Chasing Dust Devils
Research Helps Scientists Better Understand Dust Devils on Mars and Earth
"It’s a bit of an art — learning how to drive across dry lake beds at great speed," says Matt Balme. "And it can be quite hairy as well."
During the next three years, Balme will spend a lot of time careening across playas and other open, desert areas to better understand dust devils on Mars — how much dust they lift into the atmosphere and how this affects the martian climate. Understanding the martian climate is helpful in determining whether or not the Red Planet could have once been a habitat for life. Additionally, the study will shed important light on how dust behaves on Mars, which is useful information in developing future Mars exploration missions.
Dust devils look like miniature tornados, tossing up dust and debris, but the atmospheric conditions that cause them are very different. Dust devils are unrelated to storm systems. Instead, they form when ground temperatures are warmer than the surrounding air and moving air triggers a swirling column called a convective vortex.
Balme, a research scientist with the Planetary Science Institute (PSI) and a research fellow at Great Britain’s Open University, is the lead investigator on a NASA-funded study designed to link meteorological data to the number and intensity of dust devils in an area. This data will then be plugged into martian climate models to help scientists better understand the role of dust devils in shaping the martian climate.
Similarly, the study will help scientists better understand the effect of Earth’s dust devils on local or regional air quality and their role in transporting aerosols long distances.
The study also will generate data to test a host of equations that describe how dust devils form, that were recently developed by Nilton Renno, a University of Michigan professor who is working with Balme. The equations, which apply to everything from dust devils to cyclones, can determine the intensity of a convective vortex based on temperature, humidity, and the depth of the troposphere — the lower part of the atmosphere. These theories could help meteorologists predict the intensity of hurricanes and cyclones.
Since no grid of weather stations exists on Mars, scientists depend on computer models to study the climate. Balme hopes to make those models more accurate by using data from terrestrial dust devil observations as analogs for the martian ones.
That’s where racing across playas comes into play. The test setup includes three meteorology stations arranged in a triangle and a mobile sampling platform attached to the front of a 4WD truck. The weather stations will measure wind speed, air temperature and air pressure.
Researchers will race ahead of dust devils in the triangular area covered by the weather stations. Then they’ll slam on the brakes and quickly lower the instruments to ground level to sample the dust devil’s wind speeds, temperature, dust load, and size as the whirlwind passes over them.
"The good thing about having it attached to the truck is that if you miss the dust devil, you can quickly pick up the instruments and chase it down again," Balme said.
Mars is an extremely dusty place, Balme noted, and models suggest that dust devil activity might support the persistent dustiness of the planet’s atmosphere. With more accurate modeling, scientists will better understand the effect of dust devils on the martian climate.
Some data scaling is in order, however, because the martian atmosphere is so much thinner than Earth’s. While 50 mph is near the maximum velocity for terrestrial dust devils, their martian cousins may spin at as much as 200 mph.
Balme cautions that no direct measurements have been made of dust devil speeds on Mars. So this is a theoretical number. But much higher wind speeds are needed to lift an equivalent amount of dust into the thin Martian air compared to Earth.
Mars also is much colder than Earth. While dust devils often are associated with hot deserts, they’re just as likely to form in cold deserts, such as those found on Mars, Balme said. The triggering factors don’t depend on absolute temperature, only on a difference in temperature.
"We’ve done lots of chasing," Balme added. "But chasing just gives you more information about dust devils." What’s needed now is to link dust devil formation — how many occur and how intense they are — to the climatic conditions that cause them.
PSI Associate Research Scientist Steve Metzger will lead the field team in Nevada’s Eldorado Valley, south of Las Vegas, while Renno will lead the tests at a crop-duster airstrip near Eloy, Ariz., about halfway between Phoenix and Tucson.
"The big thing about this project is we’re not just going in and measuring dust devils," Balme said. "We want to be measuring the ambient meteorology at the same time. So having two different test areas gives us two different climatic regions to study."
PSI Research Scientist Asmin Pathare also will be involved in the field studies, and several researchers from Great Britain’s Open University will be working on the project.
The Open University group will contribute an instrument that looks vertically into a dust devil to measure ultraviolet radiation. It will provide data on the exact location where measurements are taken within a dust devil and on the amount of dust it’s carrying. The Open University scientists include Martin Towner, Manish Patel, Tim Ringrose, and Steven Lewis.
NASA’s Mars Fundamental Research Program is funding the research.
In addition to the valuable scientific results, "It’s going to be good fun," Balme said. "I really like going into the field and chasing dust devils!"