Martian Dust Devils: a Hazard

Dust devils at Mars, like those at Earth, are columns of warm air, carrying aloft dust and sand grains, and moving horizontally, as a whole, on the ground. Dust devils are considerably larger than their terrestrial counterparts however. They may reach miles (kilometers) high, up to 5 to 6 miles (8 to 10 kilometers), and hundreds yards (meters) wide (even up to 0.6 to 1.2 miles -1 to 2 kilometers). Sand and dust are moving inside the dust devil faster than 90 ft per second (30 m/s) as Martian dust devils' horizontal speed might be of the order of 10 mph (17 km/h, 28 m/s). Figures for large dust devils at Earth are: 40 mph (64 km/h, 107 m/s) with a 100-foot (30 m) width and a half-mile -1 km) high (speed for regular dust devils is 25 mph (40 km/h, 67 m/s) only)

How does the dust devils form? They form at Mars the same way than at Earth, that is from a strong surface heating. The air near the surface becomes hotter than the air above it. A convection motion begins, bringing cells of hot air up, as cells of cold air down. Suffices that a gust of horizontal wind blows and the convection cells are turned on their sides, to spin. This forms the vertical column of the dust devil. The dust devil is further powered by hot air continuing to rise in the column, and the whirlwind spins faster enough then to pick up sand grains. Sand grains in turn are moved by the dust devil to dislodge the Martian dust, which is pushed high aloft! Each spring or summer an average of a half dozen dust devils may be spotted in the middle of the day at a given location at Mars. Dust devils are appearing about 10 a.m. as the ground heats and abating about 3 p.m. when it's cooling back. Summer daytime temperatures are reaching 68° F (20° C), from nighttime lows of -130° F (-90° C). Dust devils occur at all Martian latitudes as was proved by photographs from orbit

An exploration team exposed to a dust devil would have to endure the storm during about 15 minutes. With the atmospheric pressure on Mars only1 percent that at sea level on Earth, the winds however would not be felt much by the astronauts. The main hazards and inconveniences would more surely comes from other factors. Dust devils at Earth have been studied being electrically charged. Due to the "triboelectric charging" (from the Greek "tribo", "rubbing"), two unlike materials rubbing together leads to that one material gives up some of its electrons (negative particles) to the benefit of the other. The first material looses electrons. It becomes positively charged. The second material receives an increase of negative electrons. It becomes negatively charged. Sand grains and dust work like that, with the dust particles increasing in their number of electrons. The rising central column of hot air pushes the charged dust upward as the negatively charged sand grains remains near the base of the whirlwind. These charges getting separated, this creates an electric field. At Earth, the difference of charge is on the order of 20 kVolts/meter, that is not much, but the size of the Martian dust devils would likely yield a much larger field! It's the effects of such fields which would pose hazards to the astronauts and their artefacts. At last, as dust devils are in motion, this generates a magnetic field (a magnetic field is generated by an electric field in motion)

First, lightning. The electrical field in the dust devil does not produce flashes of lightning by itself. This would simply need an astronaut, or a vehicle, for example! The presence of an artefact or a human would induce a "filamentary" or a local arcing. It has to be noted too that the thin atmosphere at Mars does not need as much electrical intensity to be ionized than at Earth. Miniature lightning bolts, on the other hand, might strike the astronauts' suits and their rover. A way to protect the vehicle and the habitats against arcing lightnings would be to make the shapes rounded, without corners. What next? Radio static. Radio static would ensue from charged sand grains hitting bare-wire antennas. Next? Once the whirlwind gone, it would leave behind an increased adhesion of dust to spacesuits and other objects and habitats, due to electrostatic cling. Last but not least, with gigantic towering dust devils, much dust would be thrown into the high atmosphere, carrying its negative charges along! Such a potential lightning source would pose a hazard to rockets launching from Mars! The rocket exhaust would ionize the air molecules, leaving behind a trail of charged molecules of air. When Apollo 12 launched in November 1969 amidst a thunderstorm condition, the lightning bolt just followed down the exhaust trail, hitting the rocket at the same time. Turbulences of rising hot air alone might present a hazard to spacecraft landing and takeoff maneuvers too, jolting the craft

A last point: all such hazards might exist on a larger scale due to the famed dust storms which occur at Mars and which can shroud the whole planet for weeks! All this makes that scientists are placing a high priority on understanding the physical and electrical properties of Martian dust devils and other atmospheric phenomena before sending astronauts. Dust devils hazards might be put under control through the use of a LIDAR ("Light Detection and Ranging"), that is a radar where laser light plays the role of an usual radar's radio waves to measure the distance of an object, as clinging dust might be removed inside an airlock to prevent it to be tracked inside a station