The building blocks of DNA have been arriving here from meteorites in greater quantity than previously thought, lending weight to the idea that the seeds of life on Earth originated in space.
While certain components of DNA were first found in meteorites in the 1960s, scientists weren't able to rule out the possibility that they originated from terrestrial contamination.
But a new analysis of a dozen meteorites has found that they contain the amino acids adenine and guanine - components of DNA nucleobases - as well as other molecules that are highly unlikely to have originated on Earth.
In two of the meteorites studied, the team found for the first time trace amounts of three molecules related to nucleobases that are almost never found in terrestrial life.
These nucleobase-related molecules, called nucleobase analogs, form the first strong evidence that the compounds in the meteorites came from space.
"You would not expect to see these nucleobase analogs if contamination from terrestrial life was the source, because they're not used in biology," says NASA astrobiologist Michael Callahan.
"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."
This conclusion's corroborated by a comparison of the meteorites with the Antarctic ice in which they were found.
Not only were the amounts of nucleobases found in the ice much lower than in the meteorites, none of the nucleobase analogs were detected in the ice sample. Similarly, a soil sample collected near one of the non-Antarctic meteorite’s landing sites also lacked the nucleobase analog molecules found in the meteorite.
The nucleobases appear to have been built within the meteorites through a non-biological process. Indeed, the team was able to replicate this in the lab through chemical reactions containing hydrogen cyanide, ammonia, and water.
"For the first time, we have three lines of evidence that together give us confidence these DNA building blocks actually were created in space," says Callahan.