In the 1970s, as the Voyager mission cruised toward the outer planets, scientists predicted that the spacecraft would find moons like our own. The moons around Jupiter, for example, are about the size of our moon or smaller, so it stood to reason that they, too, would be cold, still, and speckled with craters. Instead, Voyager found the first, surprising evidence of volcanic activity somewhere besides our planet. It was very hard for people to accept that such a small moon like Io could still have active volcanism, because Io should have cooled a long time ago, Lopes said.

In the 40 years since, planetary scientists have moved from monitoring eruptions on Earth to finding them sprinkled across the solar system. Soon, perhaps, they will get a closer look at what exactly makes these extraterrestrial blasts tick.

The team targeting Io knows about a phenomenon the Voyager scientists didnt, called tidal heating. Io orbits between Jupiter and two of the planets other moons, Europa and Ganymede, and this configuration means that Io is subject to the gravitational forces of all three. The constant tugging heats up Ios interior, melting rock into lava. As the moon stretches and shrinks over the course of a brisk 42-hour orbit, cracks emerge on its surface, and the lava escapes through.

Its changing the shape of the whole planet, says Alfred McEwen, a planetary geologist at the University of Arizona who is leading the mission concept to Io. Lava, loosed from the interior, flows like muddy waters in a flash flood and fills in craters, regularly smoothing out the moons terrain. Many of the exoplanets that astronomers have discovered so far orbit close enough to their stars to experience the same kind of tidal heating, which makes Io a particularly suitable analogue for understanding worlds beyond our neighborhood, McEwen says.

Closer to home, theres Venus, where the surface is a mosaic of volcanic features, from peaks to plains, shaped from eons of roiling activity. We see huge fields of small volcanoes in places on Venus that remind us of the little guys we see in Iceland, says James Garvin, the chief scientist at NASAs Goddard Space Flight Center and the lead on one of the Venus missions. The planets volcanoes, numbering in the hundreds, are thought to have petered out long ago, but scientists have found evidence that some activity might be under way right now.

A few years ago, an infrared camera on a European spacecraft peered through the planets thick atmosphere and caught spots on the surface suddenly heating up and cooling down again. Smrekars mission to Venus would send a spacecraft to orbit the planet, map its topography, and determine whether theres still some churning going on. Another mission, led by Garvin, will drop a probe through Venuss atmosphere into a potentially volcanic area, moving down as if we were descending in a helicopter ourselves, he says. The probe would have the capability to analyze atmospheric gases and pick out signatures of recent eruptions.

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A Solar System of Fire and Ice - The Atlantic