Something Surprisingly Sweet Lurks Near the Heart of the Milky Way

Scientists have spotted something sweet in the heart of the Milky Way.

Erythrulose, a sugar found in raspberries, kiwis and many red fruits, is also believed to exist in a giant molecular cloud of gas and dust near the center of our galaxy, some 26,745 light years from Earth. This is the first time that a sugar has been observed in interstellar space. The results were published in Natural astronomy.

The question is: how did this sugar get there? For life as we know it, sugars are essential, both as energy storage molecules and as ingredients of biological building blocks such as DNA and RNA. But they’re also relatively fragile and not necessarily easy to make from scratch, either in deep space or on prebiotic Earth. Molecular clouds, however, offer a potential shortcut for making sugars, because they are “huge chemical factories,” he says. Izaskun Jiménez-Serralead author of the study and an astrochemist at the Spanish National Research Council and the Astrobiology Center of the National Institute of Aerospace Technology in Spain. Molecular clouds can also be star nurseries, incubating new stars and planets in their depths.

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Called G+0.693-0.027, the cloud in which Jiménez-Serra and her team discovered sugar is rich in chemicals and “an excellent astronomical laboratory where we can look for new molecular species,” she adds. The dust that permeates the cold, dark depths of the cloud is essential. Dust provides surfaces on which atoms and molecules can group together, allowing them to become larger and more complex; it also blocks ultraviolet rays and other high-energy lights that might otherwise destroy larger compounds as they grow. Deeper in the clouds, more dust blocks more radiation, temperatures drop, and ices of water and carbon dioxide cover the dust grains, along with increasingly complex molecules.

Izaskun and his colleagues used two giant radio telescopes in Spain – the 40-meter Yebes antenna and the 30-meter IRAM – to pierce the dusty veil of G+0.693-0.027 and discern some of its subtle cosmic chemistry. Compared to higher-energy light, radio waves pass through giant clouds of gas and dust unscathed – and some of those radio waves come from the molecules made by the clouds themselves. Driven from their dust grain perches by shock waves from nearby supernovae and other effects, these molecules can emit a faint but detectable radio glow as they spin. Each molecule imprints its own barcode-like pattern on this light, and this pattern can be seen when astronomers analyze the light into its constituent colors.

These patterns are “like a strange comb where the positions of the teeth on the comb represent the frequencies at which a molecule diffuses,” explains Nick Indrioloan astronomer studying the interstellar medium at the Space Telescope Science Institute in Baltimore, who was not involved in the study. Finding individual molecules can be complicated because there may be hundreds of other molecules in the molecular cloud that are also sending their signals at the same time.

In order to identify a given molecule, scientists must first discover what its unique light pattern is by vaporizing the molecules in a laboratory on Earth. Sugars are “difficult” to measure, Jiménez-Serra explains, because they are syrupy liquids. A recently developed technique stabilized a sugar by mixing it with talc to create a solid which, when vaporized with a laser, gave a diagnostic light pattern.

Armed with this crucial information, Jiménez-Serra and his team scoured their data from G+0.693 to 0.027 in search of sugar. They found abundant evidence of erythrulose, which contains four carbon atoms, but surprisingly little evidence of sugars made from three carbon atoms in the same region, defying the traditional assumption in astrochemistry that these molecules would form by adding one carbon atom at a time. Instead, the team posits that erythrulose could have formed from glycolaldehyde and ethylene glycol, two molecules that were also found in the cloud and which each had a pair of carbon atoms. Researchers are now working on follow-up experiments to look for more complex sugars and test how these delicate molecules respond to ultraviolet light.

“More than 300 molecules have been identified in space,” explains Indriolo. So far, most of them are proving toxic to humans, but as astronomers delve deeper into the hidden hearts of molecular clouds, they are discovering more compounds that are complex precursors to life. It seems that the recipe for biology arises even in one of the most inhospitable places imaginable. “It was only hypothesized that sugars could form in regions of space, which would eventually give rise to new stars and planets,” says Indriolo. “But now we know that sugars can form in these regions.