ITHACA, N.Y. -- Infrared measurements of Comet Hale-Bopp by Cornell University and NASA investigators are yielding valuable clues about the makeup of the celestial visitor and, perhaps, the origins of the solar system.
Using a combination infrared spectrometer and camera designed and built by Cornell University researchers and attached to the 200-inch telescope at Palomar Observatory, Cornell and NASA scientists have made ground-based measurements in an effort to learn what kind of stuff the comet is sloughing off as it approaches perihelion, that is, its closest approach to the sun.
In a paper published in the journal Science (28 March 1997), Thomas L. Hayward, Cornell senior research associate in astronomy at the Center for Radiophysics and Space Research, and Martha S. Hanner, a senior research scientist at NASA's Jet Propulsion Laboratory in Pasadena, report results of their infrared observations at 8- to 13-micron wavelengths.
The spectra show that Hale-Bopp has an abundance of tiny (sub-micrometer size) silicate grains. Some of these grains are crystalline, in contrast to the more amorphous structure of the rest. This means that the grains were subjected to strong heating sometime in their history, before they were incorporated into the frozen comet nucleus about 4.5 billion years ago.
"Did the heating occur in the solar nebula, or did it occur in an interstellar cloud prior to the formation of the solar nebula? We can't say," Hayward said.
It is known that comets formed in the cold outer region of the solar nebula where intact interstellar grains could have been incorporated. "But we were surprised to see such grains even when the comet was over four astronomical units from the sun. We thought the grains would be icy at that distance." An astronomical unit is the average distance from the Earth to the sun, about 93 million miles.
Most comets can be studied only when they are within 1 or 2 astonomical units, when dust grains are warm. But Hale-Bopp was unusually active and could be detected easily in the infrared when it was still far from the sun.
The researchers made their observations of Comet Hale-Bopp from June through September last year, when the comet was still beyond the orbit of Mars. Since then, they made more measurements last month and are returning to Palomar this month as well. The comet will pass closest to the sun on April 1.
The researchers used an instrument dubbed SpectroCam-10, mounted on the 200-inch telescope at Mount Palomar, where Cornell has 25 percent observing time. The instrument detects thermal radiation emitted by the warm dust grains. "As the comet gets closer to the sun, we can see the underlying (infrared) emissions gradually change shape. We can see the grains getting warmer," Hayward said. The brightness indicates how much dust is being blown off, while the warmth is related to the size of the grains -- all fundamental information that can be used for future comparisons.
Their measurements also showed that the comet has periods of outbursts, where a burst of material is ejected out into the sunlight. "There was a lot of variability in the comet's day-to-day brightness," Hayward said, "caused by an unusually active area on the comet's nucleus that emits a burst of material when it rotates into sunlight." Since January, amateur and professional astronomers alike have observed a jet periodically shooting out from this active area. The solar radiation hits the dust grains emitted in these outbursts and pushes them away from the sun, giving the comet the tail most familiar to viewers on Earth.
These outbursts and jets, sometimes increasing the comet's general brightness by a factor of two to four, are similar to short-term brightness increases observed in Halley's Comet, the researchers said.
The information gives astronomers a basis for comparison to other comets that, taken together, could yield clues to the origins of the solar system. It is thought that comets may be remnants of the processes that formed the sun, planets and satellites. The ultimate goal: Trying to understand what the grains are made of and their processing history. That would indicate the history of this comet, which in turn tells us what the early solar system was like.
"Our hope is that these dust grains, from under the surface of the comet's nucleus, represents what the nucleus was like billions of years ago when it was formed," Hayward said. "That could help tell us what the solar system was like as it was forming. This is just another piece of the puzzle."