Scientists have spotted something sweet at the heart of the Milky Way.
Erythrulose, a sugar found in raspberries, kiwis and many red fruits, also apparently exists in a giant molecular cloud of gas and dust near the center of our galaxy, some 26,745 light-years from Earth. This marks the first time a sugar has been seen in interstellar space. The results have been published in Nature Astronomy.
The question is: How did that sugar get there? For life as we know it, sugars are vital, both as energy-storage molecules and as ingredients in biological building blocks such as DNA and RNA. But they’re also relatively fragile and not necessarily easy to make from scratch, whether in deep space or on the early prebiotic Earth. Molecular clouds offer a potential shortcut for manufacturing sugars, however, because they’re “huge chemical factories,” says Izaskun Jiménez-Serra, lead author of the study and an astrochemist at the Spanish National Research Council and the National Institute of Aerospace Technology’s Center for Astrobiology in Spain. Molecular clouds can also be stellar nurseries, incubating new stars and planets in their depths.
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Called G+0.693-0.027, the cloud where Jiménez-Serra and her team discovered the sugar is rich with chemicals and “an excellent astronomical lab where we can look for new molecular species,” she adds. The dust that pervades the cloud’s cold, dark depths is key. Dust offers surfaces for atoms and molecules to glom onto, allowing them to become larger and more complex; it also blocks ultraviolet radiation and other high-energy light that could otherwise tear apart bigger compounds as they grow. Deeper in the clouds, more dust blocks more radiation, temperatures drop, and water and carbon dioxide ices coat the dust grains, along with molecules of increasing complexity.
Izaskun and her colleagues used two giant radio telescopes in Spain—the Yebes 40-meter dish and the IRAM 30-meter dish—to pierce G+0.693-0.027’s dusty veil and discern some of its subtle cosmic chemistry. Compared with higher-energy light, radio waves pass through giant clouds of gas and dust unscathed—and some of those radio waves come from the cloud-manufactured molecules themselves. Knocked from their dust-grain perches by shock waves from nearby supernovae and other effects, such molecules can emit a faint but detectable radio glow as they spin. Each molecule imprints its own barcodelike pattern onto that light, and the pattern can be seen when astronomers parse the light into its constituent colors.
These patterns are “like a weird-looking comb where the positions of the teeth on the comb represent the frequencies at which a molecule broadcasts,” says Nick Indriolo, an astronomer studying the interstellar medium at the Space Telescope Science Institute in Baltimore, who was not involved in the study. Finding individual molecules can get complicated because there can be hundreds of other molecules in the molecular cloud that are also sending out their signals at the same time.
In order to identify any given molecule, scientists have to first discover what its unique light pattern is by vaporizing the molecules in a lab on Earth. Sugars have been “difficult” to measure, Jiménez-Serra says, because they are syrupy liquids. A recently developed technique stabilized a sugar by mixing it with talcum powder to create a solid, which, when vaporized with a laser, yielded a diagnostic light pattern.
Armed with that crucial information, Jiménez-Serra and her team scoured their data from G+0.693-0.027 in search of sugar there. They found abundant signs of erythrulose, which contains four carbon atoms, but surprisingly little evidence of sugars made from three carbon atoms in the same region—defying a traditional assumption in astrochemistry that these molecules would form by adding one carbon atom at a time. Instead, the team posits, the erythrulose might have formed from glycolaldehyde and ethylene glycol—two molecules that were also found in the cloud and that each had a pair of carbon atoms. The researchers are now working on follow-up experiments to look for more complex sugars and test the delicate molecules’ response to ultraviolet light.
“Over 300 molecules have been identified in space,” Indriolo says. So far, most of these happen to be toxic to humans, but as astronomers delve deeper into the hidden hearts of molecular clouds, they’re finding more compounds that are life’s complex precursors. The recipe for biology, it seems, arises even in one of the most inhospitable places we can imagine. “It was only hypothesized that sugars can form in the regions of space that will eventually give rise to new stars and planets,” Indriolo says. “But now we know that sugars can form in these regions.”
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