The radioactive heating of water-rich meteorites may be the answer to one of the most puzzling questions about the origins of life on Earth: why are proteins formed almost entirely from the left-handed forms of amino acids, rather than their right-handed mirror-image forms?
In a recent paper in Meteoritics & Planetary Science, a team led by Daniel Glavin of NASA’s Goddard Space Flight Center, Greenbelt, Maryland, reported that some carbon rich meteorites are high in left-handed amino acids, while others contain an equal balance between these and their mirror-image counterparts. Among the nine meteorites that were part of the study – including the formidable Murchison meteorite (see right) which struck Australia in 1969 – there are none which are enriched in the right-handed forms.
Scientists have long wondered why life chose to rely on left-handed amino acids (their lefty designation comes from this naming convention), since the basic chemistry that drives cell metabolism would be viable in either case. If, as many astronomers suspect, asteroids or comets were a primary source of organic moleclues just after Earth formed, then “they may have delivered these left-handed amino acids and biased the prebiotic soup,” Glavin says. “This would suggest that we were predestined for left-based life.”
As far back as 1997, astrobiologists had known of two meteorites that were oddly enriched in left-handed amino acids. The new study found this holds true for a much boader range of meteorites, with some as much as 20 % enriched.
Even more interestingly, the study also discovered that all space rocks with a high proportion of left-handed amino acids also contain hydrated minerals.
This indicates that they were formed in warm, water-rich environments, presumably in the interiors of progenitor asteroids heated by the radioactive decay of heavy elements. Those that contained equal amounts of both forms of amino acids did not show signs of water alteration.
“This suggests that water was very important in amplifying left over right, Glavin says”, though he notes that the details of this process have yet to be determined.
The new find also means that amino acids throughout the solar system should have the same left-handed bias. Thus if life ever formed elsewhere in the solar system it too would have been predestined to favor left-handed versions of these molecules. “We’re not likely to encounter our mirror image anywhere in our Solar System, ” Glavin says.
UPDATE: This post has been corrected to remove confusion over the R/S and D/L naming conventions for chiral molecules.
Posted on behalf of Rick Lovett
Image courtesty National Museum of Natural History, Washington DC