Hugh Pickens writes "NASA and the European Space Agency are expected later this week to settle an ongoing debate on whether to send a robotic mission to Jupiter's moon Europa or Saturn's moon Titan. Both are difficult places to get to — a mission to either would cost several billion dollars/euros to build and execute — and both have become alluring targets in the quest to learn whether Earth alone supports life. On the one hand, Europa is believed to have liquid oceans beneath its frozen crust which (on Earth at least) are a source of life-supporting chemistry. Scientists would like to scan Europa's surface for bits of material that may have seeped up from beneath the ice. 'Imagine if there were microbes entrained in material that has exuded onto the surface of Europa and they've been sitting there for maybe three million years,' says planetary scientist Dr. Brad Dalton. On the other hand, Titan has two enticing features in the search for life: liquids on the surface, and a thick atmosphere that can be used to slow down a spacecraft and help put it into orbit. Titan's surface water is locked into the crust as ice, but scientists suspect there may be a subsurface ocean where water mingles with ammonia. The mission will not get to the launch pad before 2020. 'It's unfortunate that there has to be a decision,' says NASA/JPL astrobiologist Dr. Kevin Hand. 'It's important to go to both. They are both such amazing and tantalizing worlds in terms of finding life.'"