When you’re talking about a gas giant planet with rings, it’s often Saturn in the limelight. After all, you can see that planet’s bright disk of icy particles from Earth with just a modest telescope.
But in 1979 the Voyager 1 spacecraft saw that Jupiter has rings too, albeit a much fainter system primarily made up of dust.
These rings seem to come from material shed when meteors impact four of Jupiter’s tiny inner moons: Ejected stuff from Thebe and Amalthea create the two thick, outer “gossamer” rings, while Adrastea and Metis feed the thin, narrow main ring and the faint inner halo.
—Image courtesy NASA/JPL/Cornell University
The last detailed studies of Jupiter’s rings happened in the 1990s, when NASA’s Galileo spacecraft was orbiting the planet.
But now the wealth of highly detailed data gleaned from Cassini’s ongoing studies of Saturn is paying off for Jupiter: Using pictures of a wave-like pattern detected in Saturn’s rings, scientists went back to older Galileo shots of Jupiter’s rings and found a similar wave.
For Saturn, the current theory is that “something” disrupts the rings, briefly titling them out of the equatorial plane, until the planet’s gravity snaps the rings back into place.
This gravitational action starts a ripple through the rings—which Cassini sees as a pattern of light and dark that changes over time.
When Showalter and co. looked at Galileo shots of Jupiter’s rings from 1996, they also saw this waving pattern.
—Image courtesy NASA/JPL
What’s more, they were able to calculate the length of each oscillation, allowing them to see that the wave moving through the rings must have been started by two separate events: Something disrupted the rings in 1990 and again in 1994.
Well. The question remains, what is this mysterious “something”?
For folks who have been following news of recent impacts on Jupiter, you might remember that the year 1994 should set off alarm bells. That was the year the pieces of broken comet Shoemaker-Levy 9 smacked into the giant planet, leaving big black marks in its atmosphere.
—Image courtesy NASA
This got Showalter and his colleague Joe Burns at Cornell University thinking: Could the comet impact also have started a wave in the rings?
The scientists were already pretty sure you’d need some sort of outside force to make the wave, since the only other possible mechanism would be if the planet’s internal mass had somehow shifted. For that to happen, you’d need something like huge convective upwelling in the atmosphere, and you’d need it to happen much faster than seems reliably possible.
The rings could start waving if a massive thing such as a large asteroid passed close enough to Jupiter to change its inertial axis—but that would require a body bigger than 62 miles (100 kilometers) wide. Objects that size don’t swing by the planet very often, and if one had done so in the ’90s, we’d have probably seen it.
So that brings us back to comets.
Showalter and Burns think what went down on Jupiter was this: In 1992 Shoemaker-Levy 9 swung so close by Jupiter that the planet’s gravity ripped the comet apart. The debris then went into a long orbit around Jupiter and eventually hit the planet in 1994.
While we watched the big pieces slam into the atmosphere, a diffuse band of smaller chunks passed through the rings, creating a widespread gravitational disturbance that kicked off the wave.
The comet debris would have had enough mass to get such a ripple going, the researchers say, while not being so huge that the pieces simply punctured the rings.
If this idea holds, it could turn out that waves in planetary rings are a type of record book for past impact events on gas giants, something that’s traditionally been hard to pin down on worlds that don’t exactly get cratered up like the moon or Mercury.
“The rings are catching impacts and even telling us the frequency and size of impacting objects,” Burns said.
I’m not sure if a ring study is part of the upcoming Juno mission to Jupiter, but maybe it’s something for the science team to consider?