The star dunes of Mars
Surface of Mars covered by constantly shifting sand blown by the planet’s winds
No, this isn't the title of a new sci-fi novel, although it would certainly make an awesome title for a book and maybe a movie. Star dunes are a form of sand dune, wind-blown accumulations of sand, some of which can reach enormous size and cover vast areas.
Both Earth and Mars have star dunes, as well as other types of sand dunes, as does Saturn's largest moon, Titan. It is quite possible that, as we explore other moons in our solar system, we might discover some of them have sand dunes as well.
FORMATION
First of all, regardless of where they are located, sand dunes are formed by the interaction of wind and topography with a quantity of sand particles. There are typically four basic types of sand dunes, each dependent on how, and to some degree, where, they are formed. Barchan dunes, also called crescentic dunes, are crescent or half-moon shaped dunes that require a flat landscape, winds blowing from only one direction and a limited amount of sand. This rare type of dune, at least on Earth, points against the wind, has steep faces (the leading face of the dune), and shallow, elongated trailing sides, often resembling "horns".
Linear, or longitudinal dunes, are large, parallel features, typically in a long, straight line, that form when there is not a lot of sand, and the wind blows in one constant direction, allowing the dunes to migrate in the direction the wind is blowing. Transverse dunes are sand dunes that form at right angles to the wind.
Star dunes are giant (among the tallest dunes that can be formed), pyramid-shaped formations of sand composed of multiple, interlaced arms marked by sinuous crests, and slip faces of various directions. They are formed by complex interactions between a multidirectional wind regime and the surrounding topography. It is believed that star dunes form as a result of the combination of individual linear dunes, with the propagation of the dune arms only occurring under favorable wind regimes. The morphology of the arms appears to be controlled by the frequency of wind reorientation, with high frequency wind reorientation leading to smaller arm dimensions.
WHERE DO THEY OCCUR?
Star dunes typically form in areas of low precipitation, variable wind directions and in association with topographical barriers. They are found around the world, with the greatest numbers found in East Asia, West Asia, the Americas and southern and northern Africa.
The major areas where star dunes occur are: the Namib Sandsea (southwestern Africa); the Erg Fachi Bilma and the Grand Erg Oriental (the largest area of sand dunes on Earth) in the northeast Sahara; the Rub' al Khan Desert and the Nafud Desert in the Arabian peninsula; the Badain Jaran Desert in inner Mongolia, China; the Gran Desierto de Altar in northwest Mexico; the Mojave Desert in southwest United States and northwest Mexico; the Mongol Els Basin in northern Mongolia, China; the Zungol Els and Chongoryn Els in southeast Mongolia, China.; and the Great Sand Dunes in Colorado, U.S.
Surprisingly, this type of sand dune is not found in inland Australia, the Kalahari and Indian sandseas, or in the southeast Sahara. They have great diversity of forms, ranging from simple pyramids, complex features, checkerboard patterns, symmetrical and asymmetrical forms, stars along linear dunes, and stars along transverse dunes, with some even having characteristics of more than one type. It is estimated that star dunes make up about 8.5 per cent of all the sand dunes on Earth.
FIRST DISCOVERED
Star dunes were first discovered in 1978, by the Viking 1 orbiter. The star dunes discovered indicate a progressive change from barchan to star form, as each successive dune has travelled up into a valley, into areas of more complex wind regimes. The Martian star dunes exhibit dark streaks emanating from them, demonstrating that the dunes were active at or near the time of the Voyager 1 flyby. Star dunes can be very tall, very old, and capable of movement. On Earth, the base of the Lala Lallia ("highest sacred point" in the local Berber language) star dune in the Sahara Desert in southeast Morocco is estimated to be 13,000 years old, and is moving at a rate of 50 cm a year. The tallest star dune on located in china's Earth, Badain Desert, rises to a height of 300m.
LOCATIONS
Topographic barriers are an important factor in determining the locations of star dune areas. Such barriers significantly increase the complexity of regional wind regimes, as well as creating barriers to sand movement, thereby increasing the quantity of sand accumulation available for dune formation, and leading to the uplift of air masses. Star dunes are more common on Mars than on Earth, due to the presence of so many impact craters.
The wind swirling around the crater rims, blowing from every direction before reaching the sand inside the craters, creates some amazing shapes and patterns. However, although much of the Martian landscape consists of vast dune fields, star dunes and linear dunes on Mars are fairly isolated occurrences, with neither form being part of major sandseas on the planet
IMAGERY
NASA'S Mars Reconnaissance Orbiter, which has been orbiting Mars since 2006, with its High-resolution Imaging Science Experiment (HIRISE) cameras, has produced some amazing images of the various sand dunes on Mars.
While there are many types of dune formations on Mars, the HIRISE images of the star dunes within the Victoria Crater, and in a crater in the Tyrrhena Terra region, are particularly beautiful examples of star dune formation. On Mars, as on Earth, in most regions, the winds blow in a prevailing direction, at least for part of the year. Dune formation on Mars is dominated by the wind patterns that occur during the Martian summer season, during which time it is primarily northern winds assailing the equatorial region, and a cyclonic rotation around the planet's north pole.
As mentioned, star dunes also occur in Saturn's largest moon Titan. However, these dunes are different from those on Earth and Mars, they are electrostatically charged. The dunes on Titan point eastward, exactly opposite to the moon's prevailing winds, which, based on atmospheric circulation models of Titan, suggest blow westward. It is theorized that the dunes on Titan are not only shaped by wind, but also by electrostatic forces.
The moon's nitrogen-suffused atmosphere, which receives only 1 per cent as much sunlight as does earth, gives the planet an average temperature of -179 Celsius. At such low temperatures. the moon's water ice surface and hydrocarbon rains condense. These factors, combined with the moon's low gravity, help assist in the build up of immense electrical charges among the windblown grains of "sand", creating ultra-sticky sand dunes that are electrostatically charged, and which clump together by "static cling".
JUST THE FACTS
Finally, I'll leave you with a very interesting fact about sand dunes in general. Sedimentologists (those who study sediments, including silt, sand and clay), using ultra-sensitive moisture-detecting probes, have discovered that dunes can actually 'breathe' and 'hydrate'. Research has shown that sand dunes routinely inhale and exhale, taking in tiny amounts of water vapor.
As the wind blows over a dune, it removes the top layer of sand, creating a change in both pressure and surface moisture, resulting in evanescent waves of atmospheric oxygen and humidity to move downward. This exchange with the atmospheric boundary layer allows the sand grains to transport oxygen and moisture from the dune surface into the center of the dune; with the pathways constantly being reinforced as the wind blows.
This 'breathing' and 'hydration' by the dune enables microbes to live deep in the sand dunes, even when no liquid water is available.
Perhaps astrobiologists should be probing the Martian sand dunes for signs of ancient or existing microbes as an indication of life on the Red Planet.
THIS WEEK'S SKY
Mercury (mag.6.7, in Pisces - the Fish) is not observable, being 6 degrees below the eastern horizon at dawn.
Venus (mag. -3.9, in Cetus - the Sea Monster), soon to pass behind the Sun in superior conjunction, is not observable, as it is 1 degree below the eastern horizon at dawn.
Mars (mag. 1.2, in Aquarius - the Water-bearer) is no higher than 3 degrees above the east-southeast horizon at dawn, and is not observable.
Jupiter (mag. -2.0, in Aries - the Ram) remains the only bright planet observable this week, becoming visible around 8:30 p.m., 11 degrees above the western horizon, before sinking towards the horizon and setting around 9:45 p.m.
Saturn (mag. +1.1, in Aquarius), near Mars in the east-southeast sky, is not observable, reaching only 6 degrees above the horizon at dawn.
Uranus (mag. +5.8, in Aries) is too close to the Sun (19 degrees separation) to be observed.
Neptune (mag. +8.0, in Pisces) sitting 3 degrees below the southeast horizon at dawn, is not observable.
Having reached perihelion on April 21, Comet P12/ Pons-brooks, now receding from the Sun back into space, can be found in the constellation of Taurus - the Bull. Currently at mag. +4.42 (Apr. 22), the comet, potentially still visible to the naked eye from a dark site, will have faded slightly to mag. +4.52 by the end of the month.
The Full Moon on April 23 was known as the "Pink Moon" by the indigenous peoples of North America, in reference to the pinkcoloured, wild, ground phlox, one of the earliest plants to bloom in the spring. Contrary to popular belief, the Full Moon does not turn pink in colour.
My apologies. In last week's article, I mistakenly stated that "each time zone incorporates approximately 30 degrees of global longitude...", when in fact, it should have stated that each time zone incorporates 15 degrees of global longitude.
Until next week, clear skies.