The Difficulties of Exploring Europa

ALL THESE WORLDS ARE YOURS EXCEPT EUROPA. ATTEMPT NO LANDINGS THERE.

-- 2010: Odyssey Two

Arthur C. Clarke wrote these words three decades ago, when Europa's uniqueness as a potential alien habitat was first realized. Unfortunately, we have obeyed this fictional command.

Partly that's because Europa's surface is a terrible place. We often say this about Mars, but Europa is worse in practically every way. It is over three times farther from the Sun, and so receives ten times less sunlight. As a result, while a warm day near the Martian equator can exceed 0°C, Europa is at best 140 degrees below. Whereas Mars has less than 1% of Earth's atmospheric pressure, Europa's "atmosphere" is a billion times more rarefied. Neither provides much protection from ultraviolet or particle radiation, but at Europa the bombardment of charged particles from elsewhere in the Jupiter system leads to crippling radiation doses that accumulate millions of times faster. All of these factors make Europa's surface a difficult place for spacecraft (or life) to operate.

Yet deep beneath this surface the environment is very different. Several lines of evidence point to a global salt-water ocean on Europa, hidden from direct view by a frigid icy shell. Like the mid-ocean ridge ecosystems on Earth, Europa's oceanic realm could host life forms for whom the surface conditions are of little concern. We currently have no idea whether such life exists or not. But if ocean water ever reaches Europa's surface, then perhaps its frozen residue could be examined for chemical clues by a carefully aimed lander. …

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