0
Facebook
Volume 85 Issue 4 | pp. 41-42
Issue Date: January 22, 2007

Comet Wild 2's Complex Chemistry

Particles and gas harvested during Stardust mission were formed in a turbulent early solar system
Department: Science & Technology
The Stardust spacecraft followed this orbit as it caught up with comet Wild 2, then returned a capsule filled with comet particles to Earth.
Credit: NASA/Ames Research Center
This movie taken from a NASA DC-8 aircraft shows the Stardust sample return capsule entering the atmosphere in the early morning hours of Jan. 15, 2006.
Credit: NASA/Ames Research Center

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.

Treasure Chest
The capsule sent back to Earth by the Stardust spacecraft came to rest on the Utah desert floor.
Credit: NASA/Ames Research Center
8504sci2_capsulecxd
 
Treasure Chest
The capsule sent back to Earth by the Stardust spacecraft came to rest on the Utah desert floor.
Credit: NASA/Ames Research Center

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.

Heavenly Body
Composite image of comet Wild 2 shows a halo of dust.
Credit: NASA
8504sci2_release
 
Heavenly Body
Composite image of comet Wild 2 shows a halo of dust.
Credit: NASA

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."

Happy Trails
Particles of dust from comet Wild 2 embedded themselves in the Stardust spacecraft's aerogel collectors, forming these tracks.
Credit: NASA/Ames Research Center
8504sci2_trackscxd
 
Happy Trails
Particles of dust from comet Wild 2 embedded themselves in the Stardust spacecraft's aerogel collectors, forming these tracks.
Credit: NASA/Ames Research Center

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.

 

More on this Story

  • Comet Wild 2's Complex Chemistry
  • Particles and gas harvested during Stardust Mission were Formed in a turbulent early solar system.
  • Stardust@home
  • 'Dusters' Look For Interstellar Dust Tracks
  • Video: Spacecraft Trajectory *
  • Stardust followed this orbit as it caught up with comet Wild 2, then returned a capsule filled with comet particles to Earth.
  • Video: Stardust Capsule Reentry *
  • This movie taken from a NASA DC-8 aircraft shows the Stardust sample return capsule entering the atmosphere in the early morning hours of Jan. 15, 2006.
  • Image: Treasure Chest
  • The capsule came to rest on the Utah desert floor.

  • * Macromedia Flash Player 8 is required to view videos.

 
Chemical & Engineering News
ISSN 0009-2347
Copyright © American Chemical Society