(c) Anthony K. Grafton 2003

Our Solar System, Part 5: The Many Moons of Jupiter

Jupiter is by far the largest planet in our solar system. In fact, Jupiter has twice the mass of all the other planets in our solar system combined. Some scientists note that Jupiter can be thought of as a “failed” star since it is made up primarily of hydrogen and helium (the main elements that make up our sun) but never acquired enough mass to become a star.

Almost everyone is familiar with Jupiter’s Great Red Spot, which can be seen using relatively small telescopes. But fewer people know about the many moons around this giant planet. According to the most recent data, Jupiter has a total of 39 moons. Most of these moons are small and relatively uninteresting, but four of them, Callisto, Ganymede, Io, and Europa, can be seen from Earth with a normal pair of binoculars and hold special interest for scientists.

The four largest moons of Jupiter have certain historical as well as scientific value. The great scientist Galileo Galilei, who perfected the telescope about 400 years ago, was the first person to see these moons around Jupiter. His observations of these moons provided the first concrete evidence that our Universe was not Earth-centered, as was then widely believed. By communicating his findings and supporting the idea that the Earth orbits the sun (and not the other way around), Galileo was eventually condemned by religious authorities. Now, four hundred years later, there is a probe from Earth named for Galileo orbiting around Jupiter and studying its moons.

Although the four largest moons of Jupiter are all interesting to scientists, by far the most fascinating of these is the moon called Europa. Data from the Galileo probe indicates that Europa is covered with frozen water, and below this icy surface lies a vast ocean of liquid water. Because liquid water seems to be necessary for life (as we know it) to exist, many scientists are hopeful that Europa’s ocean may harbor life forms of some kind. Planning has already begun for unmanned missions to Europa that will drill through the ice to examine the waters of the dark ocean below. In fact, scientists are so interested in the possibility of life on Europa that next year, after its mission is complete, the Galileo spacecraft will be intentionally crashed into Jupiter to prevent the possibility that it could contaminate Europa.

Jupiter is usually one of the brightest objects in the night sky. Can you find it and use binoculars or a small telescope to see its four largest moons?