SAN FRANCISCO: The thought is almost too frustrating to bear: the possibility that the solar system’s most promising spot for extra-terrestrial 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.

A 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 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 neighbourhood.

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 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.

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.—Dawn/The Christian Science Monitor News Service.