Space-Dust Science

To make a planet, start with an interstellar cloud of ice-covered dust and gas. Then add random turbulent processes to create structure within that cloud. A portion of the mixture can become dense enough to collapse under its own weight and begin star formation. The material that doesn’t collapse forms a disk that rotates around the protostar. Eventually, the dust and gas in the disk start to aggregate, forming planets, as well as asteroids and comets.

The dust grains have carbonaceous or silicate cores encased by ices composed of frozen compounds such as water and carbon dioxide, and the chemistry that occurs on their surfaces is key to understanding the chemistry of newborn planets. But “it’s only recently that the astronomy and astrochemistry communities have embraced the role of grain surfaces,” said Thomas Orlando, a chemistry professor at Georgia Institute of Technology and co-organizer of a symposium on astrochemistry at the American Chemical Society national meeting in San Francisco last month. “There’s now a full realization that the grain surface is very important in the formation of even the simplest molecules and could be at the heart of the formation of bigger molecules,” such as polyaromatic hydrocarbons (PAHs), Orlando said.

The molecular species that astrochemists look at tend to be simple—H₂, CO, HCN, CH₄, C₂H₂, and NH₃, for example—but groups are also working to identify and understand more complex compounds. Much chemistry on interstellar dust particles is driven by radiation, in particular ultraviolet light and X-rays from protostars and cosmic-ray particles. Astrophysicists and astrochemists both observe the compounds present in space—principally through ground-based and satellite telescope observations—and try to emulate astronomical conditions in laboratory experiments.

The formation of H₂ in interstellar space is one example of a chemical process in which dust plays a critical role. Hydrogen is the most abundant element in the universe, and H₂ is the most abundant molecule in space. Hydrogen plays a key role in reactions leading to bigger molecules.

The association of two hydrogen atoms to form H₂ is an exothermic reaction, and a third atom or molecule takes up the excess energy under typical conditions. In the interstellar medium, however, hydrogen-atom densities can be as low as 1–10 atoms/cm³ in diffuse clouds. “It’s improbable to have three atoms interact in the gas phase because of the low density,” said Gianfranco Vidali, a physics professor at Syracuse University, during the symposium.

Instead, the likely mechanism of H₂ formation involves hydrogen atoms sticking to interstellar dust grains, where the hydrogens can move about and interact. Vidali is studying these reactions by using atomic and molecular beams to irradiate amorphous silicates at 10–15 K. By developing a better understanding of how atoms and molecules adsorb onto, diffuse around, react with, and desorb from the surface (in some cases, a single photon can cause enough of a temperature jump to get an atom to desorb), he aims to provide a better model for how these reactions occur.

The ices that build up on dust also come from surface reactions, said Eric Herbst, a physics professor at Ohio State University. The simple picture of an interstellar dust particle is one of silicates or carbonaceous material in the middle and water, CO, and CO₂ ices on the surface, along with more complex organic species. Even though CO might be made in the gas phase and accreted onto the dust, “you don’t have much water in the gas phase,” Herbst said. Instead, water comes from the reaction of oxygen and hydrogen atoms on grain surfaces. The same would be true for turning atomic carbon into CH₄ or CO into formaldehyde. As with H₂ formation, adsorption and desorption processes are critical to understanding the species present in the ices and how they react, he said.

Researchers are also studying how to identify different ice mixtures—say, water, methanol, and CO₂ in varying ratios—to see what happens when compounds are deposited onto surfaces or when mixtures are heated. The goal is to build a database of spectroscopic profiles that can be compared with telescope observations to help scientists pinpoint what mixtures of species they might be looking at and how those compounds originated, said Perry A. Gerakines, a physics professor at the University of Alabama, Birmingham.

But not everything is icy around a star in its early stages of formation. Near the protostar, temperatures can be hot enough to put silicates into the gas phase. Particles at a distance retain their icy nature.

Karin I. Öberg, currently a postdoctoral researcher at the Harvard-Smithsonian Center for Astrophysics, described experimental work she did as a graduate student aimed at deducing the chemistry triggered in methanol-rich ices irradiated with UV light. She and colleagues at the Raymond & Beverly Sackler Laboratory for Astrophysics at Leiden University, in the Netherlands, found that some molecules, such as ethanol and dimethyl ether, form in a constant ratio. This is the type of information that can be used to determine whether gas-phase molecules in astrophysical environments have an icy origin. Other mixtures of molecules have varying ratios that depend on initial conditions, such as ice composition and temperature; these properties can be used as clues to explain astrophysical observations.

Not all of the interest in ice chemistry focuses on simple organic molecules. For some, these compounds are merely building blocks of chemicals of greater interest: PAHs. Over the past 15 years, observations have shown that interstellar clouds “all have a skin of PAH emission glowing all around them,” said Louis J. Allamandola, director of the Astrophysics & Astrochemistry Laboratory at the National Aeronautics & Space Administration (NASA) Ames Research Center, in Moffett Field, Calif. As a class, PAHs are more abundant than all other known interstellar polyatomic molecules combined. Until now, researchers have rarely considered PAH species in ices within interstellar clouds, “but it’s hard to imagine that they surround clouds but are not in the clouds,” Allamandola said.

Experiments show that the ionization potentials of PAHs are lower in water ice, Allamandola said, making ionization-driven reactions an important component of modeling ice chemistry. In experiments with pyrene, he and colleagues have found that ion chemistry dominates in ices below 50 K, and at higher temperatures radical chemistry takes over.

Temperature changes might also be critical for PAH chemistry in ices, noted Murthy S. Gudipati, a principal scientist at NASA’s Jet Propulsion Laboratory, in Pasadena, Calif. When naphthalene, for example, is ionized, it gets oxidized to naphthol—but only when it is warmed to 125 K. “This shows that ionized molecules may form some kind of complex with available radical and ionic species in ice at lower temperatures,” but mobility of molecular species at higher temperatures is needed to form other products, Gudipati said. He and his coworkers are studying laboratory reactions of such species in an effort to model what happens to them in space.

Ultimately, one of the goals of astrochemistry research is to better understand the compounds available during and after the formation of stars and what roles they might play in a prebiotic world on Earth and other planets, both in our solar system and around other stars. “Understanding the chemistry of solar system ices is an integral part of understanding the evolution of stars and galaxies,” Gudipati said.