The predominance of 'lefthandedness' amongst amino acids in meteorites isn't, after all, an indication of biological activity, say scientists.
A NASA team analyzing meteorite fragments that fell on a frozen lake in Canada says it's actually caused by liquid water inside an asteroid, making the search for extraterrestrial life more tricky.
"Our analysis of the amino acids in meteorite fragments from Tagish Lake gave us one possible explanation for why all known life uses only left-handed versions of amino acids to build proteins," says Dr Daniel Glavin of NASA's Goddard Space Flight Center.
Amino acid molecules come in two forms which are mirror images of each other. Life on Earth is based on so-called left-handed versions - but there's no reason why right-handed versions wouldn't work perfectly well.
All ordinary methods of synthetically creating amino acids result in equal mixtures of left- and right-handed amino acids. This leaves the question open as to how the left-handed versions came to predominate from what were presumably once equal quantities of left and right molecules.
In January, 2000, a large meteoroid exploded in the atmosphere over northern British Columbia, Canada, raining fragments across the frozen surface of Tagish Lake. Many pieces were collected within days and kept frozen, ensuring that there was very little contamination from terrestrial life.
The team ground up samples, mixed them into a hot-water solution, then separated and identified the molecules in them using a liquid chromatograph mass spectrometer.
"We discovered that the samples had about four times as many left-handed versions of aspartic acid as the opposite hand," says Glavin. "Interestingly, the same meteorite sample showed only a slight left-hand excess (no more than eight percent) for alanine, another amino acid used by life."
The implication of this large left-hand excess in one and not the other, he says, is that it wasn't created by life but instead was made inside the asteroid itself.
The team confirmed that the amino acids were probably created in space using isotope analysis.
"We found that the aspartic acid and alanine in our Tagish Lake samples were highly enriched in carbon 13, indicating they were probably created by non-biological processes in the parent asteroid," says Dr Jamie Elsila of NASA Goddard.
Some have argued that left-handed amino acid excesses in meteorites were formed by exposure to polarized radiation in the solar nebula - but this isn't enough to explain the sheer size of the left-hand aspartic acid excess.
But the team believes a small initial left-hand excess could be amplified by crystallization and dissolution from a saturated solution with liquid water. Some amino acids, like aspartic acid, have a shape that lets them fit together in a pure crystal - one composed of just left-handed or right-handed molecules. For these, a small initial left- or right-hand excess could become greatly amplified.
Other amino acids, like alanine, have a shape that prefers to join together with their mirror image to make a crystal, using equal numbers of left- and right-handed molecules. As these crystals grow, any small initial excess would tend to be washed out.
The result complicates the search for extraterrestrial life. "Since it appears a non-biological process can create a left-hand excess in some kinds of amino acids, we can't use such an excess alone as proof of biological activity," says Glavin.