10. Saturn: lord of the rings


Ringed planet with icy moons

Ringed planet with icy moons

. Saturnís yellow-brown clouds are swept into bands by the planetís rapid rotation. Two of its white moons (left), Tethys (above) and Dione, are covered with water ice. The shadows of Saturnís main rings and Tethys are cast onto the cloud tops. The outer A ring is separated from the central B ring by the Cassini Division. This gap is so tenuous that the edge of Saturn can be seen through it. The faintest of Saturnís main rings, the C-ring or crepe ring, is barely visible against the planet. This image was obtained from the Voyager 1 spacecraft on 3 November 1980. (Courtesy on NASA and JPL.)


Clouds and hazes in Saturnís atmosphere

Clouds and hazes in Saturnís atmosphere

. Enhanced colors bring out the details of Saturnís banded clouds in this image, taken in infrared light. The blue color indicates a clear atmosphere down to a main cloud layer. Different shadings of blue indicate variations of the cloud particles, in size or chemical composition. The cloud particles are believed to be ammonia ice crystals. The red and orange colors mark clouds reaching up high into the atmosphere, and the densest parts of the two storms near Saturnís equator appear white. The green and yellow colors indicate a haze above the main cloud layer. Saturnís counter-flowing east-west winds have aligned the clouds and haze within fixed latitude bands that become more pronounced near the planetís polar regions. The dark region around the south pole (bottom) marks the location of a large hole in the main cloud layer. The rings are made up of chunks of water ice, with a white color that has been browned in some cases, somewhat like dirty snow on a winter road. Two of Saturnís satellites were also recorded, Dione (lower left) and Tethys (upper right). They appear in different yellow and green colors, indicating different conditions on their icy surfaces. This false-color image was taken on 4 January 1998 using the Hubble Space Telescope. (Courtesy of NASA and the Space Telescope Science Institute.)


Temperature and pressure at Saturnís cloud levels

Temperature and pressure at Saturnís cloud levels

. The fading radio signals when the Voyager 1 and 2 spacecraft passed behind Saturn in 1980 and 1981, respectively, revealed the temperatures (bottom axis) and pressures (right axis) in its upper atmosphere. The temperature reaches a minimum of about 80 degrees kelvin at a level called the tropopause where the atmospheric pressure is 0.1 bars, or 100 millibars. By way of comparison, the pressure of the Earthís atmosphere at sea level is 1.0 bar. The altitudes (left axis) are relative to the 0.1 bar level, and the dots are spaced to indicate tenfold changes in pressure. Solar radiation causes the temperature to increase with height just above the tropospause. At lower levels, the temperature and pressure increase systematically with depth. Three possible clouds layers of ammonia, NH3, ammonium hydrosulfide, NH4SH, and water ice, H2O, are shown. The altitudes of the predicted cloud layers are based on an equiligrium gaseous mixture that is of solar composition. An increase in abundance of a condensable gas by a factor of three would lower the altitude of the cloud base by about 10 kilometers.


Inside Saturn

Inside Saturn

. Giant Saturn has a thin gaseous atmosphere covering a vast global ocean of liquid hydrogen. At the enormous pressures within Saturnís interior, the abundant hydrogen is compressed into an outer shell of liquid molecular hydrogen and an inner shell of fluid metallic hydrogen. The giant planet may have a relatively small rock-ice core.


Saturnís rings open up

Saturnís rings open up

. These images were taken from the Hubble Space Telescope during a four-year period, from 1996 to 2000 (left to right), as Saturn moved along one seventh of its 29-year journey around the Sun. As viewed from near the Earth, Saturnís rings open up from just past edge-on to nearly fully open as it moves through its seasons, from autumn towards winter in its northern hemisphere. (Courtesy of NASA and the Space Telescope Science Institute.)


Edge-on view of Saturnís rings

Edge-on view of Saturnís rings

. When the Earth is in the plane of Saturnís rings, an observer on the Earth views the rings edge on. Because the rings are so thin, they are then barely visible. Saturnís largest satellite, Titan, is seen just above the rings (left); it is enveloped in a dark brown haze and casts a dark shadow on Saturnís clouds. Four other moons are clustered near the other edge of Saturnís rings (right), appearing bright white because their surfaces are covered with water ice. From left to right, these icy satellites are named Mimas, Tethys, Janus and Enceladus. This image was taken on 6 August 1995 from the Hubble Space Telescope. (Courtesy of NASA and the Space Telescope Science Institute.)


Cross-section of rings and satellites

Cross-section of rings and satellites

. All of Saturnís main rings lie inside the Roche limit (dashed curve) within which the planetís gravity will tear a large satellite apart. The A and B rings have been observed for centuries. The more tenuous C ring was discovered in the 19th century, and definite observations of the transparent D ring awaited the arrival of the Voyager 1 spacecraft on 12 November 1980. The icy satellite Enceladus probably feeds the tenuous E ring, also revealed from Voyager. For clarity, the thickness of the rings has been exaggerated.


Saturnís shepherd satellites

Saturnís shepherd satellites

. Two shepherd satellites confine Saturnís narrow F ring. The outer shepherd gravitationally deflects ring particles inward, and the inner shepherd deflects ring particles outward. (Courtesy of NASA and JPL.)


Ringlets

Ringlets

. When viewed with high resolution, approximately 100 concentric features are seen within Saturnís rings, including some in the Cassini Division. The ring system would probably separate into countless ringlets if we could detect fine enough detail. A small satellite, discovered by Voyager 1, is seen (upper left) just outside the narrow F ring. The Voyager 1 spacecraft took this mosaic of Saturnís rings on 6 November 1980. (Image courtesy of NASA and JPL.)


Beneath the rings of Saturn

Beneath the rings of Saturn

. When the Voyager 1 spacecraft dove beneath Saturnís rings, it could view sunlight transmitted through the rings, presenting a reversed image of the sunlit side. Both the C ring and Cassiniís Division appear bright because they are sparsely populated with small particles that efficiently scatter light in the forward direction, whereas the A and B rings appear dark because their densely-packed particles absorb all the incident sunlight. This perspective is not available from Earth, where we always see the sunlit side of the rings. (Courtesy of NASA and JPL.)


Dark spokes

Dark spokes

. Several dark spoke features streak across the central third of Saturnís B ring. They sweep around Saturn with a uniform velocity in apparent defiance of Keplerís laws. Electromagnetic forces probably levitate the dark, electrically charged particles above the main rings, permitting Saturnís magnetic field to carry them around the planet. This image was taken from the Voyager 2 spacecraft on 22 August 1981. (Courtesy of NASA and JPL.)


The Roche limit

The Roche limit

. A large satellite (top) that moves well within a planetís Roche limit (dashed curve) will be torn apart by the tidal force of the planetís gravity. The side of the satellite closer to the planet feels a stronger gravitational pull than the side farther away, and this difference works against the self-gravitation that holds the body together. A small solid satellite (bottom) can resist tidal disruption because it has significant internal cohesion in addition to self-gravitation.


Titanís smoggy haze

Titanís smoggy haze

. The surface of Saturnís largest moon, Titan, is hidden beneath a thick haze that completely envelops the satellite. Divisions in the layer of haze (blue) occur at 200, 375 and 500 meters above the edge, or limb, of the satelliteís atmosphere (orange). This false-color image was taken from the Voyager 1 spacecraft on 12 November 1980. (Courtesy of NASA and JPL).


Molecules in Titanís atmosphere

Molecules in Titanís atmosphere

. Emission features in the infrared spectrum of Titanís reflected sunlight identify the molecular constituents of its atmosphere. Sharp peaks in this spectrum, acquired from the Voyager 1 spacecraft in 1980, are attributed to methane, CH4, acetylene, C2H2, ethane, C2H6, and more complex hydrocarbon molecules. The wavelength is given in units of microns, or 10-6 meters, abbreviated mm.


Model of Titanís atmosphere

Model of Titanís atmosphere

. A study of the bending and fading of homebound radio signals when the Voyager 1 spacecraft passed behind Titan led to this plot of the temperature and pressure in the satelliteís atmosphere. The temperature (bottom axis) decreases with height until about 40 kilometers altitude, and then increases rapidly at higher altitudes (left axis). The entire atmosphere is well below the freezing temperature of water, at 273 degrees kelvin, but the lower atmosphere is just warm enough to allow the condensation of liquid nitrogen. The high-altitude smog might cover clouds of methane, and liquid ethane and methane could rain down to the surface. The pressure (right axis) is given in units of millibars or 0.001 bars, and it reaches 1000 millibars or 1 bar near the surface of Titan.The air pressure at sea level on Earth is also 1 bar.


Surface of Titan

Surface of Titan

. The obscuring haze in Titanís atmosphere is transparent enough at some infrared wavelengths to allow rough mapping of its surface. Dark and bright surface features are seen in these infrared images, taken from the Hubble Space Telescope at wavelengths near a millionth, or 10-6, of a meter as Titan circled Saturn in its 16-day orbit. The bight area is about 4 million meters across, roughly the size of Australia, suggesting that Titan has regions that are elevated above its possible oceans, but it is not certain what the landforms represent. (Courtesy of NASA and the Space Telescope Science Institute.)


Three icy moons of Saturn

Three icy moons of Saturn

. Cameras aboard the Voyager 2 spacecraft obtained these pictures of three ice-covered moons of Saturn in 1980. They are Mimas (left), Enceladus (middle) and Tethys (right) with respective radii of 196, 255 and 530 thousand meters. The surfaces of all three satellites are covered with water ice. They contain the cratered scars of past impacts as well as relatively young, smooth regions. (Courtesy of NASA and JPL.)


The Saturnian moon Dione

The Saturnian moon Dione

. Many large impact craters (left) and bright regions (right) are found on Saturnís satellite Dione. The bright areas could be attributed to impact debris, ridges and valleys, or even surface frost deposits. This image was taken from the Voyager 1 spacecraft on 12 November 1980.


Saturnís moon Rhea

Saturnís moon Rhea

. The icy, cratered surface of Saturnís satellite Rhea is shown in this Voyager 1 image, taken on 12 November 1980. The craters and landscape resemble those on Mercury and the Earthís Moon. Rhea probably froze and became rigid, behaving like a rocky surface, very early in its history. (Courtesy of NASA and JPL.)


Two-faced Iapetus

Two-faced Iapetus

. Saturnís outermost large moon, Iapetus, has a bright, heavily cratered terrain and a dark landscape, as shown in this Voyager 2 image taken on 22 August 1981. The bright regions are probably made of water ice, and the dark substance may be composed of organic substances. Iapetus apparently plows through the dark material as it orbits Saturn, covering the lead, frontal hemisphere of the satellite. (Courtesy of NASA and JPL.)


Leaving Saturn

Leaving Saturn

. When the Voyager 1 spacecraft sped past Saturn on 16 November 1980, it looked back to take this picture of the ringed planet from a perspective that cannot be enjoyed from Earth. Saturnís shadow falls upon the rings, and the planetís bright crescent can be seen through the dark Cassiniís Division and other parts of the rings. (Courtesy of NASA and JPL.)


Summary Diagram

Summary Diagram

. Summary Diagram.