Where did the ingredients for life on Earth come from? Many scientists think the basic chemical building blocks for biology were delivered via comet, but the building blocks -- and the building process -- remain a mystery.
Now, a team led by French researchers thinks they may have found lab-based evidence that a class of complex organic molecules could have evolved in the ice of star-forming clouds -- and could be a potential source for the organic matter that allowed life on Earth to emerge.
The findings, published in in Proceedings of the National Academy of Sciences, could also offer a chemical tip sheet for the European Space Agency's comet-chasing Rosetta mission, giving it hints as to which chemicals to look for.
The scientists were interested in what are called "evolved interstellar ices." These ices hang out in dense molecular clouds that give birth to a star and its surrounding solar system. As the star forms and the planets and comets and asteroids coalesce out of this debris, these ices are incorporated within them.
The ices contain many familiar molecules: water, carbon monoxide, carbon dioxide, methanol, ammonia and methane -- some of which are found on Earth in biological contexts. Comets or asteroids packed full of these ices could have delivered life-giving molecules to Earth, the thinking goes.
However, it takes a lot of chemical reactions to get from simple molecules to the complex strings that make up a macromolecule such as RNA (which is what scientists think coded the genetic instructions for living things before the emergence of DNA). How and where did all that chemistry happen?
As it turns out, a lot of chemistry is probably happening in the supposedly dead emptiness of space. It may not sound as conducive to cooking up complex organic molecules as the bubbling "primordial soup" we picture on early Earth, but space can be a chemically dynamic place, too. Ultraviolet radiation, cosmic rays and heat can cause atoms in the ice to stick together, fusing into ever-more-intricate molecules over time.
So, the chemistry happening in this space ice could be doing a lot of grunt work before these molecules ever reach a planet, producing the kinds of complex organic compounds that would be essential for life to emerge. And since stars in general form out of these ice-filled molecular clouds, it could be happening all over the place, the study authors wrote. It could be the same way that organic molecules are delivered to life-friendly exoplanets, too.
"Possibly at the origin of the organic matter in our solar system and incorporated into planetesimals," they wrote, "this material may be considered as a potential source for prebiotic chemistry on [rocky] planets, following a process that may be quite universal."
The problem is, it's not easy to study the evolution of these ices in the vastness of outer space. So the scientists set up an experiment in the laboratory where they could simulate the conditions of outer space, and watch what compounds were made when they subjected the basic starting materials (i.e. water, methanol, ammonia) to the harsh interstellar chemical processes.
Previous work had turned up many intriguing organic compounds, including amino acids -- essential molecules that today serve as the building block of proteins. But in this new experiment, the scientists actually found a whole other class of compound called aldehydes -- 10 of them, including two sugar-related molecules called glycolaldehyde and glyceraldehyde.
These two sugar-related compounds, the researchers wrote, are "considered as key prebiotic intermediates in the first steps toward the synthesis of ribonucleotides in a planetary environment."
So this lab-based experiment reveals that these interstellar ices could hold the potential starting materials for RNA -- and thus, so could any comet or asteroid that coalesced out of these ices (provided they haven't been altered beyond recognition in the intervening billions of years).
Evidence in the lab is one thing. The authors say the next step would be to actually find these compounds in their natural setting, perhaps by using the new Atacama Large Millimeter Array telescopes to look for glyceraldehyde. Both compounds could be searched for in comets as well in certain carbonaceous chondrite meteorites, they added.
The findings could also soon provide useful guidance for the Rosetta mission, whose Philae lander touched town on Comet 67P/Churyumov-Gerasimenko late last year. Philae's solar-powered battery has been drained, but if it manages to wake up from its slumber once it gets closer to the sun, its Cometary Sampling and Composition experiment has a device that's designed to look for such organic molecules.