On January 7th 1610 Galileo Galilei gazed through his newly improved telescope and observed what he thought were three stars next to Jupiter. He thought they were really quite strange though because they were all lined up in a straight line around the great planet. He commented on this in his journal. The next night he looked again. He wrote down his astonishment that they were in different positions, and again around Jupiter. By 13th January, he wrote in his journal that they must be not stars, but little planets moving around Jupiter itself and following the giant planet as it travelled through the heavens.
The very first of these moons he named Jupiter 1, and went on to name them after Cosimo II, the son of Ferdinand and Christina (Ferdinand became Grand Duke of Tuscany in 1587), but as were the other three he discovered renamed in the mid 1800s, this, the first, was renamed Io. The other 3 moons which Galileo discovered are Europa, Ganymede and Callisto after the lovers of Zeus.
An astronomer called Simon Marius also independently discovered these same moons of Jupiter, and did so apparently just over one month before Galileo did. However he did not publish his observations as quickly as Galileo did, and so it was Galileo who was given the credit for the discovery. Marius had the last word though because he chose the names which Kepler had suggested in 1614 and these were the finally accepted ones. Perhaps a future spacecraft will be called Marius!
The discovery of these 4 moons of course was one of the great discoveries that provided the evidence for the heliocentric theory (the Earth orbiting the Sun) which later landed Galileo in a tricky situation, but that's an interesting story in itself and not quite as it is often told, another blog perhaps.
There was another Galileo, one that was launched on Friday 13th October 1989. It spent 14 years in space. it reached Jupiter in 1995 and spent the next 8 years in orbit around Jupiter. The story of this Galileo is amazing in itself. The NASA historian has written the following book which is free to
download, http://history.nasa.gov/sp4321.pdf
Galileo spent those 8 years in orbit around Jupiter studying Jupiter and its moons and especially the 4 Galilean moons.
The Voyager missions had already discovered that Io is actively volcanic, but Galileo sent us the close - ups.
Voyager went to find the story and Galileo stunned the world!
Io is one hundred times more actively volcanic than Earth.
It has mountains that are higher than Everest.
Its average surface temperature is around minus 130 degrees C
The temperature of its volcanoes can be around 1650 degrees C
It truly is ice and fire.
The above photo was taken by Galileo (credit NASA) in 1997.
The plume you can see on the left rim is 86 miles high!
The inset bottom right is of the shadow of the airborne plume just to the left of the terminator line (separating light and dark areas of Io). You can see the volcano structure in the inset. This plume is on every Voyager photo from 1979 and is thought was active for at least 18 years up to the day this photo was taken! The volcano has been named 'Prometheus'.
Io is five times the distance of Earth to the Sun, how is possible for anything on it to reach those temperatures?
Well, it is about the same distance from Jupiter as our Moon is from us. Next time you look at the Moon imagine you are looking at Io, it's a little big bigger than our Moon. The difference, is that Jupiter is a giant!
Earth's gravity allows our Moon to orbit Earth about every 27.2 days. For Io to keep a quarter of a million miles away from its parent planet it has to whizz around Jupiter in just under 2 days. (1.77 days) Bear in mind that the distance it travels around its orbit is about 12 times the distance our Moon travels around Earth! The gravitational forces are huge on Io, it is pulled this way and that like a rope in a tug of war being pulled by Jupiter on one side and the moons Europa, Callisto and Ganymede on the other (Ganymede is the largest moon in the entire Solar System).
This continuous pulling on Io causes friction within, and causes it to heat up. In the same the way, continuously bending a wire this way and that causes it to grow hot. This is the cause of the intense geological activity on Io, with over 400 active volcanoes with plumes that can reach 300 miles above its surface.
Here's a question: Is what we have discovered in the moons of Jupiter and the moons of the other outer planets, something like a micro 'Solar System' within a Solar System? Jupiter has its own heat source, it gives out more heat than it receives from the Sun. It is orbited by some extraordinary moons, a growing number of which are thought to have subsurface oceans. Does the definition of planet need to be stretched to include moons around the giant planets. The next generation of exploration will, I think, prove that we need to look again at what we mean by the word planet!
One thing to ponder, the yellow moon Io is largely yellow due to all the sulphuric gases erupting from its volcanoes. It is often affectionately referred to as the 'Pizza Moon' or even 'Pizza Planet', but the similarity with a pizza ends with the name.
It actually smells like rotten eggs!
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