Sand Dunes
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Sand Dunes and Martian Winds
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In the thin atmosphere of Mars the winds can blow up to 100 km/hr. The dust storms on Mars can be quite large with some engulfing the whole planet. Here is a NASA comparison of a dust storm on Mars to one on Earth over the coast of Africa. Note that the center T-shape of each storm is similar. Click on image or here for larger image.
Without surface water and plants to help keep the dust and sand in place, Martian sand dunes continually travel across the planet. The Planetary Society had a microphone project contained in the Martian Polar Lander so that we could hear the winds and sounds of Mars. Unfortunately the lander failed to land safely.
Shifting sand dunes like those found in the Hebes Chasma area on Mars (left) can be found near Moses Lake and Pasco, Washington.
Whenever winds pick up dust and sand eventually the particles are redeposited as the wind loses its force or velocity. As a small pile of sand slows down the wind more sand is deposited creating a larger pile. This larger obstruction slows the wind further and eventually the pile of sand becomes a sand dune.
Scientists have catagorized the various types of dunes based upon their shape and how they were formed.
Types of Sand Dunes
The size and shape of the dune depends on the ability of winds to pick up and carry sand and the direction the winds blow. The steepness of the sides is determined by the angle where the sand is balanced between falling downslope due to gravity and and the support of underlying sand grains. This angle of repose as it is called is up to 35°. The types of dunes are Barchans, Parabolic, Transverse and Longitudinal. The parabolic dunes at right are near Moses Lake. USGS Aerial Photo.
- Barchans, crescent shaped dunes, are formed by winds blowing in a constant direciton over low areas of sand supply. The points of these dunes are downwind with the steeper and taller middle facing into the wind.
- Parabolic dunes have a crescent shape whose points face into the wind. Often the points are anchored by vegetation and the sand dune is blown inside out. On beaches where sand is abundant, winds are moderate and tend to blow from the same direction you may see blowout parabolic dunes.
- Transverse dunes form in areas where winds blow in a constant direction with an abundant supply of sand. These make long crests and troughs at right angles to the wind and look like sea waves.
- Longitudinal dunes flow in the same direction as the wind due to strong winds and a low sand supply.
The image at left shows the sand dunes of Nili Patera, Syrtis Major. What type of dunes are they?
"This dramatic image shows a field of dark sand dunes in the Nili Patera region of Syrtis Major. The shapes of these dunes indicate that wind has been steadily transporting the dark sand from the right/upper right toward the lower left. This picture was taken on the first day of the Mars Global Surveyor (MGS) Mapping Phase during the second week of March 1999. It shows an area 2.1 kilometers (1.3 miles) wide at the full commanded resolution of 3 meters per pixel. Illumination is from the upper left." Caption and Photo Credit: NASA/JPL/Malin Space Science Systems
The sand dunes in the photo at right are just south of Moses Lake and are used for recreation by those with dune buggies. The sand is black from basalt grains mixed in with white and gray from lakebed silt.
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NASA has a Geomorphology from Space website that has pages discussing Martian Sand Seas using LANDSAT and other images.
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Geology Terms
Here are some basic terms used on this page. Find more geology terms in the Glossary.
- Basalt
- Volcanic rock caused by partial melting of the Earth's crust.
- Erratic
- Large rock or boulder carried by water or glaciers and left behind.
- Geomorphology
- The study of the changes in landforms due to volcanoes,
earthquakes, weather, floods, etc. - Gravel
- Rounded rock fragments larger than sand.
- Loess
- Fine dirt deposited by wind usually from arid or glaciated areas.
- Rain Shadow
- A mountain or mountain range that blocks rain clouds just as an object might block sunlight to form a shadow. Areas in the shadow are more dry as a result.
- Sediment
- Collection of sand, silt, gravel and organic material that sinks to the bottom of a river, lake or ocean. Some or all of these materials may be present.
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