Complex sugars cooked up from 'comet ice'
Scientists have detected ribose - a sugar needed to make RNA and DNA - in laboratory experiments which simulate the very early Solar System.
They shone UV light on a simple, frozen mixture of chemicals mimicking the ices that form in space, between stars.
As it condensed and then warmed up, the ice produced "substantial quantities" of ribose, alongside other molecules.
Published in Science, the research is the first to show that sugars can be produced in such a simple way.
It suggests that these critical molecules could form when similar ices condense around dust grains and comets in the vicinity of a young star.
Previously, nobody knew how a complex sugar like ribose could emerge from the messy, icy environment of a solar nebula - the disc-shaped cloud that preceded our Solar System.
Some of life's other building blocks are better understood. Amino acids, which are strung together to make proteins, have been detected in previous, similar laboratory simulations and also detected in samples from comets and meteorites.
Sugars are more of a mystery - partly because they have proved difficult to detect.
Cornelia Meinert, from the Université Nice Sophia Antipolis in France, said she and her team were probably not the first to manufacture these molecules in astrochemical experiments; sugars, including ribose, may have been there all along - undetected.
"In all the experiments that were run for the last 20-30 years around the world, the sugars were probably there," Dr Meinert told the BBC News website.
"We have a fancy technique called multidimensional gas chromatography - and this was the reason why we are now able to detect them."
Just a spoonful
So what is the recipe for making ribose in space?
Dr Meinert and her colleagues mixed methanol and ammonia with water, and subjected the cocktail to low pressure and very low temperature (-195C) in a vacuum chamber. They then allowed it to condense on a very cold surface, just as "pre-cometary" ice might settle around dust grains.
As it condensed, they hit the mixture with intense UV light - such as the young Sun would have emitted - and let it to warm up to room temperature.
The resulting residue, when they tested it using multiple "fancy techniques", contained not only ribose, but a veritable cookbook of complex molecules.
"You might think that there are not a lot of organics formed in these ices - but in fact it's the opposite," Dr Meinert said.
"We see a lot of different compounds and classes of compounds: amino acids, acids, alcohols, aldehydes - and the sugars. This means that the sample is very complex."
Likelihood of life
Importantly, these products could all be dissolved in water; without that solubility, they could never be incorporated into fledgling life-forms.
The results are consistent with evidence of organic molecules recently gathered from the very surface of a comet, Dr Meinert said.
The Philae lander, famously dropped onto Comet 67P by the Rosetta spacecraft in late 2014, detected what one scientist described as "frozen primordial soup" - including some of the precursors for making amino acids and sugars.
Now it seems that sugars themselves - including complex ones like ribose, made from a ring of five carbon atoms - could also be surprisingly common in space.
As Dr Meinert explained, this has potential implications for the likelihood of life in the wider Universe: "These ices are everywhere - so in other star forming systems [as well as ours] you should find amino acids and sugar molecules."
Astrobiologist Dr Lewis Dartnell, a research fellow at the University of Leicester, said the French team had come up with "a very exciting result".
"[It shows] the complexity of astrochemistry and the repertoire of organic molecules that are created beyond the earth in interstellar regions," he said.
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