One of the more memorable scenes from sci-fi cinema is a frantic cat-and-dog chase though a cluttered asteroid belt with Darth Vader's fighters relentlessly hunting down Hans Solo's spaceship in The Empire Strikes Back.
The real asteroid belt encircling our sun looks nothing like this. The asteroids are so dispersed that if you were smack dab in the middle of the belt you really wouldn’t see anything.
But a billion miles away, there's a never-ending demolition derby taking place among myriad flying boulders that have been bumping and grinding inside Saturn's magnificent rings since the early days of the solar system.
Pictures from NASA's Cassini orbiter, which has been circling Saturn since 2004, reveal the action close-up. Cassini's data show that icy debris can range from the size of a peanut to that of a car or even a house. These pieces smash together and leave a telltale trail of debris.
The most dramatic view came when Saturn's rings were tilted edge-on to the sun last August. Long shadows cast by ring structure showed the normally extraordinarily flat rings (at a thickness of about 50 feet) being flipped up a few miles above the ring plane. Embedded moons were seen twisting and shaping the rings and even sweeping paths through them. They cause the rings to ripple like football fans' "stadium wave." This gives the rings a fluidity never before seen.
What's sobering is that the rings have remained this chaotic for billions of years: smashing apart bodies, building them up through agglomeration, and then smashing them apart again. This is supported by Cassini observations, showing the water-rich particles aggregating into denser clumps. This means the mass of the rings could be three times greater than thought and therefore able to last for the entire history of the solar system.
Though the rings are only about 200,000 miles across, they can be looked at as a scale model of the gargantuan 50-billion mile-diameter protoplanetary disk that encircled our sun over 4.5 billion years ago. The dynamical processes that go on in the rings are probably similar to what went on in the early solar system's formative years. The planets were forged from gentle collisions between bodies that ultimately cause planets to grow though accretion.
One big difference is that larger bodies can't agglomerate around Saturn. The ring material is too deep within Saturn's gravitational field. Once a body begins to snowball up, it can only reach a certain diameter before Saturn's tidal pull will tear down the object back into its constituent pieces.
The Hubble and the Spitzer space telescopes have studied circumstellar disks around other stars. The first generation disks agglomerate planets. But collisions among these bodies grind down objects to make a second-generation dusty disk. One of the best known examples is the thick dust ring around the young star Beta Pictoris.
One of the biggest revelations in astronomy in the past few decades is that we live in an evolving universe. The early stars made heavier elements for successive generations of stars and planets, the galaxies went through a grand assembly, as did planetary systems. But due to the seesaw pull of gravity, the Saturnian rings are in a peculiar state of "chaotic equilibrium." The beautiful structure keeps rippling and morphing but is not evolving toward anything.
The rings should be around for several billion years more to entice future generations. It's not impossible they could be the destination of interplanetary tourist voyages in the distant future. The rings will eventually evaporate under the seething glow of an expanding red giant sun. Saturn will grow the longest comet tail ever seen in the solar system's history.
Image credit: NASA/ESA
Tags: NASA Cassini Equinox, Saturn
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