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Dark Rings By Jove

by Tony Phillips
for Science at NASA
Huntsville - Nov 11, 2002

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.

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