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Orbit: 1,221,830 km from Saturn
Orbital period: 16 days
Gravity: 1.352 m/s²
Circumference: 16,178 km
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.Titan is the largest moon of Saturn. It is the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found.

Discovered by Christian 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.

 Titan's surface

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 fly-by than in the previous 300 years. Then in late  2004, the Cassini orbiter began a  series of close encounters with Titan, taking data with many instruments.  And  in January 2005, the  Huygens probe actually landed on the surface of Titan and sent back images from the  surface.

Titan is similar in bulk composition to Ganymede, Callisto,  Triton and (probably) Pluto,  ie 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 slightly compresses its interior.

 Saturn's moon Titan

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.  But it is this thick hazy atmosphere that makes it so  hard to see  Titan’s surface.

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. It may also drive some of Titan’s peculiar chemistry.

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. Analysis  of the Huygens data will tell us a great deal about the details of the  atmospheric chemistry.

 Titan's surface showing erosion

Prior to Cassini’s arrival, it seemed likely that the clouds   would  produce a rain of ethane or methane  onto the surface perhaps producing an “ocean” up to 1000 meters deep.  However, this seems not to be the case  at least at the present time. There is little doubt that some active processes are occuring on Titan; there are few if any craters visible indicating that the  surface must be very young. But it may be that the  “lakes” are more slushy than liquid or that the basins are not filled with  liquid at all times. Preliminary  results from Huygens indicate that while  Titan’s rivers and lakes appear  dry at the moment, rain may have occurred not long ago.  There is clear evidence  for “precipitation, erosion, mechanical abrasion and other fluvial activity”.   In addition, Cassini has found  evidence of a peculiar  kind of volcano on Titan that may account for some of the unusual features  of Titan’s atmosphere.

We are beginning to get some understanding of Titan’s surface by combining  the data from all the sources available. Large  ground based observatories operating in the infra-red can see some details  as can the Hubble Space Telescope. These show  a huge bright “continent” (preliminarily called “Xanadu”) on the hemisphere of Titan that faces forward in its orbit and some darker  regions that are suggestive of oceans or lakes.  Cassini’s much higher  resolution infrared images (below right, click for animation) show the same  structures in more detail.  And the close-ups from Huygens (left) show what  appear to be drainage channels and shorelines.

These observations also confirm that Titan’s rotation is in fact  synchronous like most of Saturn’s other moons.

Cassini’s IR camera has detected a strange and as yet unexplained bright  spot on Titan’s surface.

Titan is a difficult object to study.  The Cassini instruments are  specifically designed to penetrate the haze, its radar mapper can see right thru  it and the Huygens images show the surface clearly.  But the orbiter images are  still frustratingly vague and the Huygens images are few in number and cover  only a tiny area. Analysis of this data will take some time; Titan is a very  strange place.




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