Some dust in Earth’s atmosphere may hail from beyond Neptune

THE WOODLANDS, Texas — Grains of dust from the edge of the solar system could be finding their way to Earth. And NASA may already have a handful of them, researchers report. 

An estimated 40,000 tons of space dust settle in Earth’s atmosphere every year. The U.S. space agency has been flying balloon and aircraft missions since the 1970s to collect samples of the dust. Astronomers had long thought the dust mostly comes from smashups involving comets and asteroids — especially from ones inside the orbit of Jupiter.

In fact, some of the dust grains may come from the Kuiper (KY-pur) Belt. That’s a distant region beyond Neptune of icy objects. Like the planets, they too orbit the sun. Lindsay Keller is a planetary scientist at the Johnson Space Center in Houston, Texas. He reported the findings, here, on March 21 at the Lunar and Planetary Science Conference.

Studying those particles could reveal what distant, mysterious objects in the Kuiper Belt are made of, and perhaps how they formed.

“We’re not going to get a mission out to a Kuiper Belt object to actually collect [dust] samples anytime soon,” Keller said. “But we have samples of these things in the stratospheric dust collections here at NASA.”

One way to find a dust grain’s home is to probe the particle for microscopic tracks. These tracks are typically left as heavy charged particles from solar flares punched through a dust grain. The more tracks a grain has, the longer it has wandered in space — and the more likely it originated far from Earth, Keller explains.

Probing the dust’s likely source

To gauge precisely how long a dust grain had been traveling, Keller first needed to know how many tracks a grain would typically pick up each year. Measuring that rate required a sample with a known age and known density of tracks. Moon rocks brought back on the Apollo missions would work. But the last track-rate estimate had been done in 1975. And it had been done with less precise tools than are available today.

So Keller and planetary scientist George Flynn reexamined that same Apollo rock with a modern electron microscope. (Flynn works at the State University of New York in Plattsburgh.) The two found that the rate at which rocks pick up flare tracks was just one-twentieth of what the previous study had estimated. That means it takes longer for dust flakes to pick up tracks than astronomers had assumed.

Keller and Flynn then counted the number of tracks in 14 atmospheric dust grains. Some must have spent millions of years out in space, they found. That was far too long for them to have come just from between Mars and Jupiter.

Grains specifically from the Kuiper Belt would have wandered 10 million years to reach Earth’s atmosphere, the researchers calculated. That’s “pretty solid evidence that we’re collecting Kuiper Belt dust right here,” Keller says.

Four of the dust grains contained minerals that had to have formed through interactions with liquid water. That’s surprising. The Kuiper Belt is thought to be too cold for water to be liquid.

“Many of these particles, if they in fact are from the Kuiper Belt, tell you that some of the minerals in Kuiper Belt objects formed in the presence of liquid water,” Keller says. That water probably came from collisions between Kuiper Belt objects that produced enough heat to melt ice, he says.

“I think it’s incredible if Lindsay Keller has shown that he has pieces of Kuiper Belt dust in his lab,” says Carey Lisse. He is a planetary scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. But, he adds, researchers must do more work to confirm that the dust really came from the Kuiper Belt. The dust could have been just sitting on an asteroid for millions of years. “Lindsay needs to get a lot more samples,” Lisse says. “But I do think he’s on to something.”

Lisse works on NASA’s New Horizons mission. That spacecraft found plenty of dust in the outer solar system. The spacecraft measured the dust’s abundance near Pluto when it flew past the dwarf planet in 2015. Based on those results, Lisse finds it unsurprising that some of that dust has made its way to Earth. But it is “really cool,” he says. “We can actually try to figure out what the Kuiper Belt is made of.”