Battered and bedraggled from numerous mishaps in its seven years in space, the Galileo orbiter finally made it to Jupiter in December 1995, deposited its atmospheric probe, and continued on a series of eccentric, loopy orbits among the big planet’s 16 moons. Although a balky tape recorder kept the flow of data to a trickle, what came through was choice: a wealth of information about Jupiter’s atmosphere and the moons--Io, Europa, Ganymede, and Callisto--that Galileo Galilei discovered in 1610.
Galileo’s suicide probe plunged 80 to 90 miles into the Jovian atmosphere before melting, with the orbiter tracking its speed and angle of descent. Those measurements revealed that the probe was buffeted all the way down by 400-mile-an-hour winds--far faster than previously thought. (A NASA scientist confirmed this result independently by using Earth-based telescopes half a billion miles away to measure the probe’s meager 25-watt signal.) Winds that deep and fast could not be powered by heat from the sun, as are winds on Earth; they must be driven, as some researchers had already suspected, by Jupiter’s own internal heat.
Jupiter is not only hot but surprisingly dry, according to the suicide probe: it found water to be far more scarce in the Jovian atmosphere than would behoove a planet believed to have been assembled, at least in part, from water-rich comets. Either it is the origin of Jupiter that researchers don’t understand, or it is the weather--and in particular the process of convection, by which heat works its way up through the atmosphere and in so doing distributes water around a planet. We were really eager to see convection happen on another planet, because we thought we’d get insights, says Andrew Ingersoll, a planetary scientist at Caltech. We got so many insights, we were floored.
In the first of three flybys this past year, Galileo managed to get images of what appear to be giant thunderstorms rising 20 miles above Jupiter’s cloud cover. Ingersoll believes the suicide probe may have been caught in a sort of mirror image of one of those storms--a giant dry downdraft, similar to those that occur over Earth’s deserts. In other words, the probe may have discovered a local, weather-related dry spot near Jupiter’s equator rather than a planetwide lack of water. Data from the other two flybys and from the seven additional ones planned for 1997 should show whether Ingersoll is right.
Meanwhile the orbiter’s rounds are giving scientists their best glimpse yet of the Galilean moons. Tempestuous little Io has been busy since the Voyager spacecraft first saw some of its many volcanoes in 1979; eruptions have left it shrouded in a volcanic haze, and Galileo even caught one volcano near the equator in the act of spitting out a 60-mile-high plume of gas. Perhaps more surprising, though, was the evidence for internal activity on Ganymede. Io is close enough to Jupiter for its insides to be heated by the giant planet’s tidal pull; Ganymede is not. Yet in June Galileo determined that Ganymede has its own magnetic field, which suggests that some hot and electrically charged fluid is moving around inside it. One theory is that Ganymede was once closer to Jupiter, was tidally heated then, and hasn’t quite cooled down.
Then there is Europa, which is the next moon out from Io, and which to some researchers has become the most interesting moon of all-- because its icy crust may conceal an ocean of liquid water underneath. At least that is how it appears from Galileo’s images: the pattern of cracks and craters on the surface strongly suggests a crust that melts and reconstitutes itself every so often. Bands of dark-light-dark coloration that stretch about 600 miles across the surface may even, according to one theory, be the product of geysers of water mixed with rock. If an ocean indeed exists, it would conceivably generate a small magnetic field, which Galileo may yet detect. Already some fertile minds are proposing that such an ocean might harbor microbial life.
All in all it has been a fertile mission, in spite of the technical glitches, and it is far from over. Galileo’s dance card is full for 1997, with visits to Europa and Callisto, the fourth Galilean moon. If money can be found to continue the project, scientists are toying with the idea of focusing exclusively on Europa and then, in 1999, easing the orbiter into the radiation belt around Io for one last look at the volcanic moon. After that, Galileo’s sensitive equipment would be fried and the scrappy little spacecraft would finally be silenced.