On Volcano Moon Io, it Snows Sulfur Every Day

The small Jupiter moon's atmosphere undergoes a dramatic transformation when it passes into the gas giant's shadow.

Jupiter's moon Io is not only interesting in that it's the most volcanic place in the solar system, it also has a seriously weird atmosphere that collapses and re-inflates as it passes into Jupiter's shadow every single day.

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As Io orbits Jupiter, the extreme Jovian tides warp the moon so much that huge quantities of energy are generated, causing molten rock from the moon's interior to spew onto the surface, driving perpetual volcanic activity.

But that's not the only way the gas giant impacts Io.

Every Io day (1.7 Earth days) for 2 hours, Io passes into Jupiter's shadow, blocking the sun from heating Io's thin atmosphere (known as an "exosphere"). In new observations by the Gemini North telescope in Hawaii with the Texas Echelon Cross Echelle Spectrograph (TEXES) instrument, Io's exosphere has been shown to "collapse" for those 2 hour eclipses, causing the sulfur dioxide gas pumping from its volcanoes to freeze from the atmosphere and fall onto Io's surface as a frost.

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Though this atmospheric collapse and sulfur dioxide snow has been theorized before, this is the first time the phenomenon has been observed from Earth and was only possible by Gemini and TEXES's sensitivity to the faint infrared glow produced by Io's exosphere as it passes into Jupiter's shadow.

"This research is the first time scientists have observed this phenomenon directly, improving our understanding of this geologically active moon," said Constantine Tsang, of the Southwest Research Institute (SwRI).

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As Io descends into darkness, its atmosphere deflates when the temperature drops from -235 degrees Fahrenheit (-148 degrees Celsius) to -270 degrees Fahrenheit (-168 degrees Celsius). This cycle repeats every Ionian day for 2 hours, causing the moon's pockmarked surface to be covered in a sulfur-rich frost. As Io emerges from the eclipse, the sulfur dioxide ices sublimate back into the atmosphere as a gas once more, re-inflating the exosphere.

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"This confirms that Io's atmosphere is in a constant state of collapse and repair, and shows that a large fraction of the atmosphere is supported by sublimation of SO2 ice," said John Spencer, also from SwRI. "Though Io's hyperactive volcanoes are the ultimate source of the SO2, sunlight controls the atmospheric pressure on a daily basis by controlling the temperature of the ice on the surface. We've long suspected this, but can finally watch it happen."

Interestingly, Io pumps these volcanic gases into its orbit around Jupiter, which then interact with the gas giant's magnetic field, even filling the Jovian system with ions that contribute to the massive planet's radiation belts. These are phenomena that NASA's Juno mission is measuring, so studies into the atmospheric dynamics on Io could be critical to supplement Juno's eventual science treasure trove.

Source: SwRI

GALLERY: These Are the Missions That Have Journeyed to Jupiter

NASA's Juno spacecraft is due arrive at Jupiter on Monday, becoming only the second satellite in history to orbit the solar system's largest planet. Here's a look at the previous journeys to Jupiter.

Image: Juno spacecraft gets a tally-ho on Jupiter and its four largest moons -- Io, Europa, Ganymede and Callisto – on June 21, with 6.8 million miles to go. Credit: NASA/JPL-Caltech/SwRI/MSSS

Exploration of the outer solar system began in March 1972 when NASA's Pioneer 10 spacecraft blasted off from Florida to begin a 21-month long trip to Jupiter. The probe became the first to travel through the asteroid belt beyond Mars and the first to make direct observations of an outer planet in the solar system. Its primary mission was to study Jupiter's winds, magnetosphere, radiation belts and moons, especially Io. After flying past Jupiter in December 1973, Pioneer 10 headed off in the direction of the star Aldebaran, though it will take the intrepid robot 2 million years to get there. NASA lost Pioneer 10's signal in 2003.

Image: Progressively larger images of Jupiter were relayed back to Earth by the Pioneer 10 spacecraft in 1973. The most dramatic moment was after closest approach and after the spacecraft was hidden behind Jupiter. Credit: NASA

NASA's debut mission at Jupiter was followed in April 1973 by launch of Pioneer 10's sister satellite, Pioneer 11, which not only studied Jupiter, but also took advantage of the planet's gravitational pull to slingshot itself out to Saturn. During Pioneer 11's flyby of Jupiter in December 1974, the spacecraft made the first observations of the planet's polar regions and took the first close-ups of Jupiter's Great Red Spot, a massive storm that was first observed by telescopes on Earth 350 years ago. At closest approach, Pioneer 11 passed 26,570 miles over Jupiter's cloud tops, three times closer than Pioneer 10. Pioneer 11 was last heard from in 1995. It is headed toward the constellation Aquila (The Eagle) and will pass near one of its stars in about 4 million years.

Image: First image of Jupiter's polar region, taken by Pioneer 11, in 1974. Credit: NASA

Both Pioneer 10 and Pioneer 11 were outfitted with gold-plated aluminum plaques with diagrams of a man and a woman, the location of the sun and Earth in the Milky Way galaxy and other information as a message to any potential extraterrestrials who find the probes.

Credit: Pioneer Project/ARC/NASA

Armed with knowledge about Jupiter's intense radiation environment, NASA launched a pair of follow-on missions, Voyager 1 and Voyager 2, in 1977. (Voyager 2 actually blasted off first – on Aug. 20, 1977 -- followed 16 days later by Voyager 1.) Voyager 1 flew by Jupiter in March 1979, then reached Saturn in 1980. It became the first spacecraft to reach interstellar space in 2012.

Voyager 2 sailed past Jupiter in July 1979 and Saturn in August 1981. Voyager 2's mission was extended so that it could visit Uranus in 1986 and Neptune in 1989. It is presently in the outermost layer of the heliosphere, where the solar wind is slowed by the pressure of interstellar gas.

Astronomers used the twin Voyager spacecraft to discover that Jupiter has faint rings, lightning and a Great Red Spot that changes color. They also learned that Jupiter's moon Io has active volcanoes.

Image: Plume from erupting volcano on Jupiter's moon Io imaged by Voyager 1 in 1979. Credit: NASA

In October 1989, NASA's space shuttle Atlantis blasted off with the Jupiter-bound Galileo space probe aboard. It was only the second (and last) shuttle flight to deploy a planetary spacecraft. After an upper-stage rocket burn, a pass by Venus and two Earth flybys to pick up speed, Galileo finally reached Jupiter on Dec. 7, 1995. Unlike all previous missions to Jupiter, Galileo put itself into orbit for what turned out to be seven years of close-ups studies of the giant planet and its entourage of moons.

Data collected by Galileo provided evidence of an ocean buried beneath the icy surface of Europa, and possibly one on Callisto as well. It also showed that Jupiter has thunderstorms many times larger than Earth's and that Ganymede, the largest moon, has its own magnetic field.

Image: The puzzling geology of Jupiter's ocean-bearing moon Europa comes into sharp focus in images taken by the orbiting Galileo spacecraft in the late 1990s. Credit: NASA/JPL-Caltech/SETI Institute

Galileo carried a companion probe that was released about six months before the orbiter's arrival for an independent free flight into Jupiter. Slicing into the planet's atmosphere at 106,000 mph, the probe descended about 100 miles into planet's atmosphere, relaying data for 58 minutes before it was crushed, melted and/or vaporized in the intense heat and pressure. During its descent, the probe measured winds of 450 mph – stronger than anything on Earth. Initially, the probe's data showed that Jupiter was drier than expected, but scientists later realized it descended in what was later referred to as a "hot spot."

Image: Artist's impression of the Galileo probe descending into Jupiter's atmosphere on Dec. 7, 1995. Credit: NASA

Several missions, including NASA's Ulysses solar probe, the Saturn-orbiting Cassini spacecraft and the Pluto-bound New Horizons, took advantage of Jupiter's immense gravitational pull to pick up speed or adjust their flight paths to reach their intended destinations. In exchange, NASA got some bonus science at Jupiter -- and some glorious pictures.

Image: Departing view of Jupiter taken by the Saturn-bound Cassini spacecraft on Jan. 15, 2001. Credit: NASA/JPL/University of Arizona