Oddly shaped gaps
found in Saturn's rings hint at the existence of
long sought "moonlets" and support the theory
that the rings are the broken remains of an icy
moon shattered long ago in a violent collision,
Scientists think a comet or
asteroid collided with one of Saturn's moons
about 100 million years ago. Such an impact
would have created debris in a range of sizes,
but until now, scientists only had evidence for
chunks of rock that were miles in diameter and
smaller particles that were about 65 feet (20
km) across or less. The medium-sized moonlets-so
named because their size would be between that
of a moon and smaller particles-predicted by
theory were missing.
But in July 2004, NASA's
spacecraft was hovering directly above Saturn's
ring system when it detected
strange gaps resembling
S-shaped propellers in the planet's bright
A-ring. Scientists think the gaps were formed by
chunks of rock 300 feet (100 m) wide as they
plowed through smaller particles in the ring.
The finding is detailed in the March 30 issue
of the journal Nature.
Propellers in space
propellers are the result of differences in
the speed of material orbiting in Saturn's rings
and because of the average size of the moonlets
themselves, scientists think. Because the
strength of gravity decreases with distance,
material circling closer to a planet moves
faster than material that is orbiting farther
As a result, small ring particles flanking
the two sides of an orbiting moonlet would
appear to be moving in opposite directions to a
viewer standing on a moonlet. Picture three
trains moving on parallel tracks but at
different speeds. The train on the far left is
moving fast; the middle train is moving slightly
slower and the train on the far right is moving
slowest of all. If an observer in the middle
train were to look out her window, the train on
her left would appear to be moving forward while
the train on her right would look like it was
The moonlets in Saturn's rings are like the
middle train. But because they are so large, the
moonlets impede the movement of smaller ring
particles to the left and right of them. This
creates gaps on their left and right sides.
"Disturbances on one side [of the moonlet]
get carried ahead but those on the other get
carried behind," explained study leader Matthew
Tiscareno from Cornell University. "That's what
draws it out into the propeller shape."
The gaps taper off farther away from the
moonlets as smaller ring particles gradually
refill the empty space.
Scientists think that only intermediate sized
rocks can create the propeller shapes. Small
particles aren't massive enough to have any
effect on their neighbors, while
Saturnian moons like Encke and Pan-which are
4 miles (7 km) and 19 miles (30 km) wide,
respectively-are so large that their gravity
prevents the gaps from closing back up. Through
their sheer size, the moons achieve what the
moonlets can't: they harrow out rings of empty
space that stretch around the entire planet.
More space propellers
The propellers were predicted from computer
models but had never been observed in nature
before now. They're probably found under other
conditions as well, scientists think. In fact,
Saturn itself might have created such gaps
around the Sun as it formed in the early solar
According to the
standard theory, planets form from swirling
discs of gas, dust and debris around nascent
stars. Large chunks of rock and ice in the disc
collide and clump together, forming
protoplanets and eventually planets.
"The planets in our solar system, the
precursors anyway, probably went through this
stage," said study team member Derek Richardson
from the University of Maryland.
Galaxies like our own
Milky Way are also swirling discs of matter
that have large objects, such as stars and
planets, embedded within them, so could
propellers also form in galaxies?
Probably not, Richardson said.
"The analogy in a galactic disc would be a
large star with lots of little stars getting
strongly perturbed by it," he said. "You don't
really see that kind of process operating in
galaxies. Stars are far apart."