|Meteorites contain a large variety of nucleobases, an essential building block of DNA. (Artist concept credit: NASA's Goddard Space Flight Center/Chris Smith)|
Scientists have detected the building blocks of DNA in meteorites since the 1960s, but were unsure whether they were created in space or resulted from contamination by terrestrial life. The latest research indicates certain nucleobases - the building blocks of our genetic material -- reach the Earth on meteorites in greater diversity and quantity than previously thought.
The discovery adds to a growing body of evidence that the chemistry inside asteroids and comets is capable of making building blocks of essential biological molecules. Previously, scientists found amino acids in samples of comet Wild 2 from NASA’s Stardust mission and in various carbon-rich meteorites. Amino acids are used to make proteins, the workhorse molecules of life. Proteins are used in everything from structures such as hair to enzymes, which are the catalysts that speed up or regulate chemical reactions.
The findings will be published in the online edition of the Proceedings of the National Academy of Sciences. In the new work, scientists analyzed samples of 12 carbon-rich meteorites, nine of which were recovered from Antarctica. The team found adenine and guanine, which are components of DNA nucleobases.
Also, in two of the meteorites, the team discovered for the first time trace amounts of three molecules related to nucleobases that almost never are used in biology. These nucleobase-related molecules, called nucleobase analogs, provide the first evidence that the compounds in the meteorites came from space and not terrestrial contamination.
"You would not expect to see these nucleobase analogs if contamination from terrestrial life was the source, because they're not used in biology," said Michael Callahan, astrobiologist and lead author of the paper from NASA's Goddard Space Flight Center in Greenbelt, Md. "However, if asteroids are behaving like chemical 'factories' cranking out prebiotic material, you would expect them to produce many variants of nucleobases, not just the biological ones, because of the wide variety of ingredients and conditions in each asteroid."
Additional evidence came from research to further rule out the possibility of terrestrial contamination as a source of these molecules. The team analyzed an eight-kilogram (17.6-pound) sample of ice from Antarctica, where most of the meteorites in the study were found. The amounts of nucleobases found in the ice were much lower than in the meteorites. More significantly, none of the nucleobase analogs were detected in the ice sample. The team also analyzed a soil sample collected near one of the non-Antarctic meteorite’s fall site. As with the ice sample, the soil sample had none of the nucleobase analog molecules present in the meteorite.
Launched in Feb. 7, 1999, Stardust flew past an asteroid and traveled halfway to Jupiter to collect particle samples from the comet Wild 2. The spacecraft returned to Earth's vicinity to drop off a sample-return capsule on January 15, 2006.