Metals from space descend on Boulder, Colorado, at dusk and dawn


Morning sun against the foothills of Boulder, Colorado, business area and campus.
Enlarge / Morning sun against the foothills of Boulder, Colorado, business area and campus.

Fred Langer Photography | Getty Images

Every day, the Sun rises and sets on Boulder, Colorado. And, like clockwork, a layer of sodium and other elements trickle down through the sky and hit the ground, a team of researchers at the University of Colorado-Boulder found. These elements hail originally from space and, in various forms, hit the atmosphere before making their trek to the Earth’s surface.

The team published this discovery in Geophysical Research Letters. A decade ago, Xinzhao Chu, the lead author of the research, discovered these metal layers at the McMurdo research station in Antarctica. However, near the Earth’s polar south, these elements appear sporadically, rather than daily. This is the first time researchers have discovered a case where the layers drop at regular intervals.

These layers are not visible to the human eye. So, at the Table Mountain Observatory near Boulder, the team made use of a lidar system—which operates similarly to radar but uses lasers instead of radio waves—to detect the minuscule sodium particles. While the lidar data from the region was taken a few years back, the team analyzed them last December and discovered our atmosphere’s metal cycle.

It came from outer space

According to Chu, this mysterious process starts with cosmic dust and small meteoroids hitting the atmosphere and breaking apart into neutral and ionized metal atoms. From there, they are pushed around by our planet’s electrical fields and winds in the thermosphere, and the ionized metals end up being neutralized by free electrons in the ionospheric plasma.

“They only probably convert one percent of the ions into neutral metals, but our lidars are sensitive enough to observe them,” Chu told Ars.

Eventually, gravity brings them down to Earth, usually in the form of complex particles formed with other materials in the atmosphere. Every day, tons of this debris hits the atmosphere in this way, so the sky ends up with a steady supply of these elements. According to Chu, studying these metal layers might also help scientists make better estimates about the amount of cosmic dust that reaches the Earth, though other methods would also be necessary. Some have this amount pegged at 5,200 tons every year, but the exact amount is unknown, said Jackson Jandreau, a CU-Boulder PhD student.

“It’s actually an open question—how many tons come in?” he told Ars.

There are likely other places around the world that see these layers descend at regular or irregular intervals, Chu said. The researchers’ main guess behind Boulder’s regularity is because of wind activity, though this is not confirmed.

Making an impact

According to Chu, studying these metals could also help researchers pick out planets that could harbor life. Generally, when looking for other potentially life-sustaining planets, researchers look for qualities that are similar to Earth’s. The process Chu’s team described is only possible because of the structure of Earth’s atmosphere and magnetic fields. The Earth’s atmosphere also protects the planet from the harshness of space. If scientists see similar elements in the atmospheres of other planets, that might be another signifier of a hospitable world.

Back on Earth, the role these elements play in the environment is still poorly understood. According to Jandreau, they are probably not among the most important influences on the Earth. All the same, they might serve some functions. He noted, for instance, that some of these elements might fall and act as fertilizers in the world’s oceans. Even if their roles are fairly small, “that doesn’t mean they’re a process you can neglect, or anything,” he said.

Geophysical Research Letters, 2021. DOI: 10.1029/2021GL093729 (About DOIs).



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