Two of the most interesting moons (or for that matter any celestial bodies) in our solar system are Titan, one of Saturn's 62 moons, and Europa, one of the four Galilean moons of Jupiter (which are the four largest moons of Jupiter and the earliest discovered by Galileo in the 1600's).

Titan is the largest moon of Saturn, and is the only natural satellite known to have a dense atmosphere. It is also the only celestial body other than Earth for which clear evidence of surface liquid has been found. As I have discussed in previous blogs, water is thought to be a requirement for life, or at least life as we know it. Titan has a diameter roughly 50% larger than Earth's moon, is 80% more massive and it is larger by volume than the smallest planet, Mercury.

The atmosphere of Titan is largely composed of nitrogen; minor components lead to the formation of methane and ethane clouds and nitrogen-rich organic smog. The climate (which includes wind and rain) creates surface features like those of Earth, including sand dunes, rivers, lakes, seas (probably of liquid methane or ethane) and shorelines. Much like the Earth, Titan is dominated by seasonal weather patterns.

It is rather easy to see why some people believe that Titan has life on it now or has had life on it in the past. The first probe to visit the Saturnian system was Pioneer 11 in 1979, which determined that Titan was probably too cold to support life. The craft took the first photographs of the moon, but these were rather poor; the first-ever close-up of Titan was taken on Sept. 2, 1979.

Titan was examined by both Voyager 1 and 2 in 1980 and 1981, respectively. Voyager 1's course was diverted on purpose to make a closer pass of Titan. In none of these examinations of Titan was direct evidence of life found.

A Photo of Titan, One of Saturn's Moons

Slightly smaller than Earth's moon, Europa is made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the solar system. The surface is marked by cracks and streaks with craters relatively infrequent. The apparent youth and smoothness of the surface has led to a belief that a water ocean exists below the surface, which could conceivably serve as a basis for extraterrestrial life. Obviously oxygen in the atmosphere leads people to think that life may there be as well.

Although only fly-by missions have visited this moon, the intriguing characteristics of Europa have led to several interesting exploration proposals. The Galileo mission, launched in 1989, provided the bulk of current data on Europa. A new mission to Jupiter's icy moons, the Europa Jupiter System Mission (EJSM), is proposed for a launch in 2020. The possibility of extraterrestrial life has ensured a high profile for the moon and has led to pressure for future missions.

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