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Tuesday, April 16, 2024

A Powerful ISS Instrument Will Hunt for Minerals in Dusty Lands

What blows through the Sahara doesn’t stay in the Sahara. The vast African desert regularly burps up clouds of dust that fly into Europe, turning snow-capped mountains orange. They travel clear across the Atlantic Ocean, fertilizing the Amazon rain forest with phosphorus. The stuff can even reach the United States

But for all their bluster, the Sahara’s dust emissions—and the grime from any other desert region—are not well accounted for in climate models. While satellites can track the plumes as they move around the atmosphere, scientists don’t have enough data to definitively show how dust could cool or warm the planet, either accelerating or slowing human-caused climate change

“Our data sets are based on 5,000 samples of soil, and that's not nearly enough coverage,” says Natalie Mahowald, an Earth system scientist at Cornell University. “Nobody wants to go to the middle of the desert to figure out what soils are.” So Mahowald has been collaborating with NASA on the Earth Surface Mineral Dust Source Investigation mission, or EMIT, which launches to the International Space Station next month. Their instrument will use a powerful technique known as spectroscopy, which astronomers have used for decades to determine the composition of faraway objects, but turn it earthward to analyze our own lands. That will finally give scientists a global portrait of where dust is coming from, what it’s made of, and how those particulates might be influencing the climate. “Remotely sensing it makes just way, way more sense,” Mahowald says. 

Any material’s molecules absorb and then emit electromagnetic radiation in unique ways. So astronomers can use a spectrometer to analyze the light coming off a distant planet, isolating individual elements like hydrogen or carbon based on their distinct signatures. That planet may be billions upon billions of miles away, yet its atmospheric composition is betrayed by the light bouncing off it. It’s a bit like being able to take someone’s fingerprint, even if you’re never able to touch them.

The EMIT spectrometer, which will be attached to the underside of the ISS, will image the Earth in 50-mile-wide swaths, hunting for the unique signatures of specific minerals. Iron oxide, for instance, will look different to the spectrometer than clay, even though to the human eye one desert region’s surface might look similar to another. “We need to measure the fingerprints of the minerals in arid land regions,” says Robert O. Green, EMIT’s principal investigator and a researcher at NASA’s Jet Propulsion Laboratory. “We’ll have enough mineral maps within a year to then start providing new initialization information for the climate models.”

Plugging that new data into existing models will give climate scientists a better understanding of dust’s role in our planet’s temperature. Traditionally, researchers have represented dust as a kind of simplified average, a yellow haze. “But if you look at soils, they can be all different colors: black, red, white—a very reflective color,” says Mahowald, who’s the deputy principal investigator of EMIT. “Anything that's darker is going to absorb more radiation and warm us, and anything that's lighter will reflect the radiation and cool.”

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Mapping the mineral composition of the world’s dust-producing regions will give Mahowald and Green a better understanding of what those areas are contributing to the global dust flow, and let them parse how that changes over time. So, for instance, a particular desert may be growing as the planet warms, but other regions may actually become wetter, which will reduce their contribution to atmospheric dust. (It seems counterintuitive, but a warmer atmosphere actually holds more water.)

By analyzing the chemical composition of the grime, scientists can also learn more about its biogeochemistry, and how that influences the carbon cycle. The iron in dust fertilizes the oceans, encouraging the growth of phytoplankton, which absorb CO2 as they photosynthesize. This both provides food for oceanic vegetarians and helps keep carbon out of the atmosphere. On land, phosphorus in dust fertilizes the Amazon and other forests, which similarly builds ecosystems and sequesters carbon. “The chemical composition can be really important for the biogeochemistry as well,” says Mahowald. “So we're going to find out a lot more about all of that.”

Dust also seeds clouds, acting as nuclei for water vapor. Clouds, of course, give us rain and snow, but they also act like floating mirrors, bouncing some of the sun’s energy back into space. But the way that dust particles attract water depends on what they’re made of, yet another reason it’s critical to better understand the minerals in dust-producing regions.

That’s especially important as humans continue to disturb the land, for instance clear-cutting forests like the Amazon, transforming them from wet regions into dry ones that burp up dust. “Human changes in land use, as well as climate changes that affect land use, may significantly alter dust distributions,” says Andrew Gettelman, a senior scientist at the National Center for Atmospheric Research, who wasn’t involved in EMIT. “Understanding the relationships between land surfaces and aridity, and land use change and dust, is actually going be quite important.”

While it’s beyond the scope of the EMIT mission, Green says that in the future, spectroscopy could also pinpoint sources of carbon dioxide and methane leaks, since those gases have unique signatures. Spectroscopy might map lithium-bearing minerals in playas around the world, providing new resources for the batteries needed to decarbonize our civilization. It can even see underwater, potentially analyzing coral health, and differentiating between harmful and benign algal blooms

“Spectroscopy is the most powerful analytical method discovered,” says Green. “It's why we know the nature of our universe. It's why we know the Big Bang. Everything in astrophysics is based on spectroscopy. We're just now taking that tool and pointing down to look at the chemicals and properties of our planet.”

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