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.
The simple sugar molecule glycolaldehyde was found in this dust and gas cloud, Sagittarius B2. The colors indicate radio emissions of different strengths.
(R. Gaume, M. Claussen, C. De Pree -- National Science Foundation)
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.