice by the highest-resolution camera, permitting detailed analysis of how the surface had been changed by the asteroid impact.
Malin Space Science Systems released today a gallery of the newly formed impact craters. A particularly beautiful example is shown at right. Malin explains the image. "What you see are these dark bands, dark rays of materials. This is caused by the interaction of the shock wave being dragged by the impacting object as it moves through the atmosphere. That shock wave interacts with the surface, then it hits the ground and sends out a big blast, and that's a big wave that moves out, and then that blast collapses and there's an in-rushing wind as well. That's what's caused all these very beautiful patterns, that are basically a light surface of dust that's been blown away by the blast of the impact. You can also see to the lower left, a series of small, much darker spots. Those are where ejecta from the impact came flying out of the hole and formed a string of secondary craters." The new craters are not just a curiosity. They yield important clues for understanding the space environments of both Mars and Earth, as Malin explains. "We've found 20 craters in about 30 percent of the planet that we were able to re-photograph in about five years. So that averages out to about 12 craters per year forming across the surface of Mars. If you were to live on Mars for about 20 years, you would live close enough to one of these events to hear it. That raises the question, is this a hazard for astronauts that we have to consider? The answer is probably yes. There is some hazard, it's probably a low hazard, but it's one we'd have to think about, in terms of these objects hitting Mars at a fairly substantial rate. Finally, this work is also applicable in understanding the hazards of large impacts to Earth, because it validates models that we have used to take our lunar and terrestrial impact crater history, spread out over billions of years, and collapse that down to being able to make some statement about the cratering rate over the last few years, and potentially into the future." The timing of today's announcements of fresh gullies and craters on Mars as discovered in Mars Global Surveyor images is poignant, because Mars Global Surveyor has not been heard from in a month and is likely lost after nearly nine continuous years of operations at Mars. "There could be a miracle, it could come back to life, but most people on the project have, I think, pretty much accepted that we're not going to hear from the spacecraft again," Christensen remarked Monday on The Planetary Society's Planetary Radio show. Today's announcement drives home the message that, despite its age, Mars Global Surveyor was still performing its duty to survey Mars and make new discoveries that were enabled, in part, by its longevity. Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter will carry on its work, but cannot replace it. |
|||||||||||
 |
 |
|||||||||||
[ what is this? ]