• Io is pronounced EYE-oh, also pronounced EE-oh by some authorities.
• Closest of the four Galilean moons to Jupiter.
• About the same size as earth's moon.
• Has active, violent volcanoes.
• Average distance from Jupiter is 421,800 km.
• Revolution is 1 day 19 hours.
• Diamter is 3,630 km.
• Composed of silicates and sulfur.
• Geology and chemistry:
  • Not dominated by the silicate soils of the inner planets.
  • Nor by the ices of the moons of the giant planets.
  • Geology and chemistry of volcanic, sulfur-rich compounds.
• In 1979, close-up pictures from Voyagers 1 & 2 showed:
  • Io appeared to be mottled with orange, yellow, red, and white patches.
  • These patches appeared to be pocked with blackish spots.
• Only a single week prior to Voyager's photographic flyby, California planetologist Stan Peale and and his colleagues had this prediction:
  • Any satellite close to a planet is slightly stretched into a sort of foootball shape with the long axis toward the planet, because of the gravitational pull of the planet.
  • In Io's case, the additional gravitational attractions of neighboring moons keep forcing Io to vary its distance form Jupiter, causing the stretching to increase and decrease.
  • This constant flexing of Io heats it sinterior, just as a tennis ball will get warm if you keep flexing it.
  • The unique action of neighboring moons on a given moon explains some unusual geologic features on certain other moons as well.
• Icy moons like Io were previously believed geologically dead by researchers.
• Io, far form being dead, is a volcanic powerhouse!
  • Voyager 2's flyby revealed that at least 6 of the 8 vents revealed by Voyager 1 (4 months earlier) were still active.
  • Two new ones also appeared on the scene.
  • Most of the blackish spots were determined to be volcanic calderas (irregularly shaped craters left by volcanic eruptions).
  • Io maintains continuous simultaneous major eruptions.
• Normal surface of Io has a daytime temperature of about -148ºC (-234ºF).
• Numerous warm spots have temperatures around 327ºC (621ºF).
• Voyager data suggest outbursts as hot as 427ºC (801ºF).
• Outbirsts could thus involve molten sulfur (melting point 112ºC, or 234ºF)
• Io's volcanism explains the absence of ice on its surface.
  • Jupiter's other big moons are all ice-rich.
  • Intense volcanism has melted crustal materials.
  • These crustal materials have been erupted.
  • Then covered by later eruptive debris.
  • Then recycled through a repetitive sequence.
  • Volatile materials, such as water, boiled off long ago.
  • Heavy materials, such as silicate rock, sank into the lower layers of Io.

Interestingly, after the volcanoes were discovereed by Voyager, observers at Mauna Kea Observatory in Hawaii were able to detect them with advanced, heat-sensitive, infrared detectors. This technique confirms that volcanoes continue to erupt at different vents on Io, year after year.

Io has some of the most stunning visual effects in the solar system. Imagine a full "day" on Io, experienced from a single point on its surface. One side of Io always faces Jupiter. On that side, Jupiter dominates the sky, always hanging in the same place. Jupiter subtends an angle of about 20 degrees - 40 times the angular size of the moon in our sky.

Because Jupiter covers a large part of Io's sky, the sun spend nearly two and a half hours of each day in total eclipse behind Jupiter. When the eclipse ends, the sun comes out from behind Jupiter and begins to warm the landscape, which has cooled markedly during the eclipse. At this moment, another of Io's unusual phenomena occasionally becomes visible: the so-called post-eclipse brightening.

In the mid-1960s astronomers monitored the brightness of Io before and after these eclipses, hoping to discover whether Io had an atmostphere. They hoped that during the cold eclipse period, frost might condense and form whitish deposits if the temperature dropped low enough. Calculations suggested that if several large volcanoes are erupting during an eclipse, enough sulfur dioxide might condense on orange or red backgrounds to brighten the overall appearance of Io during the few minutes after the eclipse ends. Sure enough, astronomers observed a brightening of Io, which fadeda bout 15 to 20 minutes after the eclipse ended, as the surface warmed. A mystery surrounds these observations. The post-eclipse brightening is seen after some eclipses, but not after all of them. Some astronomers claim the whole thing is due to faulty data; others suggest it may depend on the state of volcanic eruptions and the abundance of vapors they produce.

Some 10½ hours later, when Io has moved a quarter of the way around Jupiter, its sky develops a yellowish glow. this effect was first seen from Earth. The explanation is that a large, thin gas cloud of sodium, sulfur, and other atoms surrounds Io. The presence of this cloud has to do with Io's location inside Van Allen radiation belts around Jupiter.

Energetic atoms trapped in these belts strike Io's surface and knock other atoms loose. More may "come unglued" due to Io's volcanic eruptions. As these atoms escape Io, they diffuse into a cloud many Io-diameters wide, stretching forward and backward along Io's orbit. Sunlight striking the atoms excites them. Inparticular, if sunlight of a certain wavelength strikes the sodium atoms, they absorb and then re-emit this color - a glow called the sodium "D" line, familiar to us as the yellow color in most candle flames. the sodium cloud around Io glows with a faint yellow light - a sodium aurora that resembles a moderate aurora on Earth.

About 10½ hours later, Io is halfway around its orbit, between Jupiter and the sun. The sodium glow has slowly faded from the sky. Now Jupiter is in its "full" phase - a dazzling yellow, orange, red, and tan disk with brightly colored cloud patterns, such as the famous Red Spot storm system.

The sun has set at our location after another 10½ hours. Io has carried us three-quarters of the way around Jupiter, and the planet is in its "third quarter" phase. The yellow sodium glow is back in the sky, perhaps a bit brighter now, depending on the amount of volcanic gases emitted in the last few hours. In the cold of night, the sulfur dioxide frost may have formed again, and the landscap is dully illuminated by the yellow light of Jupiter itself.

So far, no one has stood on the plains of Io to see these views or feel the seismic tremors as its volcanoes explode. But if we humans manage to establish viable interplanetary travel before we imprison oursevles forever on a resource-exhausted planet, the day may come when heavily shielded from Jupiter's radiation, we will see the sights of Io not through spacecraft instruments or paintings, but with our own eyes.