Did space rocks deliver sugar? (Science News of the week).

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.