navigation image map next page table of contents previous page

We continue with our exploration of saturnian satellites by concentrating possibly the most intriguing of the group, and certainly one of the most interesting in the Solar System - Titan is a "maverick", being surrounded by a nitrogen atmosphere and some organic molecules that give it a distinctive reddish-orange color. Brief treatments of several of the irregular-shaped satellites are then considered. The important Cassini-Huygens mission in 2004 to Saturn is presented on this page, with images to be added from time to time as they are sent back after that spacecraft achieves orbit and begins its 4 year study of Saturn and its moons.


Between Rhea and Iapetus is the large (5,150 km [3,200 mi]) satellite Titan, which is a "maverick" among the icy group, in that it is quite different in its appearance owing to the presence of an atmosphere with a distinctive color described as a brownish-orange. In a sense it is as odd amongst its peers as Io is within the Galilean satellites.

Color-enhanced Voyager image of Titan, a satellite of Saturn with a dense atmosphere.

Some of the variations in the atmosphere can be discerned as Titan rotates. Here are four views obtained through the Hubble Space Telescope, using selected bands in the infrared that penetrate the atmosphere.:

Four views of Titan taken at different times by the HST>.

A diagramatic summary of what is known about Titan's atmosphere, prior to the Cassini mission, appears here:

Cross-sectional diagram of Titan's atmosphere.

Estimates of the relative amounts of atmospheric constituents, in percentages and parts per million, are given in this table; nitrogen is the principal constituent:

Composition of Titan's atmosphere.

Titan has been studied from Earth. Photos taken through ground telescopes over several years show variations in lighter-toned areas which are assumed to be some type(s) of clouds that come and go; these are mainly just above the surface:

Telescope views of Titan's changing atmosphere and possible surface features.

The European Southern Observatory (ESO) Very Large Telescope (VLT) in Chile has obtained sharp images using an instrument called the Simultaneous Differential Imager (SDI) camera, which obtains Near-IR images at 1.575 and 1.600 nanometers (surface sources) and 1.625 nm (atmosphere). Here are images from the VLT showing the atmosphere, with some of the variations probably related to surface influences:

ESO's VLT images of Titan's atmosphere (left) and surface.

SDI data taken on different nights can be reprocessed by differencing to give this group of images representing changes which may be due to the surface; the actual surface itself is likely to be partially obscured by the organic haze in the atmosphere:

Near-IR images of Titan's surface taken over a 6 night interval.

The color version can be enhanced to bring out as many details as possible - rendered in reds and yellows - rather than in the natural color that is a medium brownish-yellow. Titan appears unlike the others because its dense atmosphere (with surface pressures of 1.5 atm, about 50% higher than Earth's) hides its surface. Nitrogen is the main constituent of the atmosphere but the 2% of methane and associated organics cause the colors noted. The continuous clouds are somewhat analogous to the haze or smog found above urban areas such as Los Angeles. We show some of this atmospheric structure here:

Color-enhanced Voyager closeup image of the atmosphere on Titan, which consists of a haze made up of about 90% molecular nitrogen, 6% argon, with the remainder methane and organic derivatives responsible for its orangish colors.

What is below the clouds is uncertain. At Titan's atmospheric temperatures (-179° C), the ethane (and probably methane and nitrogen) may have condensed into a liquid "ocean" that is up to a kilometer deep. Below this, Titan probably consists of a layered mix of silicates and water ice. An infrared image, acquired by the Hubble Space Telescope, suggests that there may be several upwellings of (solid?) material, similar to "continents" (see yellow area in the image above). Recent studies now point to some of the variations in the atmosphere imagery are related to topographic irregularities on Titan's surface. In fact, prominences as high as 2000 meters have been postulated to account for some of the patterns in the image. A map has been prepared that purports to indicate the height variations in that surface:

Variations in height of the surface of Titan.

This atmosphere consists of about 90% molecular nitrogen, 6% argon, with the remainder being methane and organic derivatives, such as hydrogen cyanide, ethane, acetylene, and CO2. Solar radiation breaks some of these constituents into colored compounds, analogous to urban smogs on Earth. Ammonia, now a trace constituent, may once have been more abundant, until dissociating into carbon-nitrogen compounds.

This mysterious satellite (larger than Mercury and Pluto) - totally disparate, compared to the other saturnian moons - may harbor still other organic molecules of unusual nature. This surmise just cries out for follow-up study, since no other planet besides Earth or planetary satellite in the Solar System seems so promising for finding and identifying the more complex molecules that may be present.

19-66: What is the main implication of this unusual subsurface liquid and the chemical compounds found at Titan? ANSWER

19-67: Now that you have been introduced to the saturnian satellites, let's see how well you can recall and identify each of the larger ones. Examine this montage gallery of 9 satellites and name each one. ANSWER

 A montage of the 9 larger satellites of Saturn; the large colored one shows a more natural color than does the view presented earlier on this page.

The Cassini-Huygens Mission

Just as the Galileo spacecraft has added significant knowledge of Jupiter, a sophisticated mission to Saturn is underway. That probe is part of the Cassini-Huygens mission, named for the 17th Century French-Italian astronomer who pioneered telescopic observations of Saturn. The Cassini spacecraft successfully launched from Cape Canaveral at 4 AM, on October 15, 1997, to begin a 6.7 year journey to the Ringed Planet. This vehicle will take a round-about trip to gain velocity by gravity assist "kicks". By passing Venus twice, then Earth again, and Jupiter later, the spacecraft will use their gravitational pulls to increase its speed enough to reach Saturn. JPL manages the mission, with experiments from NASA, the European Space Agency, the Italian Space Agency, and other participants. A good overview of the Cassini mission is found on JPL's Cassini website. The Cassini Orbiter will explore Saturn's magnetosphere, its rings, its icy satellites, its atmosphere, the atmosphere of the moon Titan and, perhaps its surface as well.

Built at a cost of $3.27 billion dollars, the complete Cassini spacecraft, the size of a school bus (5.7 m [22 ft]; 5.83 tons), is shown here in its assembly room before being taken to its launching rocket:

The Cassini-Huygens spacecraft, before launch.

The Orbiter contains a variety of instruments, including a magnetometer, visible imager, radar, UV and IR spectrometers, ion and mass pectrometers, a plasma spectrometer, a dust analyzer, and radio science instruments. Power for the spacecraft is a RTG (Radioisotope Thermoelectric Generator). The general layout of its major components is portrayed in this schematic:

The nearly 7 years needed for Cassini-Huygens to travel the 3.5 billion kilometers (about 2.1 billion miles) involved a series of 4 gravity "kicks" (velocity boosts) by flying the spacecraft relativelly close to planets, as shown in this diagram (from the Cassini-Huygens Home Page):

Pathway of the approach of Cassini-Huygens towards Saturn.

After arrival on late June 30, 2004 Cassini will go through a series of fuel burns to progressively insert it into orbits of different radii and ellipticity around Saturn. This diagram gives the shifts during the first 7 months<:/p>

Gradual adjustment of orbit patterns by C-H through January, 2005.

After the first close encounter, Cassini-Huygens will follow a highly elliptical orbit that won't bring it close to Saturn and its moons until mid-August.

Cassini, which cost in excess of 3 billion dollars (and raised protests from some environmentalists fearful of its 77 grams of plutonium that will serve as a power source), is the most sophisticated and ambitious probe yet sent to explore the planets.

This ambitious mission was reviewed at a JPL lecture. Access this at von Karman lectures, selecting Webcast in the Format box, then choosing the topic "The Cassini-Huygens Mission to Saturn, April 2, 2003.

As of mid-June 2004, Cassini-Huygens has been operating to perfection. Here is a view of the full facing planet and its rings taken on March 27, 2004

Cassini view of Saturn as it approaches the planet in late March.

A second view taken later shows part of the rings and three of the outer satellites (a fourth being unidentified); this helps to realize the great disparity in size between Saturn's rings and its satellites:

Cassini image of part of Saturn and several satellites.

Cassini began its process of insertion into orbit around Jupiter at 6:30 PDT on June 30, 2004 by traveling through a gap in its rings. One of its antenna was deployed to act as a shield against microparticles of ice in the ring complex. A burn sequence of nearly 90 minutes was required to achieve a proper orbit for the first phase of exploration. Total success during this phase was achieved. A series of hundreds of spectacular images of the rings was obtained in this timeframe. A few of the best are shown here; click on the images to see the captions which describe what you are seeing.

The A ring, seen from the dark side (underneath), with the Encke gap as influenced by the sheperds Prometheus, Pandora, and Pan

The Encke Gap in Ring A.

Details of the Encke Gap and spiraling density waves.

Fine-line structure in the A Ring.

The A ring, with bending waves on the right and density waves on the left.

In this next image of Ring A, the closeup detail shows the individual ice block nature of the larger particles making up the ring itself.

Details of part of Ring A, showing a coarse granularity texture apparently caused by block of ice.

The next image shows the rings in a close approximation of their true color, as might be seen by an astronaut about to pass through them:

Nearly true color rendition of the main saturnian rings.

Cassini has determined that there is a gradual increase in size of the ice particles going from the inner main ring to the outer main ring:

Particle size variations in Saturn's rings.

This variation in particle size and in possible non-ice "dirt" are factors in these striking color renditions of the A and the B-C ring characteristics, as determined from UV images interpreted by a group at the University of Colorado-Boulder:

UV image of Saturn's A ring, using color to indicate variations in size and impurities (red being dirtier and smaller, just the opposite of the figure above).

UV image of variations in the B and C rings.

Another discovery, using the Visual and IR Imaging Spectrometer, is that the Cassini Gap, and by inference the Encke Gap, is not a region of minimal particles but instead contains "dirt", particles of non-ice composition. These may be remnants of the rocky portion of a satellitic fragment composed of low reflectance basaltic material.

Images showing several reflectance properties of the saturnian rings, including a concentration of low reflecting material (provisionally called 'dirt'.

The F ring is narrow and seemingly isolated, as seen in the first image below. The F ring has always merited special attention because of its twisted appearance. Cassini show that in this mosaic image. The contortions are caused by gravitational interactions with the small satellites Prometheus and Pandora:

The far-out F ring; the outer half of the A ring appears to the left.

The F ring with its shepherd satellites Prometheus (inner) and Pandora (outer).

The contorted F ring of Saturn.

Both moons are small, appearing much like asteroids. Here is a Voyager view of Prometheus (145 x 85 x 62 km; 90 x 53 x 39 miles):

The inner moon Prometheus.

Athough some data relating to the mean temperatures in the individual rings had been obtained prior to Cassini's visit, those data were refined to give more exact values. A temperature profile was obtained during passage and used to assign colors to the rings by extrapolating the strip profile to the entire rings. In the image below, blue represents a temperature range clustered around 70° C (-333 ° F), green 90° C (-298° F), and red 110° C) (- 261 ° F):

Saturn's rings colored to indicate average temperatures; see text above.

This next Cassini image shows a novelty - yet not unexpected. Under certain solar lighting conditions, sunlight is variably absorbed by Saturn's rings. The result is a series of thin light to dark bands on the saturnian surface, representing shadows. B and C ring shadows appear in this image; the black dot at the bottom is the shadow image of the moon Mimas.

Shadows of some of Saturn's rings on this planet's surface.

Cassini has also produced near-true color images of Saturn's surface. This next image shows shades of blue in the northern hemisphere. This color is due to scattering by gases, much like that observed in the Earth's sky during a clear day. The variable shading is caused by ring shadows.

Cassini view of Saturn's northern hemisphere, showing shadows and shades of blue owing to scattering by atmospheric gases.

Temperature and wind speed variations in the atmosphere above the saturnian clouds are shown in this next diagram. Highest temperatures are towards the upper atmosphere; strongest winds at this time in the saturnian year are near the equator.

Temperature and wind speed variations with latitude and altitude in the saturnian atmosphere above the cloud surfaces.

As Cassini approached Saturn, its Ion and Neutral Camera was able to take data that leads to a determination of the Magnetosphere and Magnetopause. In the image below, excitation of hydrogen ions gives rise to the orange glow that indicates the extent of the magnetosheath.

Display of Saturn's magnetosphere; data collected when Cassini was 6 million kilometers (3.7 million miles) from the planet.

There are now so many informative images from the Cassini mission that it is helpful to add another page. So proceed to page 19-19a by clicking on Next below.

navigation image map next page previous page


Primary Author: Nicholas M. Short, Sr. email: [email protected]