Astronomers have found that a planet's rings can be affected by specific events that occurred decades, not millions of years, ago.
Scientists analyzing data from NASA's Cassini, Galileo and New Horizons spacecrafts have traced telltale ripples in the rings of Saturn and Jupiter back to collisions with cometary fragments dating back only more than 10 years ago.
Jupiter's ripple is linked to a comet Shoemaker-Levy 9 in July 1994 when the comet's debris cloud hurtled through the thin Jupiter ring system on a collision course into the planet. A comet is an icy small Solar System body that, when close enough to the Sun, displays a thin, fuzzy, temporary atmosphere and sometimes also a tail.
On the other hand, Saturn's ripples were attributed to a similar object - likely another cloud of comet debris - plunging through the inner rings in 1983.
Scientists have learned about the patchy patterns in Jupiter's rings in the late 1990s from Galileo's visit to Jupiter. Unfortunately, the images were fuzzy, and scientists were not able to understand the cause of such patterns.
However, scientists got a better picture of the activity when Cassini entered orbit around Saturn in 2004 and started sending back thousands of images. A 2007 science paper by Hedman and colleagues first noted corrugations in Saturn's innermost ring, dubbed the D ring.
A group including Hedman and Mark Showalter, a Cassini co-investigator based at the SETI Institute in Mountain View, California, saw that the grooves in the D ring appeared to wind together more tightly over time.
Images taken after Saturn's August 2009 equinox from NASA's Cassini spacecraft revealed alternating light and dark bands extending from Saturn's D ring, completely across the C ring, and right up to the inner B ring edge. These brightness variations are almost certainly caused by the changing slopes in the rippled ring plane, much like the corrugations of a tin roof.
Scientists, after detailed studies, concluded that a broad swath of the rings became suddenly tilted in the early 1980s, likely because cometary debris crashed into the rings. The corrugation's radial extent implies that the impacting material was a dispersed cloud of debris instead of a single object. The corrugation's amplitude of 2 to 20 meters (7 to 60 feet) indicates that the debris' total mass was around 1 trillion kilograms (or one billion metric tons).
"We're finding evidence that a planet's rings can be affected by specific, traceable events that happened in the last 30 years, rather than a hundred million years ago," Matthew Hedman, a Cassini imaging team associate said in a statement. "The solar system is a much more dynamic place than we gave it credit for."
Playing the process backward, Hedman demonstrated the pattern originated when something tilted the D ring off its axis by about 300 feet (100 meters) in late 1983. The scientists found Saturn's gravity on the tilted area warped the ring into a tightening spiral.
Cassini imaging scientists received another clue around August 2009 when the sun shone directly along Saturn's equator and lit the rings edge-on. The unique lighting conditions highlighted ripples not previously seen in another part of the ring system. Whatever happened in 1983 was big - not a small, localized event.
The collision tilted a region more than 12,000 miles (19,000 kilometers) wide, covering part of the D ring and the next outermost ring, called the C ring.
Unfortunately, spacecraft was not visiting Saturn at that time, and the planet was on the far side of the sun, hidden from telescopes on or orbiting Earth, so whatever happened in 1983 went unnoticed by astronomers.
Hedman and Showalter wondered whether the long-forgotten pattern in Jupiter's ring system might illuminate the mystery. Using Galileo images from 1996 and 2000, Showalter confirmed a similar winding spiral pattern by applying the same math they had applied to Saturn and factoring in Jupiter's gravitational influence. Galileo was launched on a space shuttle in 1989 and studied Jupiter until 2003.
Unwinding the spiral pinpointed the date when Jupiter's ring was tilted off its axis between June and September 1994. Shoemaker-Levy plunged into the Jovian atmosphere in late July. The Galileo images also revealed a second spiral, which was calculated to have originated in 1990. Images taken by New Horizons in 2007, when the spacecraft flew by Jupiter on its way to Pluto, showed two newer ripple patterns, in addition to the fading echo of the Shoemaker-Levy impact.
"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," Showalter said. "Now scientists know that the rings record these impacts like grooves in a vinyl record, and we can play back their history later."
"Finding these fingerprints still in the rings are amazing and helps us better understand impact processes in our solar system," said Linda Spilker, Cassini project scientist, based at NASA's Jet Propulsion Laboratory in Pasadena, California.
The findings are detailed in two papers published Thursday in the journal Science.
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