A group of American and French astronomers, including several who are
playing a leading role in the Rosetta mission, has announced the discovery
of the noble gas argon in Comet Hale-Bopp. This is the first time that one of
the so-called noble gases (argon, xenon, neon etc.) has been found in a comet.

The discovery was made by an ultraviolet spectrometer that flew aboard a
Black Brant rocket on a brief suborbital lob over New Mexico in March 1997.
The instrument was designed and built by scientists from Southwest
Research Institute (SwRI), based in San Antonio, Texas, and collaborators
from the University of Colorado, the University of Maryland, and the
Observatoire de Midi-Pyrenees in France.

Its task was to study the reflected ultraviolet light from the comet in an
effort to learn more about its composition and history. However, the argon
find was only made possible by Hale-Bopp’s unusually high brightness as it
made its closest approach to the Sun.

“The argon signals are weak, but unmistakable,” said Dr. Alan Stern, director
of the SwRI Space Studies Department and principal investigator for the
ALICE instrument on Rosetta. “We had previously suspected their presence,
but were able to recently confirm the result when we cross-compared two
independent spectra obtained by our rocket instrument back in 1997.”

The presence of argon is important because noble gases do not interact
chemically with other elements and are easily lost from icy bodies like
comets through sublimation — a process much like evaporation. This means
they can be used to indicate the temperature of a comet during its formation
and its evolutionary history.

Although the spectral signature of the gas was quite weak, the data indicated
that the comet contained more argon than expected, when compared to the
composition of the Sun. The argon abundance in Hale-Bopp was so high that
the comet must have formed far from the Sun and spent almost its entire
life — about 4.5 billion years — in the cold, frigid outer reaches of the
Solar System.

“This enrichment was completely unexpected,” said Dr. Stern. “It shows that
the comet’s core temperature has never risen above 40K (-233 C), otherwise
the argon would all have sublimated and dispersed into space. Curiously,
Jupiter has also been found to be enriched in argon, suggesting that some of
the planet’s argon gas may have been delivered by argon-rich comets falling
into its atmosphere.”

Dr. Stern believes that this breakthrough is just the start of a new series of
discoveries about comets.

“We want to see if other comets are also enriched in noble gases,” he
explained. “Using even more sensitive instruments, such as the ALICE
spectrometer on board Rosetta, we look forward to comparing different
comets to one another to learn about the diversity of cometary birthplaces.
We also want to see if different parts of a comet have similar amounts of
noble gases. This will tell us if different regions have different origins.”

The ALICE experiment on Rosetta is a state-of-the-art ultraviolet
spectrometer which weighs a mere 3 kg compared with the 180 kg instrument
carried aloft by the sounding rocket. Apart from its ability to search for
noble gases in the coma or cloud of material surrounding Comet Wirtanen,
ALICE will also study the coma’s composition, look for dust in the coma
and map the surface of the comet’s nucleus in ultraviolet light.

The Rosetta spacecraft and its complement of 11 instruments, including
ALICE, is scheduled to arrive at Comet Wirtanen in 2011 after an eight-year
odyssey which will include two close asteroid flybys. Compared with
Hale-Bopp, Comet Wirtanen is a much smaller, less active body that has
been considerably altered as it orbits the Sun once every 5.45 years. Its
aphelion (maximum distance from the Sun) is currently about 768 million km,
near the orbit of Jupiter. In contrast, Hale-Bopp’s orbit is so huge that it
will not return to the Sun’s vicinity for another 2,400 years.


* The Rosetta mission

* More about Wirtanen



[Image 1:
Comet Hale-Bopp
The full-field picture on the left, taken with the Wide Field Planetary Camera 2
(in WF mode), shows the comet against a stellar backdrop in the constellation
Sagittarius. Credit: H.A. Weaver (Applied Research Corp.), P.D. Feldman (The
Johns Hopkins University), and NASA

[Image 2:
Rosetta Instrument ALICE
ALICE Principal Investigator is Dr. Alan Stern Southwest Research Institute