Meteorites contain a large variety of nucleobases, an essential building block of DNA. (Artist concept credit: NASA’s Goddard Space Flight Center/Chris Smith) |
NASA-funded researchers have found more evidence meteorites can carry DNA components created in space.
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 (21.4-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.
The
research was funded by NASA’s Astrobiology Institute at the agency’s
Ames Research Laboratory in Moffett Field Calif., and the Goddard Center
for Astrobiology in Greenbelt, Md.; the NASA Astrobiology Exobiology
and Evolutionary Biology Program and the NASA Postdoctoral Program at
the agency’s Headquarters in Washington.
