HOLD FOR RELEASE: 4 P.M. EST,
THURSDAY, MARCH 27, 1997
Contact: Larry Bernard
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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."
-30-
EDITORS: Infrared images are available at
http://www.news.cornell.edu/science/March97/comet.lb.html
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