October 2, 2019
Cosmic molecules may point to the origins of carbon-based life.
In a different reality, space might smell like almonds. After all, scientists surveying the chemicals in the cosmos have found benzonitrile; just a bit of the compound would fill your nostrils with a bitter almond scent.
But our cosmos is too vast. “Space smells like nothing,” says astrochemist Brett McGuire. “There’s not enough to get an actual whiff.”
Astrochemist Brett McGuire combines skills in chemistry and astronomy to search for complex molecules in space. Courtesy of B. McGuire
McGuire, 32, of the National Radio Astronomy Observatory in Charlottesville, Va., confirmed the presence of benzonitrile in a dark cloud in the Milky Way. He also discovered some of the other most complex molecules in space to date. By figuring out which molecules are out there, he and others hope to learn how the organic chemistry that undergirds all life on Earth — and perhaps anywhere else in the universe — gets started in space.
McGuire got his start in space as an undergraduate chemistry major at the University of Illinois at Urbana-Champaign. During a talk, Ben McCall, now a sustainability expert at the University of Dayton in Ohio, explained what he does for a living. He said something like, “I blow shit up, torture it with lasers and then I look for it in space,” McGuire recalls.
Enough said. McGuire spent that summer working in McCall’s lab, building a spectrometer to study how hydrogen gas, H2, reacts with H3+ — three hydrogen atoms with only two electrons. Some of McCall’s research included zapping gases of simple molecules with electricity — “an actual miniature lightning bolt,” McGuire says — to force atoms to recombine into new compounds that can’t be bought in a bottle.
“Brett was a very precocious young scientist,” McCall says. “This was the only time I’ve had a student who really started a new instrument from scratch as an undergrad.”
The discovery of benzonitrile in a dust cloud in the Milky Way suggests that complex molecules can form from the buildup of smaller molecules in space. (Carbon is black, hydrogen white and nitrogen blue.) Ben Mills and Jynto/Wikimedia Commons
Because space is so big and mostly empty, at least by Earth standards, it can take millions of years for two molecules flying around like billiard balls to get close enough to interact. “But it’s not just neutral billiard balls out there,” McGuire says. A charged molecule, like H3+, which has been discovered in interstellar space, can pull other molecules closer. “More or less all chemistry in space can trace itself back to H3+ at some point.”
And all that chemistry includes some tantalizingly lifelike stuff. In 2016, McGuire and colleagues reported discovering propylene oxide in a gas cloud within the Milky Way.
MOLECULE CLUE A gas cloud (Sagittarius B2) near the center of the galaxy (Sagittarius A*) is loaded with propylene oxide, a molecule that comes in mirror-image configurations. B. Saxton, NRAO/AUI/NSF from data provided by N.E. Kassim, Naval Research Laboratory, Sloan Digital Sky Survey.
That was the first molecule seen in space that, like the amino acids that make up proteins and are essential to life on Earth, has two forms that are mirror images of each other. Large rings of carbon and hydrogen, called polycyclic aromatic hydrocarbons, or PAHs, have also been spotted around dead or dying stars — though it’s been hard to tell how many carbons and hydrogens the PAHs contain.
PAHs are thought to be the seeds of dust, planets and organic chemistry in our galaxy and other galaxies, McGuire says. So how do they form? “How do you go from H3+ to things that literally click together to make the building blocks of life?” he asks.
The work of enumerating what’s out there mostly takes place in a lab on Earth. McGuire injects a puff of gas of the molecule he’s interested in into a large vacuum chamber, where the low temperature and pressure make the gas expand. Then he hits the gas with a pulse of intense microwave or radio radiation, sending the molecules tumbling. As they tumble, the molecules emit photons at a specific frequency. That light signature, called the molecule’s rotational spectrum, is what McGuire looks for when he searches for those molecules in space.
Once McGuire knows the molecular fingerprint he’s after, he turns to radio telescopes to find the same print in space. Many scientists focus on one branch of this process or the other, the laboratory spectroscopy or the interstellar astronomy; only a few have expertise in both. “Brett is one of those very few people,” McCall says.
To sniff almonds in space, McGuire and colleagues focused the Robert C. Byrd Green Bank Telescope in West Virginia on TMC-1, a dark cloud about 450 light-years from Earth “where maybe there are stars that are considering starting to form,” McGuire says. Forty hours of observing confirmed that benzonitrile, a benzene ring with a cyanide molecule stuck on the end, was there [Science].
Scientists have detected complex molecules in TMC-1, a stellar nursery in the Milky Way. The cloud lacks big, bright stars, and its dust grains glow only faintly (shown in orange). ESO
Lately, McGuire and colleagues are closing in on a bigger prize: specific PAHs in the space between stars. Knowing the makeup of PAHs in space will help reveal how they click together from smaller molecules, McGuire says. Finding these molecules would show that advanced chemistry is happening, in some cases before stars begin forming.
Benzonitrile and the more complex molecules it hints at are “the first clear marker” of carbon-based chemistry in space, says Ryan Fortenberry, an astrochemist at the University of Mississippi in Oxford who wasn’t involved in the benzonitrile finding. “Before this, we were just kind of wandering around in the wilderness,” Fortenberry says. “Now we have found the trail.”
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