[DB 1507 (37); OAB 59] The methane in the atmosphere of Saturn's moon Titan is constantly being converted into other organic molecules. Since the hydrogen gas produced by these reactions is lost into space, these processes are not reversible and the methane is permanently lost. If there were nothing replacing the methane, and if the rate of loss were constant, then all of Titan's present methane supply would disappear in about 10 million years. However, this does not indicate that Titan is less than 10 million years old. Using exactly the same reasoning, one could measure the amount of water vapor in the Earth's atmosphere, and the rate at which the water is lost as rain, and thus derive a maximum age of the Earth. But of course such a calculation would be silly because the Earth's atmospheric water vapor is resupplied by the oceans. It is very likely that the Titan question has a similar explanation — surface lakes or underground reservoirs of liquid hydrocarbons (including methane) are very likely to exist on Titan, given its cryogenic temperatures and the abundance of such compounds in its atmosphere. Such reservoirs at the surface would easily explain the continuing presence of methane in Titan's atmosphere. However, Titan's opaque atmosphere has kept our knowledge of its surface quite minimal, so no one can say for sure whether such reservoirs exist. The Cassini spacecraft, due to arrive at Saturn in 2004, should shed a great deal of light on this Unexplained Mystery. The problem of Titan's methane is a simple case of insufficient data, not an indication that the Solar System is young.