A natural sugar found in raspberries and used in fake tan lotions has been detected in an enormous cloud of dust and gas that lurks near the heart of the Milky Way.
The discovery does not suggest that the galaxy revolves around a distant civilisation of pale, safety-conscious frugivores, but shows that compounds important for life can form in the frigid expanse between the stars.
The simple sugar erythrulose appears to be produced through chemical reactions on tiny interstellar dust grains, which then rain down on nearby worlds or reach them after being incorporated into comets that eventually clatter into planets.
“This is the very first sugar to be detected in interstellar space and it is important because it tells us that these sugars are more common than we previously thought,” said Dr Izaskun Jiménez-Serra at Spain’s Centre for Astrobiology near Madrid. “It opens the possibility for life to develop on other worlds in a similar fashion to what it did in on Earth.”
Scientists have struggled to understand how simple sugars became abundant on Earth, because lab studies show they would not have formed easily on the young planet. The previous detection of sugars in ancient meteorites and on the Bennu asteroid suggested some sugars may come from space, but until now no such compounds had been directly detected in what astronomers call the interstellar medium.
Jiménez-Serra and her colleagues used two Spanish radio telescopes to observe a dust cloud called G+0.693-0.027 near the centre of the Milky Way. After finding no trace of simple sugars bearing three carbon atoms, they were not hopeful of finding others, but then spotted the signature of erythrulose, a four-carbon sugar. “To my surprise, I saw the signals,” Jiménez-Serra said.
Writing in Nature Astronomy, the researchers describe how erythrulose can form when two other organic compounds, glycolaldehyde and ethylene glycol, which are abundant in some corners of the cosmos, combine on microscopic dust grains. The reactions take place despite the temperature hovering around -250C.
As well as providing energy for life, simple sugars such as erythrulose can react to form ribonucleotides, the building blocks of what was probably the first genetic material, RNA. As early life evolved, DNA arose as a more robust store for genetic code, with RNA becoming the intermediary between genes and the proteins from which life is made.
The scientists estimate that millions of tonnes of erythrulose could have rained down on Earth when a surge of asteroids and comets battered the planet during what is called the Late Heavy Bombardment. “To have suffered this kind of rain of organics, I think that seems to have been a key step,” Jiménez-Serra said. “That material could have contributed to prebiotic soups where the first biomolecules were synthesised.”
Erythrulose is found in trace amounts in red raspberries, but the compound is also used in fake tan lotions. The sugar reacts with amino acids in dead skin cells to create brown polymers called melanoidins through the Maillard reaction. The same process gives steak its dark crust.
“We have been waiting for an actual detection like this,” said Prof Yoshihiro Furukawa at Tohoku University in Japan, who discovered sugars in the Bennu asteroid. “Sugars formed in the interstellar medium can reach Earth and other planets via cometary dust … This supply may have helped facilitate the emergence of life, if planetary environments were able to build life from such molecules, although that process itself remains unclear.”
