Giant
propeller-shaped structures have been discovered in the rings of Saturn and appear
to be created by a new class of hidden moons, NASA announced Thursday.
NASA's
Cassini spacecraft spotted the distinctive structures inside some of Saturn's rings, marking the first time scientists have
managed to track the orbits of individual objects from within a debris disk
like the one that makes up Saturn's complicated ring system.
"Observing
the motions of these disk-embedded objects provides a rare opportunity to gauge
how the planets grew from, and interacted with, the disk of material
surrounding the early sun," said the study's co-author Carolyn Porco, one
of the lead researchers on the Cassini imaging team based at the Space Science
Institute in Boulder, Colo. "It allows us a glimpse into how the solar
system ended up looking the way it does."
Photos of
the propellers taken by Cassini show them to be huge structures several
thousands of miles long. By understanding how they form, astronomers hope to
glean insight into the debris disks around other stars as well, researchers
said.
The results
of the study are detailed in the July 8 issue of the journal Astrophysical
Journal Letters.
Propellers
at Saturn
Cassini
scientists have seen double-armed propeller structures in Saturn's rings
before, but on a smaller scale than the larger, newfound features. They were
first spotted in 2006 in an area now known as the "propeller belt,"
which is located in the middle of Saturn's outermost dense ring – the A ring.
The propellers
are actually gaps in the ring material were created by a new class of objects,
called moonlets, that are smaller than known moons but larger than the
particles making up Saturn's rings. It is estimated that these moonlets could
number in the millions, according to Cassini scientists.
The moonlets
clear the space immediately around them to generate the propeller-like features,
but are not large enough to sweep clear their entire orbit around Saturn, as
seen with the moons Pan and Daphnis. [Photos of Saturn rings and moons.]
But in the
new study, researchers a new legion of larger and rarer moons in a separate
part of the A ring, farther out from Saturn. These much larger moons create
propellers that are hundreds of times larger than those previously described,
and these objects have been tracked for about four years.
The study
was led by Cassini imaging team associate Matthew Tiscareno at Cornell
University in Ithaca, N.Y.
The
propeller features for these larger moons are up to thousands of miles long and
several miles wide. The moons embedded in Saturn's rings appear to kick up ring
material as high as 1,600 feet (0.5 km) above and below the ring plane.
This is much
greater than the typical ring thickness of about 30 feet (10 meters),
researchers said.
Hidden
Saturn moons
Still, the Cassini spacecraft is too far away to see the moons amid the swirling ring material that
surrounds them. Yet, scientists estimate that the moons measure approximately
half a mile (about one km) in diameter, based on the size of the propellers.
According to
their research, Tiscareno and his colleagues estimate that there are dozens of
these giant propellers. In fact, 11 of them were imaged multiple times between
2005 and 2009.
One such
propeller, nicknamed Bleriot after the famous aviator Louis Bleriot, has shown
up in more than 100 separate Cassini images and one ultraviolet imaging
spectrograph observation during this time.
"Scientists
have never tracked disk-embedded objects anywhere in the universe before
now," said Tiscareno. "All the moons and planets we knew about before
orbit in empty space. In the propeller belts, we saw a swarm in one image and
then had no idea later on if we were seeing the same individual objects. With
this new discovery, we can now track disk-embedded moons individually over many
years."
Over
their four years of observation, the researchers noticed shifts in the orbits
of the giant propellers as they travel around Saturn, but the cause of these
disturbances have not yet been determined.
The
shifting orbits could be caused by collisions with other smaller ring
particles, or could be responses to these particles' gravity, the researchers
said. The orbital paths of these moonlets could also be altered due to the
gravitational attraction of large moons outside of Saturn's rings.
Scientists
will continue to monitor the moons to see if the disk itself is driving the
chances, similar to the interactions that occur in young solar systems. If so,
Tiscareno said, this would be the first time such a measurement has been made
directly.
"Propellers
give us unexpected insight into the larger objects in the rings," said
Linda Spilker, Cassini project scientist based at NASA's Jet Propulsion
Laboratory in Pasadena, Calif. "Over the next seven years, Cassini will
have the opportunity to watch the evolution of these objects and to figure out
why their orbits are changing."
NASA
launched the Cassini probe in 1997 and it arrived at Saturn in 2004, where it dropped the European Huygens probe on the cloudy
surface of Titan, Saturn's largest moon. Cassini was slated to be
decommissioned in September of this year, but received a life extension that
now runs through 2017.
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