Life as we know it on Earth is not the only kind possible in the universe, scientists reminded NASA in a report released last week.
Issued by the National Academy of Sciences and sponsored by the space agency, the 116-page report reviews current research into what life is and what it needs to survive, as well as the way life might differ on other worlds.
"Our investigation made clear that life is possible in forms different than those on Earth," said committee chair John Baross, an oceanographer at the University of Washington.
Despite UFO reports, scientists have not yet found any evidence for life beyond Earth.
A common heritage
A gnat and a blue whale might appear to have little in common, but the two are linked by their common use of DNA to encode genetic information and their use of the same 20 amino acids to build proteins.
Furthermore, all of the chemical reactions that keep their bodies running take place in water and require carbon-based molecules.
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Gnats and whales inherited their shared biochemistry from a common ancestor that lived billions of years ago, one that reproduced and diversified into all the life forms we see today. We, humans, are also the descendents of that common ancestor.
That all life on Earth is linked by a common heritage is one of science's most profound discoveries, but it can be a handicap when attempting to find life elsewhere in the universe.
"It is critical to know what to look for in the search for life in the solar system," Baross said. "The search so far has focused on Earth-like life because that's all we know, but life that may have originated elsewhere could be unrecognizable compared with life here."
More than one way
The report committee, made up of 11 scientists from diverse fields, worry that researchers have already limited their scope of thinking about where extraterrestrial life might be found.
The assumption that life requires water, for example, has limited the search for life on Mars to those "habitats" where liquid water is thought to be present or to have once flowed. But recent research suggests liquids such as ammonia or formamide could serve as an alternative to water for some alien organisms.
For this reason, the committee recommends that increased priority be given to a follow-up mission to probe Saturn's moon Titan, a world now known to be covered in lakes and rivers of liquid water-ammonia mixtures.
Recent research also suggests that alien life forms might use something other than DNA to encode their genetic information.
DNA on Earth works through the pairing of four chemical compounds called nucleotides. Experiments in synthetic biology have created structures with six or more nucleotides that can encode genetic information and also potentially undergo Darwinian evolution.
The report recommends that future searches for alien life include instruments capable of detecting lightweight chemical elements — such as carbon, hydrogen, oxygen, phosphorus and sulfur — in addition to the typical complex organic molecules found in terrestrial organisms.
In their report, the committee scientists appear ambivalent about NASA's current focus on human missions to the Moon and Mars.
While humans can do more than even the most sophisticated robots, they also carry the serious risk of contamination.
"No discovery that we can make in our exploration of the solar system would have a greater impact on our view of our position in the cosmos or be more inspiring than the discovery of an alien life form, even a primitive one," the committee wrote. "At the same time, it is clear that nothing would be more tragic in the American exploration of space than to encounter alien life and fail to recognize it either because of the consequences of contamination or because of the lack of proper tools and scientific preparation."
John Rummel, the senior scientist of astrobiology at NASA, called the committee's findings a "very interesting report by a very capable group of scientists that should provide a good basis on which to proceed."
Rummel said NASA is committed to minimizing the risks of human contamination on Mars and is examining ways to best do that. "You need to characterize those regions on Mars where Earth contamination could persist and potentially grow and thrive, and avoid those," he said.
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