Procyon-Vulpecula’s Space And Astronomy Advent Calendar

Procyon-Vulpecula’s Space and Astronomy Advent Calendar

December 16th

WORLDS OF FIRE AND ICE Original full-sized image link: http://solarsystem.nasa.gov/images/JEGIC_520x306.jpg In 1610, Galileo Galilei looked up at Jupiter with the telescope, which had just been invented, and saw that it was surrounded by four moons which orbited around it. This proved that not everything had to orbit the Earth, as Aristotle and the Church had taught for centuries. The four Galilean moons of Jupiter were central to a major change in our understanding of our place in the Universe 400 years ago - and one of them may lead another major revolution in our understanding of our place in the scheme of things, if life is discovered there. Io, the closest of the four moons to Jupiter, is the one that looks like a multicoloured pizza. It’s made of rock and metal, like Earth, and is about the size of our Moon. By all rights it should have frozen solid inside, and cooled to the point where no surface activity was possible. But Jupiter’s intense gravity keeps flexing and stretching Io, heating it up like a squeezed stress ball, and melting its interior to make Io the most volcanically active body in the Solar System. Maps of Io are impossible to make, as the surface features change in a matter of years as they all get paved over by lava and ash! Some volcanoes spew molten silicate rock, like on Earth, while others spew molten sulphur - yes, brimstone. Io really is a lot like the traditional picture of Hell! Ganymede, the third of the four moons, is the largest - and the largest moon in the Solar System. Just bigger than Saturn’s Titan, Ganymede is much bigger than Mercury (but not as massive, as Mercury is made mostly of metal and Ganymede is made of ice and rock.) Ganymede’s icy surface is a mixture of ancient, cratered plains that haven’t been touched for aeons, and recently resurfaced areas that have been folded and faulted and had slushy ice from inside welling up below. Perhaps the gravitational effects of the other moons can occasionally squeeze Ganymede enough to produce a limited amount of heating inside. Ganymede is also the only moon in th Solar System to have an appreciable magnetic field, too… Callisto, the last of the four moons, is the second largest. This ball of ice is dead and cratered, and covered in impact scars. Enormous ringed basins mark its surface, and craters look white where they penetrate the dirty surface and let fresh ice from inside well up. This moon hasn’t seen any major changes besides meteorite impacts for over four billion years - the oldest surface of any planet or moon. Callisto also lies outside the radiation belts of Jupiter, so if humans ever visit the Jovian system, Callisto will be the safest place for us to set up camp. Europa, however, is the most intriguing. Europa is the second of the four major moons from Jupiter and the smallest. It is mostly made of rock, but covered in a thin shell of ice. Jupiter’s tidal heating cracks and flexes the ice, and new ice wells up from below to fill the cracks. Europa shows powerful evidence that something warm is moving beneath the ice - maybe warmer, slushy ice, or maybe an ocean of liquid water? The Galileo probe, which orbited Jupiter from 1995 to 2003, examined Europa closely and found evidence that the icy surface was thin - maybe a few hundred metres. But we know that some sort of water must go down deeper than that - a deep, liquid ocean? Europa may contain more liquid water than all the oceans of Earth combined. Clinching evidence for the ocean came in 1998, when the Galileo probe detected changes in Jupiter’s magnetic field when Europa passed through it. This implied an electrically conducting liquid was sloshing about under Europa’s icy plates - like a saltwater ocean. And the dirty, reddish-brown colour of the cracks? That appears to be organic molecules of some sort. With warm water, an energy source, and organic matter in abundance, Europa seems like one of the most likely places in the Solar System to look for life. If that happens, the Galilean moons will again overturn our understanding of our place in the Universe. Image credit: NASA/Galileo For more on the Galilean moons, go to: lasp.colorado.edu/education/outerplanets/moons_galilean.php Or, go look at them yourself! All four Galilean moons are easily visible through binoculars. Go find out where Jupiter is in the sky, and turn your binoculars to it. You’ll almost certainly see a few specks of light on either side of it - those are probably Io, Europa, Ganymede and Callisto.