A Solar System of Fire and Ice

Earth was once the only world known to have volcanoes, but they’re all over the place, spouting lava and icy plumes.

The Galileo spacecraft caught a volcano erupting on Jupiter's moon Io in 1997.
The Galileo spacecraft caught a volcano erupting on Jupiter's moon Io in 1997. (NASA)

Rosaly Lopes spent five years carefully inspecting a churning landscape where molten rock spilled forth like the arced jets of a water fountain. Using data from an orbiting probe, she picked out eruptions across the fiery surface, eventually spotting 71 active volcanoes that no one had ever detected before.

“People used to joke with me, ‘Oh, you found another active volcano!’” Lopes told me. “‘You’re going to be in the Guinness World Book of Records’”—until one day, one of those offhand comments made its way to somebody who actually worked for Guinness World Records. Lopes ended up in the 2006 edition, recognized for discovering the most active volcanoes anywhere.

None of the volcanoes were on Earth, though. They were several hundred million miles away, on a moon of Jupiter called Io.

Today, Io is known as the most volcanically active place in the solar system. Other volcanic spots are scattered across our neighboring planets and moons, too, and probably countless more in other solar systems across the universe. Recently, NASA announced it would fund proposals for four new robotic missions, all headed for a close look at these kinds of worlds—Io, Venus, and Triton, a moon of Neptune.

Not long ago, Earth held the title for the most volcanic spot in the solar system. As a rule, volcanic activity indicates that a world is cooling off; after planets and moons form—an extreme and fiery process—they can spend billions of years ejecting heat from their interiors through cracks in the surface. Small bodies, like our moon, should go cold faster than others, and spurts on the surface can reveal the invisible contours of a world deep within. “Volcanism is like a window into the interior of the planet,” says Sue Smrekar, an astronomer at NASA’s Jet Propulsion Laboratory who is leading one of the proposed missions.

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 didn’t, called tidal heating. Io orbits between Jupiter and two of the planet’s 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 Io’s 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.

“It’s 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 moon’s 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, there’s 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 NASA’s Goddard Space Flight Center and the lead on one of the Venus missions. The planet’s 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 planet’s thick atmosphere and caught spots on the surface suddenly heating up and cooling down again. Smrekar’s mission to Venus would send a spacecraft to orbit the planet, map its topography, and determine whether there’s still some churning going on. Another mission, led by Garvin, will drop a probe through Venus’s 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.

Farther out, on Triton, the lava plumes are made of ice. In 1989, the Voyager mission revealed a world surfaced, cantaloupe-like, with bumps and ridges, and so cold that nitrogen exists as shiny frost on the surface. In quite the stroke of luck, the spacecraft, as it flew past, caught geysers of particles erupting from the surface and drifting downwind in the moon’s thin atmosphere.

Spacecraft haven’t been back since, and scientists are eager to investigate Triton in more depth—particularly the intriguing possibility that the plumes could be coming from a hidden, subsurface ocean. A briny body of water, even this far from the warmth of the sun, could harbor some of the basic components that could give rise to life in the same way that Earth’s dark seafloors have.

“[Triton] is five times further away than Saturn. Understanding even worlds that far out in the solar system could still have some of the ingredients of a habitable world would basically revolutionize our understanding of what it means to be a habitable world,” says Louise Prockter, the director of the Lunar and Planetary Institute in Houston and head of the proposed Triton mission.

NASA has doled out $3 million to each team to develop their mission concepts. Next year, the agency will pick one or two of them to move forward, toward spaceflight construction. The Venus missions would reach their target in the 2020s, while the Io mission won’t arrive until 2031 and the Trident mission until 2038, when scientists’ knowledge of these distant volcanic worlds would be nearly half a century old. By then, it’s possible that astronomers, using the most powerful telescopes, would have discovered volcanic plumes bursting from worlds deep in space, around other suns. The findings in our own solar system have shown that the cosmos is trembling with the rumble of churning worlds. Someone will have to discover all those volcanoes—Lopes’s record won’t stand forever.