Io the volcani Rosetta Stone
MOST PLANETS IN THE SOLAR SYSTEM have moons, but giant Jupiter has perhaps the most extraordinary satellite of all — the volcanic wonderland Io, a world made for superlatives. Its hundreds of active volcanoes erupt huge volumes of hot lava. Lava flows cover areas the size of small countries on Earth. And although it’s almost exactly the same size as Earth’s Moon and just as much a target for impactors, Io’s surface looks nothing like the ancient, heavily cratered lunar landscape — in fact, Io’s surface has no impact craters at all: The incredible level of volcanic activity constantly renews the surface and quickly buries all evidence of impacts.
The most astonishing thing about Io, however, is that it is volcanic at all. When the worlds of the Solar System first formed, radioactive isotopes were incorporated into their interiors. As these isotopes decayed, the interiors heated up and, in many cases, fueled surface volcanism. Over time, though, this heat was lost to space. Small bodies lose heat faster than large ones, so, for example, Earth’s Moon has no active volcanoes while the larger Earth has plenty. Most scientists therefore expected Io to be volcanically dormant — a cold, frozen ball quietly whirling around Jupiter.
But the primary source of Io’s internal heat is not radioactive. Instead, this heating comes from the gravitational tug of war between Jupiter, Io and two other moons — Europa and Ganymede. Caught in an orbital resonance, the satellites pull at one another as they pass, kneading their interiors and warming them.
Io is in the sweet spot for the most extreme heating. The flexing of Io melts part of the rocky interior and ultimately manifests at the surface as breathtakingly intense volcanism. Io’s volcanoes radiate so much energy in the infrared that astronomers can observe many of these individual eruptions using telescopes on Earth.
Io is therefore one of the most restless, mesmerising worlds
