As the Dustbuster (top) flies through space, dust particles entering its maw (above) smash into a target plate that fragments them into positively charged ions and free electrons. The rebounding positive ions are given a uniform “kick” to the left by the accelerator grid and are then steered into the detector by means of an electric field created by the reflectron rings.

Cosmic Dust in the Wind

Don’t let its seemingly vast emptiness fool you: the universe is a dirty place. Comets, supernovae, and solar winds spew microscopic particles of matter, called cosmic dust, across the universe. Instead of being a filthy nuisance, this cosmic dust may hold clues about the history of the solar system and the origins of life on Earth.

“Origins, that’s a big word at NASA these days,” says Jesse (Jack) Beauchamp (BS ’64), the Ferkel Professor of Chemistry. Along with Thomas Ahrens (MS ’58), the Jones Professor of Geophysics, Emeritus, Beauchamp has built a device to extract cosmic dust’s secrets. They call their creation the Dustbuster, and they hope it will be put on a future mission to the outer planets.

Unlike the Dustbuster you may have in your car or broom closet, this gadget isn’t a vacuum; it’s a mass spectrometer. On Earth, chemists, biologists, and those CSI guys use mass spectrometers to identify unknown molecules. It works on the principle that when a molecule or atom is charged, or ionized, its behavior in an electric or magnetic field will depend, partly, on its mass.

Cosmic dust flows through the outer reaches of our solar system at speeds of 10 to 80 kilometers per second. Any particles hitting a target plate on the Dustbuster are instantly vaporized, and the energy of the impact strips electrons from the molecules, producing positively charged ions with various amounts of kinetic energy. Inside the Dustbuster, the ions are accelerated by an electric field and guided towards an ion detector through a part called the reflectron. This part negates any differences in kinetic energy between the ions produced by the impact. Since the electric field provides each ion with the same amount of energy, the time it takes each ion to reach the detector will depend on its mass. It’s an ionic drag race—imagine a Honda Civic dueling a Hummer powered by a Civic engine. Just as the heavier Hummer will move more slowly, heavier ions will accelerate to lower velocities than lighter ions. Faster, lighter ions will arrive at the detector first, so monitoring when ions reach the finish line determines their masses.

“There’s quite a history of using mass spectroscopy in space exploration, from the Viking program onward,” says Beauchamp. On the recent Cassini-Huygens mission to Saturn, data from the Cassini Dust Analyzer (CDA) showed that Saturn’s outer ring was formed from dust spraying off of the south pole of its moon, Enceladus.

“Having seen the CDA, we were inspired to see if we could build something that was smaller in size, used less power, but had high performance,” says Beauchamp. While the CDA is 17 kilograms and 1 meter long, the Dustbuster is only about 0.5 kilograms and 20 centimeters long. Two types of Dustbusters have now been built and tested: Dustbuster I is designed to sample cosmic dust found streaming through the solar system, while Dustbuster II is designed to sample the high flux of dust from comet tails.

How can something as simple as the mass of a molecule found in a tiny dust particle tell us about the history of our solar system? Cosmic dust’s journey often begins in distant stars, from which it is shot out across the galaxy through their solar winds or, more dramatically, a supernova. Some cosmic dust accumulates inside interstellar clouds that become unstable and collapse, forming new stars and planets. Much of our solar system, including the matter in your own body, was once cosmic dust particles flying through the galaxy.

A dust particle’s composition can be read like a passport. Inside stars, many of the heavier elements, like carbon, oxygen, and iron, are forged from lighter elements, like hydrogen and helium, through a process called nucleosynthesis. (Caltech physics professor Willie Fowler, PhD ’36, won the Nobel Prize in Physics in 1983 for working out the details.) Isotopes of the elements—atoms that have the same number of protons, but a different number of neutrons—are also created through nucleosynthesis. Depending on the type of star and its stage in life, nucleosynthesis will produce different mixes of elements and isotopes, so by analyzing the cosmic dust, scientists can learn about the evolution of stars. Organic, carbon-based molecules are synthesized as the dust flies through different chemical environments in space, like on the tails of comets. Scientists are very interested in these, as such molecules may have served as precursors to DNA, amino acids, and other biological molecules on Earth.

Besides Beauchamp’s work on the Dustbusters, he has also been working on a return visit to Saturn’s moon Titan. “We have been heavily involved with looking at Titan as a model for early Earth,” says Beauchamp. Lab experiments that simulate conditions on Titan and data from the Huygens lander have confirmed the presence of simple organic molecules there. “‘Astrobiological hotspot’ is a term I like to use. It’s where you suspect there are the conditions for emergent synthesis of organic molecules,” says Beauchamp. Learning how this occurs on the surface of Titan could help explain how the molecules of life were first synthesized on Earth.

To study these astrobiological hotspots, any probe returning to Titan will need a mass spectrometer. “Mass spectrometers are extremely valuable tools for such missions,” says Kim Reh at JPL. Reh was part of a team that submitted a proposal to NASA in October for a mission to “prebiotic” moons in the outer solar system, like Jupiter’s moon Europa and Saturn’s moons Enceladus and Titan. Beauchamp was a consultant to the team. “NASA intends to review the results of this study by the end of this year and select one or two of these science targets for further study in 2008. The longer-term goal is to select a mission in 2009,” says Reh. —MT