Right next to the fifth and largest planet in our solar system, Jupiter, orbits a moon the same size as Earth's moon. On this moon there is an icy surface that scientists are trying to discover. Due to the presence of what they believe to be water, there is the possibility of life on this moon called Europa. In 1609, Galileo Galilei, using the "telescope" which allowed you to see things closer than they appeared, created a first version of the telescope. With it, he observed the skies like no one had ever done before. He discovered that the Moon is not perfectly globular, it has craters, the Sun has sunspots, Venus orbits the Sun (contrary to widespread belief in his time), and then he observed four "stars" around Jupiter ("Our Solar System" ). Within a few days he realized that these objects were not stars, they were moons. Io, Ganymede, Castillo and Europa are collectively known as the Galilean Moons or Satellites. During the 19th century, the first measurable physical studies of these moons became possible when Simon de Laplace derived satellite masses from shared gravitational perturbations, and subsequently other researchers used a new generation of telescopes to measure the mass of these moons. The data collected showed that the density decreased from the inner to the outer satellites. According to Adam Showman, "More recent observations of water ice on the surfaces of the three outer moons have led to the inference that the compositions of the satellites range from Io's predominantly silicate rock to 60% silicate rock and 40% volatile ices (in mass) to Io. Ganymede and Callisto” (77). Voyager's close flybys of Jupiter in 1979 revealed indications of extensive geologic activity such as the fractured terrains of Europa, which likely result from tidal heating and bending. . half of paper... nal Geographic. National Geographic Society, 12 December 2013. Web. 2 May 2014. Helmenstine, Anne M. "Chemistry of Colored Snow: Causes of Colored Snow." . 2 May 2014. "Our Solar System: Galileo's Observations of the Moon, Jupiter, Venus, and the Sun." Exploring the Solar System: Science and Technology: Scientific Features: Our Solar System: Galileo's Observations of the Moon, Jupiter, Venus and the Sun. NASA and Web. 28 April 2014. Showman, Adam P. and Malhotra, Renu "The Galilean Satellites". Science 1 October 1999: p77-p84