Sand dunes are among the most widespread aeolian features present on Mars, serving as unique indicators of the interaction between the atmosphere and surface. On a planetary body, dunes accumulate where a supply of sand-sized grains exists or may be abraded, is carried downwind by winds of saltation strength, and is subsequently deposited where these winds weaken below the threshold for sand transport. As a result, the study of dune processes contributes to both atmospheric and sedimentary science. Both the presence and morphology of sand dunes are sensitive to subtle shifts in wind circulation patterns and wind strengths, which are thought to be influenced by changes in Martian orbital parameters. The spatial distribution of aeolian sand relates to patterns of sedimentary deposition and erosion of source materials, giving clues to the sedimentary history of the surrounding terrain. Dunes are particularly suited to comprehensive planetary studies in part because they are abundant on the Martian surface over a wide range of elevations and terrain types, and in part because they are large enough to be studied using the wide suite of spacecraft data now available. Thus a global scale study of Martian dunes serves a dual purpose in furthering understanding of both climatic and sedimentary processes, two fundamental topics currently driving Martian science.
This consortium and website were put together to facilitate exchange of information that pertains to Martian dune studies. Currently the consortium is made up of members from the USGS Astrogeology Team, Arizona State University, NASA Ames Research Center, and Planetary Science Institute.
Previous aeolian studies of the Martian surface relied on Mariner 9 and Viking Orbiter images to examine and map aeolian morphologies. More recent studies, using high-resolution images like Mars Global Surveyor (MGS) Mars Orbiter Camera narrow angle (MOC NA) and Mars 2001 Odyssey Orbiter Thermal Emission Imaging System (THEMIS) visible range images (VIS), have enabled scientists to re-examine surficial areas from earlier investigations and see new aeolian deposits unresolved by previous instruments. As a result of the influx of high resolution data, the Martian stratigraphic column is undergoing rapid evolution as are the interpretations of much of Mars' geologic history, contributing to new insights about Martian aeolian processes and relationships. Surface images from both orbiting spacecraft (e.g., from MGS MOC NA) and Mars Exploration Rovers (MER) demonstrate how ubiquitous erosional and depositional features of aeolian origin are on the Martian surface.
Current releases of THEMIS infrared (IR) images (100 m/px resolution) provide nearly complete coverage of the Martian surface. As such, these images can robustly serve as a basis for a planet-wide inventory of moderate to large-scale dune deposits. Within a global context, dune forms and regional distributions can easily be compared to global datasets (e.g., Mars Orbiter Laser Altimeter (MOLA) elevations and Thermal Emission Spectrometer (TES) derived thermal inertia values) and models (e.g. General Circulation Models (GCMs)) to provide a better understanding of the planet-wide processes that have shaped the Martian surface.
The digital dune database makes it possible to look at dunes in a global context, comparing their geographic location and attributes to other global coverages, such as geologic maps, GCMs, MOLA and TES. Such comparisons provide significant perspective on local, regional, and global-scale aeolian processes that have shaped and continue to influence the surface of Mars.
The Mars Global Digital Dune Database has been one of the primary driving factors for the Mars-Dunes Consortium.
The MGD3 includes seven major data layers: 1) The Dune Field feature class (polygon) includes ~550 dune fields on Mars between +65 and -65. 2) The Crater feature class (polygon) includes ~ 400 craters on Mars between +65 and -65 that are occupied by dune fields in the database. 3) and 4) The Crater centroid to Dune centroid Azimuth feature class (polyline and point versions) is based on polylines that extend from crater centroid to the centroid of a dune field within the crater on Mars between +65 and -65. 5) The Raw Slipface feature class (polyline) includes >10,000 polylines that were digitized on slipfaces, based on gross morphology of dunes, to represent wind direction responsible for that morphology. 6) The Average Slipface Azimuth feature class (point) was created by averaging raw slipface azimuths for the ~ 200 dune fields in which measurements were possible. Dune fields with multidirectional winds have more than one average, resulting in ~270 average slipface azimuths. 7) The GCM feature class (polyline) represents output from the Ames Mars General Circulation Model (GCM) for the area from +70 to -70. Only output records with a shear stress value > .0225 N/m2 are included.
MGD3is complete for the +65 to -65 portion of Mars. The distribution of the dunes can be seen in the above map of that region (top). The database is in progress for the -65 to -90 portion of Mars. The distribution of possible dunes is shown in the table below. Based on possible dunes, the total dune field area in the -65 to -90 area is estimated to be about 50,000 km2. The map of +65 to +90, above right, shows the possible location of dunes. An areal estimate of dune fields is not yet available for that region.
| Latitudinal Region | Number of Dune Fields | % of Total Number of Dune Fields | Area of Dune Fields (km2) | % of Total Area |
|---|---|---|---|---|
| 30° to 65° N | 9 | 1.6 | 1507 | 2.2 |
| 0° to 30° N | 36 | 6.6 | 5165 | 7.4 |
| 0° to 30° S | 37 | 6.8 | 13,275 | 19.0 |
| 30° to 65° S | 465 | 85.0 | 49,803 | 71.4 |
| Total | 547 | 100 | 69,750 | 100 |
The following table summarizes the geographic distribution of the dunes in the database, (+65 to -65) by latitudinal band. Remember that only moderate to large dune fields are included in this database. The distribution of smaller dune fields may not follow this pattern.
The following table summarizes the quantity of dune fields in craters by number of dune fields and by area of dune fields for the Latitudinal bands and for the Argyre, Hellas and Valles Marineris regions.
| Latitudinal Region | Number of Dune Fields | Area of Dune Fields (km2) | Number of Dune Fields in Craters | Area of Dune Fields in Craters (km2) | % of Dune Fields in Craters (by area) |
|---|---|---|---|---|---|
| 0° to 30° S | 37 | 13,275 | 17 | 4326 | 32.6 |
| 30° to 65° S | 465 | 49,803 | 368 | 39,990 | 80.3 |
| Argyre and surrounding rough terrain | 47 | 5347 | 12 | 2453 | 45.9 |
| Hellas and surrounding rough terrain | 18 | 1310 | 2 | 87 | 6.6 |
| Valles Marineris | 20 | 8949 | 0 | 0 | 0 |
THEMIS IR images were chosen because they provided planet-wide coverage of Mars at a resolution (100 m/pixel) capable of revealing many dune features. Due to small particle size, dunes have a lower thermal inertia than surrounding rock, accompanied by a large diurnal temperature change. The dunes are relatively warm in the daytime images, appearing light in tone, making them easy to detect. In nighttime images the pattern reverses with the relatively cool dunes appearing dark.
The higher resolution THEMIS VIS images are used, when available, to verify that the features are dunes and to classify the dune types.
Because of the higher spacial resolution, MOC images are used to identify and analyize slip faces and dune types.
Though HiRISE images have not yet been used on the MGD3, these images shoudl offer even higher resolution that will better identify dune types, and should be able to show improved detail within the dunes.
CRISM images, like HiRISE, have not yet been used for the MGD3. Also like HiRISE though, CRISM will offer greater detail and a better understanding of the dunes on Mars.
