Saturn VI

   Titan is the fifteenth of Saturn's known satellites and the largest:
        orbit:    1,221,830 km from Saturn
        diameter: 5150 km
        mass:     1.35e23 kg
   In Greek mythology the Titans were a family of giants, the children of Uranus and Gaia, who sought to rule the heavens but were overthrown and supplanted by the family of Zeus.

   Discovered by Huygens in 1655.

   It was long thought that Titan was the largest satellite in the solar system but recent observations have shown that Titan's atmosphere is so thick that its solid surface is slightly smaller than Ganymede's. Titan is nevertheless larger in diameter than Mercury and larger and more massive than Pluto.

   One of the principal objectives of the Voyager 1 mission was the study of Titan. Voyager 1 came within 4000 km of the surface. We learned more in the few minutes of that encounter than in the previous 300 years.

   And yet our knowledge is frustratingly incomplete. Titan is surrounded by a thick, opaque atmosphere; the surface cannot be seen at all in visible light (below left). (The Cassini mission will map Titan's surface with radar as Magellan did at Venus.) All that the Voyager images show is a slight variation in color between the northern and southern hemispheres. Some surface detail is visible in the infrared with HST.

   Titan is similar in bulk properties to Ganymede, Callisto, Triton and (probably) Pluto. It is not known whether it has any internal structure like Ganymede or is uniform like Callisto.

   Titan is about half water ice and half rocky material. It is probably differentiated into several layers with a 3400 km rocky center surrounded by several layers composed of different crystal forms of ice. Its interior may still be hot. Though similar in composition to Rhea and the rest of Saturn's moons, it is denser because it is so large that its gravity compresses its interior.

   Alone of all the satellites in the solar system, Titan has a significant atmosphere. At the surface, its pressure is more than 1.5 bar (50% higher than Earth's). It is composed primarily of molecular nitrogen (as is Earth's) with no more than 6% argon and a few percent methane. Interestingly, there are also trace amounts of at least a dozen other organic compounds (i.e. ethane, hydrogen cyanide, carbon dioxide) and water. The organics are formed as methane, which dominates in Titan's upper atmosphere, is destroyed by sunlight. The result is similar to the smog found over large cities, but much thicker. In many ways, this is similar to the conditions on Earth early in its history when life was first getting started.

   Titan has no magnetic field and sometimes orbits outside Saturn's magnetosphere. It is therefore directly exposed to the solar wind. This may ionize and carry away some molecules from the top of the atmosphere.

   At the surface, Titan's temperature is about 94 K (-290 F). At this temperature water ice does not sublimate and thus there is little water vapor in the atmosphere. Nevertheless, there appears to be a lot of chemistry going on; the end result seems to be a lot like a very thick smog.

   There are scattered variable clouds in Titan's atmosphere in addition to the overall deep haze. These clouds are probably composed of methane, ethane or other simple organics. Other more complex chemicals in small quantities must be responsible for the orange color as seen from space.

   It seems likely that the ethane clouds would produce a rain of liquid ethane onto the surface perhaps producing an "ocean" of ethane (or an ethane/methane mixture) up to 1000 meters deep. Recent ground-based radar observations have cast this into doubt, however.

   Recent observations with the Hubble Space Telescope show remarkable near infrared views of Titan's surface (right and top). Voyager's camera couldn't see through Titan's atmosphere but in the near infrared the haze becomes more transparent, and HST's pictures suggest that a huge bright "continent" exists on the hemisphere of Titan that faces forward in its orbit. These Hubble results don't prove that liquid "seas" exist, however, only that Titan has large bright and dark regions on its surface. The landing site for the Huygens probe has been chosen in part by examining these images. It will be just "offshore" of the largest "continent" at 18.1 degrees North, 208.7 degrees longitude.

   The observations by HST also indicate that Titan's rotation is in fact synchronous like most of Saturn's other moons.

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Bill Arnett; last updated: 2000 Oct 20