Explorations to Come

During the Galileo mission, NASA decided that Europa was one of its highest-priority objectives in planetary exploration. Galileo Project Scientist Torrence Johnson consistently cited our explanation of the cycloids as the first convincing evidence for an ocean under the surface. That ocean made Europa the sexiest planet in the solar system. From a logical point of view, the thick-ice canon should have put a damper on the enthusiasm, because thick ice meant the ocean would be hard to reach and there would not be much life there anyway. But the same players continued to talk up fantasies and speculation about life in the ocean. They needed to sell the next mission to Europa.

Mars, of course, has long had its own advocates. The case for life there is equally provocative, and it has the advantage of being easier to reach, at about a tenth the distance. More recently, the hot ticket has been the Cassini mission orbiting Saturn, so the momentum shifted toward the moons Enceladus and Titan. For a while Enceladus was touted as the new Europa, because it almost certainly has some liquid water and even active geysers. But we are still too far from understanding Enceladus to know whether it is in the race. Titan, with a real atmosphere and some provocative chemistry, is a definited contender with some powerful and smart advocates.