Last week, in the dim reaches of the outer solar system, the Galileo spacecraft hurled itself, silently and unseen, into Jupiter's gloomy atmosphere. It was promptly crushed and vaporized.
Credit: SETI Institute
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 Galileo's exit strategy was motivated by the wish to avoid contaminating one of Jupiter's 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, there's always the possibility of life.
|Remarkable frozen texture on Jupiter's moon, Europa.
This result is now so well known it has entered the realm of conventional wisdom. Consequently, it's easy to overlook the astrobiology revolution brought about by Galileo's satellite surveillance. This upheaval goes beyond the intriguing possibility that a microbe-laced ocean might lurk beneath Europa's 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 Jupiter's 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 Aristotle's 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 Jupiter's disk, and one to the west. That, per se, was not so peculiar. But something about this asterism caught Galileo's highly discerning eye. The stars were unusually and uniformly bright, and formed a straight line with Jupiter that was parallel to the ecliptic.
|Giant Red Spot in background, one of Jupiter's moon in foreground with eclipse shadow cast on gas giant Credit: NASA/JPL Cassini
The following night, Galileo once again pointed his glass toward Jupiter (undoubtedly a painstaking task, given his instrument's 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 mankind's 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 Europa's 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 Nature's great variety, and mankind's 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.
Related Web Pages
Europa Diary I: Landing on Alien Terrain
Europa Diary II: Life on Ice
Europa Diary III: On Polar Bear Time
Europa Diary IV: Walking on Thin Ice
Alaska Europan Photo Gallery (Credit: Matt Pruis/ Jere Lipps )
Ice on Europa
Interpreting Europa's Features
Infrared Spectroscopy: An Overview
Galileo Project Home