Winds of Mars: Aeolian Activity and Landforms


Compiled by
Steven Williams

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The planet Mars has always captured our imagination. In ancient times, its wandering motion and blood-red color were portents of ill tidings and earned it the name of the God of War. Its looping path against the background stars vexed Ptolomy and other Sun-centrists, inspired Copernicus to place the Sun at the center of the solar system, and spurred Kepler to discover his laws of motion. Early telescopic observations revealed surface markings that changed with time; wishful thinking turned those variabilities into evidence of a vast martian civilization. We know better today, but little green men from Mars remain a staple in pulp science fiction and in tabloid sensationalism.

Evidence of martian winds can be seen with difficulty in the telescope. McLaughlin, in 1954, was the first to speculate that periodic albedo changes on Mars might be due to wind action rather than changes in vegetation. Early space probes viewed only a limited section of the heavily cratered part of the martian surface, and some investigators drew the conclusion that all of Mars was like our Moon and, hence, uninteresting. Others predicted that winds and dust in the atmosphere might prove significant. The breakthrough came in 1971 with the arrival of the Mariner 9 spacecraft in Mars orbit. A global dust storm was raging and the entire surface was obscured, but as the storm slowly abated, an astonishing variety of geologic features was revealed. Martian science was forever changed and an important lesson was learned: Don't think you know everything about a planet based on what you see on a small portion of its surface! In 1976, Vikings 1 and 2 arrived at Mars, each having a lander module and an orbiting instrument platform. They were phenomenally successful; most of what we know about Mars comes from the Viking mission. We are still analyzing the results more than a decade later, and there remains much to be done.

The slides in this set were selected to provide an overview of the types of aeolian activity and landforms found on Mars. It is by no means all-inclusive; thousands of Mariner 9 and Viking photographs would have to be shown. There is a glossary of martian placenames and terms after the slide descriptions. Not cited are the hundreds of scientific journal articles that deal with the topic of martian aeolian activity and landforms; however, there is included a short reference list of some of the general texts on the topic of Mars. Many pertinent references are cited therein if you are inclined to pursue a more in-depth study of this fascinating topic.

Section 1. Aeolian Activity (slides 1–5)
Modification of the martian surface by the wind is a process occurring presently, as evidenced by atmospheric dust storms, dust devils, and perhaps even tornados.

Section 2. Wind Streaks (slides 6–9
Wind streaks are perhaps the most striking and widespread aeolian feature on Mars. Some have been observed to undergo changes in short amounts of time, indicating that the aeolian erosion and/or deposition that formed them is presently active.

Section 3. North Circumpolar Dunefiels (slides 10–14)
Encircling the northern polar cap of Mars is a large dunefield, or erg. The composition and source of the material in the dunefield have not been positively determined. Located within it are transverse dunes of many types.

Section 4. Other Dunes (slides 15–16)
The north circumpolar erg is not the only place dunes are found on Mars. Further examples are shown in the next two slides. Very few martian dunes are found in the open; most dunes are “trapped” in some way by local topography. The odds of finding an isolated dune are low because the dunes migrate very quickly until they get trapped by an obstacle of some sort, where they can remain for a long time.

Section 5. Wind Erosion Features (slides 17–19)
There are other prominent wind erosion features on Mars in addition to the dark streaks. On Earth, when erodible rocks and sediments are exposed to a strong unidirectional wind, they are sculpted into streamlined shapes that have been likened to inverted boat hulls. These wind-shaped hills are called yardangs, a term derived from the Turkistani word yar, meaning steep bank. Terrestrial yardangs range in size from a few meters to tens of kilometers and are best developed in arid areas where they would not be destroyed by running water.

Section 6. Laminated Polar Terrain and Other Aeolian-Related Features (slides 20–24)
The polar caps of Mars can be seen with a small telescope and their seasonal changes have been observed for hundreds of years. The part of the cap that varies is frozen carbon dioxide (“dry” ice). A small residual cap remains throughout the year; the larger northern cap is water ice, the southern is probably also water ice, but with a frosting of dry ice. Beneath the ice are thick sedimentary deposits: in the south, atop heavily cratered terrain, in the north, on top of plains units.

One of the most startling observations made by Mariner 9 was of the southern residual polar cap and the deposit beneath it, which was found to be composed of thin layers or laminations. The layering is thought to be due to cyclical climatic variations. The individual layers are too thick to be due to annual changes in climate (Mars has seasons just like the Earth, but they are more severe because the orbit of Mars is more eccentric than that of Earth and the martian atmosphere is too thin to prevent large temperature swings). However, the laminations are thin enough to be related to climate changes caused by naturally occurring cyclical variations in the orientation of Mars’ axis of rotation, akin to a mechanism proposed by Milankovitch as a trigger for terrestrial ice ages.

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