Io: Moon On Fire

The diameter of Jupiter’s volcanic moon, Io, could fit within the footprint of the continental United States–about 2,175 miles across. Compared to the Earth’s moon, which is quiet and dark except for reflected sunlight, Io is on fire. The fiery pockmarks that dot its surface are volcanic openings. Surrounded by yellow and red rings of sulfur, these bursting lava floors paint its Hadean surface in the colors of Dante’s Inferno. The infernal blacksmith pounds this moon to a kneaded ball ablaze. The layers of purgatory brew up often enough to make Io by far the most volcanically active body in the solar system. A single volcano on Io may spew a hundred times more lava than all earthly counterparts combined. To say that Ionian geology is igneous is to underestimate the image of a Jovian moon that is burning from the inside out, from its iron core to its sulfurous crust.

High resolution close-up of Io’s volcanic surface
Credit: NASA/Galileo

Named for a Roman priestess to Jupiter’s wife, Juno, Io’s mythological incarnation mirrors its astronomical fate. In myth, Io got too close to Jupiter and suffered the wrath of the Olympian king by attracting the scorn of his wife. The priestess fell from favor; she survived only to be cursed as a white cow, first disguised to escape Jupiter’s wife, then chased by Mercury, finally tortured by a gadfly sent by Juno herself. Gravity may be the equivalent gadfly for Io, the moon.

"I’m increasingly aware that some of the most interesting things in astronomy and astrophysics, for instance, can change the way people understand the universe, how it got started and where it’s going. I found those Voyager pictures of the moons of Jupiter incredibly exciting, these beautiful color pictures showing volcanoes on the surface". -Robert C. Richardson, Nobel Laureate, Physics, Cornell, (1996)

Orbiting close to Jupiter’s titanic gravitational pull, Io also falls into a curious resonance with an opposite tug from other moons of the Jovian family. One reason Io’s surface looks so tortured has to do with a near enough miss from both the other moons, Europa and Ganymede, that its crust splits and cavernous mountains rise suddenly out of these fault-lines. The tallest Ionian mountain measured so far (52,000 feet) is roughly twice the height of Mount Everest. But the real mountains on Io are not permanent landmarks–instead, they arise from volcanic sprays that reach up hundreds of miles and can be observed even from good terrestrial telescopes as plumes of sulfurous gases, dust and searing infrared heat. On Earth, such a rising column of lava would jet from sea-level straight into space.

Unlike earthly volcanoes, Io’s vents are not places one can always find on an old map, since they move around as the surface splits and heals periodically during its orbits. Where a plume may appear next, might be right under a visitor’s foot. The reason for this sudden mobility is part of Io’s placement: as Jupiter pulls Io one way, there is a regular spacing between Europa and Ganymede that pulls the opposite direction. Such opposing pushes and pulls may give rise to a volcano just about any place where the ground weakens. No such wandering volcanoes are known on Earth.

The irregular circles around these vents may be compared to the textures expected after a souffle falls. The fuming pits are called caldera. The red rings show more recent eruptions. Over the course of four months, the red sulfur rings can change their shape. Following an eruption, this reddening may indicate that the yellow sulfur has boiled recently. The red edges are not sharp, seemingly air-brushed with a wide jet of paint. A casual glance reveals a sphere weathered to a barely finished coat of peeling paint. The blackened crust spots are thought to be formed by oxygen and silicon. While on Earth, lava has a distinct silicate portion, Io’s sulfurous lava will become much stickier and viscous the hotter it gets and flow more easily as it cools. Io’s dominant mossy or olive tint is likely the coolest part of its crust. The wide spectrum of colors points to ongoing chemical reactions, ones that any alchemist might marvel to have spontaneously combust in a crucible. The recipe for Io may read: mix sulfur, oxygen, silicon and iron, add fire and ice, cook under the gravitational pressure of Jupiter and its family of nearby moons. Wait a few billion years, the crucible yields Io.

Most critical to astrobiologists studying Jupiter’s moons, the eccentricity or oval shaped orbits of Jupiter’s moons are pumped or oscillated by tidal forces as they orbit. This input of Jupiter’s gravitational energy heats up the inner moons particularly like Io without relying only on the Sun’s radiant heat, and thus gives an interesting way to provide one of the three ingredients for life–an energy source–even if far from the Sun.

What remains to be found among the Giant Planets like Jupiter and Saturn are some candidates that combine all three ingredients for primitive life: energy, liquid water and some atmosphere. Only Saturn’s moon, Titan, has an appreciable atmosphere, and only Jupiter’s Europa or Ganymede have any indications of water ice. But uniquely powerful tidal forces around the Giant Planets do offer some promising, non-radiant and non-volcanic heat sources.

In this picture, one looming Ionian mystery remains: how can the fragile, dominant element –sulfur– support mountains of such enormous heights as those observed towering above Mount Everest’s scale? The ash should collapse under its own mountainous weight.

But whether Io offers a gooey, brittle surface or not, the most surprising part may have to do with its daily temperature swings so far from the Sun. It has long been supposed that beyond about ten percent further than earth’s distant to the sun, heat would be hard to find. The outer reaches beyond Mars are either rocky-frozen or gaseous entirely. But the Jovian family of moons have energy sources not dreamt of in our philosophy–tides, volcanoes, and a daily temperature that may offer a thermal mix of extremes. Io’s volcanic plumes top out the moon’s local heat index, reaching temperatures in a plume of over 2,000 degrees Fahrenheit , to over 4,000 degrees on the surface near a lava melt. Stray far beyond a vent though, the temperature can plunge to minus 260 degrees.

Only a third the size of Earth and five times as far from the Sun, Io generates twice our total terrestrial heat bill.

The largest volcano on Io–and the largest in the solar system–spans a cavernous 125-mile lava pit. The caldera would swallow a state the size of Arizona in oozing magma. Named Loki, after the Norse thunder god, this single volcano generates the equivalent heat to a third of the entire Earth’s global balance. Underneath the crust lies a core of iron and iron sulfide. This molten center is likely about half the moon’s total observed size. This iron core rotates internally as Io’s orbit passes through the intense magnetic fields that circumscribe Jupiter. For NASA’s Galileo spacecraft to fly within a few hundred miles of Io, the probe frequently fell into safe-mode–a form of self-protection for its sensitive electronics to survive both the magnetic and radiation of the Jovian system up-close. The light from a single volcanic blaze will saturate its camera. Living to tell-the-tale after going to the edge of an Ionian volcano is one journey fraught with danger and sacrifice. The high-rising volcanoes of Io may distract from the real risk of a flyby. What causes the volcanic calderas and sulfur plumes has much to do with the unseen–the invisible radiation, magnetism and tides that Jupiter and its family of moons have grown accustom to as part of their daily orbits.

To flirt with Jupiter is to find a new source of energy far from the Sun. This mini solar system defines its own rules –the gadfly sent to curse Io has much to do with the laws of orbital mechanics, but there is no disguising this moon as any plain white bovine. Instead, the fiery ball offers a palette of chemistry, color, and thermal collage.

For those expecting to find the consistency and constancy of our own Moon, the only constant on Io is change.

Related Web Pages:

Galileo Home Page
Io Blasts Mile-High
Galileo’s Spyglass
Life and Giant Planets
Jupiter: Moon Festival
Jupiter’s Perfect Storms
Salty Volcanoes on Jovian Moon