Meet Mr. McCall,
Space Detective
Benjamin McCall | University of
Illinois
Unlike any TV detective ever, Benjamin McCall is collecting fingerprints
that will guide him on a hunt in space that might ultimately help explain
the origins of the first seeds of life.
The fingerprints are taken, using spectroscopy, from new molecules he constructs
in his lab. With them, he will search by telescope for celestial substances that
match up in the interstellar clouds that form the nurseries where complex organic
molecules, and stars and planets, are born.
Simulation of the ground state of CH5+, courtesy of Joel Bowman.
Space chemists over the last 30 years have found some ordinary things,
like vinegar and sugar, in the distant clouds of gas and dust among the
Milky Way’s
200 billion or so stars. But McCall is pushing the search toward much harder-to-identify
molecules, fleeting ions that he has to construct himself.
First, though, he must complete a new instrument at the Roger Adams Laboratory
at the University of Illinois. The instrument will measure the characteristic
colors, or spectra, of these ions, providing the fingerprints to guide
him into the interstellar medium.
“We approach astrochemistry from two directions,” McCall said. “Laboratory
spectroscopy of astronomically important molecules, and astronomical spectroscopy
of molecules in the interstellar medium.”
He’s in a class apart. Only a handful of Americans hold joint appointments
in both astronomy and chemistry departments, and he’s one of them.
His work will help shape the new field called astrochemistry.
His colleagues at Illinois have been on the field’s leading edge since
it emerged. That celestial vinegar was detected in the ’90s with
a radio telescope by Illinois astronomers, in a cloud known as Sagittarius
B2, 25,000 light-years from Earth. But that search used spectroscopic fingerprints
that are easier to come up with.
“You can go to the grocery store for vinegar,” McCall said.
The newer search involves hydrocarbon molecules known as carbocations (pronounced
carbo-CAT-eye-ons). At the pressures humans are used to, these ions have
a lifespan of less than a billionth of a second, so he must concoct them
in plasma that has a pressure closer to that of the medium of space, where
they can live for hours. The plasma glows something like a neon sign as
it rushes into a vacuum chamber.
To take their spectroscopic fingerprints – checking their colors, or spectra – he
must remove the ions from the more abundant neutral molecules. “We will
suck the ions out of that plasma, using high voltages applied to metal plates,” he
said. “They will form a beam of ions, and then we have something
to work with. We can turn it, focus it. Do our spectroscopy on it.”
His lab uses the infrared part of the spectrum, which probes both rotations
and vibrations of the molecules. “In space, we’ll be looking
at the molecule rotating only. So there’s some quantum mechanical
massaging required to convert the infrared data into the spectrum that
we will observe with the radio telescopes,” he said.
In the second phase of their work, his team will use a new array of telescopes,
the Combined Array for Research in Millimeter Wave Astronomy, or CARMA,
run by a consortium that includes Illinois. CARMA is located in a remote
corner of California.
Investigating in the deep chill of space is hard. Chemistry unfolds slowly.
Densities are so low and temperatures so cold that no molecules are in
a hurry to react and make new products.
So McCall has narrowed his search to the most likely actors, the speedy
carbocations.
And there in the chemistry of the clouds, he hopes, will be the payoff: knowledge of the network of chemical reactions that created the products for the universe of things as we know them, including us.
Nov. 1, 2007
