Never touted as easy, hunting for aliens just got a little bit harder.
It seems an excess of "left-handed" molecules, long assumed to be a signature of life, can be created inside asteroids through a non-biological process. That puts a damper on missions that intended to look for this chemical signature as evidence of biological activity on other worlds.
Molecules have handedness, or chirality, if their mirror-images cannot be superimposed upon each other, rather like your right and left hands. Life on Earth is built almost exclusively on left-handed amino acids, so scientists have assumed that a strong left-handed bias is a fundamental part of biochemistry.
Instruments on the European Space Agency's ExoMars and Rosetta missions are designed to search for an excess of left-handed molecules as an indicator of life.
But a new study found that meteorites collected from Canada's Tagish Lake also have a excesses of left-handed aspartic and glutamic acids, two amino acids that are common in terrestrial life.
Natural path
The researchers propose that, in the solar system's early days, heating as a result of radioactivity could have melted ice trapped deep inside asteroids. Liquid water then dissolved already present amino acids, which crystallised into mostly left-handed groupings.
Previous studies had seen small chiral excesses in amino acids from meteorites. But the new study is the first to propose a natural path for large left-handed enrichments in nonbiological materials, says team leader Daniel Glavin of NASA's Goddard Space Flight Center in Greenbelt, Maryland.
"As evidence mounts that [left-handed] excess occurs naturally across bodies in the solar system, any strategies designed to search for life based on looking for this excess require serious rethinking," says Alberto Fairen of the SETI Institute in Mountain View, California.
Even without chirality, though, the search for extra-terrestrial life can continue. "A swimming pool full of [left-handed] amino acids is not alive," says Harald Steininger, a project scientist for the MOMA instrument on ExoMars. "Life shows in many different aspects, and chirality is only one of them."
Journal reference: Meteoritics & Planetary Science, DOI: 10.1111/j.1945-5100.2012.01400.x
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