Dark Rings

Many years ago Pioneer 11 flew through Jupiter’s rings, but no one knew it
at the time. This week NASA’s Galileo spacecraft did it again … and
scientists were ready.

In 1974, NASA’s Pioneer 11 spacecraft plunged through the rings of Jupiter.

And no one noticed.

Jupiter’s dark rings–as wide as Saturn’s yet nearly invisible–hadn’t been
discovered yet. Indeed, it wasn’t until five years later that cameras
onboard Voyager 1 caught sight of them for the first time. On Mar. 5, 1979,
the spacecraft swung behind Jupiter, and from inside the planet’s shadow the
faintly sunlit rings were visible–but just barely.

Ever since, researchers have wished for another flyby like Pioneer 11’s.
NASA’s Voyager, Cassini and Galileo spacecraft have photographed the rings
many times, but always from a distance. No probe had actually entered the
rings for 28 years.

Until this week.

On Nov. 5, 2002, Galileo took the plunge and flew through Jupiter’s rings
again. And this time scientists were ready.

“We’ve been looking forward to this flyby for a while,” says Joe Burns, a
planetary scientist at Cornell University and a member of the Galileo
imaging team. “It’s an opportunity to study the particles that make up these
rings and to learn about their environment.”

Galileo is nearing the end of its twice-extended 7-year mission to Jupiter.
Risky maneuvers like flying through Io’s volcanoes and Jupiter’s rings were
saved for last. This week’s ring encounter and close-approach to Jupiter is
one of the final things Galileo will do before it plunges into Jupiter
itself next year.

Unlike Saturn’s rings, which are made of bright, icy chunks as large as
houses, Jupiter’s rings consist of fine dust akin to the particles in
cigarette smoke. The dust grains are dark (they reflect barely 5% of the
sunlight that hits them) and they are spread so thin that the rings are
almost transparent. This is what makes the rings so hard to study.

The origin of Jupiter’s rings was revealed by Galileo’s cameras more than
five years ago. “The dust comes from small rocky moons orbiting Jupiter,”
says Burns. These moons are constantly pelted by meteoroids, which burrow
into the ground and explode. Jupiter’s rings are the debris from those
impacts.

In fact, Jupiter has several rings: The main ring is the brightest. It’s
close to Jupiter and made of dust from the satellites Adrastea and Metis.
Two wide gossamer rings encircle the main ring. These come from the
satellites Thebe and Amalthea. There is also an extremely tenuous and
distant outer ring that circles Jupiter backwards. No one is certain, but
that ring might be made of captured interplanetary dust.

When Galileo approached Jupiter last Tuesday, it passed through one of the
gossamer rings. The spacecraft’s close approach to Amalthea on the same day
was much-anticipated by scientists who will figure out the mass of that moon
from its gravitational tug on Galileo.

Saturn’s rings probably formed from the total breakup of an icy moon about
the size of Amalthea (100 km wide). Jupiter’s rings, on the other hand, are
merely dust from the surface of such moons. “Saturn’s rings are millions of
times more massive than Jupiter’s,” notes Burns.

Meteoroids have been striking Jupiter’s moons and kicking up dust for
billions of years. So why isn’t there more “stuff” in Jupiter’s rings? Why
are Jupiter’s rings so much less massive than Saturn’s?

Burns explains: “Dust grains ejected into Jupiter’s rings don’t stay in the
rings forever. The grains spiral in toward Jupiter and eventually
disappear.”

They lose orbital energy for several reasons: Sunlight is one. Dust grains
absorb and re-emit sunlight, losing momentum in the process. Scientists call
this “Poynting-Robertson drag.”

Plasma collisions are another reason. Jupiter’s magnetosphere (a magnetic
bubble that surrounds the planet) is filled with electrified clouds called
plasmas. The dust grains in the rings are themselves charged–like the
static-charged dust that accumulates on your computer screen. When charged
grains collide with plasma clouds, the grains can lose orbital momentum.

The “age” of Jupiter’s rings depends on which of these mechanisms dominates.
Plasma collisions might de-orbit ring particles in only a few years.
Poynting-Robertson drag, which Burns favors, takes longer, perhaps 100,000
years. (The age of Saturn’s rings is likewise controversial. Read
Science@NASA’s “The Real Lord of the Rings” for more information.)

Jupiter’s rings are constantly replenished by meteoroid impacts, so they
won’t disappear any time soon. Next year’s rings, however, might be made of
different “stuff” than this year’s. In that sense, Jupiter’s rings might be
younger than you are.

When Galileo flew through the rings this week, the spacecraft’s suite of
electromagnetic sensors and its Dust Detector were working well. (The
spacecraft itself, bombarded by radiation from Jupiter, went into safe mode
near the end of the ring encounter, but not before data had been collected.)
Burns hopes the unprecedented in situ measurements will finally solve the
puzzle.

Or they might reveal more surprises. Jupiter’s dark rings remain, after all,
unexplored territory.

Note: Galileo left Earth aboard NASA’s space shuttle Atlantis in 1989. JPL,
a division of the California Institute of Technology in Pasadena, manages
the Galileo mission for NASA’s Office of Space Science, Washington, D.C.