Comet Hale-Bopp, which blazed across the sky in 1997, may be brimming with some of the primordial material from which the sun and the planets were formed more than 4 billion years ago.
California Institute of Technology scientists who tuned radio telescopes onto the comet's nucleus as it cut across the solar system found vents spewing a volatile mixture of gas and dust into space.
The images suggest that 15 to 40 percent of Hale-Bopp's mass is pristine interstellar material, unchanged in billions of years. The rest has been transformed extensively during the comet's passage through space.
The images are among the finest ever obtained of a comet with radio telescopes.
The findings were published in Thursday's issue of the journal Nature.
Comets are often called dirty snowballs. Most orbit far from the sun in the deep freeze beyond Pluto's orbit. That enables them to remain structurally the same over billions of years.
"Nothing has changed much out there since that time. Therefore it's a way of sampling some of the chemistry, or very close to what it was, when the solar system formed," said Paul Weisman, a scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif., who was not involved in the work.
The Cal Tech researchers found two icy jets erupting with forms of primordial deuterium-hydrogen, the poisonous gas hydrogen cyanide and a form of hydrogen called heavy water. The material may be rising from deep within the comet, said Geoffrey Blake, a Cal Tech professor of cosmochemistry and planetary sciences.
Actual proof that comets contain the primordial material of the solar system may have to wait until a spacecraft can dig inside one.
A NASA probe set to launch in 2003 will try to do just that. If it survives the landing, in 2006, it will drill into Hale-Bopp to sample what's inside.