Io is only slightly larger than Earthís Moon at 2250 miles in diameter and was nothing more than a star until the two Pioneer spacecraft flew past it during 1973-74. The Pioneers revealed Io to be a small, fuzzy disc, but it displayed unusual colors from red to orange to yellow, which hinted that it was no ordinary moon. Voyager 1 arrived at Jupiter during March 1979 with far better optics and as luck would have it, Voyager 1 passed very close to Io. As Voyager 1 approached Io it took photographs of a strange world that resembled a bad pizza. There were strange dark spots and one huge, red, heart-shaped feature among several blotches of orange, yellow, and white. There were no craters to be seen anywhere when it had been theorized that such a frigid world at -250ļF should be covered with well-preserved craters. 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 and a total of eight actively erupting volcanoes were discovered during the Voyager 1 flyby! The riot of colors 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 the hot, black molten sulfur.
Voyager 2 flew by Jupiter during July 1979 and immediately detected changes in Ioís volcanic activity since March. One volcano, Pele, stopped erupting, while two others increased in intensity. Several surface features changed appearance in hardly four months! It was determined that powerful gravitation tugs by Jupiter and Ioís neighboring large moons: Europa, Ganymede, and Callisto, raised tides on its surface as high as 300 feet! Such flexing heats the interior of Io to such a great extent that the lava spewed from the volcanoes and fissures is as hot as 3000ļF, twice as hot as Earthís lava! Io is so hot that there is no water ice to speak of, unlike the other moons that orbit Jupiter. By that time scientists were already planning another mission to Jupiter; Io was just one more reason to return. The spacecraft orbiter, later known as Galileo, orbited Jupiter for more than its planned two years and had several opportunities to explore Io up close.
Galileo, arriving at Jupiter on December 7, 1995, flew close to Io upon arrival, but no photographs were taken due to a balky tape recorder. This was unfortunate because Galileo was not allowed to fly close to Io again during its two-year primary mission due to the high radiation discovered near Io that was capable of disabling the spacecraft. Once the primary mission came to a successful completion, NASA was willing to risk Galileo for close encounters with Io, which rewarded them with close-up views of volcanic activity that showed huge changes in its lava flows and plumes. Two memorable changes were noted with the first being the huge volcano, Pele, between April and September 1997. The huge, bright red ring of fresh sulfur deposits around the volcano was broken by the eruption of a smaller volcano known as Pillan. It deposited a grayish ash probably composed of silicate dust over an area the size of Arizona. A year later the gray deposit was covered over by another layer of 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 flew over Tvashtar again on February 22, 2000 and the geyser had ceased, but another nearby fissure was erupting with the beautiful orange glow of lava. A huge volcanic eruption followed later in 2000 with a plume reaching 250 miles high, the highest ever observed! It was so intense that it could be seen from Earth with an infrared filter at the Keck Observatory. Tvashtar was still the most highly active region on Io when New Horizons flew past it during February 2007 on its way to Pluto. A huge plume over 200 miles high was seen near the limb along with long rivers of lava glowing on the night side.
Juno is the next planned mission to Jupiter and is scheduled for launch on August 5, 2011 with arrival on August 2, 2016. It will be a polar orbiter that will concentrate mainly on Jupiter and its weather. Unfortunately it will orbit too close to Jupiter to observe Io in great detail. However, scientists have designed a spacecraft that would thoroughly explore Io in fine detail much like Cassini has been exploring the geysers of Saturnís moon, Enceladus. The flight plan is similar to Cassiniís, therefore a dedicated spacecraft for Io is doable with less risk as it will involve proven technology. The spacecraft is called the Io Volcano Observer (IVO) and its goal is threefold:
1. Understand Ioís volcanic processes.
2. Understand the process of tidal heating.
3. Understand the loss of material from Io and follow that material to the magnetosphere, plasma torus, and neutral clouds.
The Io Volcano Observer will be a focused mission and will test the longevity of the next generation power source, the Advanced Stirling Radioisotope Generator (ASRG) as the current world supply of nuclear power for spacecraft, Plutonium 238, dwindles. This testing will assure that IVO would enjoy an extended mission. This would also allow for long term measurements of Jupiterís radiation belts, which will help mission planners with future spacecraft to Jupiter, especially the multi-billion dollar flagship that will someday fly to Europa.
If approved, launch of the Io Volcano Observer will occur during January 2015 with arrival at Jupiter during 2021. Upon arrival, IVO will go into orbit around Jupiter with plans to fly by Io at least six times. It will fly as close as 60 miles above the surface, lower than the tallest volcanic plumes, and will sample the thin atmosphere. Great care will be taken to avoid the plumes, but this mission will hopefully determine what the volcanoes are emitting and in what concentrations. The spacecraft will be able to determine how the volcanic emissions are dispersed in Ioís atmosphere and in orbit around Jupiter, especially within the powerful radiation belt near Io. IVO will be able to monitor the thermal characteristics of the surface and plumes through time along with the tidal pull on Ioís surface. Radiation poses a threat to IVO, especially the cameras, but all of the equipment will be radiation hardened. IVO will not go into orbit around Io, so it will not be constantly immersed in the radiation. IVO would also study Jupiter and the other moons, which will compliment the data from Juno.
It is not a sure bet that the Io Volcano Observer will be approved, but it is an enriching, focused mission that will try to unravel the mysteries of a world tortured by volcanoes. Io is a fascinating world where geology is much like the weather on Earth. If we lived on Io, there would need to be a geology report every day with all the constantly changing volcanic activity. IVO will certainly monitor a world that changes before our very eyes.