rs, then new hopes that life might have survived from a watery past were negated by Viking's mid-70s on-the-surface biology experiments. There are new reasons why Mars might once have harbored life, though headlined conclusions two years ago about fossilized microbes in a Martian meteorite have since been challenged.
Giving up on our own solar system's "zone of habitability," astrobiologists next turned to possibilities for life in planetary systems around other stars. But before leaving our solar system altogether, let's not forget Europa. The outer solar system, beyond Mars and the asteroid belt, is indeed cold, only dimly warmed by the distant Sun. Basking as we do in the Sun's radiance and sustained by an ecology of photosynthesis, however, we are biased about the primacy of the Sun. Modern oceanography teaches that we may be exceptional: teeming life exists on and beneath the ocean floor, and may flourish in rocks beneath the Earth's surface. Energized by geothermal systems, life might even have originated below the Earth's surface, only later evolving upwards to take advantage of the ubiquitous sunlight.
Voyager's reconnaissance of Jupiter
A breakthrough in understanding the variety of planetary energy sources came two decades ago. As the first Voyager spacecraft sped toward Jupiter, California researchers Stan Peale, Pat Cassen, and Ray Reynolds1 were developing the theory of Jupiter's tidal forces on the Galilean satellites. The three inner moons -- Io, Europa, and Ganymede -- were known to be trapped by each other's gravitational forces in an orbital resonance, which keeps them in fixed orbital and rotational configurations, despite the tidal forces of mighty Jupiter, which would otherwise dominate their motions just as tidal (differential gravitational) interactions between the Earth and the Moon have pushed the Moon outward over the eons.
Peale et al. showed that tremendous amounts of tidal heat must be dissipated within Io and -- to a lesser extent -- within Europa. These moons are literally being powerfully wrenched. Days before Voyager reached Jupiter, Science published the predictions of Peale et al. of prolific volcanism on Io. Indeed, the first close-ups of Io showed its colorful, crater-free landscape dominated by sulfurous deposits from unending eruptions of numerous volcanoes.
Almost lost amid public excitement about Io's volcanic plumes were Voyager's mysterious views of Europa. Europa was least favorably situated for close-up imaging, so even the best pictures barely exceeded our own Moon as viewed through powerful binoculars. The smooth, ice-covered world was criss-crossed by enigmatic lines -- most of them straight, but some forming strange curlicue shapes. Europa's secrets would have to await Galileo's much higher-resolution investigations, some 14 years later.
Meanwhile, researchers continued to speculate about Europa. Though the tidal pulling and tugging on Europa's interior is much less than for Io, the rate of heating is hardly trivial. Given other evidence that Europa contains abundant water, tidal heat just might keep the water liquid. Beneath its icy crust, Europa could have an ocean. So, unexpectedly, the solar system has a second potentially habitable zone -- out near Jupiter.
Europa reveals her mysteries
As Galileo approached Jupiter in 1995, some researchers were expecting a Europan "ocean;" some even had the temerity to discuss possible life ("fish") in the Europan seas. Others calculated that the tidal heat would be lost too rapidly to sustain high enough temperatures for liquid water. We simply knew too little about Europa, and the processes within it, to decide. Galileo's engineers and scientists had to make tough choices about what to study during the spacecraft's orbital tour, because the main antenna's failure to deploy left a shrunken radio "pipeline" for sending data back to Earth. Europa was given priority.
What we have seen of Europa during the first 15 Galileo orbits (11 in the prime orbital tour) has been stunning, if only to exemplify the artistry of nature. Like Jack Fro