Jupiter's Moons: 2 Aristotle: 0

Sep. 25, 2003
by Seth Shostak - Senior Astronomer

Last week, in the dim reaches of the outer solar system, the Galileo spacecraft hurled itself, silently and unseen, into Jupiters gloomy atmosphere. It was promptly crushed and vaporized.

A kamikaze-like ending to an orbiting spacecraft is hardly unusual. Many satellites eventually flat-line by self-immolation in the thin air a hundred miles above Earth. But Galileos exit strategy was motivated by the wish to avoid contaminating one of Jupiters large moons. In eight years of surveying the jovian system, the Galileo mission had produced a highly provocative result: some of these moons -- and especially the milky-white Europa -- showed evidence of hidden, ancient oceans. And with liquid water, theres always the possibility of life.

This result is now so well known it has entered the realm of conventional wisdom. Consequently, its easy to overlook the astrobiology revolution brought about by Galileos satellite surveillance. This upheaval goes beyond the intriguing possibility that a microbe-laced ocean might lurk beneath Europas rigid, frigid crust. The real paradigm shift was when we realized that Earth-like planets, with surface water and heavy atmospheres, are not the only type of world that might be habitable.

Remarkably, this is the second time that Jupiters moons have provoked a revolution in cosmic perspective.

The first began nearly 400 years ago, when Galileo Galilei turned a 20-power spyglass on the heavens. His initial targets were the Moon and stars. Galileo soon made the astounding and dismaying discovery that the Moon was hardly the perfectly smooth body advocated by Aristotelian theory. Its surface was ruptured and riven by mountains and valleys: it was another world, not a divinely unblemished companion to Earth set in place for our pleasure.

Galileo also noted that, wherever he pointed his telescope, he could see an order of magnitude greater number of stars than was visible to the eye. There was obviously more to heaven and Earth (especially the former) than was dreamed of in Aristotles philosophy.

But the Moon and the stars were not enough. The discovery that made Galileo a household word, and eventually a namesake for NASA spacecraft, occurred on January 7, 1610, when the ambitious math professor from Padua noticed some small stars around Jupiter. There were three such stars; two to the east of Jupiters disk, and one to the west. That, per se, was not so peculiar. But something about this asterism caught Galileos highly discerning eye. The stars were unusually and uniformly bright, and formed a straight line with Jupiter that was parallel to the ecliptic.

The following night, Galileo once again pointed his glass toward Jupiter (undoubtedly a painstaking task, given his instruments very narrow field of view). He was stunned to find that Jupiter was now east of the stars even though its motion across the sky should have placed it to the west assuming the small constellation consisted of fixed, background stars. Galileo was puzzled, but relentless. Two nights later, he saw that Jupiter had sidestepped the stars to the west, and now only two were visible.

It took Galileo no more than a week to decipher this celestial dance. The small stars (he eventually counted four) were satellites of Jupiter.

The philosophical implications of this were big, really big. Here was a world that was not only festooned with moons -- and remember that until then, only the Earth was known to be so favored but festooned with four! (As an aside, the current tally for Jupiter is 61 moons.) In addition, Galileo had discovered a system in which moons were orbiting a planet that was, itself, in orbit. In other words, there was no single "center" to the universe, no single master. The still-controversial Copernican idea that Earth was only one of several rocky attendants to the Sun -- the deflating premise that mankinds home and hearth was not the nexus of existence -- was endorsed in heavy ink by one week of telescopic observation.

The Galileo spacecraft, ten generations later, has done something similar, although it may take time to sink in. It has provided several lines of evidence for a vast, moon-girdling ocean beneath Europas 10 mile-thick ice glaze. Similar oceans may exist on Ganymede and Callisto, all kept warm and liquid by the changing gravitational tugs among these moons and their host planet. Who, a generation ago, would have thought that moons no bigger than our own could harbor massive seas?

That no one did is a testament to Natures great variety, and mankinds occasional lack of ingenuity. Our concept of habitable worlds was, not so long ago, hardly more generous than the uncompromising Earth-centered view of Aristotle. The Galileo spacecraft has changed all that. The manner of its demise -- intended to allow future missions to explore Europa without fear of prior contamination -- is dramatic testimony to its success.