All About Dunes
Why Study Dunes on Mars?
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.
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.
The Mars Global Digital Dune Database (MGD3)
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.org 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.
|Dune Field Attribute Table|
|Attribute Name||Attribute Description|
|Dune Field Lon+Lat ID||ID number based on dune field centroid latitude and longitude.|
|Dune Field Longitude (East)||Position of dune field centroid in decimal degrees East longitude.|
|Dune Field Latitude (aerocentric)||Position of dune field centroid in decimal degrees latitude (aerocentric).|
|Dune Type||Dune type as described in McKee, 1979.|
|Confidence||Indicates our confidence level in identifying the feature as a dune (scale 1 to3).|
|Image Types Used||Types of images used in building database, (THEMIS IR, THEMIS VIS, and/or MOC NA).|
|Dune Field Area (km2)||Area of dune field polygon in km2, calculated in Sinusoidal projection.|
|Mean Dune Height by Type (m)||Mean dune height calculated by averaging groups of dune fields with similar dune types present.|
|Volume Method 1||Volume estimate of dune field in km3 (automated method using MOLA 128 gridded elevation raster).|
|Volume Method 2||Volume estimate of dune field in km3, (area * mean dune height).|
|Dune Field Average Elevation (m)||Average elevation of dune polygon (MOLA, 463 m/px)|
|Environment||Divides dune fields into 2 main groups, those within craters, C, and those outside craters, N. A second descriptor is added for those in Hellas basin, H, those in Argyre basin, A, and those in Valles Marineris, VM.|
|Mars 5M Chart||Mars Chart (1:5 million) number for quadrangle in which dune field is located.|
|IR Images||THEMIS IR images used to locate dune.|
|VIS Images||THEMIS VIS images used to classify dune.|
|MOC Images||MOC NA images used to classify dune and measure slipfaces.|
|Additional attributes when
Slipfaces were measured
|Slipface ID||The Dune_lat_lon_ID with a, b, c or d appended when multiple averages are calculated for a single dune field. This occurs when winds are multidirectional.|
|Raw Slipface Azimuth||Azimuth of individual digitized slipfaces given in decimal degrees.|
|Slipface 1 Azimuth||Average of raw slipface measurements in the direction with largest number of raw slipfaces.|
|Slipface 1 Count||Number of raw slipfaces used to calculate the Slipface 1 Azimuth.|
|Slipface 2 Azimuth||Average of raw slipface measurements in the direction with second largest number of raw slipfaces.|
|Slipface 2 Count||Number of raw slipfaces used to calculate the Slipface 2 Azimuth.|
|Slipface 3 Azimuth||Average of raw slipface measurements in the direction with third largest number of raw slipfaces.|
|Slipface 3 Count||Number of raw slipfaces used to calculate the Slipface 3 Azimuth.|
|Slipface 4 Azimuth||Average of raw slipface measurements in the direction with fourth largest number of raw slipfaces.|
|Slipface 4 Count||Number of raw slipfaces used to calculate the Slipface 4 Azimuth.|
|Additional attributes when
dune field occupies crater
|Crater “BarlowID”||ID number based on crater centroid latitude and longitude|
|Crater Area (km2)||Area of crater polygon in km2, calculated in Sinusoidal projection.|
|Crater Diameter (km)||Diameter of crater polygon in km, calculated based on area.|
|Crater centroid to Dune centroid Azimuth||Azimuth is calculated for polylines that extend from crater centroid to dune centroid.|
|GCM Attribute Table|
|Attribute Name||Attribute Description|
|Solar Longitude (Ls)||The position of Mars relative to the Sun measured in degrees from the vernal equinox (start of northern Spring).|
|UDT||Universal Daylight Time is local time at the Mars prime meridian.|
|LMT||Local Mean Time is the local time on Mars relative to a division of the Martian day into 24 equal parts.|
|GCM_Longitude_East||The position of the GCM grid point in decimal degrees East longitude.|
|GCM_Latitude_aerocentric||The position of the GCM grid point in decimal degrees latitude (aerocentric).|
|Wind_stress||GCM model output wind stress in Newtons/meter2.|
|Wind_velocity||GCM model output wind velocity in meters/second.|
|Wind_Azimuth||GCM model output wind azimuth in decimal degrees.|