What's Under Europa's Icy Crust?
Looking for Life on a Jovian Moon
In 1610, with his then-new telescope, Galileo Galilei discovered four moons orbiting Jupiter. Now the second of these Galilean satellites, Europa, is under scrutiny by a spacecraft named in his honor, NASA's Galileo. Since its two-year orbital tour of Jupiter ended late last year, Galileo's researchers are focusing on Europa in an extended-mission phase called the Galileo Europa Mission (GEM).
Among Europa's investigators are several in Southwest Research Institute's (SwRI) Space Studies Department in Boulder, Colorado. Since 1977, I have been a member of the Imaging Team, which oversees operation of Galileo's camera. Other members are Dr. William Merline and University of Colorado graduate students Beau Bierhaus and Shawn Brooks. We are in the front seat of one of NASA's most ambitious and rewarding missions of planetary exploration. Bolstered by Galileo's Europa results, one view is that this small world -- the size of our own Moon -- just may be the most likely place in the solar system, besides Earth, to find flourishing life.
Life in the solar system
Searchers for life elsewhere in the universe have naturally sought environments like Earth. Liquid water seems essential for both the origin and the sustenance of life, so candidate planets would be in a star's "zone of habitability," where it is neither so cold that any water would freeze nor so hot that it would boil. Scientists looked first to our twin planet, Venus, next toward the Sun and then to Mars, next farther away, for evidence of clement climates and life.
About 40 years ago, radio telescopic data -- confirmed later by the Mariner 2 spacecraft -- revealed Venus to be a veritable hellhole; thanks to a runaway atmospheric greenhouse effect, Venus is radically too hot to harbor life. Prospects for life on Mars have roller-coastered since Mariner 4's first pictures of a bleak, crater-scarred landscape, which dashed hopes for Martian life. Later missions revealed signs of once-flowing rivers, 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 pow