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09 April 2016

The "R" in RNA can easily be made in space, and that has implications for life beyond Earth

BUILDING BLOCKS OF LIFE COULD DEVELOP IN INTERSTELLAR SPACE


New research suggests that the sugar ribose -- the "R" in RNA -- is probably found in comets and asteroids that zip through the solar system and may be more abundant throughout the universe than was previously thought.  

The finding has implications not just for the study of the origins of life on Earth, but also for understanding how much life there might be beyond our planet.

Scientists already knew that several of the molecules necessary for life including amino acids, nucleobases and others can be made from the interaction of cometary ices and space radiation. But ribose, which makes up the backbone of the RNA molecule, had been elusive -- until now. 


The new work, published Thursday in Science, fills in another piece of the puzzle, said Andrew Mattioda, an astrochemist at NASA Ames Research Center, who was not involved with the study.

"If all these molecules that are necessary for life are everywhere out in space, the case gets a lot better that you'll find life outside of Earth," he said.

RNA, which stands for ribonucleic acid, is one of the three macromolecules that are necessary for all life on Earth -- the other two are DNA and proteins.

Many scientists believe that RNA is a more ancient molecule than DNA and that before DNA came on the scene, an "RNA world" existed on Earth. However, ribose, a key component in RNA, only forms under specific conditions, and scientists say those conditions were not present on our planet before life evolved. So, where did the ribose in the first RNA strands come from?

To see if these molecules could have been delivered to Earth by asteroids and comets, a team of researchers re-created the conditions of the early solar system in a French lab to see whether ribose could easily be made in space. 

They started with water, methanol and ammonia because these molecules were abundant in the protoplanetary disk that formed around the sun at the dawn of the solar system, and are also abundant in gas clouds throughout the universe. They were put in a vacuum and then cooled to a cryogenic temperature of 80 degrees kelvin (minus-328 degrees Fahrenheit).


The resulting ices were then heated to room temperature, which caused the volatile molecules to sublimate, leaving a thin film of material.

"The simulation is of cometary ices only, not cometary dust grains," said Uwe Meierhenrich, a chemist at the University of Nice Sophia Antipolis in France and one of the authors of the study.

The experiment took about six days to complete and yielded just 100 micrograms of the artificial cometary ice residue in the lab.

Artificial cometary ices have been created hundreds of times before in labs around the world, but until now researchers have not had the tools to detect sugars like ribose in the samples. 

Cornelia Meinert, also of the University of Nice Sophia Antipolis, explained that it's not just sugar and sugar-related molecules that are created in these experiments, but also amino acids, carboxylic acids and alcohols.

"We are confronted with a very complex sample containing a huge diversity of molecules," she said. "The identification of individual compounds is therefore very difficult."

Meinert said it wasn't until the group was able to use a new technique called multidimensional gas chromatography that they were able to detect ribose in these samples at all.

The researchers say that the ice samples they made in the lab could easily be made in other parts of the solar system.

"Our ice simulation is a very general process that can occur in molecular clouds as well as in protoplanetary disks," Meinert said. "It shows that the molecular building blocks of the potentially first genetic material are abundant in interstellar environments."

Scott Sandford, a astrochemist who has done similar work with cometary ices at NASA Ames Research Center, said adding sugars to the list of molecules that can be forged in space is an important step in understanding what building blocks of life may be available to foster life in other worlds.

"Insofar as these materials play a role in getting life started on planets, the odds are good that they'll be present to help," he said.

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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.