The Year in Science: Space 1997

A Flower-Petal Path Among the Moons

By Kathy A. Sviti|Thursday, January 01, 1998
The news from the moons of Jupiter in 1997 was as follows: Ganymede definitely has a magnetic field; Callisto does not; Io has more volcanoes than ever; and Europa may have a liquid ocean surging underneath a thin crust of ice.

Still going strong eight years after its launch, the Galileo spacecraft had close encounters with the three outer Galilean moons this past year and snapped new photos of innermost Io from a distance. The probe’s trajectory—the repeated use of one moon’s gravity to change the orientation of the probe’s ellipse around Jupiter and to fling it toward the next moon—is one of its niftiest traits and a big improvement over the one-time flybys of the Pioneer and Voyager days. It gives Galileo’s handlers not only a closer look at its targets but also the novel luxury of a second look if they want one.

In 1996, for instance, Galileo had found evidence that Ganymede had a magnetic field—but not firm enough evidence to persuade everyone that the field was really Ganymede’s and not Jupiter’s. Last May, on the spacecraft’s fourth encounter with the moon, researchers got the proof they needed. The energetic-particle detectors saw particles behaving as if they were on closed magnetic-field lines around Ganymede, says project scientist Torrence Johnson of the Jet Propulsion Laboratory. On Earth such particles would produce a shimmering aurora as they spiral down the field lines and crash into the polar atmosphere; on Ganymede, which has no atmosphere to speak of, they simply slam into the surface.

Yet the mere existence of the moon’s magnetic field has surprising implications. It means Ganymede probably has a core of electrically conducting fluid that is in motion. In Earth’s core the fluid is iron, and gravity readings by Galileo, which give a picture of Ganymede’s interior, suggest that it has something similar. Though it is the largest moon in the solar system—about 1,000 miles wider than our own moon—it is much smaller than Earth. It should have shed its internal heat and frozen solid 3 billion years ago. Apparently it has been reheated somehow, and this reheating has caused its heavy metals to sink into its core. That was really unexpected, says Johnson.

The only known method for this reheating is tidal pull; if a moon’s orbit is not perfectly circular, the planet’s gravity will act on it in an uneven way, stretching its insides back and forth and thereby heating them. That process certainly heats Io and Europa, which lie closer to Jupiter. It does not seem to heat Callisto, which lies a little farther out; although a dearth of small craters suggests that some form of erosion is smoothing Callisto’s surface, it is apparently dead inside and doesn’t have an iron core. One possible explanation, Johnson says, is that Callisto is the odd moon out in an unusual gravitational symbiosis among the other three satellites—one that conspires with Jupiter’s gravity to heat them but not Callisto.

The hottest moon, of course, is Io, the most volcanically active object in our solar system; Galileo detected more new volcanoes on Io this past year, bringing the total to 81. But the most interesting moon right now is Europa. Images taken in February provided signs of a liquid ocean, presumably of water, beneath an icy surface. There are parts of the surface that look as if they have melted, with huge 10- to 20-kilometer ice rafts that have broken off the edge of the crust, moved around, and then frozen in place again, Johnson says. You can’t do that without having liquid water very close underneath the surface.

The images don’t prove that Europa has an ocean today—let alone that it is inhabited, as every dreamer dreams—but they do strongly indicate that it had one in the geologically recent past, which is almost as good. It is very hard to come up with a scenario where the moon is very hot and active for 99.9 percent of its history and then just froze yesterday, Johnson says. Anyway, Galileo gives him and his colleagues the chance to find firmer proof of the first ocean other than Earth’s. At the end of 1997, the probe was scheduled to take two more passes at Europa. And in 1998, nasa is planning on seven more.
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