Saturn's largest satellite is the mysterious world called Titan. It has slightly greater diameter, density, and mass than Callisto. It is probably half rock, half ice. What is special about Titan is that it has a thick atmosphere with a surface air pressure about 1.5 times thicker than the Earth's. Even though Titan's mass is even smaller than Mars', it is so cold (just 95 Kelvin) that it has been able to hold on to its primordial atmosphere. The atmosphere is made of cold molecular nitrogen with hazy methane clouds that block our view of the surface. Other organic molecules have been detected in its atmosphere. They are formed from sunlight interacting with the atmospheric nitrogen and methane. Some of the organic molecules then settle down to the surface as very dark deposits. The picture of Titan, Triton and the Moon at the end of this sub-section shows hazy Titan as viewed in visible wavelengths from the Voyager spacecraft. Unfortunately, Voyager's cameras were precisely tuned to the wrong wavelengths so it could not peer through the methane-nitrogen mixture. Therefore, all it saw was an orange fuzz ball.
The Cassini spacecraft is now orbiting Saturn and flying by its numerous moons as part of a four-year mission with special attention focussed on Titan (45 flybys scheduled during the four-year mission). Titan is probably like the early Earth's chemistry. Its very cold temperatures may then have preserved a record of what the early Earth was like before life formed. Using infrared wavelengths and radar Cassini has been able to peer through the hazy atmosphere. The picture below is a mosaic of 16 images taken at infrared wavelengths coming from the surface and that pass through the atmosphere easily to Cassini's camera.
Another probe called Huygens, built by the European Space Agency, hitched a ride on Cassini and parachuted down to Titan's surface in January 2005. The color picture below (left) is Huygen's view from the surface of Titan. The probe settled 10 to 15 centimeters into the surface. The mechanics of Titan's hydrogeological cycle is similar to the Earth's but the chemistry is different: instead of liquid water, Titan has liquid methane and instead of silicate rocks, Titan's rocks are dirty water ice. Liquid methane below the surface is released to the atmosphere to replenish that lost to the formation of the photochemical smog that eventually gets deposited in the soil. Methane rain washes the higher elevations of the dark material and it gets concentrated down in valleys to highlight the river drainage channels (see picture below right). Later images from Cassini have revealed huge methane lakes.
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Titan,
Triton, and our Moon to the same scale.
Triton has many black streaks on its surface that may be from volcanic venting of nitrogen heated to a gaseous state despite the very low temperatures by high internal pressures. The nitrogen fountains are about 8 kilometers high and then move off parallel to the surface by winds in the upper part of its thin atmosphere. Another unusual thing about Triton is its highly inclined orbit (with respect to Neptune's equator). Its circular orbit is retrograde (backward) which means the orbit is decaying---Triton is spiralling into Neptune. Triton's strange orbit and the very elliptical orbit of Neptune's other major moon, Nereid, leads to the proposal that Triton was captured by Neptune when Triton passed too close to it. If it was not captured, Triton was certainly affected by something passing close to the Neptune system.
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last updated: June 2, 2007