Josh Colwell's Planetary Rings Page

    Origins. While Saturn's rings are the most spectacular in the solar system, each of the four large planets in the outer solar system is surrounded by a ring system. The origin of many of these rings appears to be closely tied to the population of moons orbiting the planets. The impacts of interplanetary debris, primarily in the form of particles only a few hundred micrometers in size, onto these moons knocks dust off the surface. Because many of the moons are small and have weak gravity, these particles don't fall back to the surface but go into orbit around the planet, making a dusty ring. The origin of Saturn's main ring system is less clear. It is so massive, that a moon at least as large as Saturn's moon Mimas would need to be completely disrupted to create the rings. The total mass of Saturn's rings is not well known, and is one of the goals of the Cassini mission. It could be significantly larger than the mass of Mimas. It would take a particularly large impact to disrupt a moon as large as Mimas (roughly 400 km across), though research suggests that this may have been possible in the early epochs of the solar system. Another possibility is that a large cometary object was torn apart during a close encounter with Saturn and left debris in orbit, forming the rings.
    Saturn's Rings. The particles in Saturn's rings are made primarily of water ice. The particles in the main rings (A, B, C, and the Cassini Division) range in sizes from centimeters to several meters, though Cassini has revealed evidence for supersized ring particles that are ~100 m across. These particles are also likely coated with (icy) dust from collisional grinding and meteoroid impacts.

Image: Dave Seal, JPL. Shows the relative size of some of Saturn's moons (top) and the relative positions and spacings of the main rings and inner moons (bottom). The "main rings" usually refers to rings A, B, C, and the Cassini Division (which is not empty, but a distinct ring region similar to the C ring).

    Information on gaps and ringlets in Saturn's rings can be found at wikipedia. More detailed information is in the book Saturn from Cassini-Huygens. The book is a collection of peer-reviewed review chapters on everything in the Saturn system except the moon Titan, which is discussed in a companion book.

    UVIS Studies of Saturn's Rings. The Ultraviolet Imaging Spectrograph (UVIS) is one of twelve science instruments on the Cassini spacecraft orbiting Saturn. Larry Esposito at the University of Colorado, Boulder, is the Principal Investigator of the UVIS instrument. UVIS studies two aspects of the rings with two different telescopes: the Far Ultraviolet Spectograph (FUV) makes images of the rings in ultraviolet light, and the High Speed Photometer (HSP) observes stellar occultations. I used data from the FUV obtained when Cassini entered orbit around Saturn on July 1, 2004, to create this false color image of the rings which was featured in numerous news outlets including Time Magazine as a 2004 Picture of the Year, and (my favorite) on The Daily Show with Jon Stewart.
A false color ultraviolet image of Saturn's A ring and Cassini Division
Image: NASA/JPL/University of Colorado. This image was constructed from a hundreds of FUV spectra obtained of the rings, from the outer edge of the B ring to the outer edge of the A ring. To create the image, I summed all data taken within concentric radial bins of the rings to improve the signal to noise ratio. I then reprojected the data into the ring shape shown above. Mapping different regions of the FUV spectrum to red, green, and blue, produced the image above. The color red was mapped to emission from atomic Hydrogen gas (which fills interplanetary space). Red regions indicate gaps and more transparent regions of the rings. Brighter blue regions are more reflective in the FUV part of spectrum. Because water ice is the dominant spectrally active constituent of the ring particles, these regions have more pure water ice and fewer non-ice contaminants.

We are studying the FUV data to identify variations in water ice abundance and grain size properties across the rings. Differences between different ring regions will constrain models of ring origin and evolution.

The UVIS HSP stellar occultation data provides the highest resolution measurements of the structure of the rings. By measuring the brightness of a star up to 1000 times per second as the rings pass in front of the star, the HSP measures ring structure at scales approaching the size of the largest ring particles (a few meters).


Back to Josh Colwell's home page.

Last updated: August 24, 2009