Huwerl Thornton, Jr.
The next moon we are going to explore is Europa. Europa was discovered by Galileo in 1610. Europa is the second of the Galilean satellites. The Galilean satellites are the four satellites Galileo discovered orbiting Jupiter. Those four satellites were named Io, Europa, Ganymede, and Callisto. They were named after the four lovers and companions of the Greek god Zeus in Greek mythology. The Roman name for the chief god in mythology was Jupiter. Europa was a Phoenician princess who was abducted to Crete by Zeus. Zeus had taken on the form of a white bull. It is also Jupiter's fourth largest satellite. It is slightly smaller than the Earth's moon. It is similar in composition to the terrestrial planets. Europa is mostly composed of silicate rock. It has a thin outer layer of ice. The Galileo spacecraft recently sent back data that suggests Europa has an internal structure that is layered. It is also believed to have a small metallic core.
Europa's surface is unlike anything in our solar system. It is extremely smooth. There are few features that are more than 100 meters high that have been seen. Europa has very few craters. There have only been three craters larger than 5 kilometers in diameter that have been found. Scientists believe that this is an indication that Europa has a young and active surface. The precise age of Europa's surface is yet to be determined. The images of Europa's surface strongly resemble pictures of sea ice on Earth.
(See Figure 2)
(image available in print form)
Figure 2
Image Courtesy of NASA
The most striking feature of Europa is the series of dark streaks that criss-cross the entire moon.
(See Figure 3)
The larger streaks are about 20 kilometers across with diffuse outer edges and a central band of lighter substance. The most current theory of their origin is that they were produced by volcanic eruptions or geysers. Europa has been determined recently by the Hubble Space Telescope to have a weak atmosphere. Out of the many satellites in the solar system, only five others, Io, Ganymede, Callisto, Titan, and Triton are known to have atmospheres. The atmosphere of Europa is composed of oxygen, however, it is different from the oxygen in Earth's atmosphere. It is believed to not be biologic in origin. Scientists believe that the atmosphere is most likely generated by sunlight and charged particles hitting Europa's icy surface producing water vapor which is then split into hydrogen and oxygen. The hydrogen escapes leaving the oxygen.
(image available in print form)
Figure 3
ImageCourtesy of NASA
There have been several spacecraft that have flown by Jupiter to give us more data about the Galilean satellites. Pioneers 10 and 11 flew by Jupiter in 1973 and 1974. They sent back images that were low in resolution but yielded tremendous amounts of data. Voyagers 1 and 2 flew by Jupiter in 1979. The Voyager spacecrafts sent back tens of thousands of images of much better quality than the previous Pioneer spacecrafts. The Galileo spacecraft was launched and entered into orbit of Jupiter in 1995. The Galileo spacecraft has produced the most extensive data yet of Jupiter and its moons. The Voyager spacecrafts did not get a good look at Europa. However, the Galileo's primary mission is to focus on Europa. Galileo's first two close encounters of Europa generated images that supported earlier theories that Europa's surface is very young. The images also showed that there is some sort of activity occurring. There are also images that show regions of Europa that look a lot like pack-ice on polar seas during the spring thaws on Earth.
(See Figure 4)
Scientists believe that there is a layer of liquid water beneath the ice surface of Europa. Scientists believe that this subsurface ocean is anywhere between 50 and 100 kilometers deep.
(image available in print form)
Figure 4
Image Courtesy of NASA