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Mars Global Surveyor
Mars Orbiter Camera

New Gully Deposit in a Crater in Terra Sirenum:
Evidence That Water Flowed on Mars in This Decade?

MGS MOC Release No. MOC2-1618, 6 December 2006

(A) MOC2-1618-a
Two pictures, left shows gullies in a crater as seen in 2001; right shows gullies in the same crater as they appeared in 2005. A new, light-toned (looks white in the enhanced image) deposit formed in one of the gullies before the 2005 image was acquired.
Annotated ImageNo Annotation
(B) MOC2-1618-b
Mosaic of MOC images covering the entire unnamed crater in which the new gully deposit appeared.
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(C) MOC2-1618-c
Larger (expanded) view of the light-toned gully deposit running down through the floor of a gully and then forming a 5 or 6 fingered deposit at the base of the slope.
Annotated ImageNo Annotation
(D) MOC2-1618-d
Four views of the light-toned gully deposit, seen under different sun illumination conditions, showing that the new deposit remains bright under a variety of sun incidence angles.
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NASA/JPL/Malin Space Science Systems

Has liquid water flowed on Mars in this decade?

In June 2000, we reported the discovery, using the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC), of very youthful-appearing gullies found on slopes at middle and high latitudes on Mars. Many examples were presented in our captioned web releases and in a paper published in the journal, Science. Since that time, tens of thousands of gullies have been imaged by all of the Mars orbiting spacecraft: Mars Global Surveyor, Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter.

During the years since the original June 2000 report, the MGS MOC was used to test the hypothesis that the gullies may be so young that some of them could still be active today. The way the test was conducted was very simple: re-image gullies previously seen by MOC and see if anything changed.

In two cases, something changed. One of those cases is presented here. The other is detailed in an accompanying release, “New Gully Deposit in a Crater in the Centauri Montes Region”.

When new MOC images are received on Earth, the science operations team at Malin Space Science Systems (MSSS) examines each one, to see if there is anything noteworthy. Taken directly from MSSS Staff Scientist Ken Edgett’s notes, here is what was said when a particular image of martian gullies arrived in April 2005:

       Goal of the observation was to re-shoot the odd, very thin bright "apron" below 
       gullies near s=630, l=1655 of E11-03412 (also seen in E14-00243); we did get 
       that bright feature again and it looks like it probably has not changed -- 
       however, something else DID change-- in the S05 image between lines 3610-3740, 
       samples 1035-1310, there is a "bright gully" with small bright "apron"-- 
       this feature is not bright in the E11 image (and out of the frame in the 
       E14 image)-- this feature MIGHT indicate that new material (ice? fines?) 
       was transported through this gully since the E11 image was acquired.
       (K. S. Edgett, 26 April 2005)

A gully on the wall of an unnamed crater in Terra Sirenum, at 36.6°S, 161.8°W, was initially imaged by MOC on 22 December 2001 (A; top, left). It showed nothing noteworthy at the location where a change would later be observed, but a group of nearby gullies exhibited an unusual patch of light-toned material. As part of our routine campaign to re-image gully sites using the MOC, another image of this location was acquired on 24 April 2005 (image S05-01463). A new light-toned deposit had appeared in what was otherwise a nondescript gully (A, top, right). This deposit was imaged again by MOC on 26 August 2005, at a time when the Sun angle and season were the same as in the original December 2001 image, to confirm that indeed the light-toned feature was something new, not just a trick of differing lighting conditions. In August 2005, the feature was still present.

(A) The first figure (top, above) shows a comparison of the gully site as it appeared in MOC image E11-03412 on 22 December 2001 (left) with a mosaic of 2 MOC images acquired after the change occurred: S09-02603 (26 August 2005) and S10-01184 (25 September 2005). Sunlight illuminates each scene from the northwest (top left); the 150 meter scale bar represents 164 yards.

(B) The second figure (middle, left) is a mosaic of MGS MOC images that cover the entire unnamed crater in Terra Sirenum. The location of the light-toned gully deposits, old and new, are indicated. This is a mosaic of images acquired by MOC in 2005 and 2006; the 500 meter scale bar equals ~547 yards.

(C) The third figure (middle, right) shows an enlargement of a portion of MOC image S09-02603, from August 2005, showing details of the new, light-toned gully deposit. The new material covers the entire gully floor, from the point at which the gully emerged from beneath a mantled slope, down to the spot at which the channel meets the crater floor. At this break in slope, the gully material, as it was emplaced, spread out into five or six different fingers (digitate as in digits ). The 75 meter scale bar represents a distance of about 82 yards.

(D) To confirm that the new, light-toned gully deposit is not just a trick of changing illumination conditions as the Sun rises to different levels in the sky each season, the MOC team repeatedly imaged this site throughout 2005 and 2006. Four examples are shown here, acquired in April 2005, August 2005, February 2006, and April 2006. The “i=” indicates solar incidence angle—that is, the height of the Sun in the local sky, relative to a case where the Sun would be directly overhead (0°). Thus, the higher the incidence angle, the lower the Sun would appear in the sky to an observer on the ground.

These MOC images show that a material flowed down through a gully channel, once, between December 2001 and April 2005. After the flow stopped, it left behind the evidence—the light-toned deposit—that it occurred. The deposit is thin; thin enough that its thickness cannot be measured in MOC’s 1.5 meters-per-pixel images. However, it does exhibit a digitate termination, suggesting that the material flowed in a fluid-like manner down the ~25° slope before splaying out into multiple small lobes when the slope suddenly dropped to near zero where the crater wall meets the crater floor. This deposit, and a similar one in a crater in the Centauri Montes described in an accompanying release, together suggest that the materials involved were low-volume debris flows containing a mixture of sediment and a liquid that had the physical properties of liquid water. In this case, we propose that the water came from the subsurface, emerged somewhere beneath the mantle covering the original crater wall, and then ran down through a previously-existing gully channel. No new gully was formed; an old one was re-activated.

The light tone of the new gully deposit, and that of the older, neighboring gullies, is intriguing. We cannot know from these images whether the light tone indicates that ice is still present in and on the surface of the deposit. Indeed, ice may not be likely—under present conditions on the surface of Mars, it would be expected to have sublimed away fairly shortly after the new deposit formed. However, the light-toned material could be frost that forms and re-forms frequently as trapped water ice sublimes and “exhales” from within the deposit. Alternatively, the light-tone may result if the deposit consists of significantly finer grains (e.g., fine silt) than the surrounding surfaces, or if the deposit’s surface is covered with minerals such as salts formed as water evaporated from the material.

Do these images prove that water has flowed on Mars? No, they cannot. However, they provide the first very tantalizing evidence that this may have occurred. While the surface environment on Mars is extremely dry, drier than the most arid deserts on Earth, liquid water from beneath the martian surface may have come out of the ground and flowed across this little portion of the red planet, in this decade.

Accompanying captioned releases regarding gullies on Mars:

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Malin Space Science Systems and the California Institute of Technology built the MOC using spare hardware from the Mars Observer mission. MSSS operates the camera from its facilities in San Diego, California. The Jet Propulsion Laboratory's Mars Surveyor Operations Project operates the Mars Global Surveyor spacecraft with its industrial partner, Lockheed Martin Astronautics, from facilities in Pasadena, California and Denver, Colorado.