When the moon meets the eye like a big pizza pie, it's Io
By Amanda Jermyn

Welcome to Io, the pizza moon, so named for its colorful surface of yellows, reds and browns. First discovered in 1610 by Galileo, Io is the third largest of Jupiterís moons and just slightly bigger than Earthís moon. In Greek mythology, Io was one of Zeusís lovers, so it seems appropriate that this moon orbiting Jupiter should be named in her honor, as Jupiter is the Roman name for Zeus.

Io is about 4.5 billion years old, roughly the same age as Jupiter. It is the fifth moon from the planet, which it orbits at a distance of about 262,000 miles. Io takes about 1.77 Earth days to orbit Jupiter, and its same side always faces Jupiter.

With more than 400 active volcanoes, Io is the most geologically active heavenly body in the solar system. This activity is the result of tidal heating from friction within the moonís interior as it is squeezed and stretched by gravity between Jupiter and the other Galilean moons, Ganymede, Callisto and Europa. The intense heat generated by the friction keeps much of the subsurface crust in liquid form. This molten rock seeks any available outlet to relieve the pressure, hence the multitude of volcanoes. The lava flows from the volcanoes are constantly filling in impact craters, creating a new smooth surface from the spread of molten rock. It is thought that this material is comprised mostly of silicate rock or sulfur and its compounds, which would account for Ioís colorful, mottled, pizza-like appearance.

Some of the volcanoes produce plumes of sulfur and sulfur dioxide that rise as high as 190 miles above Ioís surface. While the volcanoes can reach a scorching 3,000įF, Ioís surface averages about minus 202įF, and is coated with sulfur dioxide frost. The surface is cold because, despite its infernal interior, Io is very far from the Sun.

The tremendous tidal forces to which Io is subjected cause its surface to warp up and down by as much as 330 feet. Compare this solid tidal fluctuation to the liquid tidal fluctuations in Earthís oceans where the maximum difference between low and high tide is about 60 feet!

Another unusual feature on Io is its many high mountains, the result of tectonic stresses in the subsurface which lift up and tilt parts of the crust. The mountains are on average about 4 miles high, with the highest peaks being about 11 miles high. By comparison, here on Earth, the peak of Mount Everest is about five and a half miles above sea level.

Data from the Galileo spacecraft indicates that Io has a giant iron core that takes up half its diameter. The spacecraft also detected a large hole in Jupiterís magnetic field near Io, leading to speculation that Io might have its own magnetic field. In addition, its orbit cuts across Jupiterís powerful magnetic lines of force, turning Io into an electric generator, capable of generating an electric current of 3 million amperes. This current takes the path of least resistance along Jupiterís magnetic field lines to the planetís surface, creating lightening in Jupiterís upper atmosphere.

Io has a thin atmosphere comprised mainly of sulfur dioxide, and no water vapor, unlike the other Galilean moons. This arid moon, now the driest known object in the Solar System, formed in an area around Jupiter where water ice was abundant. So it may once have held water, and this, together with its internal warmth, could have made life possible. However, any water, and hence life, would have long since been eradicated by intense radiation from Jupiter.

Several spacecraft have flown by Io, starting with Pioneer 10 in 1973, followed by Pioneer 11, and Voyager 1 and 2. More recently, the Galileo, Cassini and New Horizons missions have provided extensive data and images of the pizza moon. It is hoped that the Juno mission, now headed towards Jupiter, as well as the Jupiter Icy Moon Explorer (JUICE), set for launch in 2022, will help expand our knowledge of Io.

Join the Springfield Stars Club on Tuesday, May 26th at 7pm at the Springfield Science Museum for a talk by Glenn Chaple on "Visual Observing of Variable Stars - An Addict's Story." A high school science teacher for 30 years, Chaple wrote for Deep Sky magazine and for Odyssey, an astronomy magazine for children. He writes a monthly column for Astronomy magazine and is the author of books on astronomy. His latest is Guide to the Universe: Outer Planets. Chaple has been a member of the American Association of Variable Star Observers since 1980. Refreshments will be served and the public is welcome. The meeting is free of charge for members, with a suggested donation of $2 per non-member. For more information please visit the Stars Club Ė Massachusetts page on Facebook at https://www.facebook.com/groups/929125583765113/ or the Stars website, reflector.org.