Cassini spied just as many regular, faint
clumps in Saturn's narrow F ring, like those pictured here, as Voyager
did, but it saw hardly any of the long, bright clumps that were common
in Voyager images. Image credit: NASA/JPL-Caltech/SSI. Full image and caption
A map of Saturn's F ring from 2006 shows one
of the few bright, extended clumps (indicated by a green box) seen
during six years of observation by Cassini. Image credit:
NASA/JPL-Caltech/SSI. Hi-res image - enlarge image
Compared to the age of the solar system -- about four-and-a-half
billion years -- a couple of decades are next to nothing. Some planetary
locales change little over many millions of years, so for scientists
who study the planets, any object that evolves on such a short interval
makes for a tempting target for study. And so it is with the
ever-changing rings of Saturn.
Case in point: Saturn's narrow, chaotic and clumpy F ring. A recent
NASA-funded study compared the F ring's appearance in six years of
observations by the Cassini mission to its appearance during the Saturn
flybys of NASA's Voyager mission, 30 years earlier. The study team found
that, while the overall number of clumps in the F ring remained the
same, the number of exceptionally bright clumps of material plummeted
during that time. While the Voyagers saw two or three bright clumps in
any given observation, Cassini spied only two of the features during a
six-year period. What physical processes, they wondered, could cause
only the brightest of these features to decline sharply?
While a variety of features in Saturn's many rings display marked
changes over multiple years, the F ring seems to change on a scale of
days, and even hours. Trying to work out what is responsible for the
ring's tumultuous behavior is a major goal for ring scientists working
on Cassini.
"Saturn's F ring looks fundamentally different from the time of
Voyager to the Cassini era," said Robert French of the SETI Institute in
Mountain View, California, who led the study along with SETI Principal
Investigator Mark Showalter. "It makes for an irresistible mystery for
us to investigate."
The researchers hypothesize that the brightest clumps in the F ring
are caused by repeated impacts into its core by small moonlets up to
about 3 miles (5 kilometers) wide, whose paths around Saturn lie close
to the ring and cross into it every orbit. They propose that the
diminishing number of bright clumps results from a drop in the number of
these little moonlets between the Voyager and Cassini eras.
As for what might have caused the moonlets to become scarce, the team
has a suspect: Saturn's moon Prometheus. The F ring encircles the
planet at a special location, near a place called the Roche limit -- get
any closer to Saturn than this, and tidal forces from the planet's
gravity tear apart smaller bodies. "Material at this distance from
Saturn can't decide whether it wants to remain as a ring or coalesce to
form a moon," French said. Prometheus orbits just inside the F ring, and
adds to the pandemonium by stirring up the ring particles, sometimes
leading to the creation of moonlets, and sometimes leading to their
destruction.
Every 17 years, the orbit of Prometheus aligns with the orbit of the F
ring in such a way that its influence is particularly strong. The study
team thinks this periodic alignment might spur the creation of many new
moonlets. The moonlets would then crash repeatedly through the F ring,
like cars in a Hollywood high-speed chase, creating bright clumps as
they smash across lanes of ring material. Fewer clumps would be created
as time goes by, because the moonlets themselves are eventually
destroyed by all the crashes.
As with any good scientific hypothesis, the researchers offer a way
to test their ideas. It happens that the Voyager encounters with Saturn
occurred a few years after the 1975 alignment between Prometheus and the
F ring, and Cassini was present for the 2009 alignment. If the moon's
periodic influence is indeed responsible for creating new moonlets, then
the researchers expect that Cassini would see the F ring return to a
Voyager-like number of bright clumps in the next couple of years.
"Cassini's continued presence at Saturn gives us an interesting
opportunity to test this prediction," said Linda Spilker, Cassini
project scientist at NASA's Jet Propulsion Laboratory in Pasadena,
California, who was not involved in the study. "Whatever the result,
we're certain to learn something valuable about how rings, as well as
planets and moons, form and evolve."
The study by French and colleagues was published in the online edition of the Journal Icarus on July 15, 2014.
NASA's Jet Propulsion Laboratory manages the Voyager and
Cassini-Huygens missions for NASA's Science Mission Directorate at NASA
Headquarters in Washington.
More information about Cassini is available at: http://www.nasa.gov/cassini - http://saturn.jpl.nasa.gov
Preston Dyches 818-354-5011
Jet Propulsion Laboratory, Pasadena, Calif.
preston.dyches@jpl.nasa.gov
Seth Shostak 650-960-4530
SETI Institute, Mountain View, Calif.
seth@seti.org
Source: JPL-Caltech
