Enough about Planet Earth for a bit. Joel Achenbach had an amusing story in The Washington Post yesterday about NASA's plans to search for life in some of our solar system's distant moons. The quandary? It took forever to decide which moon should get explored—Saturn's Titan or Jupiter's Europa. Some pros and cons:

Europa, with its liquid water, is more likely than Titan to have life as we know it, though Titan might possibly have exotic chemical processes that would represent life-as-we-don't-know-it.

"Does life require liquid water as the liquid medium, or are other liquids possible hosts for, if not life as we know it, some kind of organized chemistry?" asks Jonathan Lunine, a planetary scientist at the University of Arizona and a member of the team pushing for a Titan mission. "You'd be testing the limits of what the word 'life' really means in the cosmos."

Titan is certainly the more dynamic world, far more Earth-like than Europa, and chockablock with the kind of carbon-based molecules that fascinate organic chemists. ...

Europa, Ganymede and Titan may all have subsurface oceans, but Europa's is closest to the surface. Whether there could be something swimming down there is purely speculative, but we know that life on Earth thrives in the most improbable of places, from hydrothermal vents in the blackest depths of the sea to lakes permanently paved with ice in Antarctica.

But evidently, Europa had the better K Street firm on retainer, because that's where NASA finally settled on last week. (That's why Europa's shown in the pic above.) The mission will launch around 2020 and mosey on into the Jupiter system by 2025. Titan—and, perhaps, life-as-we-don't-know-it—is on hold. The main trouble with Titan is that it's apparently a wee bit more complicated to navigate, requiring "an orbiter, a lander that would splash down in a hydrocarbon lake, and a balloon that would cruise through the atmosphere taking snapshots." Perhaps some other time.

--Bradford Plumer