Cometary ices may have interstellar origin.

About 4.5 billion years ago, an interstellar cloud began to contract, forming a disk of gas and dust that encircled a hot, nascent stellar object. Over time, the disk condensed and heated, undergoing chemical changes as its material gathered into bigger and bigger clumps that became our solar system's planets. Comets are thought to have arisen in the outer, cooler parts of the disk and their well-preserved supply of ices are believed to reveal what con- ditions were like when the solar system began.

Evidence now suggests that some of the ices hail from a still earlier era.

Recently analyzed spectra of Hyakutake, the comet whose ghostly tail cap- tivated skywatchers last spring (SN: 6/1/96, p. 346), indicate that certain cometary ices remain unchanged from their composition in the interstellar cloud that gave birth to the solar system. Comets ferry key organic compounds and water to Earth from the fringes of the solar system. The new finding sug- gests that comets also deliver material directly from interstellar space, which may have contributed to the development of life on our planet.

In the Oct. 17 Nature, Timothy Y. Brooke of NASA's Jet Propulsion Laboratory in Pasadena, Calif., and his colleagues detail their detection of acetylene boiled off the surface of Comet Hyakutake as it neared the sun. The team reports that the abundance of the compound differs from typical solar system values but matches the estimated abundance in cold interstellar clouds.

The scientists propose that the evaporating material derived from ices frozen onto dust grains in the interstellar cloud "rather than material processed in the... disk out of which the solar system formed," they write.

Their conclusion echoes those of two earlier studies. In the Oct. 3 Nature, William M. Irvine of the University of Massachusetts in Amherst and his col- laborators report their detection in Hyakutake of two related organic com- pounds-hydrogen isocyanide and hydrogen cyanide. The relative abundance of the compounds matches that measured in interstellar clouds and differs sig- nificantly from the ratio expected in the outer part of the solar disk.

Last May, a team led by Michael J. Mumma of NASA's Goddard Space Flight Center in Greenbelt, Md., reported high concentrations of ethane relative to methane in Hyakutake, a ratio consistent with an interstellar origin.

Taken together, "these findings suggest strongly that Hyakutake contains ice from the natal interstellar cloud," says Mumma. Neither he nor any other comet observer, however, is willing to go beyond "suggest." "The problem is that we're ignorant about the chemistry of the interstellar medium and of comets," says Harold A. Weaver, who collaborates with Brooke at Johns Hopkins Univer- sity in Baltimore. To complicate matters, the solar disk may have vaporized ices that arose in the interstellar cloud, and the sun's ultraviolet radiation may have further transformed the ices. "The data taken so far aren't enough to [indicate if an interstellar origin] is right or wrong," says Weaver.