I can imagine Galileo sitting in the dark, peering through his telescope and taking careful notes on the objects he saw orbiting Jupiter.  Or maybe I saw it in some Space Channel documentary.  Either way, the fact that in 1610 Galileo used a 30 times magnifying telescope to discover these moons blows my mind. The insights on the structure and atmosphere of these moons presented at P53B: The Galilean Satellites: 400 years of Discovery II blew my mind all over again.

The Galilean satellites, or Jupiter’s moons as they are known to lay-folk like myself, were the first objects found orbiting something other than Earth or the Sun. The four Galiean moons, seen from top to bottom in NASA image at the right, are Io, Europa, Ganymede (the big daddy of the bunch) and Callisto. All are more than 3000 km in diameter. Though they may be large for moons, Jupiter dwarfs them, weighing in at five thousand times more massive and causing them extreme tidal stress. Jupiter is also radioactive, bathing its moons in high-energy electron beams and radio emissions.

The moon nearest to Jupiter, Io, has more than 400 active volcanoes, which spew silicate lava flows 10 to 100 times voluminous than any volcanic activity currently seen on Earth, making it the most geologically active solar system object. These volcanoes also create the atmosphere of the planet by spewing sulfur dioxide, says John Spencer, of the University of Hawaii. The gases in the atmosphere settle into the equatorial region of Io, causing auroras–which I’m dubbing “the equatorial lights”–in those regions, says Melissa McGrath of the NASA Marshall Space Flight Center.  The moon has either an Earth-like core or is made of a mushy, crystal-containing magma ocean and non-convecting core. Imaging data from the Galileo mission determined Io’s peak temperatures to be around 1600 C, giving it a high degree of melt and making a magma ocean more likely.

Don’t’ judge the ice-ball Europa by it’s shell, what’s really interesting about Jupiter’s second moon is it’s potential to contain life in the liquid-water oceans under its ice crust. While Europa’s icy surface dips to temperatures of 90 K, tidal forces from Jupiter and heat from  Europa’s core have most likely melted some of this ice, says Christopher Chyba, of the Woodrow Wilson School at Princeton University. The ocean water is probably brackish under the ice, which based on crater morphology is at least greater than 3 km thick, says Bill McKinnon, of Washington University. Despite it’s icy surface, Europa’s oxygen atmosphere, high availability of ‘life components’ and possible presence of energy sources, like hydrothermal vents and Jupiter’s radiation, make life there all the more likely.

Ganymede, the Jupiter’s third moon, might also contain water in an ocean “sandwiched” between ice layers, says Geoffery Collins, of Wheaton College. Also interesting about number three is the magnetic field generated from its liquid iron core. This field shields Ganymede’s equator from weathering and from Jupiter’s radiation. It also causes polar auroras similar to Earth’s.  Ganymede’s terrain is scarred with craters in some areas, while in others there seem to be tectonic forces pulling the surface apart.  It is clearly a moon caught in a war zone.

Callisto, the furthest moon from Jupiter, is also heavily cratered. Its atmosphere is made up of carbon dioxide and oxygen, similar to Earth’s, which erodes the surface. The erosion creates a dark covering that settles in the valleys and a lighter substance that settles on the edges of craters due to differential sedimentation rates, says Roland Wagner from the DLR Institute of Planetary Research

While they may be invisible to the naked eye, insights from these Galilean moons may teach us about our home world.

–Jennifer Welsh, UC Santa Cruz Science Communication Graduate Student