What's the News: Looking at images of odd undulations in the rings of Saturn and Jupiter, astronomers have discovered that comets are to blame. The finding means that a planet's rings act as a historical record of passing comets, possibly leading to a better understanding of comet populations. "We now know that collisions into the rings are very common---a few times per decade for Jupiter and a few times per century for Saturn,"
Mark Showalter, from the SETI Institute in Mountain View, California, told the Daily Mail.
"Now scientists know that the rings record these impacts like grooves in a vinyl record, and we can play back their history later."
How the Heck:
When a comet passes by the rings of a planet, its high-speed debris impacts the ring material and "changes the inclination of the particles' orbits," Bad Astronomer Phil Plait told me. "It's like a cosmic shotgun blast."
With the ring particles knocked out of alignment, they bob up and down as they orbit the planets---a phenomenon that looks in telescope images (see animation above) like ripples in a pond or corrugated metal.
The particles undulate up and down as they slowly gradually come back into alignment with the planet's equator. As Saturn spins, centrifugal force squashes the planet such that there's more mass around the equator than the poles. The result is that Saturn's gravity is non-spherical and pulls out-of-whack particles into equatorial alignment over the course of time.
Looking at photographs of Saturn and Jupiter's rings, astronomers were able to use mathematical models to rewind the process of ring disturbance, tracing Jupiter's comet to 1994 (when Shoemaker-Levy hit), and Saturn's to an as-yet-unknown comet from 1983.
What's the Context:
The unraveling of the ring mystery was only possible because of images sent back by NASA probes: Pictures of Jupiter's rings were taken in 1996 and 2000 by the Galileo spacecraft and in 2007 by the New Horizons probe. Saturn's rings were imaged in 2009 by the Cassini spacecraft.
Researchers didn't previously know about the 1983 comet that presumably hit Saturn because the planet was on the far side of the Sun from Earth at the time.
Astronomers have actually seen impact events on Jupiter's surface.
Next Up: Knowing that planetary rings record comet events opens up an entire avenue for astronomers to study the the solar system. One of the goals now is to see whether we can look at planetary ring changes and determine how often comets swing by planets and, in turn, get a better idea of how many comets exist in the outer solar system. References: M. M. Hedman, J. A. Burns, M. W. Evans, M. S. Tiscareno, and C. C. Porco. Saturn's Curiously Corrugated C Ring. Science, 2011; DOI: 10.1126/science.1202238 Mark R. Showalter, Matthew M. Hedman, and Joseph A. Burns. The Impact of Comet Shoemaker-Levy 9 Sends Ripples Through the Rings of Jupiter. Science, 2011; DOI: 10.1126/science.1202241