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.”
Apollo missions NASA’s third human spaceflight program eventually took humans to the lunar surface. Along the way, this program sought to develop the technologies needed for long-distance space travel. It got a big kick-start after President John F. Kennedy proposed in 1961 creating the national goal of “landing a man on the Moon and returning him safely to the Earth.”
asteroid A rocky object in orbit around the sun. Most asteroids orbit in a region that falls between the orbits of Mars and Jupiter. Astronomers refer to this region as the asteroid belt.
astronomy The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.
atmosphere The envelope of gases surrounding Earth or another planet.
comet A celestial object consisting of a nucleus of ice and dust. When a comet passes near the sun, gas and dust vaporize off the comet’s surface, creating its trailing “tail.”
density The measure of how condensed some object is, found by dividing its mass by its volume.
dwarf planet One of the solar system’s small celestial objects. Like a true planet, it orbits the sun. However, dwarf planets are too small to qualify as true planets. Prime examples of these objects: Pluto and Ceres.
electron microscope A microscope with high resolution and magnification that uses electrons rather than light to image an object.
gauge A device to measure the size or volume of something. For instance, tide gauges track the ever-changing height of coastal water levels throughout the day. Or any system or event that can be used to estimate the size or magnitude of something else. (v. to gauge) The act of measuring or estimating the size of something.
Jupiter (in astronomy) The solar system’s largest planet, it has the shortest day length (10 hours). A gas giant, its low density indicates that this planet is composed of light elements, such as hydrogen and helium. This planet also releases more heat than it receives from the sun as gravity compresses its mass (and slowly shrinks the planet).
Kuiper belt An area of the solar system beyond the orbit of Neptune. It is a vast area containing leftovers from the formation of the solar system that continue to orbit the sun. Many objects in the Kuiper belt are made of ice, rock, frozen methane and ammonia. The best known of the larger Kuiper Belt objects is the dwarf planet Pluto. The belt extends out from the sun at a distance of 30 to 55 astronomical units. (An astronomical unit is equal to Earth’s from the sun.)
lunar Of or relating to Earth’s moon.
Mars The fourth planet from the sun, just one planet out from Earth. Like Earth, it has seasons and moisture. But its diameter is only about half as big as Earth’s.
microscope (adj. microscopic) An instrument used to view objects, like bacteria, or the single cells of plants or animals, that are too small to be visible to the unaided eye.
mineral Crystal-forming substances that make up rock, such as quartz, apatite or various carbonates. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in regular three-dimensional patterns). (in physiology) The same chemicals that are needed by the body to make and feed tissues to maintain health.
moon The natural satellite of any planet.
NASA Short for the National Aeronautics and Space Administration. Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It also has sent research craft to study planets and other celestial objects in our solar system.
Neptune The furthest planet from the sun in our solar system. It is the fourth largest planet in the solar system.
orbit The curved path of a celestial object or spacecraft around a star, planet or moon. One complete circuit around a celestial body.
particle A minute amount of something.
physics The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. A scientist who works in such areas is known as a physicist.
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood.
planetary science The science of planets other than Earth.
Pluto A dwarf planet that is located in the Kuiper Belt, just beyond Neptune. Pluto is the tenth largest object orbiting the sun.
solar flare An explosive event that takes place on the sun when energy that has built up in 'twisted' magnetic fields (usually above sunspots) becomes suddenly released. The energy can in minutes heat to many millions of degrees, emitting a burst of energy. That energy consists of radiation across the electromagnetic spectrum, from gamma rays to radio waves.
solar system The eight major planets and their moons in orbit around our sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.