The New York Times, July 6, 2007
The hunt for extraterrestrial life should be dramatically expanded, a panel of scientists convened by the country’s leading scientific advisory group said today, to include what they call “weird life,” organisms that lack DNA or other molecules found in life as we know it.
“The committee’s investigation makes clear that life is possible in forms different from those on Earth,” the scientists concluded. Their report, “The Limits of Organic Life in Planetary Systems,” was published today by the National Research Council of the National Academies of Sciences and posted on the NAS web site, www.nationalacademies.org.
Other experts hailed the report as an important rethinking of the search for life. “It’s going to help us a lot to make sure we go exploring with our eyes wide open,” said Michael Meyer, the Mars exploration program lead scientist at NASA.
Starfish, sequoias, salamanders, and the rest of Earth’s residents seem very diverse. But they’re surprisingly similar on the molecular scale. Every species scientists have studied needs liquid water to survive, for example. They all rely on DNA to carry genetic information. They all use that information to build proteins from the same set of building blocks, known as amino acids.
NASA has traditionally looked to life on Earth to guide their search for life on other worlds. Planets and moons that have hints of liquid water have been ranked high on the list of potential sites for life-detection missions. The 1976 Viking mission to Mars searched for traces of Earth-like life without apparent success.
But there’s good reason to suspect that other kinds of chemistry could support life as well, the authors of the new report argue. Weird life could differ from life as we know it in small or big ways. DNA uses phosphorus in its backbone, for example. It might be possible to build a backbone out of arsenic instead. Instead of water, life might exist in other liquids, such as ammonia or methane.
The report even explores the possibility of life based on silicon, not carbon. Dr. Meyer, who did not help write the report, thinks that astrobiologists should limit their search to carbon-based life forms. “When we look in the universe, the only compounds we see with more than six atoms are all carbon chemistry,” he said. “So there’s a hint that looking for carbon chemistry may be a better bet. There we have some idea of what to look for.”
The weird life report calls for NASA and the National Science Foundation to support research on Earth into weird life. Chemists need to investigate “the chemical possibilities for what forms life might take,” said Steven Benner, a member of the committee and a distinguished fellow at the Foundation for Applied Molecular Evolution in Gainesville, Fla.
Scientists should also search Earth for weird life. “There’s much about Earth life we don’t understand,” said John Baross of the University of Washington, the chairman of the committee.
“There is good evidence that the life we know on Earth was preceded by a weird form of life,” said Dr. Benner. Early Earth life may have been based on RNA, a single-stranded form of DNA. While DNA-based life may have outcompeted earlier forms on the surface of the planet, RNA life may still exist in refuges. One potential hiding place is deep below the ocean floor. “It’s an incredibly primordial world down there,” said Dr. Baross. “If you’re going to look for remnants of an RNA world, those are the environments you want to go to.”
To find weird life, however, scientists will have to build new kinds of detectors. “There’s no question that the surveys of life on the planet we’ve done so far would have missed it,” said Dr. Benner.
The scientists also concluded that the possibility of weird life should prompt NASA to reorder its future missions. They single out Saturn’s moon Titan as particularly promising. The Huygens probe that visited Titan in 2005 found evidence of liquid methane raining down on its surface, as well as a mix of water and ammonia seeping up from its interior. Both could conceivably support life, although not necessarily life as we know it.
“Nothing would be more tragic in the American exploration of space than to encounter alien life and fail to recognize it,” the report concluded.
Copyright 2007 The New York Times Company. Reprinted with permission.