The Galilean Moons

The Galilean moons are the four largest satellites of the planet Jupiter. Their names are Io, Callisto, Europa, and Ganymede. The Galilean moons have played an important role in the acceptance of the heliocentric model and thus in development of modern astronomy and cosmology. Many probes, including Pioneer 1 & 2, Voyager 1 & 2, and the Galileo probe, have visited and studied the Galilean moons over the past 40 years. They are still the objects of much interest in the astronomical community.

The Galilean moons get their collective name from their discoverer, Galileo Galilei. Between 1609 and 1610, Galileo used his improved telescope to make observations at unprecedented magnifications. This allowed him to see the Galilean satellites, which are invisible to the naked eye, for the first time. Initially, Galileo believed them to be simply stars in the background. However, further observations revealed that these new bodies were, in fact, orbiting Jupiter (Baalke). This finding was in opposition to the prevailing geocentric model, which had been in place since the time of Aristotle. The geocentric model held that all objects in the sky orbited the Earth, which was considered the center of the universe; the idea of another planet having satellites had thus far been unheard of. The discovery of such satellites supported the heliocentric model of Copernicus, who had almost 100 years earlier postulated that the Sun was actually the center of the solar system with the Earth and other planets orbiting it (Baalke). At the time, however, the geocentric model was regarded by the Catholic Church as canon. Thus, Galileo's revolutionary ideas were regarded as heresy and he was soon sentenced to house arrest by the Roman inquisition, where he would spend the remainder of his life (Johnston). The discovery of the four Galilean satellites, however, had dealt a serious blow to the geocentric theory and helped catalyze the transition to the correct heliocentric model in the following years.

A few years following their discovery, the Galilean moons were individually named after the mythological lovers of Zeus, the Greek equivalent of the Roman god Jupiter. The innermost satellite was named “Io”, who in Greek mythology was seduced by Zeus and subsequently turned into a cow to hide her from the god's jealous wife, Hera (greekmythology.com). Io orbits Jupiter at an average distance of about 421600 km, taking around 1.77 Earth days to complete its revolution. It has a diameter of 3630 km, making it the third largest of the Galilean moons and slightly larger than our own Moon (Io Fact Sheet). Unlike its icy companions, Io is thought to be composed of primarily sulfur, iron, and various silicates. It's ultra-thin atmosphere contains trace amounts of sulfur-dioxide (Showman and Malhotra, 6). Due to its proximity to Jupiter, Io has a synchronous rotation, meaning its rotational period is the same as its orbital period (In fact, most of the moons in the solar system, including our own Moon and the other three Galilean moons, have synchronous rotations as well).

Its closeness to Jupiter also causes Io to be subjected to intense tidal forces, radiation, and magnetic fields. These are responsible for a number of Io's unique and interesting properties. For one, the internal heat created by these tidal forces causes Io to be the most volcanically active body in the solar system. In fact, volcanic plumes have been observed rising as far as 300 km above the planet's surface (Planetary Society). Io's passage through Jupiter's magnetic field also turns the planet into a giant electrical generator capable of creating an electrical current of 3 million amps at 400,000 volts. The electrical current also sweeps off almost a metric ton of material from Io every second, creating a ring of ionized radiation known as a plasma torus (Solar System Exploration).

The second closest of the moons was named Europa, after the Phoenician noblewoman seduced by Zeus who (coincidentally) had taken the form of a bull (greekmythology.com). It orbits at a distance of roughly 670900 km and takes 3.55 days to complete its revolution (Europa Fact Sheet). Europa is composed of primarily ice and silicate rock with a metallic iron core. Its icy surface, while one of the smoothest in the solar system, is crisscrossed by a series of dark lines called lineae. It is thought that these lines are essentially cracks in the outer crust of the planet caused by tidal forces exerted by Jupiter. Recent observations have also revealed that Europa has an extremely thin atmosphere of molecular oxygen (Showman and Malhotra, 4-5).

Though it is the smallest of the Galilean moons (only 3138 km across), Europa is perhaps one of the most interesting and potentially important bodies in the solar system. The same tidal forces that make Io incredibly volcanic cause Europa's internal layers to heat up. This heat melts the bottom of the planets icy outer layer, creating a sub-surface liquid ocean under an outer crust of ice (Showman and Malhotra, 5). The thickness of this outer crust is currently a matter of debate in the astronomical community; estimates range from as thin as 3 km to as thick as 30 km (Malik). At any rate, the presence of liquid water, let alone an ocean of it, makes Europa the best candidate for harboring extra-terrestrial life in our solar system. Many scientists theorize that organisms similar to extremeophiles on Earth, such as the ones existing at the bottom of oceanic trenches or in subsurface Antarctic lakes, might be able to survive in such an ocean. Naturally, Europa is currently of great interest to astronomers and has been the destination of many proposed research missions.

The third Galilean moon was named Ganymede after the handsome Trojan prince abducted by Zeus to be the cup bearer of the gods (greekmythology.com). With a diameter of 5262 km, Ganymede is both the largest Galilean moon and the largest moon in the Solar System. In fact, it is even larger than the planet Mercury. It orbits Jupiter at a distance of 1070000 km and takes a little over seven days to complete its revolution (Ganymede Fact Sheet). Like Europa, Ganymede is thought to be composed of mostly rock and ice. While no atmosphere has been detected on Ganymede as of yet, it is speculated that it may also possess a thin atmosphere of molecular oxygen. The surface of Ganymede is heavily cratered and has a variety of geological features including mountains, valleys, ridges and troughs. It is mottled by light and dark regions, each with their own distinguishing characteristics. The dark regions are the most heavily cratered and are believed to be very old. The younger, lighter regions are highly grooved, a phenomenon thought to be caused by tectonic forces (Showman and Malhotra, 3).

Ganymede is unique amongst the other moons in the solar system; not only is it the largest moon, but it is also the only moon with a magnetosphere. Most astronomers believe that Ganymede's magnetosphere is generated in a manner similar to Earth's: conducting material on the inside of the planet (perhaps a molten iron/sulfur core) spins at a different rate than the outer layers producing a magnetic field (Showman and Malhotra, 4). The exact nature of Ganymede's magnetosphere is still mysterious, however, as objects with similar compositions in the solar system lack magnetospheres of their own .

The last Galilean moon was named Callisto after a nymph and priestess of the goddess Artemis (greekmythology.com). It orbits Jupiter at a distance of 188300 km and takes about 16.7 days to complete its revolution. It has a diameter of 4806 km and is the least dense of the Galilean moons (Callisto Fact Sheet). It is thought to be made of roughly equal portions of rock and water ice. Like Europa, Callisto may harbor a liquid ocean beneath its outer crust. This ocean is thought to lie between 80 and 150 km below the surface (Showman and Malhotra, 2). While scientists speculate that Callisto's subterranean ocean might harbor extremophile microbes, it is generally accepted that Europa has the greatest chance of supporting life.

Callisto is credited with being the most heavily cratered object in the solar system. The presence of so many craters indicates that any geological activity on the planet ceased long ago. In fact, it is believed that the surface is an incredible four billion years old, making it the oldest landscape in the solar system (solarviews.com). This geological stability, coupled with the fact that the planet orbits outside of Jupiter's harmful radiation belts, makes Callisto the best candidate among the Galilean moons for human exploration and colonization.

Works Cited

Baalke, Ron. "The Discovery of the Galilean Satellites." Project Galileo. JPL. 8 Oct. 2008 <http://www2.jpl.nasa.gov/galileo/ganymede/discovery.html>.

"Callisto Fact Sheet." Project Galileo. JPL. 8 Oct. 2008 <http://www2.jpl.nasa.gov/galileo/callisto/>.

"Callisto." SolarViews.com. 8 Oct. 2008 <http://www.solarviews.com/eng/callisto.htm>.

"Europa Fact Sheet." Project Galileo. JPL. 8 Oct. 2008 <http://www2.jpl.nasa.gov/galileo/europa/>.

"Ganymede Fact Sheet." Project Galileo. JPL. 8 Oct. 2008 <http://www2.jpl.nasa.gov/galileo/ganymede/>.

"Io Fact Sheet." Project Galileo. JPL. 8 Oct. 2008 <http://www2.jpl.nasa.gov/galileo/io/>.

"Io." Solar System Exploration. NASA. 8 Oct. 2008 <http://solarsystem.nasa.gov/planets/profile.cfm?object=jup_io>.

Johnston, George S. "The Galileo Affair." Astronomy Notes. 8 Oct. 2008 <http://www.astronomynotes.com/history/galileoaffair.html>.

"Jupiter's Moon Io." The Planetary Society. 8 Oct. 2008 <http://www.planetary.org/explore/topics/jupiter/io.html>.

Malik, Tarig. "New Estimate fo Thickness of Crust over Europa's Ocean." Space.com. 13 Nov. 2001. 8 Oct. 2008 <http://www.space.com/scienceastronomy/solarsystem/europa_icecrust_011113.html>.

Showman, Adam P., and Renu Malhotra. "The Galilean Satellites." Lunar and Planetary Laboratory. 1 Oct. 1999. University of Arizona. 8 Oct. 2008 <http://www.lpl.arizona.edu/~showman/publications/showman-malhotra-1999.pdf>.

"Zeus's Lovers." GreekMythology.com. 8 Oct. 2008 <http://www.greekmythology.com/myths/the_myths/zeus_s_lovers/zeus_s_lovers.html>.