Many theories about the origins of life on Earth posit that prebiotic compounds may have arrived from outer space on asteroids or comets. But a new study suggests that extreme chemical reactions fired up by meteorite impacts may have jump-started life in the early oceans, rather than delivering its building blocks preformed. Meteorites striking the primordial oceans, the paper's authors say, could have supplied significant amounts of carbon, critical to life, and created a sort of chemical pressure cooker by the force of their impacts to synthesize the foundations of biological molecules.
The researchers report in Nature Geoscience today that they replicated the impact of a chondrite, a common type of meteorite, striking the ocean at about 1.25 miles (two kilometers) per second. The team did this by subjecting chemical constituents of chondrites (iron, nickel and carbon), as well as water and nitrogen, believed to be plentiful in the early atmosphere, to shock compression. The resulting pressures and temperatures, which likely exceeded 5,000 degrees Fahrenheit (2,760 degrees Celsius), yielded a variety of organic (carbon-based) compounds, such as fatty acids and amines. And when ammonia, which a previous study showed impacts could produce, was added to the starting mix, the experiment also yielded glycine (a simple amino acid).
Study co-author Toshimori Sekine, a researcher at the National Institute for Materials Science in Tsukuba, Japan, says he was surprised by the output from the experiment, adding that "there are many additional molecules we found but didn't analyze yet." Lead author Yoshihiro Furukawa, a PhD candidate at Tohoku University in Sendai, Japan, says that in light of the results, "we can say those ocean impact events [were] very effective processes for the production of various biomolecules on the early Earth." He is quick to note, though, that it is unclear how much or how many of these biomolecules would be needed to initiate life.
To ensure the organic compounds were produced by the shock of the simulated impact (and not outside sources), Furukawa's group used carbon 13, a rare isotope of the element, in the meteorite proxy. The fact that the detected molecules were enriched with carbon 13 rather than the more common carbon 12, the authors say, rules out the possibility of contamination.
"It's neat to show that you could harness the energy of impacts to create organic bonds," says Jennifer Blank, an astrobiologist at the SETI Institute in Mountain View, Calif. But she fears that theories of life's origin may never move beyond the hypothetical. "As someone in the general field, one of the frustrations, of course, is that we're never going to know the answer," she says. "But as another mechanism for contributing to the inventory of organic compounds, this is cool."
Astronomer Donald Brownlee of the University of Washington concurs, noting that while most theories propose that organic molecules arrived from space or were formed by Earthly processes, "it is interesting to consider that they could be made here because material is falling in from space." At the same time, Brownlee wonders whether a meteorite large and powerful enough to penetrate the atmosphere and strike the ocean at high speed might preclude the formation of organics. "If the body is too large," he says, "generated materials are probably destroyed by impact processes."
The study by Sekine, Furukawa and their colleagues is a kind of oceanic, kinetic-impact analogue to the Miller–Urey experiment, a legendary 1953 demonstration by the late chemist, Stanley Miller of the University of Chicago, who, along with colleague Harold Urey, showed that an electric discharge applied to suspected components of the early atmosphere yielded a bounty of amino acids. In October, marine chemist Jeffrey Bada of the Scripps Institution of Oceanography in La Jolla, Calif., and his colleagues published a reanalysis of some of Miller's samples from a different experimental setup. Bada and his collaborators found even more organic material than Miller himself had announced—22 amino acids and five amines.
Sekine cautions that the meteorite-impact theory is not ready to supplant the vaunted Miller–Urey experiment. He says that the new study's results merely "open up a door to discuss the possibility" of meteorite impacts as an originator for life on Earth. "We do need to test the possibility for the formation of more complicated amino acids," he says.