Astronomers Help Decipher Saturn Ring Structure
Researchers today announced that the Cassini spacecraft has
obtained the most detailed look ever at Saturn’s rings,
including the B ring, which has eluded previous robotic explorers.
Its structure seems remarkably different from its two neighbors,
rings A and C.
May 23, 2005
rings is a mystery. The rings are an enormous, complex structure.
From edge-to-edge, the ring system
would not even fit in the distance between Earth and the Moon.
The seven main rings are labeled in the order they were discovered.
From the planet outward, they are D, C, B, A, F, G and E.
During a recent radio experiment, Cassini mapped this structure
with clarity never before available. This is the first of many
such observations Cassini will be conducting over the summer.
"These observations are the culmination of 15 years of preparation," said
French, a member of the Cassini radio science team and
professor of astronomy at Wellesley College. "The exquisite
detail in the rings is spectacular, and we expect a rich harvest
of additional observations over the summer."
College researcher Colleen McGhee added: "As an
aid to visualizing the intricate structure of the rings, we've
converted our measurements into false-color 'images' that have
“The structure of those remarkable rings is a sight to
behold. All ring features appear to be populated by a broad range
of particle sizes that extend to many meters in diameter at the
upper end,” said Essam Marouf, Cassini radio science team
member and professor of electrical engineering, San Jose State
University, San Jose, Calif.
Marouf said that, at the lower end, particles of about 5 centimeters
(roughly 2 inches) in diameter or less seem to be scarce in ring
B and inner ring A. In rings C and outer ring A, particles of less
than about 5 centimeters (2 inches) in diameter seem to be abundant.
Cassini found that the inner and outer parts of ring B contain
rings that are hundreds of kilometers wide (hundreds of miles)
and vary greatly in the amount of material they contain. A thick,
5,000-kilometer-wide (3,100-mile) core contains several bands with
ring material that is nearly four times as dense as that of ring
A and nearly 20 times as dense as that of ring C.
The dramatically varying structure of ring B is in sharp contrast
to the relatively flat structure of ring A or the gentle, wavy
structure of ring C, where many dense, narrow and sharp-edged ringlets
permeate its outer part.
detected more than 40 wavy features called “density
waves” in ring A, many near its outer region, close to the
moons orbiting just outside the ring. The density wave observations
will tell more about the ring surface mass density, its vertical
thickness and other physical properties.
“A marvelous array of waves, caused by gravitational interactions
with nearby moons, has been uncovered throughout ring A,” said
Marouf. “We also see a major density wave in the dense ring
B. Some of these waves have been seen in Voyager and other Cassini
observations, but not in this large number and not with this exceptional
Cassini conducted this first radio occultation observation of
Saturn's rings, atmosphere and ionosphere on May 3, 2005. An occultation
means that if you watch Cassini from Earth, Cassini would appear
occulted, or hidden, behind the rings. During a radio occultation,
Cassini sends a radio signal from the spacecraft through the rings
to Earth. Scientists then observe how the strength of the radio
signal is affected as the signal passes through ring material.
The denser a ring is, the weaker the signal received. The experiment
helps scientists map the distribution of the amount of ring material
and determine the ring particle sizes.
was the first ever to use three radio signals of different frequencies
(called Ka, X and S) transmitted simultaneously
from a spacecraft to Earth-receiving stations of NASA’s Deep
Space Network. Ring particles of different sizes affect each frequency
The Cassini tour was specifically designed to optimize the geometry
of the first radio occultation experiment and seven other occultations
scheduled from May to September 2005. These observations are at
the heart of Cassini's fundamental science objectives of characterizing
and understanding Saturn and its ring system. During its lifetime,
Cassini will obtain 20 radio occultations and 80 stellar occultations,
providing far more detailed knowledge of the ring structures.
For images and information on the Cassini mission visit http://saturn.jpl.nasa.gov
and http://www.nasa.gov/cassini .
The Cassini-Huygens mission is a cooperative project of NASA,
the European Space Agency and the Italian Space Agency. The Jet
Propulsion Laboratory, a division of the California Institute of
Technology in Pasadena, manages the Cassini-Huygens mission for
NASA's Science Mission Directorate, Washington, D.C. The Cassini
orbiter was designed, developed and assembled at JPL.