Ganymede and Callisto are the largest of Jupiter’s so-called Galilean satellites, the four moons of the giant planet that were discovered 400 years ago, in January 1610 , by Italian astronomer Galileo Galilei. Ganymede, the largest moon in the solar system, even bigger than the planet Mercury, boasts its own magnetic field and bears the marks of past tectonic activity . But Callisto, of roughly equal size and with a similar makeup of rock and ice, has neither a magnetic field nor an apparent history of tectonics–the moons’ geologic histories have proceeded very differently. [More]

Related PostsTop Empowered Patient tips for 2010The top ways to empower yourself and take your health care into your hands this year.

Relate…10 ways to get motivated in 2010A new decade is about to start, and you [...]

Original post by Scientific American Topic - Exercise and Fitness