The Cassini spacecraft flew by Titan, Saturn's largest moon, at 15:30 UTC on October 26, 2004. Cassini came within 1,174 km of Titan's surface, and during this time observed Titan with eleven of its twelve instruments. This is the closest flyby of Titan by any spacecraft, and it is the second of 45 planned encounters with Titan. One goal of this encounter was to map the planned landing area of the Huygen's probe, which will land on Titan in January of 2005.
A false-color image of Titan's atmosphere. Red and green represent infrared emission; they show the areas where methane absorb light. The blue represents ultraviolet emission, which comes from the upper atmosphere and the high haze. Photo credit: Courtesy NASA/JPL/Space Science Institute.
Titan is of great interest because it is only slightly smaller in diameter than Ganymede, the largest moon of Jupiter, and yet it has a dense nitrogen atmosphere, unlike Ganymede. Why this is so is not currently understood. Another issue is the original form of the nitrogen acquired by Titan. Nitrogen could have originally been in molecular form if the environment around Saturn were cold, but if it were warm, the nitrogen would have to be acquired as ammonia trapped in ice. Which of these theories is correct can be settled by measuring the ratio of argon to nitrogen. The ratio is expected to be unity if the nitrogen was captured as molecular nitrogen, but much less than unity if the nitrogen were captured as ammonia, because argon is not very soluble in water.
One of the early conclusions from this flyby is that the surface of Titan has few craters, which implies that resurfacing is occurring on Titan.