The thought is almost too frustrating to bear: the possibility that the solar system's most promising spot for extraterrestrial life is beyond the reach of modern science.
Almost universally, scientists acknowledge that Europa, a moon of Jupiter nearly as large as our moon, is covered by an ocean of liquid water the indispensable building block for organic life as we know it. That ocean, however, is hidden beneath a crust of ice of unknown thickness.
Today, a new report suggests the worst scenario that the cap is as many as 15 miles deep. Scientific opinion is far from unanimous, though, and a mounting debate indicates that this has become one of the most pressing questions in the search for life elsewhere in the solar system. Indeed, its answer will likely determine not only what science is possible, but also whether life elsewhere is even feasible.
Moreover, with the discovery of 11 additional tiny Jovian moons late last week, it is highlighting one of the most intriguing corners of the solar system, where 55,000-foot mountains and 4 billion-year-old landscapes are all just part of the neighborhood.
"You have to look at the set [of Jupiter's four major moons] as a whole," says Robert Greeley, an astronomer at Arizona State University in Tempe, adding that each tells a different story.
The new study of Europa's ice thickness is based on an analysis of the few craters that pockmark the moon's unusually smooth surface. Scientists conclude that water has repeatedly welled up from beneath the ice sheet and refrozen, giving Europa the appearance of a roan cue ball, streaked by darker fissures.
In cratered areas, though, an analysis suggests that the ice is between 10 and 15 miles thick. In this scenario, the idea of using a lander to burrow for water is unrealistic.
"What it means is that we have to come up with more clever strategies to answer the big questions," says Paul Schenk of the Lunar and Planetary Institute in Houston, author of today's report in the journal Nature.
In what has been called the Europa Pizza Debate thin or thick crust some of Dr. Schenk's opponents don't discredit his research. Rather, they say it merely proves thick ice in one spot at one time. In other places, called chaos regions, chunks of ice have broken free and rotated upward like saw's teeth.
"They look like icebergs floating in a matrix of mush," says Dr. Greeley. "That suggests thinness."
What makes the debate even possible is Jupiter, the gas giant that could swallow 1,231 Earths, and gives off so much radiation that standing on Europa's surface more than 400,000 miles away would be similar to standing 30 feet from the core of New Jersey's Salem nuclear reactor. Jupiter's power is so vast that some have called it a solar system in miniature, circled by 39 satellites, including the four major moons, Io, Europa, Ganymede, and Callisto.
What keeps Europa's underlying seas unfrozen is a process called tidal heating, and its effects are even more pronounced on Io, which is at roughly the same distance from Jupiter as our moon is from Earth. There, the strength of Jupiter's gravitational field is literally turning this moon inside-out.
Caught in a tug-of war between Jupiter and the outer moons, the crust of Io rises and falls in 330-foot tides of solid rock. These tidal forces have turned the Io's insides molten. Aside from Earth, it is the only place known to have active volcanoes hundreds of them. Molten rock here builds up in flows so massive that the crust buckles, tilting chunks of earth that reach 55,000 feet into the sky.
Farther out and past Europa lies Ganymede, the largest moon in the solar system larger than Mercury or Pluto. Beyond Ganymede, the slightly smaller Callisto is the most cratered body in the solar system, meaning that unlike Io and Europa, which constantly resurface themselves the 4 billion-year-old landscape is the oldest in the solar system.
Strange though it may seem, the search for life could eventually lead to these desolate moons someday. Evidence suggests that both hide vast oceans of liquid water, as well though the surface ice and rock is estimated to be 100 miles thick.
For now, though, attention is focused on Europa, despite the cancellation earlier this year of a probe to study the moon. Even thick-ice theories don't preclude the possibility of life beneath. In fact, the more extreme the conditions, the more a discovery might tell us about the spark of life.
Says Torrence Johnson, Galileo project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif., "That's why it's of such great importance: to study the origins of life."