Did Space Rocks Deliver Sugar?
Ron Cowen
How sweet it is!
Planetary scientists have for the first time detected
extraterrestrial sugar compounds in meteorites. The discovery bolsters
the view that rocks from space delivered ingredients that
contributed
to the development of life on Earth.
Simple sugars, sugar alcohols, and sugar acids—all of which have
now been found in two meteorites—are key components of RNA, DNA, and
cell membranes. They also serve as energy sources for terrestrial
organisms.
George Cooper of NASA's Ames Research Center in Mountain View,
Calif., and his colleagues examined the well-studied Murchison and
Murray meteorites. Both fell to Earth within the past half-century and
are known to contain organic compounds. To search for sugar compounds,
the team used a sensitive technique in which vaporized samples were
ionized and their chemical constituents separated according to mass.
In the Dec. 20/27 Nature, Cooper and his collaborators
report finding a variety of simple sugars, including dihydroxyacetone,
and sugar alcohols such as glycerols. The compounds occur in trace
amounts similar to the concentration of amino acids previously found
in the two meteorites.
Meteorites can easily become contaminated with terrestrial
compounds. But two lines of evidence suggest that the rocks carried
their sugars from space. The meteorites have a higher concentration of
simple sugars than of the large, complex sugars that are abundant on
Earth. Also, the ratio of carbon-13 to carbon-12 in the meteorites'
sugar compounds matches that expected from extraterrestrial sources.
Both meteorites are carbon-rich fragments of ancient asteroids
and
appear to have been chemically unaltered since the earliest days of
the solar system. The sugar compounds in the relic rocks therefore
indicate that simple sugars and related compounds "were present
on the early Earth, and, at the least, available for incorporation
into the first forms of life," the researchers note.
Some of these compounds might even predate the solar system,
originating in the interstellar cloud of gas and dust that gave birth
to the sun. Last year, scientists reported that they had found a
simple sugar in a star-forming cloud 26,000 light-years from Earth (SN:
6/24/00, p. 405).
It remains unclear, however, if meteorites, comets, and other
leftovers from the formation of the solar system seeded the early
Earth with substantial amounts of sugars or if these and other
organics were made on our planet, notes Arthur L. Weber of the SETI
Institute in Mountain View. Perhaps a combination of delivery from
space and homegrown biosynthesis sparked terrestrial life, he adds.
Jeffrey L. Bada of the Scripps Institution of Oceanography in La
Jolla, Calif., cautions that sugar molecules are extremely fragile.
Any carried to the early Earth couldn't have survived the planet's
harsh environment. That fragility has led some biologists to suggest
that RNA and other nucleic acids with sugar backbones were not the
first biological molecules to form (SN: 6/3/00, p. 363).
Weber says he agrees that sugars carried by meteorites wouldn't
last long on Earth. But he adds that his experiments have convinced
him that the energy stored and readily released by sugars can rapidly
drive the synthesis of amino acids as well as peptides, which could
have provided life's first chemical framework.
The findings highlight the richness of chemistry in space and on
Earth and the capability of both places to make molecules vital for
life, Bada adds.
References and Sources
References:
Cooper, G., et al. 2001. Carbonaceous
meteorites as a source of sugar-related organic compounds for the
early Earth. Nature 414(Dec. 20/27):879-883.
Further Readings:
Gorman, J. 2000. Sugarcoated news arrives from
space. Science News 157(June 24):405.
Sephton, M.A. 2001. Life's sweet beginnings? Nature
414(Dec. 20/27):857-858.
Travis, J. 2000. Molecule sparks origin-of-life
debate. Science News157(June 3):363.
Sources:
Jeffrey L. Bada
Scripps Institution of Oceanography
University of California, San Diego
9500 Gilman Drive
La Jolla, CA 92093-0212
George Cooper
NASA Ames Research Center
Moffett Field, CA 94035
Arthur L. Weber
SETI Institute
NASA Ames Research Center
Moffett Field, CA 94035-1000
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