Far-out sugar may help crystallize knowledge of drifting molecules

Monday, June 26th 2000, 12:00 am
By: News On 6

Scientists have discovered sugar in the Milky Way – but no chocolate, caramel or nougat yet.

Astronomers have spotted a simple sugar molecule called glycolaldehyde floating near the center of the Milky Way galaxy. While the discovery won't send candy manufacturers into outer space, it could help scientists understand how certain molecules – including the sugar-based building blocks of life – might form elsewhere in the universe.

"How sweet it is!" said Stephen Maran, an astronomer at NASA's Goddard Space Flight Center in Greenbelt, Md. "Now, if we could just mix it with the Gum Nebula, we could make Chiclets."

Researchers led by Jan M. Hollis, another Goddard astronomer, announced the sugar discovery this month. They have submitted their work for publication in Astrophysical Journal Letters.

Astronomers know of about 120 chemical compounds that drift between the stars, but glycolaldehyde is the first sugar discovered there. It's a molecule made of just eight atoms – two of carbon, four of hydrogen and two of oxygen. But those atoms can form the backbone for creating more complex sugars – such as ribose, a building block for life-critical molecules such as RNA, and glucose, a sugar found in fruit.

Complex sugars couldn't form in space if the simple molecule weren't already there, said Dr. Hollis.

"If you didn't find glycolaldehyde, that would be the end of that."

In May, the scientists used a radio telescope on Kitt Peak, Ariz., to scrutinize the light coming from a gas and dust cloud called Sagittarius B2. The cloud, which lies about 150 million billion miles from Earth, is one of the famous places that astronomers study when looking for interstellar molecules. Such cold and dense molecular clouds are the raw material from which stars and planets form.

In these clouds, dust particles provide a surface on which chemical reactions happen, with atoms joining to create molecules. Simple molecules, such as water, are the most abundant, said Dr. Hollis. But sometimes, astronomers discover more complex molecules, such as methanol, a simple alcohol, or acetic acid, known more commonly on Earth as vinegar.

Like most interstellar molecules, glycolaldehyde announced its existence through faint radio messages.

As molecules tumble through space, they occasionally switch from one energy level to another, giving off a specific amount of radiation in the form of radio waves. Dr. Hollis' team scanned Sagittarius B2 for radio emissions similar to those that glycolaldehyde emits on Earth. When the team found radio waves at six particular frequencies, known to correspond to those of glycolaldehyde, they knew they had found the molecule.

The team also looked for glycolaldehyde in the Orion Nebula and in the dust clouds surrounding several other stars, but Sagittarius B2 was the only place they found it.

"We were just happy to get it in this one," Dr. Hollis said.

By itself, glycolaldehyde is the simplest known sugar, containing only two carbon atoms. Table sugar, or sucrose, has twelve carbon atoms. But the simple molecule still raises some intricate questions.

For instance, glycolaldehyde is the third version of a two-carbon, four-hydrogen, two-oxygen molecule discovered in space. Molecules that have the same molecular formula but different atomic structures – like a Tinkertoy that's taken apart and put back together in a different arrangement – are called isomers. Scientists had already found two other isomers of C{-2}H{-4}O{-2}, methyl formate and acetic acid, in space. And it's not clear why the conditions in space would favor three different isomers to form.

"I'm speculating here, but it may indicate that interstellar chemistry may work a little bit differently than people had thought up to now," said Ed Churchwell, an astronomer at the University of Wisconsin-Madison.

Most astronomers have believed that space molecules join together atom by atom. But the atoms might first form molecule fragments, which then join together like partially assembled Tinkertoys, Dr. Churchwell said.

"That's a pretty exciting prospect," he said.

Knowing how isomers are put together in space might also shed light on how molecules assembled themselves early in Earth's history.

Some astronomers think that the conditions inside molecular clouds could resemble the conditions that prevailed on early Earth. So scientists are looking for other important space molecules, such as glycine. The amino acid is the simplest building block for proteins, which carry out the inner workings of cells.

Glycine can be made by mixing ammonia and acetic acid, two compounds already known to exist in molecular clouds. Dr. Hollis and other researchers have searched for glycine in space, but with no success so far.

Searches for other molecules have had better luck. NASA's Stardust spacecraft, now on its way toward a 2004 rendezvous with Comet Wild-2, recently snagged tarlike molecules in space. Other results, from a European infrared space telescope, suggest that polycyclic aromatic hydrocarbons – chains of ringlike molecules found in car exhaust and cigarette smoke – also exist in interstellar space.

But for now, Dr. Hollis' team wants to learn more about the sugar – where exactly in Sagittarius B2 it floats, and how much of it is there.

"We suspect it's coming from a source that's relatively small," he said, "from the hot core region of that particular molecular cloud." Further observations could help answer those questions, he said.

But he won't be able to use the same telescope he used to discover the glycolaldehyde. The National Radio Astronomy Observatory's 12-meter telescope, which also discovered dozens of other space molecules, is scheduled to be shut down next month. The money saved will go instead to developing a new array of 64 radiotelescopes in northern Chile.

At least, Dr. Hollis said, the 12-meter telescope performed well before its curtain call.

"It went out with a pretty nice send-off."