MOUNTAIN VIEW, CALIFORNIA--Biochemists would love to divine the birthplaces of amino acids, the basic components of proteins. Some probably formed on Earth billions of years ago as lightning and ultraviolet light from the sun laced the primitive atmosphere. Other amino acids may have come from space, but only if they withstood the fiery plunge to the planet's surface. Now, new research suggests that some amino acids may have survived that trip aboard two seemingly fragile hosts: comets and dust.
Planetary scientists have known that some meteorites are hardy enough to shield organic compounds from burning up. For instance, fragments of the carbon-rich Murchison meteorite, which fell onto Australia in 1969, contain extraterrestrial amino acids that stayed cool beneath a red-hot outer rind. But researchers thought that the brute force of hitting the atmosphere would destroy amino acids in comets, many of which slam into Earth at tremendous speeds and explode in the air.
However, lab experiments now show that the high pressures of impact may stabilize some amino acids and prevent heat from breaking their chemical bonds. A team led by geophysicist Jen Blank of the University of California, Berkeley, encased watery solutions of five amino acids inside steel disks and fired steel projectiles at them with a 12-meter-long gun. The impacts, at speeds up to 1.5 kilometers per second, pummeled the amino acids with up to 200,000 times Earth's atmospheric pressure and temperatures as high as 600°C. Between 40% and 70% of the amino acids survived the assault, Blank reported recently at the First Astrobiology Science Conference here. "This is the closest anyone has achieved in the lab to recreating the conditions of a cometary impact," she notes.
Another team reported that amino acids may hitch rides to Earth on micrometeorites, which add some 40,000 tons of extraterrestrial dust to the planet each year. Geochemists Daniel Glavin and Jeffrey Bada of the Scripps Institution of Oceanography in La Jolla, California, demonstrated that amino acids can vaporize off dust grains as they heat up in the atmosphere. The compounds would then recondense in the cold air behind the grains, creating tiny tails of solid amino acids that drift to the surface. Because interplanetary dust was rampant in the young solar system, he says, "This may have been a good way to get some of the prebiotic molecules onto the early Earth."
The experiments suggest that extraterrestrial delivery of amino acids to Earth is "plausible," says biochemist David Deamer of the University of California, Santa Cruz. "There's just no doubt that some of the amino acids survive the impacts," Deamer says. "It's become surprisingly convincing."