Dark Sands

Scorched dunes on Mars. Image credit NASA, JPL-Caltech, MSS. Click to enlarge.

Jul 18, 2019

Blackened dune fields can be found on Mars.

Martian atmospheric density is less than 1% that of Earth. It is almost entirely composed of carbon dioxide, although nitrogen and argon make up about 3%, with trace elements less than .01%. The temperature varies from a maximum of 20 Celsius to a minimum of -140 Celsius. The atmosphere is so thin that winds blowing at nearly 300 kilometers per hour exert almost no pressure. Using the formula q = 0.5ρv^2, where q is the pressure, ρ is the density of the atmosphere and v is the wind velocity, the dynamic pressure of a 97 kilometer per hour wind on Mars is equivalent to a 12 kilometer per hour wind on Earth.

Since the winds exert little force, what formed the many dune fields on Mars? Why do they look fresh and sharply delineated? Some are textured with fingerprint-like patterns or, as in the image at the top of the page, are dark and scorched. In many cases, the dunes lie on top of quartz deposits that exhibit polygonal fracturing over a wide area.

Finding ripples or dunes on other planets causes planetary scientists to assume that similar activity created what look to be the same kind of formations on Earth. Since sand and dust from wind and water erosion are presumed to have created the dune fields found here, then Mars “must have” had an environment at one time that provided the same conditions for them to form there.

Most sand dunes and ripples do not move around Mars. Some research has suggested that a small dune on Mars might take more then a thousand years to move a meter. Recent observations from the HiRise camera system do seem to show changes in some dunes, though, possibly due to the action of planet-wide dust storms. Although, the same press release also states that some of them in other locations have not moved for 100,000 years.

The dunes in the above image are blackened, possibly due to the vast electric discharges that created them. When electricity passes over a solid body, such as a planet, the current pulls charged material from the surface where the arc makes contact. Neutral dust and stones will be pulled along with the ionized particles as well.

As mentioned, the Martian atmosphere is 100 times thinner than Earth and 75 degrees colder, on average. However, its dust storms are far larger and are accompanied by multi-kilometer high vortices, or “dust devils”. When the Viking spacecraft landed on Mars, planetary scientists were surprised by all the suspended dust. They thought the sky would be dark, since the atmosphere is so thin that it should be too weak for small particles to blow around, let alone to raise such monstrous dust devils. Why are they so powerful?

NASA scientists studied the dust devils in Arizona in order to understand what possible causes generate them on Mars. They discovered an electric field of up to 10,000 volts per meter associated with dust devils on Earth. This means that, on both planets, they are an atmospheric electric discharge phenomenon similar to the electric winds produced by air ionizers. Perhaps it is ionic winds that are changing the shape of the dunes under consideration.

The advantage of the electrical interpretation is that it directly explains the nature of the topography dominating the craters on Mars. Electromagnetic forces between Birkeland currents constrained to a surface will force them into alignment. Ionic winds can lift material and carry it along in the direction of the current flow. Where a discharge channel bifurcates, the branches tend to remain parallel to each other and may rejoin. Orthogonal coronal discharges from parallel Birkeland currents generate the dune ripples.

It is most likely electrical effects that carved the craters on Mars, as well, and in so doing, formed the drifts of finely pulverized debris in their bottoms.

Stephen Smith

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