Comet Wild 2's Complex Chemistry

The fleets of probes we humans have sent out in recent decades to explore the solar system are nothing short of dazzling, but there's nothing like bringing back to Earth actual extraterrestrial material to study.

So when a capsule filled with particles snatched from the comet Wild 2 by NASA's Stardust spacecraft landed last January with a soft thud in the Utah desert, the scientific community immediately pounced on it.

Less than a year later, dozens of scientists have pooled their results to make a grand presentation in Science. They found a collection of wildly heterogeneous dust grains and captured gases. Trapped in the spacecraft's light, porous aerogel collectors were some 1,000 particles between 5 and 300 µm across. Among the particles are inorganics such as the silicate minerals olivine and pyroxene and a host of organics, some contained in the dust and others in the form of gas (Science 2006, 314, 1711, 1716, 1720, 1724, 1728, 1731, and 1735).

Comets are known to have coalesced in the cold outer regions of the solar system, but Wild 2 doesn't just contain materials that originated there. Olivine is common in the solar system but is believed to have formed at high temperatures in regions near the newly formed sun. And a few particles even came from interstellar space, formed long before our solar system existed, notes Michael E. Zolensky, a Stardust mission scientist at NASA's Johnson Space Center in Houston.

Theories of the solar system's formation abound, but as a result of the new findings, one candidate has gained additional support. That theory postulates that the early solar system, rather than being a neat, concentric disk of dust and gas, actually flung material far and wide via radial jets spewing out of the sun.

"It tells us the early solar nebula was a pretty wild place," says Scott A. Sandford, an astrochemist at NASA's Ames Research Center, in Moffett Field, Calif.

Given the relative delicateness of organic compounds compared with minerals, scientists were pleased that so many organics survived the impact with the aerogel and the journey to Earth. Jason P. Dworkin and Daniel P. Glavin of the astrobiology analytical lab at Goddard Space Flight Center, in Greenbelt, Md., found much of the organics in seemingly blank portions of aerogel that don't contain particles—implying that the aerogel captured molecules in the gas phase.

In the Stardust grains, scientists have found polycyclic aromatic hydrocarbons (PAHs), which have been found in meteorites, and also a host of nitrogen- and oxygen-rich compounds. These molecules are more "primitive" than organics generally found in meteorites, meaning they haven't been processed thermally since the comet assembled. If the compounds had been "really cooked," Sandford says, nitrogen and oxygen would have been driven out, and the compounds would have become carbon-rich. "It really implies comets have been great little storage refrigerators since 4.5 billion years ago," he says.

In particular, mission scientists have been able to identify methylamine and ethylamine, which have not been detected in comets before. It's long been a contention that comets may have seeded young Earth with prebiotic compounds, and this new finding suggests comets could have been an important source of nitrogen compounds, potentially used by early organisms as an energy source.

The group also has identified possible traces of glycine, the simplest amino acid, and β-alanine (3-aminopropionic acid). A definitive announcement must wait until the group can determine the ratios of key isotopes—¹³C/¹²C and ¹⁵N/¹⁴N—in the molecules, which will pin down their origin, terrestrial or extraterrestrial. "There are lots of amino acids in carbonaceous meteorites, so there's no reason to believe they wouldn't exist in comets," Glavin says. "We're starting to understand what a prebiologic inventory of early Earth may have looked like."

Sifting through the organics captured by Stardust presents a unique challenge, scientists say: Which molecules actually survived intact, and which are the result of processing after impact with the aerogel? Contaminants are also ubiquitous. Anticipating this, the project's designers included an aerogel sample that flew on the craft but was not exposed to the comet. Because that sample contained far less ethylamine or methylamine than the exposed sample, it's a pretty safe bet the bulk of these compounds came from the comet.

Mission scientists also frequently detect 6-aminohexanoic acid, the hydrolysis product of nylon 6. The sources of this molecule, Dworkin and Glavin determined, are the nylon bags and containers used to curate the samples. In fact, its discovery solves a long-standing mystery. For years, this spectral peak was seen in Antarctic meteorite samples that were also bagged in nylon. Even ALH84001, the famous martian meteorite once posited to contain traces of life, may show this nylon contamination. "The good news is we've convinced the curation office to stop using nylon," Glavin says.