Io (moon)

• 1. Introduction

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  Io has over 400 active volcanoes, more than any other object in the Solar System. Several of them fire plumes of sulfur and sulfur dioxide as high as 500 km above the surface. This innermost of Jupiter's four Galilean moons is slightly larger than Earth's Moon, yet it carries the highest density and the strongest surface gravity of any natural satellite known. Its surface is painted in subtle shades of yellow, red, white, black, and green, leading early observers to compare it to a rotten orange or to pizza. How does a world only a little bigger than our own Moon end up the most geologically active body anywhere near us? Why does it have less water, by atomic ratio, than any known object in the Solar System? And how did a point of light, first glimpsed through a 20-power telescope in 1610, become a place where lava flows stretch more than 500 km and mountains rise taller than Mount Everest? The answers begin with a violent tug-of-war played out in Jupiter's gravity.

• 2. The Engine Of Heat

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  Tidal heating, not the decay of radioactive isotopes, supplies Io's internal warmth, the result of its orbital resonance with Europa and Ganymede. Io completes two orbits of Jupiter for every one by Europa and four for every one by Ganymede, a Laplace resonance that keeps its orbit slightly eccentric at 0.0041. Without that forced eccentricity, tidal dissipation would round out the orbit and leave a far quieter world.

  The tidal forces acting on Io are about 20,000 times stronger than those the Moon raises on Earth. As Io swings between periapsis and apoapsis, the vertical difference in its tidal bulge could reach as much as 100 m. Friction from this flexing melts a significant portion of Io's mantle and core. The energy produced is up to 200 times greater than radioactive decay alone could yield, released as a global heat flow of roughly 0.6 to 1.6 times ten to the fourteenth watts.

  Io's volcanoes do not sit where the simplest tidal models predict. They are shifted 30 to 60 degrees to the east. A study published by Tyler and colleagues in 2015 proposed that an ocean of molten rock beneath the surface, a mixture of molten and solid rock, could generate extra heat through the friction of its own viscosity. The same ability to make heat in a subsurface ocean raises the chance of life on bodies like Europa and Enceladus.

• 3. A World Of Fire

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  Loki Patera, at 202 km across, is the largest of Io's volcanic depressions and consistently its strongest volcano, contributing on average 25 percent of the moon's global heat output. These depressions, called paterae, generally have flat floors bounded by steep walls and average 41 km in diameter. Unlike calderas on Earth and Mars, they usually do not sit atop shield volcanoes, and at least half are bounded by faults or mountains. Some hold lava lakes whose crust overturns continuously, as at Pele, or episodically, as at Loki.

  Lava flows mark another major terrain, some stretching more than 500 km. Many of the largest, like those at Prometheus and Amirani, grow by the build-up of small breakouts atop older flows, resembling the inflated flows at Kilauea in Hawaii. The leading edge of the Prometheus flow moved 75 to 95 km between Voyager in 1979 and the first Galileo observations in 1996. A major eruption in 1997 produced more than 3,500 square kilometres of fresh lava and flooded the floor of Pillan Patera.

  Temperature measurements of Io's hotspots suggest at least 1,300 K and some as high as 1,600 K, pointing to basaltic lava with mafic to ultramafic, magnesium-rich compositions rather than the molten sulfur once inferred from Voyager images. The plumes come in two kinds. The largest, like those at Pele, Tvashtar, and Dazhbog, release dissolved sulfur and sulfur dioxide from erupting magma, dragging silicate pyroclastics with them and forming red rings more than 1,000 km in diameter. The other kind, at Prometheus, Amirani, and Masubi, forms when lava vaporizes underlying sulfur dioxide frost, producing bright circular deposits and ranking among Io's most long-lived plumes.

  On the 27th of December 2024, the JIRAM instrument aboard Juno observed an unprecedented outburst in the southern hemisphere, centred near 73 degrees south, 140 degrees east. It covered about 65,000 square kilometres, three times the size of Loki Patera, and produced a total power of 140 to 260 terawatts, likely the most energetic volcanic event ever recorded on Io, surpassing the Surt eruption of 2001 at about 80 terawatts.

• 4. Inside The Moon

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  Io's metallic core makes up about 20 percent of its mass, an iron or iron-sulfide center wrapped in a silicate-rich crust and mantle. With a density of 3.5275, Io is the densest regular moon in the Solar System, far above Ganymede and Callisto at around 1.9, and slightly above the Moon and Europa. Models from Voyager and Galileo measurements give the core a radius between 350 and 650 km if it is almost entirely iron, or 550 to 900 km for a mix of iron and sulfur. Galileo's magnetometer found no intrinsic magnetic field, suggesting the core is not convecting.

  Forsterite, a magnesium-rich mineral, makes up at least 75 percent of the mantle, whose bulk composition resembles L-chondrite and LL-chondrite meteorites. To support the observed heat flow, 10 to 20 percent of the mantle may be molten. Re-analysis of Galileo magnetometer data in 2009 revealed an induced magnetic field requiring a magma ocean 50 km below the surface, with further analysis in 2011 offering direct evidence. That layer, estimated at 50 km thick and about 10 percent of the mantle, may reach 1,200 degrees Celsius. Above it lies a lithosphere of basalt and sulfur, at least 12 km thick and likely less than 40 km.

  That picture is now contested. According to an international study published in the journal Nature in 2024, no magma ocean would exist on Io despite its many volcanoes and its tidal interaction with Jupiter. Using data from two recent Juno flybys, the scientists argued for an almost solid mantle beneath the surface. The 2024 outburst observations likewise support a model of regional magma reservoirs linked through a largely solid outer shell, like a large-scale sponge, rather than a globally molten layer.

• 5. Mountains And A Vanished Sea

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  South Boösaule Montes rises to 17.5 km, the tallest of Io's 100 to 150 mountains, which average 6 km in height and 157 km in length. Nearly all of them are tectonic, not volcanic. They form when compressive stresses at the base of the lithosphere, driven by the continuous burial of volcanic material, uplift and tilt chunks of crust through thrust faulting. Where mountains cluster, volcanoes are scarce, and where volcanoes dominate, mountains thin out, hinting at large regions of the lithosphere under compression or extension.

  Plateaus are the most common mountain form, large flat-topped mesas with rugged surfaces, while others are tilted crustal blocks with one shallow slope and one steep. Only a handful appear volcanic, resembling small shield volcanoes with slopes of 6 to 7 degrees near a central caldera, averaging just 1 to 2 km in height. Landslide deposits are common at the base of these mountains, and scalloped margins reveal sulfur dioxide sapping that weakens the crust.

  Io holds the least water of any known body in the Solar System. Its high density and compositional mapping point to little or no water, though small pockets of ice or hydrated minerals have been tentatively identified, most notably on the northwest flank of Gish Bar Mons. This dryness likely traces to the early Solar System, when Jupiter ran hot enough to drive volatiles like water out of Io's vicinity, but not hot enough to do so farther out where icy moons formed.

• 6. Tethered To Jupiter

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  Io acts as an electric generator, developing 400,000 volts across itself and driving a current of 3 million amperes. Releasing ions, it inflates Jupiter's magnetic field to more than twice the size it would otherwise have. Jupiter's magnetosphere sweeps gas and dust from Io's thin atmosphere at a rate of one tonne per second, carrying off ionized sulfur, oxygen, chlorine, sodium, potassium, sulfur dioxide, and sodium chloride dust.

  A neutral cloud of sulfur, oxygen, sodium, and potassium surrounds Io, reaching as far as six times its radius and filling its Hill sphere, the region where Io's gravity beats Jupiter's. Over a 20-hour span these particles spread into a banana-shaped cloud extending up to six Jovian radii. Some atoms become fast neutrals, keeping a velocity of 70 km per second against the 17 km per second orbital speed, and are flung away in jets.

  Io orbits within the Io plasma torus, a doughnut-shaped ring of ionized sulfur, oxygen, sodium, and chlorine that co-rotates with Jupiter at 74 km per second. The torus has three parts: a warm outer region, a vertically extended ribbon, and a cold inner region spiraling toward Jupiter. After an average of 40 days, particles from the warm torus escape and help inflate Jupiter's magnetosphere from within. During an encounter in 1992, the Ulysses spacecraft detected streams of dust-sized particles, averaging 10 micrometres and made mostly of sodium chloride, that Galileo later traced to Io.

  The Io flux tube couples Io's atmosphere to Jupiter's polar upper atmosphere, producing the auroral Io footprint and darkening the Jovian poles. When Io is visible from Earth, radio signals from Jupiter increase considerably.

• 7. The Thinnest Air

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  Sulfur dioxide makes up most of Io's extremely thin atmosphere, with traces of sulfur monoxide, sodium chloride, and atomic sulfur and oxygen. Maximum pressure ranges from about 0.3 to 3 nanobar, peaking on the anti-Jupiter hemisphere along the equator in the early afternoon when surface frost is warmest. On the night side it dips to 0.0001 to 0.001 nanobar. Temperatures run from the surface value at low altitudes up to 1,800 K higher up, where thin air lets plasma and Joule heating take over.

  Sunlight-driven sublimation of surface frost sustains much of the atmosphere, which is densest over the anti-Jupiter hemisphere and confined largely within 40 degrees of the equator. Because density is tied to surface temperature, the atmosphere partially collapses at night or in Jupiter's shadow, with an roughly 80 percent drop in column density. Observers have noted a post-eclipse brightening, as if the moon is briefly frosted, fading after about 15 minutes as the frost sublimates. In 2013 the Gemini Observatory directly measured this collapse and reformation.

  During eclipse, high-resolution images reveal an aurora-like glow, brightest near Io's equator rather than its poles. Lacking a magnetic field of its own, Io lets electrons traveling along Jupiter's field strike its atmosphere directly. The glow is strongest where the field lines run tangent to the surface, near the equator, because the gas column there is longest. These equatorial aurorae rock with the changing tilt of Jupiter's magnetic dipole.

• 8. From Galileo To Juno

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  Galileo Galilei first recorded Io on the 7th of January 1610 through a 20-power refracting telescope at the University of Padua, though his low magnification merged Io and Europa into one point of light. The next day, the 8th of January 1610, used by the IAU as the discovery date, he saw them as separate bodies. Simon Marius later claimed to have found the moons a week earlier, but Galileo published first in Sidereus Nuncius and is credited with the discovery. Marius's naming scheme, drawn from a 1613 suggestion by Johannes Kepler, named each moon for a lover of Zeus. Io was a priestess of Hera, daughter of the River Inachus. For centuries Io was simply Jupiter I, a fifth-magnitude point used to measure longitude, validate Kepler's third law, and make the first measurement of the speed of light.

  Close study began with spacecraft. Pioneer 10 and Pioneer 11 passed on the 3rd of December 1973 and the 2nd of December 1974, refining Io's mass and revealing a thin atmosphere and intense radiation belts. In 1979 Voyager 1 flew within 20,600 km, returning a multi-colored, crater-free landscape. Navigation engineer Linda A. Morabito then spotted a plume in one image, and analysis found nine, proving Io volcanically active, just as a paper by Stan Peale, Patrick Cassen, and R. T. Reynolds had predicted. Voyager 2 passed on the 9th of July 1979 and found seven plumes still active, with only Pele shut down.

  Galileo arrived at Jupiter in 1995 after a six-year journey, discovering Io's large iron core on its close flyby on the 7th of December 1995. Cassini observed a new plume at Tvashtar Paterae in December 2000. New Horizons, bound for Pluto, flew by on the 28th of February 2007, imaging a large Tvashtar plume. Juno, in orbit since the 5th of July 2016, made close flybys on the 30th of December 2023 and the 3rd of February 2024, both at 1,500 km, to map Io's gravity and look for surface changes. JIRAM has since confirmed activity at Amirani, Prometheus, Seth, Culann, Tvashtar, and others, some with eruptions exceeding 45 years. The Jupiter Icy Moons Explorer, launched in April 2023, will monitor Io's volcanoes from a distance before settling into orbit around Ganymede in 2031.