Saturn

It is difficult to say what is impossible, for the dream of yesterday is the hope of today and reality of tomorrow.
- Robert Goddard

## Table of Contents

- Saturn (This Page)
  - Introduction
  - Saturn Statistics
  - Animations of Saturn
  - Views of Saturn
  - Rings of Saturn
  - Saturn's Moon Summary
  - References
- Satellites of Saturn
  - Atlas, Calypso, Dione, Enceladus, Epimetheus, Helene, Hyperion, Iapetus, Janus, Mimas, Pan, Pandora, Phoebe, Prometheus, Rhea, Telesto, Tethys, Titan, and Possible New Satellites of Saturn.
- Chronology of Saturn Exploration
- Voyager Saturn Science Summary
- The Cassini Mission
- Saturn Image and Animation Index

## Additional Saturn Resources:

- Saturn's Ring System
- Saturn Ring Plane Crossings of 1995-1996 (JPL)

Introduction

Saturn is the sixth planet from the Sun and is the second largest in the solar system with an equatorial diameter of 119,300 kilometers (74,130 miles). Much of what is known about the planet is due to the Voyager explorations in 1980-81. Saturn is visibly flattened at the poles, a result of the very fast rotation of the planet on its axis. Its day is 10 hours, 39 minutes long, and it takes 29.5 Earth years to revolve about the Sun. The atmosphere is primarily composed of hydrogen with small amounts of helium and methane. Saturn is the only planet less dense than water (about 30 percent less). In the unlikely event that a large enough ocean could be found, Saturn would float in it. Saturn's hazy yellow hue is marked by broad atmospheric banding similar to, but fainter than, that found on Jupiter.

The wind blows at high speeds on Saturn. Near the equator, it reaches velocities of 500 meters a second (1,100 miles an hour). The wind blows mostly in an easterly direction. The strongest winds are found near the equator and velocity falls off uniformly at higher latitudes. At latitudes greater than 35 degrees, winds alternate east and west as latitude increases.

Saturn's ring system makes the planet one of the most beautiful objects in the solar system. The rings are split into a number of different parts, which include the bright A and B rings and a fainter C ring. The ring system has various gaps. The most notable gap is the Cassini [kah-SEE-nee] Division, which separates the A and B rings. Giovanni Cassini discovered this division in 1675. The Encke [EN-kee] Division, which splits the A Ring, is named after Johann Encke, who discovered it in 1837. Space probes have shown that the main rings are really made up of a large number of narrow ringlets. The origin of the rings is obscure. It is thought that the rings may have been formed from larger moons that were shattered by impacts of comets and meteoroids. The ring composition is not known for certain, but the rings do show a significant amount of water. They may be composed of icebergs and/or snowballs from a few centimeters to a few meters in size. Much of the elaborate structure of some of the rings is due to the gravitational effects of nearby satellites. This phenomenon is demonstrated by the relationship between the F-ring and two small moons that shepherd the ring material.

Radial, spoke-like features in the broad B-ring were also found by the Voyagers. The features are believed to be composed of fine, dust-size particles. The spokes were observed to form and dissipate in the time-lapse images taken by the Voyagers. While electrostatic charging may create spokes by levitating dust particles above the ring, the exact cause of the formation of the spokes is not well understood.

Saturn has 18 confirmed moons, the largest number of satellites of any planet in the solar system. In 1995, scientists using the Hubble Space Telescope sighted four objects which might be new moons.

Saturn Statistics
Mass (kg) | 5.688e+26
Mass (Earth = 1) | 9.5181e+01
Equatorial radius (km) | 60,268
Equatorial radius (Earth = 1) | 9.4494e+00
Mean density (gm/cm^3) | 0.69
Mean distance from the Sun (km) | 1,429,400,000
Mean distance from the Sun (Earth = 1) | 9.5388
Rotational period (hours) | 10.233
Orbital period (years) | 29.458
Mean orbital velocity (km/sec) | 9.67
Orbital eccentricity | 0.0560
Tilt of axis (degrees) | 25.33
Orbital inclination (degrees) | 2.488
Equatorial surface gravity (m/sec^2) | 9.05
Equatorial escape velocity (km/sec) | 35.49
Visual geometric albedo | 0.47
Magnitude (Vo) | 0.67
Mean cloud temperature | -125°C
Atmospheric pressure (bars) | 1.4
Atmospheric composition Hydrogen Helium | 97% 3%

Animations of Saturn

- Saturn's spokes in rings.
- 60-HST exposures showing a storm in the atmosphere of Saturn.

Views of Saturn

Saturn With Rhea and Dione
NASA's Voyager 2 took this photograph of Saturn on July 21, 1981, when the spacecraft was 33.9 million kilometers (21 million miles) from the planet. Two bright, presumably convective cloud patterns are visible in the mid-northern hemisphere and several dark spoke-like features can be seen in the broad B-ring (left of planet). The moons, Rhea and Dione, appear as blue dots to the south and southeast of Saturn, respectively. Voyager 2 made its closest approach to Saturn on August 25, 1981. (Courtesy NASA/JPL)
Saturn With Tethys and Dione
Saturn and two of its moons, Tethys (above) and Dione, were photographed by Voyager 1 on November 3, 1980, from a distance of 13 million kilometers (8 million miles). The shadows of Saturn's three bright rings and Tethys are cast onto the cloud tops. The limb of the planet can be seen easily through the 3,500-kilometer-wide (2,170 mile) Cassini Division, which separates ring A from ring B. The view through the much narrower Encke Division, near the outer edge of ring A is less clear. Beyond the Encke Division (at left) is the faintest of Saturn's three bright rings, the C-ring or crepe ring, barely visible against the planet. (Courtesy NASA/JPL)
Nordic Optical Telescope
This image of Saturn was taken with the 2.6 meter Nordic Optical Telescope, located at La Palma, Canary Islands. (© Copyright Nordic Optical Telescope Scientific Association -- NOTSA)
Saturn's Rings Edge-On
In one of nature's most dramatic examples of "now-you see-them, now-you-don't," NASA's Hubble Space Telescope captured Saturn on May 22, 1995, as the planet's magnificent ring system turned edge-on. This ring-plane crossing occurs approximately every 15 years when the Earth passes through Saturn's ring plane.

The rings do not disappear completely because the edge of the rings reflects sunlight. The dark band across the middle of Saturn is the shadow of the rings cast on the planet (the Sun is almost 3 degrees above the ring plane.) The bright stripe directly above the ring shadow is caused by sunlight reflected off the rings onto Saturn's atmosphere. Two of Saturn's icy moons are visible as tiny starlike objects in or near the ring plane.

- Comparison of ring edge-on with more traditional observation.
- Three image sequence of ring-plane crossing.
- Three image sequence of ring-plane crossing with labels.
- Ring-plane crossing in August 1995.

Storm on Saturn
This image, taken by the Hubble Space Telescope, shows a rare storm that appears as a white arrowhead-shaped feature near the planet's equator. The storm is generated by an upwelling of warmer air, similar to a terrestrial thunderhead. The east-west extent of this storm is equal to the diameter of the Earth (about 12,700 kilometers or 7,900 miles). The Hubble images are sharp enough to reveal that Saturn's prevailing winds shape a dark "wedge" that eats into the western (left) side of the bright central cloud. The planet's strongest eastward winds, clocked at 1,600 kilometers (1,000 miles) per hour based on Voyager spacecraft images taken in 1980-81, are at the latitude of the wedge.

To the north of this arrowhead-shaped feature, the winds decrease so that the storm center is moving eastward relative to the local flow. The clouds expanding north of the storm are swept westward by the winds at higher latitudes. The strong winds near the latitude of the dark wedge blow over the northern part of the storm, creating a secondary disturbance that generates the faint white clouds to the east (right) of the storm center. The storm's white clouds are ammonia ice crystals that form when an upward flow of warmer gases shoves its way through Saturn's frigid cloud tops.
HST Views Aurora on Saturn
The top image shows the first image ever taken of bright aurorae at Saturn's northern and southern poles, as seen in far ultraviolet light by the Hubble Space Telescope. Hubble resolves a luminous, circular band centered on the north pole, where an enormous auroral curtain rises as far as 2,000 kilometers (1,200 miles) above the cloudtops. This curtain changed rapidly in brightness and extent over the two hour period of HST observations.

The aurora is produced as trapped charged particles precipitating from the magnetosphere collide with atmospheric gases. As a result of the bombardment, Saturn's gases glow at far-ultraviolet wavelengths (110-160 nanometers). These wavelengths are absorbed by the Earth's atmosphere, and can only be observed from space-based telescopes.

For comparison, the bottom image is a visible-light color composite of Saturn as seen by Hubble on December 1, 1994. Unlike the ultraviolet image, Saturn's familiar atmospheric belts and zones are clearly seen. The lower cloud deck is not visible at UV wavelengths because sunlight is reflected from higher in the atmosphere.
Last View of Saturn
Two days after its encounter with Saturn, Voyager 1 looked back on the planet from a distance of more than 5 million kilometers (3 million miles). This view of Saturn has never been seen by an earth based telescope, since the earth is so close to the Sun only the sunlit face of Saturn can be seen. (Copyright © Calvin J. Hamilton)
Rings of Saturn
This color-enhanced image shows the dark spoke-like features in the rings. The spokes seem to form very rapidly with sharp edges and then dissipate. The A ring appears as the outermost bands but in this image appears as two bands divided by the Encke's division. The Cassini's division divides the A and B bands. (Credit: Calvin J. Hamilton)
False Color Image of Saturn's Rings
Possible variations in chemical composition from one part of Saturn's ring system to another are visible in this Voyager 2 picture as subtle color variations that can be recorded with special computer-processing techniques. This highly enhanced color view was assembled from clear, orange and ultraviolet frames obtained August 17, 1981 from a distance of 8.9 million kilometers (5.5 million miles). In addition to the previously known blue color of the C-ring and the Cassini Division, the picture shows additional color differences between the inner B-ring and and outer region (where the spokes form) and between these and the A-ring. (Courtesy NASA/JPL)
Saturn's F-Ring
Saturn's outermost ring, the F-ring, is a complex structure made up of two narrow, braided, bright rings along which "knots" are visible. Scientists speculate that the knots may be clumps of ring material, or mini moons. The F-ring was photographed at a range of 750,000 kilometers (470,000 miles). (Courtesy NASA/JPL)
Saturn Family
This montage of images of the Saturnian system was prepared from an assemblage of images taken by the Voyager 1 spacecraft during its Saturn encounter in November 1980. This artist's view shows Dione in the forefront, Saturn rising behind, Tethys and Mimas fading in the distance to the right, Enceladus and Rhea off Saturn's rings to the left, and Titan in its distant orbit at the top. (Courtesy NASA/JPL)
Saturn's Satellites and Ring Plane Structure
This image shows Saturn's satellites approximately to scale as well as Saturn's ring structure. (Courtesy Dave Seal, JPL)

Rings of Saturn

The following is a summary of the rings of Saturn.

Name | Distance* | Width | Thickness | Mass | Albedo
--- | --- | --- | --- | --- | ---
D | 67,000 km | 7,500 km | ? | ? | ?
C | 74,500 km | 17,500 km | ? | 1.1x10^18 kg | 0.25
Maxwell Gap | 87,500 km | 270 km
B | 92,000 km | 25,500 km | 0.1-1 km | 2.8x10^19 kg | 0.65
Cassini Div | 117,500 km | 4,700 km | ? | 5.7x10^17 kg | 0.30
A | 122,200 km | 14,600 km | 0.1-1 km | 6.2x10^18 kg | 0.60
Encke gap | 133,570 km | 325 km
Keeler gap | 136,530 km | 35 km
F | 140,210 km | 30-500 km | ? | ? | ?
G | 165,800 km | 8,000 km | 100-1000 km | 6-23x10^6 kg | ?
E | 180,000 km | 300,000 km | 1,000 km | ? | ?

*The distance is measured from the planet center to the start of the ring.

Saturn's Moon Summary

Saturn has 18 officially recognized and named satellites. In addition, there are other unconfirmed satellites. One circles in the orbit of Dione, a second is located between the orbits of Tethys and Dione, and a third is located between Dione and Rhea. The unconfirmed satellites were found in Voyager photographs, but were not confirmed by more than one sighting. Recently, the Hubble Space Telescope imaged four objects that might be new moons.

Several generalizations can be made about the satellites of Saturn. Only Titan has an appreciable atmosphere. Most of the satellites have a synchronous rotation. The exceptions are Hyperion, which has a chaotic orbit, and Phoebe. Saturn has a regular system of satellites. That is, the satellites have nearly circular orbits and lie in the equatorial plane. The two exceptions are Iapetus and Phoebe. All of the satellites have a density of < 2 gm/cm3. This indicates they are composed of 30 to 40% rock and 60 to 70% water ice. Most of the satellites reflect 60 to 90% of the light that strikes them. The outer four satellites reflect less than this and Phoebe reflects only 2% of the light that strikes it.

The following table summarizes the radius, mass, distance from the planet center, discoverer and the date of discovery of each of the confirmed satellites of Saturn:

Moon | # | Radius (km) | Mass (kg) | Distance (km) | Discoverer | Date
--- | --- | --- | --- | --- | --- | ---
Pan | XVIII | 9.655 | ? | 133,583 | M. Showalter | 1990
Atlas | XV | 20x15 | ? | 137,640 | R. Terrile | 1980
Prometheus | XVI | 72.5x42.5x32.5 | 2.7e+17 | 139,350 | S. Collins | 1980
Pandora | XVII | 57x42x31 | 2.2e+17 | 141,700 | S. Collins | 1980
Epimetheus | XI | 72x54x49 | 5.6e+17 | 151,422 | R. Walker | 1966
Janus | X | 98x96x75 | 2.01e+18 | 151,472 | A. Dollfus | 1966
Mimas | I | 196 | 3.80e+19 | 185,520 | W. Herschel | 1789
Enceladus | II | 250 | 8.40e+19 | 238,020 | W. Herschel | 1789
Tethys | III | 530 | 7.55e+20 | 294,660 | G. Cassini | 1684
Telesto | XIII | 17x14x13 | ? | 294,660 | B. Smith | 1980
Calypso | XIV | 17x11x11 | ? | 294,660 | B. Smith | 1980
Dione | IV | 560 | 1.05e+21 | 377,400 | G. Cassini | 1684
Helene | XII | 18x16x15 | ? | 377,400 | Laques-Lecacheux | 1980
Rhea | V | 765 | 2.49e+21 | 527,040 | G. Cassini | 1672
Titan | VI | 2,575 | 1.35e+23 | 1,221,850 | C. Huygens | 1655
Hyperion | VII | 205x130x110 | 1.77e+19 | 1,481,000 | W. Bond | 1848
Iapetus | VIII | 730 | 1.88e+21 | 3,561,300 | G. Cassini | 1671
Phoebe | IX | 110 | 4.0e+18 | 12,952,000 | W. Pickering | 1898
Possible New Satellites of Saturn

References

Thomas, P., J. Veverka, D. Morrison, M. Davies. and T. V. Johnson. "Saturn's Small Satellites: Voyager Imaging Results." Journal of Geophysical Research, November 1, 1983, 8743-8754.

Soderblom, Laurence A. and Torrence V. Johnson. "The Moons of Saturn." Scientific American, January 1982.

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##### Copyright © 1997 by Calvin J. Hamilton. All rights reserved.