Glossary

Index of Definitions

  • ansa
  • FDS
  • occultation
  • optical depth
  • phase angle
  • reseau
  • resonance
  • Roche Limit
  • shepherd satellite

Ansa

An ansa is the portion of a ring that appears farthest from the disk of a planet in an image. This is the location in an image where we see the finest radial resolution on a ring. The word comes from the Latin word for “handle,” since the earliest views of Saturn’s rings suggested that the planet had two handles extending out on either side. The plural is either “ansae” or “ansas.”

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FDS

FDS stands for “Flight Data System.” It is the name for the internal clock on Voyagers 1 and 2. Most Voyager observations, including images, are identified by the FDS clock time when the observation took place. Within any planetary encounter, FDS counts are unique and increase with time. However, the FDS clock was reset after the Saturn and Uranus encounters, so FDS values are sometimes re-used for different planets.

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Occultation

The phenomenon of one celestial body apparently passing in front of another. By observing the rings as they occult a star or spacecraft, it is possible to probe the optical depth of the rings at very fine spatial resolution. Typically, a star passes behind the rings as seen from Earth or a spacecraft, and instruments record the amount of light passing through the rings as a function of time. If the rings are nearly opaque, then very little light gets through; if the rings are nearly transparent, then most of the light gets through. Thus, the varying light level recorded represents the radial structure of the rings.

Voyager also performed radio occultation experiments, in which radio telescopes on Earth recorded the radio signal from Voyager as it passed behind the rings. Since radio signals are not hindered by the smallest ring particles, this experiment tells us about the largest particles in the ring system.

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Optical Depth

Optical depth is a measure of the transparency of a ring system. When a ring is “optically thick” (i.e., the optical depth is large), the ring is nearly opaque and very little light passes through. When a ring is “optically thin” (i.e., the optical depth is small), very little material is present and most of the light passes through.

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Phase Angle

The phase angle is the Sun-Target-Observer angle. A phase angle of zero indicates that the Sun is, in effect, shining over your shoulder so you see a fully-lit target (like a full Moon). A phase angle of 180 degrees indicates that you are looking back toward the Sun at the “dark side” of a target (like a new Moon). Bodies much larger than the wavelength of light tend to be brightest at low phase angles. However, fine dust is very efficient at forward-scattering light, so dust tends to be brightest at high phase angles. The manner in which a ring scatters light can tell us a great deal about its particle properties.

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Reseau

Within the optics of the Voyager cameras was a grid of black dots called “reseau markings.” These appear superimposed upon each image sent back from Voyager, and are used to correct for geometric distortions in the camera. They are often suppressed in published images by replacing them by the local average brightness. Nevertheless, you can almost always find them if you look closely.

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Resonance

When you push on a pendulum at just the rate that the pendulum naturally swings, the effects add up and lead to a very large effect. This phenomenon is called “resonance.” In a similar vein, ring particles have natural response frequencies based on their orbital motion. Nearby satellites have small gravitation effects on the ring particles, but at resonance locations the effects can be quite pronounced. Many of the structures in planetary ring systems have been associated with the resonant effects of nearby moons.

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Roche Limit

The Roche Limit was first described by Edouard Roche in 1848. It is the closest distance a body held together by self-gravity can come to a planet without being pulled apart by the planet’s tidal (gravity) force. As a result, large moons cannot survive inside the Roche Limit. On July 7, 1992, Comet Shoemaker-Levy 9 broke apart into 21 pieces due to tidal forces when it passed within Jupiter’s Roche Limit; on the subsequent pass, each of the comet’s pieces collided with Jupiter.

If a planet and a moon have identical densities, then the Roche Limit is 2.446 times the radius of the planet. The Roche Limits for the ringed planets are:

Jupiter - 175,000 km (108,000 miles)
Saturn - 147,000 km ( 92,000 miles)
Uranus - 62,000 km ( 39,000 miles)
Neptune - 59,000 km ( 37,000 miles)
This limit represents the rough boundary between each planet’s ring system and its innermost larger moons.

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Shepherd Satellite

The gravitational influence of a moon in orbit near the edge of a planetary ring can have the effect of repelling the ring material. This “shepherding” effect has been found to confine a number of rings in the solar system, and the moons that do the shepherding are called shepherd satellites.

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