News of recent lab experiments — demonstrating how chemical “cosmic barbecues” around nearby dying carbon-rich stars might have created life’s prebiotic building blocks — glossed over one key point. That is, would enough of such material have survived the journey from interstellar space through Earth’s atmosphere to a violent impact on the surface of our young planet?
The idea that long chain precursors of biologically-crucial nucleic acids, like RNA and DNA, might have actually formed in the seemingly inhospitable wilds of the interstellar medium certainly makes for interesting reading. But is it really relevant for the evolution of life on an earthlike planet?
As detailed in The Astrophysical Journal, researchers from Lawrence Berkeley National Lab in California and the University of Hawaii at Manoa have for the first time demonstrated that cosmic hot spots near these stars could be ideal for the creation of bio-relevant, nitrogen-containing ringed molecules.
The team recreated the conditions in the inner circumstellar envelopes of such stars that could provide the energy to allow for such chemistry. Such reactions aren’t likely in our galaxy’s plethora of blood-cold molecular clouds.
Thus, first they injected gas made of a nitrogen-containing, single-ringed carbon molecule and two acetylene carbon-hydrogen molecules into a 700 Kelvin hot nozzle. Then using radiation from the Advanced Light Source (ALS) at the Berkeley Lab, the team probed the gas. They found that their hot nozzle had transformed the initial gas into complex, nitrogen-containing ring molecules of quinoline and isoquinoline.
As a result, the team is now convinced that these hot stellar environments can form such key molecules, which ultimately are ejected into the interstellar medium and dispersed via the stellar winds.
If so, what’s the best way to look for these molecules in space?
“These molecules are highly flexible and contain many atoms, so if they are in space, it would be almost impossible to detect them via radio astronomy,” said Professor Ralf Kaiser, the paper’s second author and a physical chemist at the University of Hawaii at Manoa. “The best way [to find them] is to probe our solar system’s cometary matter and meteorites.”
If such nucleic acids originated outside our solar system near a neighboring hot star, could they have survived the journey to Earth’s surface without be ripped apart or destroyed?
Once molecular clouds collapse and transit to star-forming regions, says Kaiser, this bio-relevant matter can enter circumstellar disks, where planetary bodies form.
“Organics such as nucleic acids, which were initially formed in interstellar space, can be incorporated into meteoritic parent bodies,” said Kaiser. “So, at least a fraction of the organics formed in the interstellar medium can survive impact on Earth.”
Ideas about where RNA and DNA actually first arose has somewhat loosened in the last decade or so. In contrast, in my 2001 book Distant Wanderers I wrote: “Most biophysicists now believe that RNA emerged from ocean sediments containing sulfur-rich clay compounds called thioesters. Chemical bonding in thioesters may have fueled a sort of prebiotic metabolism leading to the chains of proteins that form RNA. DNA then evolved from RNA.”
Thus, could these compounds have just as easily formed in situ at the bottom of Earth’s early oceans?
Kaiser says local in situ formation of such molecules is more than tricky.
“Glycerol – the most fundamental unit of prebiotic cell components — can only be formed under a narrow range of conditions,” said Kaiser. “And it cannot form at either low or high pH ranges or in the presence of calcium or magnesium salts which would be present at very tiny concentrations on early Earth.”
In contrast, Kaiser says molecular formation in the interstellar medium is much more versatile. He notes that previous other laboratory experiments have provided compelling evidence that bio-relevant precursor molecules such as glycerol and linked amino acids can be easily formed in interstellar ices.
But this subject’s most compelling question is also the hardest: Would the fraction of nucleo-bases that formed in space; that is, nitrogen-containing ringed molecules which help make up RNA and DNA have been enough to give Earth’s earliest organics that extra biological oomph? Or would life here have also needed local production of these complex compounds?
“There are too many uncertainties from this one study to answer,” said Kaiser. “There is simply so much speculation now, that we don’t want to guess.”