Images suggest water still flows in brief spurts on Mars
Thu Dec 7, 2006 at 13:45 UTC
NASA photographs have revealed bright new deposits seen in two gullies on Mars that suggest water carried sediment through them sometime during the past seven years. "These observations give the strongest evidence to date that water still flows occasionally on the surface of Mars," said Dr. Michael Meyer, lead scientist for NASA's Mars Exploration Program, Washington.
Liquid water, as opposed to the water ice and water vapor known to exist at Mars, is considered necessary for life. The new findings heighten intrigue about the potential for microbial life on Mars. The Mars Orbiter Camera on NASA's Mars Global Surveyor provided the new evidence. The deposits appear in images it took in 2004 and 2005 but not in a 1999 image of one site or a 2001 image of the other site.
- New Gully Deposit in a Crater in Terra Sirenum
- New Gully Deposit in a Crater in the Centauri Montes Region
- Other Martian Gullies with Light-Toned Deposits
The atmosphere of Mars is so thin and the temperature so cold that liquid water cannot persist at the surface. It would rapidly evaporate or freeze. Researchers propose that water could remain liquid long enough, after breaking out from an underground source, to carry debris downslope before totally freezing. The two fresh deposits are each several hundred meters, or yards, long.
The light tone of the deposits could be from surface frost continuously replenished by ice within the body of the deposit. Another possibility is a salty crust, which would be a sign of water's effects in concentrating the salts. If the deposits had resulted from dry dust slipping down the slope, they would likely be dark, based on the dark tones of dust freshly disturbed by rover tracks, dust devils and fresh craters on Mars.
The light-toned deposits that formed in two gully sites on Mars during the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) mission in the 1999 to 2005 period are considered to be the result of sediment transport by a fluid with the physical properties of liquid water. The young, light-toned gully deposits were found in a crater in Terra Sirenum and in a crater east of the Hellas basin in the Centauri Montes region.
In their study of how the light-toned gully deposits may have formed, the MOC team considered their resemblance to light- and dark-toned slope streaks found elsewhere on Mars. Slope streaks are most commonly believed to have formed by downslope movement of extremely dry, very fine-grained dust, through processes thought by some to be analogous to terrestrial snow avalanche formation. The four images below (Figure A-D) shows the light-toned gully deposit in the crater in the Centauri Montes region, compared with typical slope streaks and several examples in which the slope streak gives a strong impression of having been produced by fluid-like flow.
Image Credit: NASA/JPL/Malin Space Science Systems
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Figure A.
Image Credit: NASA/JPL/Malin Space Science Systems
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Figure B.
Image Credit: NASA/JPL/Malin Space Science Systems
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Figure C.
Image Credit: NASA/JPL/Malin Space Science Systems
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Figure D.
(A) Light-toned gully deposit that formed between August 1999 and February 2004 in a crater in the Centauri Montes region of Mars.
(B) Typical dark slope streaks, as seen in a valley in central Arabia Terra. Hundreds of MOC images have shown that the darkest streaks are the youngest; commonly, the darkest streaks are less than a few years old.
(C) Dark slope streaks on a slope surrounding a pedestal crater in Tikhonravov Crater in Arabia Terra. These streaks, particularly the two near the center of the image, show evidence that the downslope movement of debris was diverted around obstacles, particularly large boulders.
(D) Dark slope streaks, including some that diverted around obstacles, on knobs in the Aeolis region of Mars.
Image Credit: NASA/JPL/Malin Space Science Systems
High resolution image
As shown in this image, slope streaks can be lighter or darker than their surroundings. The majority of them are dark, and nearly always when light streaks are present, dark ones are nearby. This observation differs from the two light-toned gully deposits observed in Terra Sirenum and the Centauri Montes regions. In the case of the light-toned gully deposits, no dark slope streaks are present anywhere within the craters that they formed. As far as can we can tell, based on MGS MOC and Mars Odyssey Thermal Emission Imaging System (THEMIS) coverage, there are no dark slope streaks within hundreds of kilometers of the two gully deposits.
Image Credit: NASA/JPL/Malin Space Science Systems
High resolution image
This image shows that light and dark slope streaks occur in very specific regions (brown-shaded areas; particularly Tharsis, Amazonis, and near Elysium), and the gullies occur in different regions (pink-shaded areas; particularly middle northern and southern latitudes). In other words, the two fresh, light-toned gully deposits observed by MOC occur very far from the regions in which light and dark slope streaks are found. Indeed, the regions in which slope streaks occur are those that are most thickly covered with mantles of dust. Experience from driving the Sojourner and Mars Exploration Rover, Spirit, on dusty surfaces shows that when the uppermost coating of fine, bright dust is disturbed, the underlying surface is darker. Thus, dark slope streaks are considered to be areas where dust has been disturbed and slid downhill by dry, granular flow. The dust is extremely fine, like the flour used to bake bread.
The location of the two craters in which MGS MOC observed new, light-toned gully deposits are indicated. Except for a small portion of Tempe Terra (northeast of Tharsis), gullies and slope streaks do not occur together in the same regions. Slope streaks occur in regions that have been known since the Viking orbiter missions to be thickly mantled with dust. Gullies occur at middle and high latitudes, where dust mantles are not so thick. The pink-shaded gully area is based upon the MOC team's survey of over 96,000 MOC narrow angle camera images. The brown-shaded area, representing light and dark slope streak occurrences, comes from a combination of the MOC team's assessment and the published results of O. Aharonson, N. Schorghofer, and M. F. Gerstell (2003) Slope streak formation and dust deposition rates on Mars, Journal of Geophysical Research, v. 108, no. E12, doi:10.1029/2003JE002123.
Image Credit: NASA/JPL/Malin Space Science Systems
High resolution image
New dark slope streaks are forming on Mars all the time. MGS MOC images have captured hundreds of before-and-after examples. The MOC narrow angle camera has acquired hundreds of images that repeat locations where slope streaks occur. In so doing, MOC captured hundreds of cases where new slope streaks formed. New streaks are always dark, not light. Several examples on hillslopes in the Elysium/Cerberus region are shown here, spanning a period from April 1998 through January 2005.
Image Credit: NASA/JPL/Malin Space Science Systems
High resolution image
New slopes streaks are always dark. No new light slope streaks have been observed by MOC, despite monitoring over the past 7-9 years at dozens of locations. Curious as to whether new light streaks form under present conditions, just as new dark streaks do, the MOC team monitored dozens of light slope streak sites. But no new light slope streaks have been observed. This time series includes images from 2000, 2003, and 2005; no new streaks formed during this period.
Image Credit: NASA/JPL/Malin Space Science Systems
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Figure E.
Image Credit: NASA/JPL/Malin Space Science Systems
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Figure F.
Not only have no new light slope streaks been observed, dark streaks have been seen to superpose (i.e., formed or were deposited on top of) light streaks. This observation reinforces the view that light slope streaks are not formed by the downslope movement of dust -- when dust moves, it creates a dark streak. Light streaks might therefore be old dark streaks, perhaps trapping light-toned dust better than the adjacent, intermediate-toned slopes.
(E) Light and dark slope streaks on the wall of an unnamed valley in central Arabia Terra. Dark streaks in some cases superpose light streaks.
(F) Light and dark slope streaks in Arabia Terra in an example where a new, large dark streak formed sometime between January 2001 and August 2003. The new dark streak formed on top of older light slope streaks.
The MGS MOC team's experience, gained from examining all of the more than 96,000 MOC narrow angle camera images -- and seeking evidence for changes among gullies, slope streaks, and other variable features on the planet's surface -- is that slope streaks form on very dusty slopes, the newest ones are dark, and older ones lighten with time as new dust is deposited on them. The origin of light slope streaks is not known, although we suspect they are related to dark slope streaks. Dark slope streaks may be dark because they have lost their light dust cover or because disturbed surfaces often have micro-relief that casts miniature shadows on the surface. Slope streaks that are lighter than their surroundings may have trapped newly-fallen dust within this microrelief, and may do so more effectively than adjacent, smooth slopes.
Slope streaks have attributes that indicate they experienced fluidized motion, but the fluid was less viscous (more fluid) than water-lubricated dirt. As with snow avalanches, the "fluid" was probably atmospheric gas ingested and intimately mixed with the dust as it began and continued to move downslope. Such air-fluidized flows stop more abruptly than do water-fluidized flows on comparable slopes.
In developing geologic interpretations from inspection of images, many factors are considered: size, relief, shape and pattern, color or brightness, texture, and association (context). Context is often the final discriminator between features that look similar. It is undeniable that the gully deposits resemble slope streaks, but they do not share the same context. If they are slopes streaks formed by downslope movement of dry, unconsolidated dust, then they are extremely rare features, because the new gully deposits (a) do not occur in regions where slope streaks occur, (b) are not found near any dark slope streaks, while typical light slope streaks have dark ones nearby, and (c) formed during the MGS MOC mission, while no new light slope streaks were observed to have formed anywhere else on the planet during the mission. Conversely, their presence within craters with gullies and the existence of similar light-toned features on other gullied (and in some cases on adjacent) slopes, with which they share geomorphic attributes, may be coincidental but is probably not. The gullies themselves provide the context for the gully deposits, and argue for a genetic relationship.
Mars Global Surveyor has discovered tens of thousands of gullies on slopes inside craters and other depressions on Mars. Most gullies are at latitudes of 30 degrees or higher. Malin and his team first reported the discovery of the gullies in 2000. To look for changes that might indicate present-day flow of water, his camera team repeatedly imaged hundreds of the sites. One pair of images showed a gully that appeared after mid-2002. That site was on a sand dune, and the gully-cutting process was interpreted as a dry flow of sand.
Today's announcement is the first to reveal newly deposited material apparently carried by fluids after earlier imaging of the same gullies. The two sites are inside craters in the Terra Sirenum and the Centauri Montes regions of southern Mars.
"These fresh deposits suggest that at some places and times on present-day Mars, liquid water is emerging from beneath the ground and briefly flowing down the slopes. This possibility raises questions about how the water would stay melted below ground, how widespread it might be, and whether there's a below-ground wet habitat conducive to life. Future missions may provide the answers," said Malin.
Besides looking for changes in gullies, the orbiter's camera team assessed the rate at which new impact craters appear. The camera photographed approximately 98 percent of Mars in 1999 and approximately 30 percent of the planet was photographed again in 2006. The newer images show 20 fresh impact craters, ranging in diameter from 2 meters (7 feet) to 148 meters (486 feet) that were not present approximately seven years earlier. These results have important implications for determining the ages of features on the surface of Mars. These results also approximately match predictions and imply that Martian terrain with few craters is truly young.
- New Craters
- Procedure for Finding New Impact Sites on Mars Using the Mars Global Surveyor Mars Orbiter Camera
- New Impact Crater in Arabia Terra
- Before-and-After Look at Impact Craters
- Second of Two Fresh Impact Crater Sites With "Before" and "After" Narrow Angle Mars Orbiter Camera Images
- Fresh Crater in Arabia Terra with Light-Toned Ejecta
- New Impact Crater Formed Between April 2001 and December 2003
NASA's Jet Propulsion Laboratory, Pasadena, manages the Mars Global Surveyor mission for the NASA Science Mission Directorate, Washington.
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