National Geographic : 1999 Sep
S THE MOST VOLCANIC BODY IN THE SOLAR SYSTEM, S ROCKY IO IS TORTURED BY JUPITER'S MIGHTY GRAVITATIONAL PULL. WITH A SOLID SURFACE THAT RISES AND FALLS IN 300-FOOT TIDES, IO CHANGES ITS FACE DAILY WITH NEW ERUPTIONS OF GAS AND SUPERHEATED LAVA. Between April 4, 1997 (near right), and September 19, 1997 (far right), an area on lo roughly the size of Arizona was buried beneath a dark smudge of vol canic debris. A volcano called Pillan Patera had exploded, throwing hot sulfur and rock ash in an 87-mile-high plume, seen on lo's edge in another view (facing page). This view also shows a plume rising from a vol cano called Prometheus, visible at lo's equator near the moon's nightside. Prometheus remains AMESRESEARCH CE as active as it was 20 years ago, when Voy ager I sent back the first images of an active volcano beyond Earth. The dark shape extending to Prometheus's right is the shadow of the plume cast by the sun. Like a blast furnace at night, lo (below left) glows with gases released by volcanic activ ity. Charged particles from Jupiter's magnetic field collide with the gases, causing them to glow in colors that identify their composition: Sulfur dioxide shines white, oxygen red. Green areas may be oxygen and sodium. "Some of the lavas erupting on lo are extremely hot, as high as 3140°F," says plan etary geologist Alfred McEwen. "That's much hotter than present-day eruptions on Earth but similar to lava that commonly erupted on Earth in the early Precambrian period. The tidal forces of Jupiter melt lo's interior, creat ing a very high heat flow." Adds Johnson: "So when we look at lo, we're looking at a place that is in some ways a lot like Earth was long ago." Passing across the face of Jupiter (below right), rocky lo floats in sharp contrast to its gassy parent.