Shuttle LIFE

Space Topics: Saturn


Saturn's Largest Moon

Saturn's Moon Titan
When the Voyagers passed by Titan they found its surface to be hidden behind an impenetrable barrier of orange haze. This global view shows a faint dichotomy between the northern and southern hemispheres. Color: True color. Credit: NASA / JPL / Calvin Hamilton

Size: 5,150 kilometers - Saturn's largest moon - 0.4037 Earths
Orbital radius: 1,221,830 kilometers - 20.3 Saturn radii - outside Saturn's ring system
Orbital period: 15.945 days
Discovery: 1655 by Christiaan Huygens

Nearly as large as Mars and possessing an atmosphere thicker than Earth's, Titan would unquestionably be considered a planet if it orbited the Sun on its own.  When scientists first began to understand what that atmosphere was made of -- primarily nitrogen, with a large component of methane and significant quantities of more complex organic molecules -- they quickly realized that Titan would be a place where liquid methane or ethane could be raining from the skies.  Thanks to Cassini-Huygens it is now known that Titan is a youthful planet whose surface is modified and its impact craters erased by the action of flowing liquids and possibly even a methane cycle of cloud formation, rain, runoff, and evaporation, akin to Earth's water cycle. The world revealed to Huygens' cameras as it descended to Titan's surface on January 14, 2005 contains clear signs of river vallies and water-rounded rocks. And Cassini's Titan RADAR mapper is obtaining detailed views of channels and valleys, eroded impact craters, and windblown sand dunes that all speak of Titan's active geology.

The surface of Titan has historically been difficult to explore because the thick hazes of complex hydrocarbons that make its atmosphere so interesting also prevent a clear view to its surface.  Titan stands out for how deep its atmosphere is, with layer upon layer of hazes making for a complicated series of obstacles to the sight of orbiting instruments. Its highest hazes may be 300 kilometers (190 miles) above the ground, nearly four times higher than the highest clouds on any other planet. And these thick hazes are made of piles of different compounds, a "witch's brew" of carbon, nitrogen, and hydrogen compounds.

The Voyager cameras were unable to penetrate Titan's hazes, returning picture after maddening picture of a nearly featureless upper atmosphere. Cassini-Huygens was sent to Titan with cameras capable of piercing spectral "windows" in the haze, a RADAR mapping instrument designed specifically to produce maps of Titan's surface, and the Huygens probe, which was dedicated to the study of Titan's atmosphere.  Despite the quantity of data that has been returned so far, Titan still presents many enigmas to scientists.  The complex interrelationships between its atmosphere, its surface, and its interior will take lifetimes to untangle.

  Earth Titan
Average surface temperature     290 K
15 C
60 F
-90 K
-180 C
-290 F
Average surface pressure 1 bar 1.6 bars
Major gases 77% Nitrogen
21% Oxygen
0.93% Argon
~ 1% water (varies)
90-97% Nitrogen
0-6% Argon
2-5 % Methane
0.2% Hydrogen
Minor gases
(ppm = parts per million)
330 ppm carbon dioxide
18 ppm neon
5.2 ppm helium
1.5 ppm methane
1.1 ppm krypton
0.5 ppm hydrogen
0.4 ppm ozone
0.3 ppm nitrous oxide
0.12 ppm carbon monoxide
0.087 ppm xenon
0.01 ppm ammonia
~45 ppm carbon monoxide
~10 ppm ethane
~2 ppm acetylene
~0.5 ppm propane
~0.2 ppm hydrogen cyanide
~0.1 ppm ethylene
~0.01 ppm carbon dioxide
~0.008 ppm water (at high altitude)
~0.005 ppm acetonitrile
~0.005 ppm cyanogen
Cloud layers: altitude and composition 12-30 km
Concentrated Sulfuric Acid, everywhere but transparent 0-12 km
Sulfates, dust, sea salt, organic compounds, here and there, may be transparent or opaque 0-12 km
Water clouds and fog, making on average 50% opaque cloud cover
200-300 km, variable
detached haze layers 35-200 km, variable
photochemical haze layer made of carbon-nitrogen-hydrogen polymeric compounds 10-15 km
methane liquid and ice clouds, about 10% cloud cover

Only one feature on Titan was prominent on Earth-based telescopic observations to be named prior to Cassini's arrival: Xanadu, a large bright-colored splotch on Titan's leading hemisphere.  Now that Cassini has arrived, Titan has earned a complex naming scheme for its wide variety of features:

  • Albedo features (large areas that are bright or dark) are named for sacred or enchanted places, paradise, or celestial realms from all the world's cultures.
  • Craters and ringed features are named for wisdom deities.
  • Facula (small, bright features) are named for islands on Earth that are not politically independent.  Faculae (chains of bright features) are named for Earth archipelagos.
  • Flumina (linear, channel-like features) are named for mythical or imaginary rivers.
  • Lacus (small, dark features) are named for lakes on Earth.  An attempt is made to find an Earth lake with similar shape to the Titanian lacus.
  • Virgae (streaks or stripes of color) are named for rain deities.
  • Other features are named for deities of happiness, peace, and harmony from all the world's cultures.

Flybys of Titan

Even if Titan were not such an interesting target Cassini would be compelled to fly by Titan frequently.  Titan's large mass provides Cassini with gravity assists that change the shape of its orbit.  Cassini only carried enough fuel to Saturn to change its orbital velocity by a total of 400 meters per second (900 miles per hour).  The 46 Titan gravity assists provide Cassini with a total velocity change of 33,000 meters per second (74,000 miles per hour).  How the orbit is changed by a flyby depends upon the latitude and altitude of Cassini's path with respect to Titan, so the Titan flyby geometries are dictated more by orbital dynamics than they are by the desires of scientists.  Even so, Cassini will build up a fairly complete data set of Titan's entire surface and atmosphere over the course of the mission.

Flyby Date Orbit Flyby
T0 Jul 02, 2004 Rev 00 Outbound  339,120 km  
TA Oct 26, 2004 Rev 0A Inbound  1,174 km yes
TB Dec 13, 2004 Rev 0B Inbound  1,192 km  
TC Jan 14, 2005 Rev 0C Inbound  60,003 km  
  -- Huygens mission --    
T3 Feb 15, 2005 Rev 03 Inbound  1,579 km yes
T4 Mar 31, 2005 Rev 05 Outbound  2,404 km  
T5 Apr 16, 2005 Rev 06 Outbound  1,026 km  
T6 Aug 22, 2005 Rev 13 Outbound  3,669 km  
T7 Sep 07, 2005 Rev 14 Outbound  1,075 km yes
T8 Oct 28, 2005 Rev 17 Inbound  1,353 km yes
T9 Dec 26, 2005 Rev 19 Outbound  10,409 km  
T10 Jan 15, 2006 Rev 20 Inbound  2,043 km  
T11 Feb 27, 2006 Rev 21 Outbound  1,813 km  
T12 Mar 19, 2006 Rev 22 Inbound  1,951 km  
T13 Apr 30, 2006 Rev 23 Outbound  1,855 km yes
T14 May 20, 2006 Rev 24 Inbound  1,879 km  
T15 Jul 02, 2006 Rev 25 Outbound  1,906 km  
T16 Jul 22, 2006 Rev 26 Inbound  950 km  
T17 Sep 07, 2006 Rev 28 Inbound  950 km  
T18 Sep 23, 2006 Rev 29 Inbound  950 km  
T19 Oct 09, 2006 Rev 30 Inbound  950 km  
T20 Oct 25, 2006 Rev 31 Inbound  950 km  
T21 Dec 12, 2006 Rev 35 Inbound  950 km  
T22 Dec 28, 2006 Rev 36 Inbound  1,500 km  
T23 Jan 13, 2007 Rev 37 Inbound  950 km  
T24 Jan 29, 2007 Rev 38 Inbound  2,726 km  
T25 Feb 22, 2007 Rev 39 Outbound  950 km  
T26 Mar 10, 2007 Rev 40 Outbound  950 km  
T27 Mar 26, 2007 Rev 41 Outbound  950 km  
T28 Apr 10, 2007 Rev 42 Outbound  950 km  
T29 Apr 26, 2007 Rev 43 Outbound  950 km  
T30 May 12, 2007 Rev 44 Outbound  950 km  
T31 May 28, 2007 Rev 45 Outbound  2,426 km  
T32 Jun 13, 2007 Rev 46 Outbound  950 km  
T33 Jun 29, 2007 Rev 47 Outbound  1,944 km  
T34 Jul 19, 2007 Rev 48 Inbound  1,300 km  
T35 Aug 31, 2007 Rev 49 Outbound  3,212 km  
T36 Oct 02, 2007 Rev 50 Outbound  950 km  
T37 Nov 19, 2007 Rev 52 Outbound  950 km  
T38 Dec 05, 2007 Rev 53 Outbound  1,300 km  
T39 Dec 20, 2007 Rev 54 Outbound  950 km  
T40 Jan 05, 2008 Rev 55 Outbound  950 km  
T41 Feb 22, 2008 Rev 59 Outbound  950 km  
T42 Mar 25, 2008 Rev 62 Outbound  950 km  
T43 May 12, 2008 Rev 67 Outbound  950 km  
T44 May 28, 2008 Rev 69 Outbound  1,348 km  
T45 Jul 31, 2008 Rev 78 Outbound  3,980 km  

Map of Titan

Global map of Titan (simple cylindrical projection)
Global map of Titan (simple cylindrical projection)
Global map centered at 180 degrees longitude (the anti-Saturnian point). The map is 2,048 pixels wide, and Titan's diameter is 5,150 kilometers, so the map resolution is 7.9 kilometers per pixel at the equator. Source Credit: NASA / JPL / Space Science Institute / Fridger Schrempp

Listen to the Sounds of Titan

Data from the Huygens landing site

Cassini RADAR images of Titan