One moon, Europa, looks potentially more hospitable to life than desolate Mars. Dark bands on its surface are cracks in the icy crust much like those seen in ice floes on Earth’s polar seas. The new pictures show that in places, fragments have rotated away from each other, which could mean that there’s something slippery underneath – either warm ice or water. Scientists already know that Europa has the other factors necessary for life: organic molecules and heat below the surface.
If Europa is ice, then another moon, Io, is fire. A blue geyser seen flaring from the surface is an eruption of the volcano Ra Patera, inactive when Voyager probes passed by 17 years ago. The 60-mile-high plume is probably sulfur dioxide; parts are gaseous and the rest has frozen into a snowlike substance. Galileo’s pictures show that Ra Patera’s eruptions have coated more than 15,000 square miles with volcanic debris.
Volatility appears to be the rule on Io. The new pictures also show red areas – probably created by deposits from other superhot volcanic eruptions – and relatively new lava surfaces elsewhere. Scientists suspect Io is in for even more change because of tidal activity induced by the gravity of Europa and another moon, Ganymede, as well as by Jupiter itself. Io ““gets stretched, pulled and stretched back and forth, so it’s frictionally heated, like pulling a piece of metal,’’ says Alfred McEwen, a Galileo imaging team member from the Lunar and Planetary Laboratory at the University of Arizona.
Ganymede yielded a surprise, too, although not from photographs. A device aboard Galileo that is used for measuring electromagnetism – called a plasma wave spectrometer – detected a magnetosphere, or magnetic field, around the moon. This is puzzling; usually only planets have magnetic fields. No one knows why Ganymede is different.
Over the next year and a half, Galileo will continue to transmit high-resolution imagery from Europa as it orbits Jupiter. Researchers are especially eager for pictures of craters made by asteroids or other objects impacting the ice plates. The more craters, the older the surface, so their numbers indicate when the ice broke apart. As for Io, they’re hoping to catch eruptions with high effusion rates, volcanic activity with rapid lava flow that has been linked to both mass extinctions and climate change on Earth. Io’s volcanoes might help explain changes in our own atmosphere. If so, the show stealer could be more than just another pretty face in the cosmos.
