Catching space dust in the stratosphere
The smallest space dust can be caught before it ever touches the ground. Since the 1970s, high-flying aircraft have collected interplanetary dust particles about 20 km above the Earth.
Research planes fly far higher than airliners, into the thin, dry stratosphere. Under their wings they carry plates coated with sticky silicone oil. Over many hours of flying, a few tiny grains of space dust stick to them. These grains, called interplanetary dust particles, are often fluffy clumps of minerals and carbon that probably came from comets.
Key facts
- Altitude
- About 20 km, in the lower stratosphere
- Particle size
- Mostly about 0.005–0.05 mm
- Collectors
- Plates coated in silicone oil, carried under the wings
- Curated since
- 1981, in a NASA collection open to researchers worldwide
Why fly so high?
Near the ground, the air is full of dust from soil, sea spray, plants, fires and industry, and a few grains of space dust are lost among billions of others. The lower stratosphere, at about 20 km, is far cleaner. Weather does not reach it, and most Earthly dust is washed out of the air long before it gets so high. The smallest space grains, slowed by the upper atmosphere and settling gently, drift through this layer on their way down.
Grains caught there have never touched soil or water, so they are as fresh as space dust can be. They are known as interplanetary dust particles, or IDPs, and are sometimes called Brownlee particles after the American astronomer Donald Brownlee, who pioneered their collection in the 1970s.
How the collection works
Converted spy planes and other high-altitude research aircraft carry flat collector plates in pods under their wings. Each plate is coated with a thin layer of thick silicone oil. During a flight the collectors are opened to the air for many hours, and any particle that hits them sticks. Back on the ground, the plates are examined in a clean room, and particles are picked out one by one with fine glass needles, washed free of oil, photographed and catalogued.
NASA has curated a cosmic dust collection since 1981 and publishes catalogues of the particles so that researchers anywhere can request them. Not every particle is from space. The collectors also catch volcanic ash, aircraft exhaust and tiny spheres of aluminium oxide from the exhaust of solid-fuelled rockets, so each one must be checked.
Two main kinds of IDP
Chondritic smooth IDPs are compact and contain clay minerals formed by water. They resemble the matrix of CI and CM carbonaceous chondrites and probably come from water-rich asteroids.
Chondritic porous IDPs are fluffy aggregates of tiny grains with lots of open space between them, rather like a ball of soot. They contain no water-bearing minerals but are rich in carbon and in GEMS, which stands for glass with embedded metal and sulphides. They also hold more presolar grains, tiny crystals made in other stars, than almost any other material. Their fragile structure and primitive make-up point to comets, and comparison with dust brought back from comet Wild 2 supports that link.
Dust from a particular comet
Most of the time, the dust caught in the stratosphere comes from the general zodiacal cloud and cannot be traced to one parent. But when the Earth passes through a known comet's dust trail, flights can be timed to catch it. In 2003, collectors were flown during the Earth's encounter with the dust trail of comet 26P/Grigg–Skjellerup. Some of the particles collected then turned out to be exceptionally rich in presolar grains, consistent with very primitive cometary dust.
Studying a grain smaller than a hair's width
IDPs are far too small for ordinary laboratory methods. Researchers slice them with diamond knives into sections less than a ten-thousandth of a millimetre thick and study them with transmission electron microscopes, ion probes that measure isotopes in areas a few hundred nanometres across, and the brilliant X-rays of synchrotrons. From a single grain, they can identify individual crystals, measure the isotopes of hydrogen, carbon, nitrogen and oxygen, and map its organic matter.
IDPs and micrometeorites
IDPs and micrometeorites are parts of the same rain of dust. IDPs are mostly smaller and less heated; micrometeorites are larger and are collected after they land. Together they cover sizes from a few micrometres to about 2 mm. Some micrometeorites from Antarctic snow, including ultracarbonaceous ones, resemble IDPs closely, suggesting they come from the same kinds of comet.
Did you know?
Silicate grains from other stars were first identified in the Solar System inside interplanetary dust particles, in 2003.
A typical IDP weighs around a billionth of a gram, yet scientists can measure dozens of elements and isotopes in it.
Some IDPs contain grains of a sticky, carbon-rich material that has never been heated, preserving organic molecules from the cold outer Solar System.