Scientists
Solve Mystery of Jupiter's Rings
September 16, 1998
by Dennis Cauchon
Astronomers have discovered why the giant planet
Jupiter has rings around it -- and the answer surprised
even the experts.
Using photos from the Galileo spacecraft now circling
the solar system's largest planet, scientists determined
that Jupiter's three rings are made of moon dust. The
dust was kicked up over millions of years as asteroids
and comets crashed into four small moons near Jupiter's
surface.
Revealed: Dust kicked up from asteroids
and comets crashing into small moons near
Jupiter's surface is source of rings around
planet, scientists say with certainty. |
The discovery is especially important for two
reasons, according to scientists at Cornell University
and the National Optical Astronomical Observatories:
- It goes a long way toward explaining all the
rings around Neptune and Uranus and some of the
rings around Saturn.
- That Jupiter's rings are made up of moon dust
is one of the few things about the solar system's
origin that scientists believe they know with
certainty.
"Definitive results are hard to get in science.
Usually, the evidence is circumstantial. But in this
case, we really know," said Michael Belton, head of the
Galileo imaging team.
Galileo image: Scientists analyzed
pictures of Jupiter and its ring system
acquired by NASA's Galilelo spacecraft. In
this image, the sun is behind the planet. |
The Galileo team solved the mystery by delicately
turning the 5,000-pound spacecraft, which was launched
by the space shuttle in 1989, so as it passed through
Jupiter's shadow, it faced the rings. During this
several hours of shade in November 1996, Galileo saw the
rings lighted by the sun, providing unprecedented
details of the tiny dust particles.
On Earth, the astronomers were surprised to see the
three rings stopped abruptly at different moons.
Belton said he thought at first that the data was
faulty.
Astronomer Joseph Burns, the Galileo team's ring
expert, confessed that he initially had no idea why the
rings stopped at moons.
Burns figured out the answer on a plane returning to
Cornell in Ithaca, N.Y., where he teaches, from
Galileo's headquarters at the Jet Propulsion Laboratory
in Pasadena, Calif.
"My first thought was, 'Can I get off this plane?
I've got it!'" Burns recalled.
A second flyby in Jupiter's shade in September 1997
confirmed the astronomers' theories: The depths of the
three rings matched the depths of the moons' orbits, and
the material in the rings matched the dark red dust on
the moons' surfaces.
Previously, scientists had thought the rings were
caused by interplanterary volcanic dust from Io, a
bigger, more distant moon.
Instead, the dust came from the four small moons
closest to Jupiter's surface: Metis, Adrastea, Amalthea
and Thebe. Three of these moons were unknown until the
Voyager spacecraft passed by in 1979. There are more
moons than rings, the scientists say, because some of
the moons supply dust to more than one ring.
The four moons are just 12 to 120 miles wide, and
they get pounded by debris. Every hour or so, a
sand-sized particle hits the moons. Every few million
years, an object as wide as a half-mile leaves a big
crater.
These objects strike the moons at a speed 100 times
that of a .22 bullet. "It's like shooting a bullet into
sand: the sand bursts up, and because there's little
gravity, it escapes into the ring," said Maureen Ockert-Bell,
a Galileo researcher.
The dust specks are few and far between, perhaps one
every 30 yards.
The solar system's most famous rings, the inner rings
around Saturn, probably have a different origin than the
ones discovered at Jupiter, astronomer Joseph Veverka
said. Those rings include snowball-sized ice chunks, he
said. However, Saturn's less visible outer rings could
be similar in origin to Jupiter's.
The Cassini spacecraft, launched in November 1997,
will examine those rings when it arrives at Saturn in
2004.
Galileo is studying Europa, another moon of Jupiter,
where some scientists suspect there is a frozen ocean
that could harbor life.
