— Amino acids have been discovered in a meteorite whose parent asteroid should have destroyed the compounds.
— Because the concentrations of the amino acids are low, the discovery may not be applicable to the origin of life.
Planetary scientists have found amino acids, building blocks of life, in an unexpected place: a meteorite whose parent asteroid formed at temperatures so high that such fragile organic compounds should have been destroyed. One explanation for the surprising discovery is that some amino acids might form through a mechanism that does not require the presence of water, upping the chances of finding life beyond the solar system, says Daniel Glavin of NASA's Goddard Space Flight Center in Greenbelt, Md.
"Amino acids are forming in environments that we really didn't think were possible," Glavin says. He and his colleagues found the material in a fragment of the asteroid 2008 TC3, the first celestial object that has ever been spotted before slamming into Earth's atmosphere and raining meteorites onto the planet's surface (SN: 4/25/09, p. 13). The researchers describe their discovery in an article posted online December 13 in Meteoritics & Planetary Science.
Asteroid 2008 TC3 has an unusually violent history, notes Glavin and study coauthor Peter Jenniskens of the SETI Institute in Mountain View, Calif. The roughly 4-meter-long (13-foot-long) asteroid is believed to be a fragment of a fledgling planet that formed at the birth of the solar system and was heated to temperatures exceeding 1,100 degrees Celsius (2,012 degrees Fahrenheit) — hot enough to melt iron. The rich amalgam of materials in the chunks of the asteroid that fell to Earth suggests that 2008 TC3 was then subject to a series of violent collisions with other asteroids that fused different pieces of space rocks.
That's why Glavin and his collaborators didn't expect to find anything but terrestrial amino acids in the gram of material they got when 2008 TC3 broke apart in Earth's atmosphere in October 2008, leaving remnants scattered in the Nubian desert in Sudan. The sample they analyzed was classified as a ureilite meteorite, a type that comes from parent asteroids devoid of water, and thus unable to form amino acids by known mechanisms.
But the team did discover amino acids in the sample that are either rare or nonexistent on Earth. More importantly, the two possible forms of the compounds — a left-handed structure and its mirror image — were equally common. In contrast, amino acids made by life on Earth are predominantly left-handed.
"The pattern of amino acid abundances... are hard to explain via terrestrial contamination," comments Conel Alexander of the Carnegie Institution for Science in Washington, D.C. Because of the high heat, Glavin adds, the extraterrestrial origin of these amino acids also can't be explained by a familiar process in which two types of highly reactive organic compounds — aldehydes and ketones — interact with ammonia, hydrogen cyanide and water to produce the protein building blocks.
One possibility, favored by Glavin, is that once the asteroid cooled below 500 degrees C (932 degrees Fahrenheit), carbon monoxide, molecular hydrogen and ammonia gases could have reacted with grains of iron or nickel to produce amino acids. That mechanism has long been speculated to occur in asteroids but has never been documented outside the laboratory.
A new way of naturally producing amino acids "really increases the likelihood, in my opinion, of life existing elsewhere in the universe" and may have also helped seed the solar system's terrestrial planets with prebiotic compounds, Glavin says.
A less likely possibility, he notes, wouldn't require a new mechanism to explain the amino acids. In this scenario, collisions would vaporize and then transfer amino acids from other asteroids to 2008 TC3.
Alexander cautions that the new results may not be directly applicable to the origin of life, especially because the concentrations of the amino acids in the sample are low and because ureilite meteorites constitute a minority of the meteorites that fall to Earth. Nevertheless, he adds, "it does show that synthesis of amino acids in nature can occur in unexpected places and ways, and that we should keep a very open mind about how and where prebiotic chemistry can occur."