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Io: Fire World
Robotic probes may someday provide close-up views of some of the most remarkable vistas in the solar system, from the canyons of Mars to the ice-geysers of Triton. For a true hot-spot, they might show us the surface of Io, one of the moons of Jupiter. It is an eerie landscape of active volcanoes, tall mountains, and plains covered with frozen sulfur.
|Discovery||1610, Galileo Galilei||1610, Galileo Galilei||1610, Galileo Galilei||1610, Galileo Galilei|
|Distance from Jupiter||262,000 miles
1.1 million km
|1.2 million miles
1.9 million km
|Orbital Period||1.8 days||3.6 days||7.2 days||16.7 days|
Other than the Sun, portions of Io offer the hottest surface in the solar system. Several hundred volcanoes dot the surface, and they belch sulfur-rich lava that is hundreds of degrees hotter than the hottest lava on Earth.
Io's interior is heated by a tug-of-war between Jupiter and the planet's other big moons. Io is "locked" so that the same hemisphere always faces the planet, just as the same hemisphere of our own moon always faces Earth. But as the other moons move past Io, their gravity tugs at it, too. That heats Io's interior enough to melt some of its rock, which "bubbles" to the surface.
Europa: Water World?
For decades, Mars was considered the most likely home for life in the solar system. As observations continued to show a sterile, desolate world, though, scientists began turning their attention to Europa. Europa's icy crust appears to cover a large ocean of liquid water, where life may have gained a foothold.
Tidal gravity may have created the ocean by heating Europa enough to melt some of its ice.
There is abundant evidence of an ocean. The Galileo spacecraft found two types of terrain that may be related to water. "Chaotic" terrain looks like icebergs breaking off glaciers on Earth. The other terrain consists of smooth plains marked by ridges that are hundreds of miles long. The ridges may form as Jupiter's gravity rips apart the thin ice sheet. In addition, the motions of salty water below the surface may generate Europa's weak magnetic field.
Ganymede and Callisto: Ice Worlds
Ganymede is the solar system's largest moon -- larger than the planet Mercury. It consists of about half ice and half rock and metal. Grooves and ridges that crisscross its surface indicate that it has undergone great changes over the eons.
During several passes, the Galileo spacecraft saw mountains of ice, plus sheets of ice that erupted from volcanoes. It also saw deep canyons and broad, smooth plains created by the motions of Ganymede's crust.
Like Europa, Callisto's icy surface may conceal an ocean. The case for an ocean is more tentative, but it is bolstered by a huge basin on one side of the moon. It was created by a powerful impact billions of years ago.
But there is no jumble of rocks and mountains on the opposite side of Callisto, as there is with big impact basins on our own moon. A deep ocean could have cushioned the impact, preventing shock waves from piling up rocks half a world away.
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The Astro Guides for the Solar System and Beyond the Solar System are supported by the National Aeronautics and Space Administration under Grant Nos. NNG04G131G and NAG5-13147, respectively.