By Sid Perkins
In recent geological ages, large extraterrestrial bodies colliding with Earth have been associated with worldwide extinctions, but new experiments show that massive impacts that occurred early in our planet’s history could have created the raw materials for life.
The hellish temperatures and pressures generated when an extraterrestrial object strikes Earth at speeds of several kilometers per second are enough to shatter and vaporize rock (SN: 6/15/02, p. 378). Yet part of such an immense burst of energy can trigger chemical reactions that generate complex organic substances from basic inorganic ingredients, says Takeshi Kakegawa, a geochemist at Tohoku University in Sendai, Japan. He and his colleagues conducted lab experiments intended to simulate a common meteorite striking one of Earth’s early oceans. The team reports its findings online December 7 in Nature Geoscience.
First, the researchers filled tiny, thick-walled canisters of stainless steel with various mixtures of carbon, iron and nickel — common constituents of meteorites— and water, ammonia and nitrogen, significant components of the ancient ocean and atmosphere. Then, the team fired the canisters at a solid target. The shock of impact briefly subjected the enclosed materials to temperatures approaching 4,700° Celsius and pressures about 60,000 times that of the atmosphere at sea level. These temperature and pressures are similar to those that would be caused by a large meteorite slamming into Earth at about 2 kilometers per second, says Kakegawa.
After each test, Kakegawa and his team cleaned off the outside of the canister, drilled a hole in it, and then extracted and analyzed the contents. In two of the team’s five tests, impacts created fatty acids like those found in cell membranes, and also generated a variety of amines, the ingredients for amino acids, Kakegawa says. In one test, the impact generated substantial amounts of glycine, the smallest of the 20 amino acids commonly found in proteins.
None of the organic chemicals generated by the impacts was a contaminant from any poor handling, Kakegawa proposes. That’s because all of the carbon in those resulting substances was the carbon-13 isotope, the same rare form that he and his colleagues used for the original mixture.
Scientists estimate that around 4 billion billion (1018) metric tons of meteorites fell to Earth between 4.4 billion and 3.8 billion years ago. Even though meteorites only contain, on average, about 0.1 percent carbon, oceanic impacts during this era could have generated at least one hundred billion (1011) metric tons of organic substances, the researchers estimate. Although these chemicals couldn’t have survived the conditions at ground zero of the impact, they probably could have formed in the more-tolerable temperatures present in the plumes of steam and vaporized rock that spewed skyward in the aftermath.
The team’s new analyses “are a nice piece of work,” says George Cody, a geoscientist at the Carnegie Institution for Science in Washington, D.C. A number of previous studies have hinted that the building blocks of life could have been generated by lightning in Earth’s ancient atmosphere (SN: 6/3/00, p. 363) or at deep-sea hydrothermal vents (SN: 9/9/00, p. 175; SN: 2/2/08, p. 67). Having multiple sources of such raw materials “makes determining the origin of life that much more difficult,” Cody adds. However, he notes, “the more we learn, the more we see how early Earth was rich with organic compounds.”
