Io is a moon of Jupiter only slightly larger than Earth’s Moon at 2250 miles in diameter and appears nothing more than a star from Earth. Nothing much was known about it until Pioneers 10 & 11 flew past it during 1973-74. The Pioneers revealed Io to have unusual colors from red to orange to yellow, indicating Io was no ordinary moon, but the views were nothing more than a small, fuzzy disc. It was not until Voyagers 1 & 2 flew by in 1979 that mind-blowing images revealed Io to resemble a bad pizza. Voyager 1, arriving during March 1979, takes credit for resolving Io into a world of strange dark spots and a huge, red heart-shaped feature among several blotches of orange, yellow, and white. There were no craters to be seen anywhere when it was theorized that such a frigid world at -250ºF should be covered with them. The strange colors, unusual surface features, and lack of impact craters once again hinted that Io was a world unlike any other.

A young Jet Propulsion Laboratory (JPL) scientist, Linda Morabito, checked some images taken by Voyager 1 after it flew past Io in order to be sure that the spacecraft was on course. The black and white image of crescent Io revealed another fainter, fuzzy crescent extending nearly 200 miles from the edge of the limb on the daylight side. There was also a huge, white blob catching the sunlight on the terminator on the night side that resembled a huge cloud towering hundreds of miles above the surface. This blob could not possibly be a cloud as Io had no appreciable atmosphere and the faint crescent above the limb could not be another moon as nothing like it could be seen in other images. Further analysis quickly revealed that these were huge volcanic plumes; a total of eight actively erupting volcanoes were discovered during the Voyager 1 flyby! The riot of color was due to the various states of sulfur ranging from white sulfur dioxide frost to the typically stable and older yellow and orange sulfur surface, to the newly deposited, warm, red sulfur from the active plumes, and hot, black molten sulfur.

When Voyager 2 flew by Jupiter in July 1979 it detected changes in Io’s volcanic activity. One volcano, Pele, had stopped erupting, while two others increased in intensity. Several surface features changed appearance in hardly four months! An orbiter known as Galileo arrived at Jupiter on December 7, 1995 and deployed a probe into Jupiter’s atmosphere. The primary mission kept Galileo from orbiting close to Jupiter to avoid its intense radiation which kept it far away from Io, but once the primary mission successfully ended, a series of extended missions allowed Galileo to make close flybys of Io starting in 1997. The views were stunning with close-ups of volcanic activity that showed huge changes in its lava flow and plumes. Two memorable changes were noted with the first between April and September 1997 at the huge volcano, Pele. The huge, bright red ring of fresh sulfur deposits surrounding the volcano was broken by the eruption of a smaller volcano known as Pillan that had deposited a grayish ash probably composed of silicates over an area the size of Arizona. A year later the gray area was covered over by another larger red fallout from Pele as it erupted again. The second change occurred in the north polar region in an area known as Tvashtar Catena. A flyby on November 26, 1999, revealed a fiery geyser shooting lava several miles high that was so bright it was overexposed in the original photograph. Galileo’s flyover of Tvashtar again on February 22, 2000, showed the geyser had ceased, but another nearby fissure was erupting with a beautiful orange glow of flowing lava. This area was so highly active that plumes as high as 250 miles were seen with the heat pulse detectable from Earth with infrared detectors!

The intense volcanic activity is due to the powerful pull of Jupiter’s gravity and the passing orbital tugs of Europa, Ganymede, and Callisto creating tides as high as 300 feet! Such flexing heats the interior of Io so much that the lava spewed from the volcanoes and fissures is as hot as 3000ºF, twice as hot as Earth’s lava! Io is so hot and flexible that there is no water ice to speak of or craters, yet there are a few sharp-peaked mountains as high as nine miles!

Juno has been orbiting Jupiter since July 4, 2016, but is nearing the end of its mission. It was originally to explore only Jupiter but has been exploring the moons during its extended mission making several close passes of Io last year further confirming that many volcanoes remain active. Io generates twenty times more heat flow than Earth. Volcanoes are an important planetary process, but how it all works is still a puzzle. Where and how is tidal heat produced inside a planet or moon, how does the heat escape to the surface, and what effect is this process on these worlds? Io demands a fully dedicated mission all its own.

The Io Volcano Observer (IVO) is a Johns Hopkins Applied Physics Laboratory (APL) spacecraft being developed that would make at least ten close flybys of Io over 3.5 years while orbiting Jupiter. It would use a suite of specialized instruments to peer beneath Io’s bright, sulfur-covered surface, capture images and video of its volcanic activity, and track Io’s heat energy from the inside out. Following the flow of heat is key to understanding where the heat is being generated within Io. Tidal heating could be spread throughout the interior or concentrated closer to the surface. IVO would measure the gravity and magnetic fields around Io to sense what is happening inside. Jupiter’s powerful magnetic field sweeps over Io and any distortions in the field produced by currents inside the electrically conductive magma would give an indication of its existence, size, and depth. IVO would be able to correlate the magma with the surface features and determine the thickness of the crust and how it moves. This would add knowledge to how the Earth and other rocky planets evolved shortly after formation when they were cooling. IVO is planned to be in a long, looping orbit staying far away from Jupiter and its dangerous radiation for most of the mission, and time its Io encounters at the best locations at the precise time to collect the clearest observations of Io’s magnetic and gravity fields, and any wobble Jupiter causes, a good indicator of Io’s interior structure. IVO would come as close as 120 miles from Io’s surface, image 90% of the surface at a resolution of 900 feet, and as good as ten feet in select smaller areas of high interest. IVO would be capable of capturing movies of the erupting lava and plumes on each flyby and measure the flow of heat. The infrared data from a thermal mapper will help in the understanding of the lava temperature and chemistry. IVO would also be equipped with a mass spectrometer for sampling gases erupting from the volcanoes. Direct measurements of Io’s thin atmosphere and active plumes will create a record of the chemical history of Io and the Jovian system. An extended mission of about two years would allow ten additional Io flybys. IVO would be built to be radiation-hardened so it can survive longer near Io. The science objectives will determine: 1). The degree and distribution of melt in Io’s mantle. 2). Io’s lithospheric structure. 3). Where and how Io is losing heat. 4). Io’s orbital evolution. 5). The current rate of volatile loss from Io.

The 2030’s could be a great decade in the exploration of Jupiter and its moons beginning with the arrival of Europa Clipper in April 2030 with it flying close past Europa around four dozen times. JUICE will arrive the following year and go into orbit around Ganymede in 2034 for at least a year after multiple flybys of Europa, Ganymede, and Callisto. Both missions will also study the environment around Jupiter for possibly the entire decade if extended missions are possible. Callisto will also be a part of the action as China is planning a mission to orbit a spacecraft around it sometime during 2035-38. Only Io would be missing the action during the great decade, but if IVO is approved, it could make for a thorough, well-rounded super-mission with four spacecraft exploring all four moons and Jupiter at nearly the same time, or at least within a period of a decade. This creates a powerful observatory mission to study the intertwined tidal heating, orbital evolution, and magnetospheric processes of the Jovian system.

Io is a world so volcanically hyperactive that it literally turns itself completely inside out within ten million years! With its youthful, active surface, Io is a fascinating world to explore that excites the imagination. A great decade is ahead for planetary exploration and Io will inspire the young scientists of tomorrow.