A cotton candy-like cloud of simple sugar drifts in the unspeakably cold center of the Milky Way about 26,000 light years away, offering a remote, yet tantalizing, hint of how the building blocks of life may have reached Earth billions of years ago.
This frigid cloud is composed of molecular glycolaldehyde, a sugar that, when it reacts with other sugars or carbon molecules, can form a more complex sugar called ribose, the starting point for DNA and RNA, which carry the genetic code for all living things.
Astronomers have known about sugar in space for some time, but new research reported last week in the Astrophysical Journal Letters showed that gaseous sugar could exist at extremely low temperatures, as are found in regions on the fringes of the solar system where comets are born.
Thus, while many scientists agree that life probably derived from a rich "primordial soup" concocted in the warm-water puddles of early Earth, the new research offers fresh evidence for another popular view -- that life, or at least some of its basic ingredients, may have flown in from interstellar space aboard a comet or asteroid.
"These are long-standing questions," said astronomer Philip R. Jewell, of the Robert C. Byrd Green Bank Telescope in West Virginia. "You want to know what sort of molecules would form in the interstellar medium. This is a clue."
A four-member team led by Jan M. Hollis, of NASA's Goddard Space Flight Center, and Jewell, used Green Bank's 115-yard-diameter parabolic reflector to examine Sagittarius B2, a cloud of dust and gas several light-years wide at the heart of the Milky Way, in the direction of the constellation Sagittarius.
Green Bank is a radiotelescope that identifies specific molecules in the cosmos by analyzing their radio emissions as they rotate end over end in space. Each molecule has its own unique signature frequencies, derived and catalogued through testing on Earth.
Jewell said the team had found glycolaldehyde in a warmer part of the cloud in 2000, but this time detected it in an area where temperatures were only 8 degrees above absolute zero, that is, minus 445 degrees Fahrenheit. All molecular motion stops at absolute zero (minus 459 Fahrenheit).
"Being that cold is interesting," said research astrophysicist Scott A. Sandford, of NASA's Ames Research Center. "At 8 degrees kelvin, molecules aren't going to be hopping off into the gas phase."
Finding complex molecules floating free in cold space so that their radio signatures could be recognized was something of a surprise, Jewell said, because at such low temperatures, they are much more likely to be found frozen solid to dust particles in the cloud.
"You need something non-thermal to get the sugar molecules off the dust grains," said Sandford, speaking from his Mountain View, Calif., office. "A shock wave could go through the cloud, cause grain collisions and blow the molecules into the gas phase." Heat will not work, he added, because it would break down the sugar molecules into simpler compounds.
Jewell said shock waves are quite likely what happened: "This is a star-forming region, and while star formation is a pretty hot process, the shock waves would pass through the center of the region and out into the colder outer areas," jarring the dust to release the sugar molecules.
It is unclear whether the glycolaldehyde, a simple "two-carbon" sugar containing two carbon atoms, two oxygen atoms and four hydrogen atoms, was frozen to the dust particles before the shock wave came by, or was formed by interstellar chemistry after the shock wave liberated simpler molecules.
In either case, however, "the conclusions are pretty exciting," said University of Arizona astrochemist Lucy M. Ziurys, director of the Arizona Radio Observatory. Ziurys, an expert in developing radio signatures for carbon molecules, has criticized the Green Bank team for not being thorough enough, but said her own students had replicated the Green Bank results.
"If sugar's in space, it's an important thing," Ziurys said in a telephone interview. "You add a few more carbons, and you end up with a sugar called ribose, and ribose is an essential component" of DNA and RNA.
What that means, however, is anybody's guess: "So suppose we have these interstellar clouds that are producing sugar molecules, and they're found throughout the galaxy," Ziurys said. "The big question is: Did the basic ingredients of life begin out in these clouds or on a planet?"
"We don't have a clue," Sandford said. "This seems to raise the odds that life could get started out there, but we don't know. That's why most of these arguments tend to be of a general nature."
In our solar system, and presumably elsewhere, the colder reaches of space are areas where particles of dust, ice and other debris bond in ever-larger clumps that eventually become comets.
Most comets in the solar system were formed about 4.5 billion years ago near the planets Uranus and Neptune and were subsequently cast into deep space well beyond Pluto. They reenter the solar system when nearby stars or large planets perturb their orbits.
Scientists long ago raised the possibility that early impacts from comets -- or asteroids from the belt between Mars and Jupiter -- may have brought Earth most of its water supply as well as the sugars and other compounds that served as the building blocks of life. The Green Bank research provides further evidence that this may have occurred.
Once liberated from their icy embrace and allowed to steep in warm water on the Earth's surface, the sugars could have combined with other carbon compounds to form ribose and, eventually, DNA and RNA.
But while this view appears to clash with more traditional thinking -- that the early Earth mixed its own soup without any help from space -- there is no reason why both phenomena could not have occurred.
"Current thinking is that sugars formed on the planet, but they could have been deposited on the planet by a comet or by interstellar dust," Ziurys said. "The important thing is that one method does not exclude the other."
And "nothing says that the stuff that fell out of the sky was the key thing, or the stuff that came from hydrothermal vents was the key thing, or the stuff that was struck by lightning was the key thing," Sandford said. "In the end, the chemical system that made life on Earth wasn't worried about 'Made in' labels. It just grabbed what it needed."