AMHERST, Mass. - University of Massachusetts astronomer Matthew Senay is part of a research team reporting that Comet Hale-Bopp, which passed by the Earth last year, provides evidence that comet ices are created inside huge interstellar clouds of gas, dust, and ice. The findings, detailed in the current issue of Science, suggest that comet ices are not, as previously thought, created in the outer regions of the solar system, where comets themselves are formed. The study also offers new evidence which implies that the temperature of the interstellar cloud that formed our solar system could not have been colder than 30 degrees Kelvin; a temperature considerably higher than scientists previously believed. The research team includes astronomers from the University of Hawaii, the National Research Council of Canada, and the Observatoire de Paris-Meudon.
Astronomers studied the chemical composition of Hale-Bopp’s nucleus during its journey past the Earth in April of 1997, and compared their findings with the chemistry of interstellar clouds, which are comprised of gas and ice-mantled dust. These clouds can become massive, fragmenting and collapsing into smaller clouds or "protostellar nebulae." Chemical reactions within the smaller clouds cause significant amounts of deuterium, a form of hydrogen sometimes called "heavy hydrogen," to latch onto the ice-encrusted dust particles. An individual protostellar nebula continues collapsing until the beginnings of a star appear at the center, with planets forming in the flattened disk of gas, dust, and ice - called the "protoplanetary nebula" - around it.
Using a radio telescope located in Hawaii, astronomers determined the ratio of deuterium to hydrogen in the hydrogen cyanide molecules ejected by Hale-Bopp. These molecules cannot be seen visually; however, they emit radio waves at specific frequencies, enabling scientists to measure molecular abundances with precision, Senay says. The team then compared this ratio with the ratio of the same substances in the hydrogen cyanide of interstellar clouds. The large deuterium to hydrogen ratio in the hydrogen cyanide ejected from the cometary ices of Hale-Bopp is evidence for interstellar ices being the forerunner of comets, Senay says.
This research provides evidence that cometary ice is created within interstellar clouds that have not yet collapsed; the ice is then incorporated into the comet during its formation.