Tuesday, August 16, 2011

Seeds of Life from Space

Last week, just in time for the annual Perseid meteor shower, NASA scientists announced new evidence that the building blocks of life may have been manufactured in outer space and delivered to our planet on a meteorite. The implications of the discovery are profound: if the chemicals that make up DNA can be formed anywhere in the cosmos, the universe is most likely teeming with life.

Every year in early August, the Earth passes through a cloud of debris in space left behind by comet 109P/Swift-Tuttle, which last came through the solar system in 1992. Some of the ice and dust shed by the comet on its 135 year journey around the Sun is pulled in each year by the gravity of the Earth and burns up in the atmosphere, creating a "shooting star" for anyone who happens to be watching. At the peak of the shower, observers far from the city lights can see about 100 meteors every hour. Meteorites -- chunks of space rock that are large enough to make it all the way to the ground -- are much less common than meteors. Even when they don't disintegrate entirely from the friction of entering the atmosphere at 100,000 miles per hour, most of them fall in the ocean or are quickly eroded on land. The exception is Antarctica, where meteorites are preserved in the ice -- and this is where the space rocks with DNA were found.

Over the past 50 years, evidence of life has turned up in many space rocks. It has been clear since the 1960's that amino acids (chemicals that build proteins) can be formed in space, and a recent examination of a dozen Antarctic meteorites led NASA scientists to suggest that adenine and guanine -- two of the four "nucleobases" that link up to form DNA -- can also come from the sky. We know that the meteorites came from space, but the difficulty has always been establishing the source of the organic molecules: they are much more likely to come from contamination after they reach the surface of the Earth. In this case, the scientists did not find any other molecules that should have accompanied the nucleobases if they had come from contamination. Instead they found additional non-biological molecules that were not present in samples of the soil and ice where the meteorites were found. The properties of the samples, along with simple experiments to show how nucleobases can be manufactured naturally in space, support the idea that these space rocks contain some of the building blocks of life.

Over the past two decades we have learned that planets are much more common than we ever imagined, and this latest research suggests that a lifeless planet can be seeded with organic molecules from outer space. If rocks from the sky contain some of the ingredients of DNA, it is not a stretch to believe that life must be common in the universe.