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News Release
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EMBARGOED FOR RELEASE AT 5 P.M. EDT
ON THURSDAY, APRIL 25, 1996
CONTACT: Emil Venere
esv@resource.ca.jhu.edu
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Hubble Telescope Yields Insights about Saturn's
Rings
Astronomers, taking advantage of a rare "edge-on" view, have used
the Hubble Space Telescope to inquire into two major questions
about Saturn's rings: when were they formed, and how long will
they last?
The observations confirmed a 30-year-old prediction that frozen
water in the rings is vaporized and then broken into its
constituents, creating a tenuous "atmosphere" of gas around the
rings.
Researchers have known for decades that Saturn's rings are
composed of many particles, each covered at least in part by
frozen water. Scientists don't know how the ring system formed,
but one theory is that a sizeable moon containing a large
quantity of water orbited too close to the planet and was torn
apart by gravitational tidal forces. The satellite's debris,
including its water, would then have been dispersed around
Saturn, evolving into the colorful ring system.
"The question is, how long ago did this happen, and how long do
the rings survive?" said Doyle Hall, an associate research
scientist in The Johns Hopkins University Department of Physics
and Astronomy, who led a team of investigators for the Hubble
Space Telescope
observations.
The findings are being reported in the April 26 issue of the
journal Science, in a paper written by Hall and astronomers Paul
Feldman of Johns Hopkins, Melissa McGrath of the Space Telescope
Science Institute and Jay B. Holberg of the University of
Arizona.
Bombardment by solar radiation, charged particles, dust-size
meteorites or other ring particles all tend to erode the icy ring
particles, creating small plumes of water vapor. The water vapor
molecules then split into hydrogen atoms and hydroxyl molecules,
which are made of oxygen and hydrogen.
By analyzing the Hubble data, astronomers learned that the ring
system is losing up to 3,000 kilograms (6,600 pounds) of frozen
water per second. But even at this rate, the researchers estimate
that the rings probably will survive at least another billion
years before they are dehydrated into dried-out remnants. Because
the erosion rate is relatively slow, the rings might have formed
a billion or more years ago, Hall said. But astronomers can't be
certain of that because they don't know what the erosion rate was
in the past, he said.
Ever since the icy nature of Saturn's rings was discovered,
astronomers have wanted to search for the hydroxyl molecules.
They got their chance in August 1995, when Saturn's rings
appeared edge-on from the Earth. Because the rings are so thin,
the edge-on view makes them almost invisible from Earth, just as
a sheet of paper would be invisible if viewed perfectly edge-on
from a distance. Without the usual bright glare of reflected
sunlight from the rings, astronomers were able to study the
tenuous envelope of gas that surrounds the rings. The astronomers
used the Hubble telescope's Faint Object Spectrograph to observe
the region directly above the rings, and detected the signature
of hydroxyl molecules in ultraviolet light.
Astronomers won't soon have another chance to spy Saturn's rings
edge-on; the orbital configuration that made the rings appear
edge-on occurs roughly every 15 years. But the next two times it
happens, Saturn will be nearly on the other side of the Sun and
out of Earth's view.
"We won't have an opportunity to do this kind of experiment from
Earth's perspective until the year 2038," Hall said.
Although ring systems surround all four of the solar system's
giant planets, Jupiter, Saturn, Neptune and Uranus, none is as
spectacular as Saturn's. The ring system surrounding Uranus, for
example, appears to be dried-out and dark compared to Saturn's
colorful rings. Perhaps that's because those rings were subjected
to the same sort of dehydrating erosion astronomers are now
seeing on Saturn, Hall said.
Knowing how fast Saturn's rings are losing water is valuable
information that may enable scientists to uncover secrets about
the nature of the other planetary ring systems, he said. The
findings also could lead to new insights into the workings of
Saturn's magnetosphere, which surrounds the planet and its ring
system. After the water is broken down into its constituents, the
hydroxyl molecules and hydrogen atoms eventually are stripped of
electrons, turning into positively charged ions that become
trapped in Saturn's magnetic field. The addition of those charged
particles may have an important effect on the magnetosphere,
which exerts profound influences on Saturn's atmosphere.
EMBARGOED FOR RELEASE AT 5 P.M. EDT ON THURSDAY, APRIL
25, 1996
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