Scientists Find Rings
Of Jupiter Are Shaped In Shadow
(30 April 2008) Scientists from the
University of Maryland and the Max-Planck Institute for Solar System Research
in Germany appear to have solved a long-standing mystery about the cause of
anomalies in Jupiter's gossamer rings.
In a new study published
in the May 1 issue of Nature, they report that a faint extension of the
outermost ring beyond the orbit of Jupiter's moon Thebe, and other observed
deviations from an accepted model of ring formation, result from the interplay
of shadow and sunlight on dust particles that make up the rings.
"It
turns out that the outer ring's extended boundary and other oddities in
Jupiter's rings really are 'made in the shade,'" said Douglas Hamilton, a
professor of astronomy at the University of Maryland. "As they orbit about the
planet, dust grains in the rings alternately discharge and charge when they
pass through the planet's shadow. These systematic variations in dust particle
electric charges interact with the planet's powerful magnetic field. As a
result small dust particles are pushed beyond the expected ring outer boundary,
and very small grains even change their inclination, or orbital orientation, to
the planet."
Hamilton and German co-author Harald Kruger studied for the
first time new impact data on dust grain sizes, speeds, and orbital
orientations taken by the spacecraft Galileo during its traversal of Jupiter's
rings in 2002-2003, as part of its deliberate manoeuvring for a death plunge
into the planet. Kruger analysed the new data set and Hamilton created
elaborate computer models that matched dust and imaging data on Jupiter's rings
and explained the observed eccentricities.
"Within our model we can
explain all essential structures of the dust ring we observed, " said
Kruger.
According to Hamilton, the mechanisms identified in this paper
affect the rings of any planet in any solar system, but the effects may not be
as evident as it is at Jupiter. "The icy particles in Saturn's famous rings are
too large and heavy to be significantly shaped by this process, which is why
similar anomalies are not seen there," he said. "Our findings on the effects of
shadow may also shed some light on aspects of planetary formation because
electrically charged dust particles must somehow combine into larger bodies
from which planets and moons are ultimately formed."
Jupiter, Galileo
and the Mystery of the Rings
Jupiter, the fifth planet from the Sun, has
63 known moons. The dust forming Jupiter's faint rings is produced when bits of
space debris smashes into the small inner moons Adrastea, Metis, Amalthea and
Thebe (closest to farthest). This dust is organised into a main ring, an inner
halo, and two fainter and more distant gossamer rings. The rings largely are
bounded by the orbits of these four moons, but a faint outward protrusion of
dust extending beyond the orbit of Thebe has, until now, mystified
scientists.
Italian scientist Galileo Galilei was the first to discover
that Jupiter had moons. Galileo first observed the planet's four largest moons
in 1610.
On December 7, 1995 NASA's Galileo spacecraft reached Jupiter
and began the first of 35 orbits around the planet. Over seven years the
spacecraft took some 14,000 images of Jupiter, its moons and rings. It also
released a probe that sent back the information on the planet's atmosphere. On
September 21, 2003 the Galileo spacecraft was put into a controlled dive to end
its mission, by plummeting through Jupiter's atmosphere. In addition to its
imaging instruments, the spacecraft carried a supersensitive dust detector,
which registered thousands of impacts from dust particles on its way through
Jupiter's ring system in 2002 and 2003. The Thebe extension was one of the many
new discoveries made by the Galileo spacecraft.
The overall Galileo
mission was managed by NASA's Jet Propulsion Laboratory (JPL), which also built
its main (orbiter) spacecraft.
(source: University of
Maryland)
