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Mars Global Surveyor
Mars Orbiter Camera

Mars Orbiter Camera (MOC) High Resolution Images:

Evidence for Recent Wind Action on Martian Sand Dunes


Mars Global Surveyor Mars Orbiter Camera Release:          MOC2-72A, -72B, -72C, -72D, -72E
Mars Global Surveyor Mars Orbiter Camera Image ID:         588303947.50805
							   P460-3; and
(A) 50805sub_cntx_1_rel_ICON.gif
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(A) Regional context for MOC image 50805 subframe. The MOC image is shown superposed at the center of a large white box that represents the area shown in more detail in (B), below. The base map is a mosaic of Viking Orbiter 2 images 065b56 and 065b58. The dark rippled surfaces are sand dunes. The bright circular feature is an old impact crater filled with north polar ice deposits. The bright surface in the upper right corner is also covered with ice. The patchy, bright-gray splotches in the lower left are clouds. The big crater is about 32 kilometers (20 miles) across and centered at 77.5°N, 271.1°W. The 10 kilometer scale bar represents a distance of 6.2 miles. North is up, illumination is from the lower right.

(B) 50805sub_cntx_2_ICON.gif
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(B) Local context for MOC image 50805 subframe. Base map is a mosaic of Viking Orbiter 2 images 065b56 and 065b58. The Viking images were taken in October 1976, at a time when the north polar region was in the middle of summer. In summer, the dunes have a low albedo--that is, they are dark. The MOC image was taken in August 1998, when the north polar region was in early spring. At that time, the dunes were relatively bright because they were covered with seasonal CO2 (carbon dioxide) frost. In order to catch these dunes with MOC during the same time of year that Viking saw them, a new image would have to be acquired in June 1999. The MOC team, of course, plans to try this if the ground track crosses this location in 1999. The 10 kilometer scale bar also represents about 6.2 miles. North is up, illumination is from the lower right.

(C) 50805_sub_rel_ICON.gif
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(C) MOC image 50805 subframe. This image was taken on August 22, 1998, and has a resolution of about 2.5 meters (8 feet) per pixel. The dunes and surrounding surface appear bright because they are covered with seasonal frost left over from the northern hemisphere winter that ended in mid-July 1998. The dark spots are areas where some of this frost has begun to sublime away, and/or where wind has exposed dark sand from underneath the frost. Many of the dark spots have thin, dark streaks that radiate in several different directions. The fact that these thin streaks show consistent directions from spot to spot suggest that they were caused by passing winds that mobilized some of the dark sand at each location. The 200 meter scale bar also represents about 656 feet (219 yards). North is toward the upper right, illumination is from the lower left.

(D) 50805_sub3_pos_neg_rel_ICON.gif
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(D) MOC image 50805 subframe taken from the top portion of the image shown in (C). The picture on the right shows the same scene as in the image on the left, except that the brightnesses have been reversed so that dark surfaces appear bright, and bright surfaces appear dark (like a photographic negative). Sometimes it is easier for the human eye to pick out fine details of black-on-white if they are reversed to be white-on-black. Thus, the image on the right is helpful for seeing some of the fine, detailed structure of the dark spots (here they are bright spots) and their associated wind streaks. As noted above, the 200 meter scale bar represents about 656 feet (219 yards). North is toward the upper right, illumination is from the lower left.

(E) 069b32_and_MOC_anot_ICON.gif
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(E) The observation of possible recent wind action on the north polar dunes led the MOC science team to attempt to image the same dune fields more than once, to look for changes. Because the image ground tracks are quite narrow and it is very unusual to pass over the same geographic feature twice within a few months time, only one attempt came close to repeating observations of the north polar dunes. The best attempt is shown here. The dune field, seen as a dark, rippled surface on the floor of a 20 kilometers-wide (12 miles-wide) crater in Viking image 069b32, was covered with bright frost at the time that MOC took pictures of it on Mars Global Surveyor's 460th and 529th orbits. Unfortunately, the two MOC images did not cross on the dune field, but instead crossed near the crater rim. The offset results from differences between the predicted MGS orbit and the actual orbit, and from uncontrolled spacecraft attitude motion. Slight variations between the predicted and actual orbit result largely from variations in the martian gravity field and slowing of the spacecraft as it impinges on the far upper atmosphere of Mars. Variations in attitude arise because the spacecraft's ability to measure its attitude is imprecise on the scale of the MOC images, and so the attitude drifts until a change can be detected, and corrected. The two MOC images are shown here at much reduced resolution, and are shown merely to illustrate the attempt to get repeat coverage of a dune field. The arrows indicate the direction the spacecraft was travelling on orbits 460 and 529, and the dates indicate when each image was obtained.

You may need to adjust the images for the gamma of your monitor to insure proper viewing.

Note: This MOC image is made available in order to share with the public the excitement of new discoveries being made via the Mars Global Surveyor spacecraft. The image may be reproduced only if the image is credited to "Malin Space Science Systems/NASA". Release of this image does not constitute a release of scientific data. The image and its caption should not be referenced in the scientific literature. Full data releases to the scientific community are scheduled by the Mars Global Surveyor Project and NASA Planetary Data System. Typically, data will be released after a 6 month calibration and validation period.

Click Here for more information on MGS data release and archiving plans.


Recognizing that Mars is a desert planet, science fiction writers, scientists, and proponents of Mars exploration have, for decades, written and talked about "The Sands of Mars". The first martian sand dunes were observed by the Mariner 9 spacecraft in 1972. Ever since then, however, it has been unclear as to whether these dunes are active in today's extremely thin martian atmosphere (100 times thinner than on Earth at Sea Level), or if the dunes are the "fossil" remnants of a past epoch when the atmosphere was thicker and sand was more easily transported.

This year, the Mars Orbiter Camera (MOC), onboard the Mars Global Surveyor (MGS) spacecraft, made some key observations that appear to indicate that some martian dunes are active today. In fact, some dunes probably experienced activity--wind blowing the sand around--as recently as mid-1998.

Dunes typically contain granular fragments of rocks and minerals. These grains are usually 0.06 to 2 millimeters (0.002 to .08 inches) in size (which geologists call sand), and they are transported by the wind either by hopping over the ground (a process called saltation) or rolling along the ground (called traction). Images from the Mariner 9 and Viking orbiters of the 1970s did not have sufficient resolution to see detailed patterns of sand movement, although a few Viking images showed faint streaks emanating from a few dune fields; these were interpreted as possible indicators of sand movement.

Mars Global Surveyor has taken many images of martian dunes. Some dunes appear to be inactive and covered with dust. Other dunes, however, show all of the characteristics of fresh, active dunes. The most exciting examples have been found among the dunes in the martian north polar region.

The north polar cap of Mars is surrounded by a zone of dark (i.e., low albedo) dunes. These were first seen by Mariner 9 as a rippled texture, and by Viking as definitive sand dunes. Between late-July and mid-September 1998, the MGS periapsis (closest point in the spacecraft orbit relative to Mars) took the MOC right over the north polar dune fields four times a day. This provided many opportunities to take high resolution pictures of these dunes--resolutions that ranged from 1.5 to 5.0 meters (5 to 16 feet) per pixel.

The very first images of these north polar dunes--one of which was released via the World Wide Web on August 7, 1998--showed that they were coated with thin, bright frost that was left-over from the northern winter season that ended in mid-July. The first images also showed small dark spots along the bases of many of the dunes.

As more and higher-resolution images of the north polar dunes were taken, it became obvious that the dark spots on these dunes were areas where the seasonal frost coating had been removed--either by sublimation or by wind erosion--and that dark material was being exposed from underneath. The dark material was presumed to be the sediment that comprises the north polar dunes.

Some of the dark spots, as in the examples shown here in (A), (B), (C), and (D), above, have thin, dark streaks emanting from them. These dark streaks are interpreted to be the result of wind action. The simplest explanation is that gusts of wind have blown the dark sand out across the frost-covered dunes, creating a streak of deposited sand over the frost. Some spots, as in the image shown here ((C) and (D)), have multiple streaks, each one indicating a different wind gust that moved in a different direction.

Because the frost that covers the north polar dunes can only be a few months old (i.e., northern winter lasted from mid-February 1998 to mid-July 1998), the dark streaks superposed on bright frost are clear indicators that dune material has been moved by the wind within recent months. The image shown here, MOC #50805 ((A-D) above), was taken on August 22, 1998. The streaks emanating from dark patches among the dunes in image 50805 must have formed sometime during 1998, and they most likely formed some time in July and/or August--once spring had begun in the northern hemisphere.

The observation of dark spots and wind streaks among the north polar dunes led the MOC science team to attempt to image the same dunes more than once. If the dunes are indeed active, then it would be possible--it was hoped--to see changes from one image to the next. Such changes could be used to (a) confirm that the dunes are active and (b) estimate the rate at which sand can be transported by wind under martian conditions. Since the MOC was turned off at the end of the Science Phasing Orbits in mid-September 1998, only about seven weeks (late-July to mid-September) were available to try to repeat an observation of a north polar dune field. Only once during this short span of time was there an opportunity to cross a dune field previously observed. The results of the attempt are shown in (E). A north polar dune field on the floor of an old impact crater was crossed by MOC twice--once on July 30, 1998, and again on September 2, 1998. However, it turned out that the two images crossed outside the dune field, near the crater rim. It is quite difficult to image the same location twice with the MOC, because it cannot be pointed in a desired direction---it only "sees" what is beneath it. Minor fluctuations in the spacecraft orbit and attitude--due to variations in the martian gravity field and to upper atmosphere drag and inaccuracies within the attitude control system--led to the offset crossing.

The 1998 observations of the north polar dunes and other dune fields on Mars are quite tantalizing and appear to indicate that many dunes are active under present martian conditions. Confirmation of this result will await the Mapping Phase of the MGS mission, when it should be possible to take additional pictures of the same dune fields already observed by MOC. These new pictures will be compared with the ones from 1998 to see if any changes occurred. The Mapping Phase of the MGS mission is scheduled to commence in late-March 1999, and run for an entire martian year, into March 2001.

The results of the initial MOC study of martian sand dune activity are given in a paper entitled "Activity of Mars Eolian Dunes: Observation of a Low-Albedo Dune Field At High Spatial Resolution by the Mars Global Surveyor Camera," by MSSS Staff Scientist Kenneth S. Edgett and MOC Principal Investigator, Michael C. Malin, presented at the Geological Society of America Annual Meeting on October 29, 1998.

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, CA. 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, CA and Denver, CO.

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