Titan: a Satellite with an Atmosphere.

Recently the ten largest satellites in the solar system, as ~ve1l as Pluto, were observed spectroscopically. Oniy Titan was found to have an atmosphere of sufficient prominence to be detected, but Triton and Pluto require further study. The composition of Titan's atmosphere is similar to that of Saturn, although the optical thickness is somewhat less. The presence of gases rich in hydrogen atoms on a small body like Titan is surprising and indicates that the atmosphere was formed after Titan had cooled off. Similar arguments, though less compelling, may be advanced for analogous conclusions in regard to the formation of the atmospheres of Mars, Venus, and the earth. I. OBSERVATION