How Extreme Temperature Swings in Deserts Stir Sand and Dust
Understanding the movement of particles, some of which enter the atmosphere, may help scientists improve climate models and forecast dust storms on Mars
The White Sands Dune Field is desolate and isolated, far removed from any human activity, traits that made the New Mexico desert an ideal spot for the U.S military to test the world’s first atomic bomb in 1945. It still serves today as an active missile range. The 275-square-mile expanse of white, gypsum sands also provide an ideal place for geomorphologist Andrew Gunn to conduct an unprecedented field experiment.
Able to work in peace, and without concerns of losing his expensive equipment to thieves, Gunn and his colleagues were trying to uncover the secrets of how sand moved. More specifically, they were studying whether daily temperature and wind changes in the Earth’s atmosphere led to predictable movements of sand and dust below. Determining a cause and effect at this patch of desert would be a key step in forecasting how particles at the planet’s surface influence the weather.
“The dune field is very strange, kind of inhospitable,” says Gunn. “It feels alien.”
Geomorphology, which Gunn studies at the University of Pennsylvania, looks at the way ice, water and air mold and transform the Earth’s landscapes. At White Sands, where temperatures can shift from -19 degrees Fahrenheit at night to 95 degrees during the day, Gunn discovered that the environment’s extreme temperature swings generate powerful winds that move dunes and pump dust into the atmosphere. The finding, published in Geophysical Research Letters earlier this year, will help scientists understand both how to build climate models here on Earth and how atmospheric processes may have shaped the surface of Mars.
Deserts cover roughly one-third of Earth’s terrestrial surface and are a critical part of the climate system. When sand turns to dust, winds or storms lift it into the atmosphere where it joins a global sediment circulation system that provides nutrients to phytoplankton in the ocean, feeds plant-life in the Amazon and even affects the formation of clouds. Several studies have explored how large weather events like thunderstorms and haboobs carry dust into the system, yet studies of how normal daily atmospheric changes affect the movement of sediment are less common.
Gunn and his team headed out to White Sands in the springs of 2017 and 2018—the windy season—armed with a hypothesis and a collection of gizmos to test it. The scientists thought that as the Earth’s surface and lower atmosphere became warmer than the air above, this would create winds that would move the sand. The researchers used a doppler lidar machine to scatter lasers into the air to measure winds roughly 1000 feet above the surface. They used a solar-powered tower with sensors, called a meteorological mast, to record heat and moisture. A sand saltation sensor detected when even a single grain of sand moved. And back in the lab, they analyzed satellite images using a machine-learning algorithm to measure dust entering the atmosphere.
They found that in the morning, sunlight heats the ground, which heats the lower atmosphere to the point that it becomes unstable and begins to convect—with hot, less-dense air rising and cooler, and denser air sinking. This convection stirs up the atmosphere and eventually drags a stream of fast-moving higher winds down to the ground.
“The idea is, basically, that dune fields create their own wind,” says Gunn.
Around noon, as surface temperatures peaked, the team discovered wind speeds reached their highest speeds while humidity in the sand had evaporated. Sand grains skipped along the surface, and dust moved up into the atmosphere. After sunset, the temperature of the air and sand dropped quickly. Wind speeds at the surface died down and the sand grains settled. Every day, the process repeated, with the desert moving a little and pumping more dust into the atmosphere.
“The transport of sand, the movement of the dunes, the emission of dust from the landscape—that is all intrinsically tied to this daily cycle,” says Gunn.
After the study at White Sands, Gunn and colleagues looked at meteorological observations taken over a decade from 45 dune fields around the world to see if they could find evidence of the same process. The findings mirrored those at White Sands. The higher the change in temperatures, the faster the winds generated at the surface of the desert.
Unexpectedly, they found that the size of the desert influenced the strength of the wind. The larger the dune field, the stronger the link between temperature shifts and wind speeds, and sand transport along the ground and into the atmosphere.
The discovery of this daily cycle of heat and the transport of sand and dust could improve climate modeling says Doug Jerolmack, an experimental geophysicist at the University of Pennsylvania and an author on the study. These models, which use dust emission data to predict cloud formation, are helpful for climate physicists and meteorologists in making accurate climate predictions. Clouds play an important and complex role in regulating the temperature of Earth’s climate, yet are difficult to model. Better data on dust could help researchers understand more about how clouds form, grow and interact with each other.
“There are a variety of things that water condenses around to make clouds, but the two major ones are dust and sea salt,” Jerolmack says. “This convective instability in the desert is now like a vertical pump, that is taking the dust and delivering it to the upper atmosphere where it can seed clouds.”
Lori Fenton, a planetary scientist at SETI Institute not involved with the study, says the same process observed at White Sands is likely to happen on Mars, perhaps with even stronger temperature and humidity swings. “On Mars, the dune sand is darker than the surrounding terrain, which would further enhance the convective instability that forms wind gusts,” she says.
Until relatively recently, scientists thought sand dunes on Mars were stationary relics from a former age. Yet ripples and dunes shown to be moving on the red planet suggest certain areas, such as Nili Patera dune field, Styrtis Major and Mawrth Vallis, are being moved by the current climate.
Martian dust storms, which start local and sometimes combine to envelope the whole planet, might be partially explained by Gunn’s findings too, as the atmospheric mechanics discovered at White Sands could be what kicks off the initial dust that creates local storms. “The formation of big planet-scale dust storms are an unsolved mystery in Mars science,” says Fenton.
NASA’s Perseverance, now roving along the surface of Mars, is expected to cross sand dunes and large ripples along its traverse of Jezero Crater to the edge of an ancient river delta. Its onboard sensors will be picking up meteorological data on surface temperature, wind profiles and dust particles—similar to Gunn’s experiment at White Sands. This will help confirm if intense heating is driving higher wind speeds on the Red Planet.
Getting accurate predictions of dust movement on Mars is important for practical reasons, too. In 2018, when NASA’s Curiosity rover got caught in a dust storm, it could no longer recharge its battery. “As we plan to send more equipment and eventually people to Mars, you want to have a good understanding of the wind regime,” says Jean-Philippe Avouac, a geologist and planetary scientist at CalTech. “If there's a lot of sand blown by the wind it’s going to damage all the equipment and that would be a major issue.”