Life's Building Blocks Are Found All Over Galaxy
By Ernie Tretkoff
A class of molecules that make up the building blocks of life is widespread in the galaxy, Emma Bakes of NASA/SETI reported at the April Meeting.
All known life depends on this class of molecules, called nitrogenated aromatics, which are part of DNA, RNA, and other chemicals critical to life, including the oxygen-producing photopig-ments in plants and the oxygen-storing pigments in animals. "They form the very foundations of all terrestrial life," said Bakes.
Scientists have recently discovered that these molecules are present throughout the galaxy, said Bakes. Observations have turned up nitrogenated aromatics in the interstellar medium, in comets, in protoplanetary disks around stars, in planetary atmospheres, and in objects in the outer solar system.
These molecules are made up of nitrogen attached to a ring of carbon, with alternating single and double bonds. This class of molecule includes purines, pyrimidines, pyrroles, and other chemicals. They are known as aromatics because many of them have distinctive odors.
Nitrogenated aromatics form in a variety of environments, including planetary atmospheres and the surfaces of icy dust grains in the interstellar medium.
Bakes suggests that these molecules could also have formed in the atmosphere of early Earth. Her recent work, mainly with computer simulations, has focused on the atmosphere of Titan, which is thought to resemble that of early Earth. Titan is Saturn's largest moon and the only moon in our solar system with an atmosphere. The dense, smog-like haze around Titan contains nitrogen, methane, and hydrocarbons, but little oxygen. Bakes' simulations indicate the presence of nitrogenated aromatics in Titan's atmosphere, suggesting that these chemicals might also have been produced in the early Earth's atmosphere.
To confirm the composition of Titan's atmosphere, the European Space Agency's Huygens probe, currently on its way to Titan aboard NASA's Cassini spacecraft, will sample Titan's atmosphere in early 2005.
Some scientists are also considering the possibility that a comet delivered the chemicals necessary for life on Earth, but Bakes said that aromatic molecules are fragile and might not survive an impact.
Though scientists acknowledge the importance of nitrogenated aromatics, they have no idea how to get from the molecules to life, said Bakes. "The million dollar question is, 'How do we get from them to us?' "
Bakes said she would bet on life being widespread in the universe because these molecules are found in so many environments. "If they are made everywhere, perhaps life is everywhere," she said. It might also be possible to have life based on an entirely different chemistry, she added