Dust to dust
Dust to dust: New lab studies
death of stars, origin of planets
April 6, 1998: Almost a century ago, American physicist Robert Millikan watched an oil droplet rise and fall as he gently applied an electrical charge. He was pursuing a fundamental measurement of nature, the size of the electron-volt. In the process, he simulated part of what happens when dust condenses into planets.
Now scientists at NASA's Marshall Space Flight Center are running a sophisticated variation of Millikan's experiment as they try to understand dust behavior in space.
"The main purpose is to study the microphysics of a charged particle when it's exposed to a plasma [electrified gas]," said Dr. James Spann of the Space Sciences Laboratory at NASA/Marshall.
Dust is an important part of the universe, from (top to bottom) galaxies, nebulas, to Jupiter's moon Io and comets. Picture information is at the end of this story.
Spann is principal investigator for the new Dusty Plasma Laboratory which he and graduate student Catherine Venturini have been developing over the last two years. Today and Wednesday (April 6, 8) they present their initial work to the Seventh Workshop on the Physics of Dusty Plasmas at the University of Colorado in Boulder.
"I call this the three-dimensional Millikan experiment," Spann said. Millikan's original experiment - which won him the Nobel Prize in physics in 1923 - worked in two dimensions as he watched an oil droplet rise and fall as X-rays ionized it.
Space is not a pristine vacuum. It is peppered with cosmic rays, atoms, molecules, and asteroids, and so on up to planets and stars. And dust. Lots of dust.
Indeed, dust is the stuff from which planets are made. A dying star disgorges heavy atoms that eventually form dust grains that, in turn, coalesce into ever-larger debris until planets are formed around new stars.
What has not been given much study is the fact that dust is readily pushed around by sunlight and by stellar winds, and easily electrified by exposure to ultraviolet light or even friction with other dust particles. When electrified, they will repel each other, like your hair standing on end after brushing on a cold, dry day. That repulsion can alter how dust grains gather when the cloud is so loose that has a weak gravity field.
Scientists have run experiments on quantities of dust, but Spann and Venturini are making the first systematic investigation on the behavior individual particles.
Right: Spann and Venturini examine the Dusty Plasma Laboratory apparatus. The windowed chamber at top holds the electrodes that position a single dust particle. The illuminating laser is at right. Additional high-resolution pictures are available.
"We're approaching it from a fundamental perspective," Spann explained. "Here's one particle. How does it change under various conditions, as the electrical charge changes, with different masses, with different shapes and materials?"
The answers should have many uses.
"Applications range from atmospheric aerosols [droplets and dust in the air] to stellar formation," Spann said. "Dust is everywhere, in planetary atmospheres, the zodiacal light [sunlight reflected back from deep space], in planetary origins."
Spann and Venturini simulate dust in space with a table top apparatus designed to isolate a single bit of dust in a vacuum and manipulate it with an electrical charge.
Spann started developing the Dusty Plasma Laboratory started in 1995 under the Center Director Discretionary Fund. Venturini, then a senior in physics at Loyola Marymount University in Los Angeles, Calif., was recruited through the NASA Academy program.
"On my first day, Jim said, 'OK, we have to put this thing together'," Venturini said. She continues her work in the Dusty Plasma Lab as she finishes her master's degree in physics at UAH. Her current work is funded by the Alabama Space Grant Consortium.
"It's not a complicated set-up," Spann said. "But it has a lot of parts that have to work together. We have a lot of things that are normally used in laboratories, but not together."
The system comprises several tubes of stainless steel welded together so they intersect at the center of the table. Atop the intersection is a small, windowed chamber which, in turn, is crowned by the dust injector and an electron gun.
Venturini explained that most of the tubing is just a manifold connected to vacuum pumps to evacuate the chamber after the dust is injected. All of the action takes place in the windowed chamber, which encloses a quadrupole trap: two hemispherical electrodes at top and bottom and a ring electrode at the middle (right). These form hilltops surrounding an unusual valley. When the electrodes are charged, the center becomes the point of zero potential. An electrified dust mote will sort of "fall downhill," repelled on all sides by the electrodes.
Then they watch the behavior of the dust particle through a TV camera coupled to a long-distance microscope (at left is a typical image). A helium neon laser selected because its red light will not impart an electrical charge to the dust illuminates. Blue light can do so; ultraviolet light almost certainly will. While that is an important factor for dust behavior in space, it would be a complication for experiments where Spann and Venturini are trying to deduce the basics of dusty plasma behavior.
For now, they use table salt, or store it. Household dust won't do since that usually comes from clothing, skin, animal fur, dead insects, and other items not found in space.
Sodium chloride is dissolved in pure water, and a tiny droplet is squeezed through a syringe into the test chamber, which still contains air. The quadrupole trap captures the droplet and suspends it in midair.
The water evaporates, leaving just a tiny crystal of salt about 3 to 5 microns wide (that's less than a 5/10,000ths of an inch). Next, the air is evacuated ever so gently so no breeze dislodges the dust grain. This takes about an hour, and uses a sapphire-coated needle valve to allow the tiniest wisps of air to escape.
Once conditions are right, the grain is bombarded with electrons and the grain's behavior is measured. Spann said the grain would charge until it either repels new electrons, or it accepts them while shedding other electrons like newcomers bumping people out of a crowded room.
Where Millikan tweaked his experiment at just a few volts, Spann and Venturini apply voltages of 100 to 1,000 electron-volts.
It's tougher than it sounds.
"The difficult part of an experiment is to make it simple," Spann said. He and Venturini are still controlling the apparatus by hand, and Venturini is writing a computer program to measure the grain's motions both to take scientific data and to nudge the grain back into position.
"What we're trying to establish in the lab is an area of expertise that could be used by experimental and modeling groups," Spann said.
More complex experiments could examine the behavior of dust grains when they absorb then re-emit infrared radiation. This could help scientists understand what they are seeing in images of so-called planetary nebula, immense dust clouds surrounding young or dying stars.
It can also help in understanding how the view is obscured in other wavelengths.
"The mid-infrared background that telescopes have to contend with is dominated by interplanetary and interstellar dust particles," Spann said, "so understanding that is very important." For example, telescopes looking into space directly opposite the sun see a dull, glowing patch of space. This is the gegenschein (also called the zodiacal light), sunlight scattered back by dust.
An effect more familiar to most people is the beautiful tail a comet forms as it approaches the sun. The tail comprises dust, ice, and gas energized by sunlight and the solar wind. The Dusty Plasma Laboratory will help in understanding the mechanisms that make the tail spread as it does.
Once they have the system mastered, they will experiment with a range of different dust motes that conduct, store, or insulate against electricity.
Then they will be ready to start investigating the cosmos on a microscale.
|The pictures at top show dust in galaxy AM0500-620 and forming "twisters" in the Lagoon Nebula, spewing from a volcano on Io, and shed by Comet Hyakutake - C/1996 B2. The pictures were taken by the Hubble Space Telescope and are credited to HST and (in the first picture) the University of Alabama. High-resolution GIF, JPG, and TIFF copies of these images are available at the linked sites. Print-quality (300dpi) images of the Dusty Plasma Laboratory are available, too.||related links|
May 7: CNN story on space dust and relation to extinctions
April 21: JPL press release detailing a recent discovery of planets forming around a star in the constellation Centaurus.
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