GALILEAN   MOONS   OF   JUPITER

The four largest moons of the planet Jupiter, shown to scale

The four largest moons of Jupiter, shown to scale. Our own Moon is between Io and Europa in size, while Ganymede is slightly larger than the planet Mercury. See also Terrestrial Extraterrestrials. Image courtesy NASA/JPL-Caltech.

The four largest moons of Jupiter were first observed in 1610 by Galileo Galilei, using his newly invented telescope. This was the first instrumental detection of any celestial bodies. These objects have since become known as the Galilean moons in honor of their discoverer.

Beginning in Galileo's time, observers have noted a resemblance between the system of the four Galilean moons and the inner Solar System. In the former, Jupiter plays the role of the Sun, and Io through Callisto stand in for Mercury through Mars. More recent astronomical research confirms this intuitive resemblance. Just as the regular planets formed in an accretion disk around the Sun, the Galilean moons assembled in a dusty disk around Jupiter (Stevenson 2001, Canup & Ward 2002, Alibert et al. 2005c).

All four moons are composed of a mixture of rock and ice, which varies according to their distance from Jupiter. The closest, Io, is mostly metals and silicate rock; the next, Europa, is about 90% silicates and 10% water ice; the third and largest, Ganymede, is 60% rock and 40% ice; and the outermost, Callisto, is about 50% rock and 50% ice (Anderson et al. 1996, 1997; Canup & Ward 2002). This distribution implies that the accretion disk around Jupiter was cool at the outer edge but hot near the planet when the Galilean satellites formed. In the inner, hotter region of the disk, mostly silicates were available, while in the cool outer region water ice was abundant (Canup & Ward 2002, Alibert et al. 2005c).

As the moons assembled out of collisions between boulders and glaciers, their orbits also evolved together. All migrated inward toward Jupiter in such a way that the three inner Galileans entered into a stable clockwork relationship with each other (see animation, below).

The orbits of the three moons converged into a three-way mean motion resonance, such that the period of the second moon is twice that of the first, and the period of the third moon is twice that of the second. Callisto, the fourth moon, orbits just outside a similar 2:1 relationship with Ganymede, having formed and migrated more slowly (Canup & Ward 2002).



Animation of the orbits of Io, Europa, and Ganymede around Jupiter

Orbits of the Three Inner Galilean Moons

An animation of the three-way mean motion resonance that guarantees the stability of the Jovian system. Io orbits Jupiter in 1.77 days (about 43 hours). The next moon, Europa, completes an orbit in twice that period (3.55 days), while the third moon, Ganymede, completes an orbit in twice the period of Europa (7.15 days). In each case the orbital period defines the day/night cycle. All four Galilean moons always turn the same hemisphere to Jupiter because of tidal drag. Wikimedia image created 2006 by Splarka.



The composition and behavior of the Galilean moons can provide considerable insight into the morphologies of extrasolar systems as well as of our own Solar System. The detection of low-mass exoplanets known as Super Earths requires a theoretical explanation of their possible composition. Such explanations are often informed by investigations into the Solar System's icy moons (e.g., Kuchner 2003, Valencia et al. 2007). In addition, a number of exoplanet pairs exhibit mean motion resonances (e.g., Gozdziewski et al. 2006, Barnes & Greenberg 2006b), a relationship whose classical example remains the three inner Galileans.

Among Jupiter's numerous companions, the Galilean moons are unique. The orbital space around the giant planet is differentiated into three zones:

  1. An inner region of tenuous rings, less massive and far less visible than Saturn's, shaped and shepherded by the four irregular Ring Moons, which range in diameter from a dozen to about a hundred miles (Metis, Adrastea, Amalthea, Thebe). This region extends out to 138,00 miles (222,000 km).

  2. A broader central region containing the circular, evenly-spaced orbits of the four Galilean moons, from Io to Callisto. All these moons are spherical like planets and relatively large and massive, with varied terrains. Their orbits extend radially from 261,450 miles to 1,170,725 miles (421,700 km to 1,888,270 km).

  3. After a substantial gap ensues a still broader outer region, which contains more than 50 tiny, irregular moons on eccentric, inclined, and mostly retrograde orbits. In radial distance from Jupiter their orbits range from about 4,583,800 miles for Themisto to about 18,780,325 miles for Moon 63, which is not yet named (7,393,216 km to 30,290,846 km). All these moons are probably captured asteroids or Kuiper Belt objects. The orbital separation of the outer moons is extreme, on the order of 0.2 astronomical units (AU). This is half the distance of Mercury from Sol, and considerably larger than the orbits of many extrasolar planets around their host stars.

Like Jupiter, these satellites orbit the Sun at a semimajor axis of 5.2 AU in a period of 11.9 years. Like all large moons in the Solar System, the Galileans are low-density objects, so that their masses are smaller than their diameters might suggest. For example, Mercury's diameter of 3005 miles (4840 km) is smaller than that of Ganymede, but Mercury's mass of 0.06 times Earth is more than twice the mass of Ganymede. On the other hand, the Earth's Moon is very similar to Io in size and mass, with a diameter of 2154 miles (3474 km) and a mass 0.0123 times Earth. (All data from solarsystem.nasa.gov.)


Io is a rocky object more similar in composition to the terrestrial planets than to the icy moons of the four giant planets. Its interior is differentiated into a molten iron core surrounded by a mantle of silicate rocks. Gravitational stresses generated by the opposing perturbations of Jupiter and the other Galilean moons make Io one of the most volcanic environments in the Solar System. Its terrain has been continually resurfaced by lava flows over billions of years. Impact craters such as those observed on Mars, the Moon, and other giant planet satellites are not visible, having been buried by magma.

Many surface regions of Io are sulfurous in appearance, with yellowish and orange hues. Traces of an extremely tenuous atmosphere have been detected, composed mostly of sulfur dioxide outgassed by volcanic eruptions. Io has more than 400 active volcanoes, so many that simultaneous plumes of gas are often visible at varied locations across its surface. Quite separate from these violent regions are rocky highlands that feature at least 150 mountains, often in the form of sheer eroded mesas.


Europa contains a metallic core, an extensive rocky mantle, and a relatively thin crust of water ice. Its surface is extremely flat, without craters or mountains; the only notable feature is a network of dark cracks. This terrain is interpreted as a smooth shell of ice, and it is often speculated – though far from proven – that a global ocean of liquid water is concealed underneath.

The notion that Europa is the most likely haven of extraterrestrial life in the Solar System has become something of an astrobiological cliche. If the moon's interior is warm enough to sustain an ocean, and if the ice crust is thin enough to permit the passage of sunlight, then perhaps aquatic organisms have evolved. Future observations may be able to falsify this hypothesis, though positive proof would probably require a mission to Europa's surface.

Ganymede is the largest and heaviest moon in the Solar System, surpassing even the planet Mercury in diameter, though not in mass. It has a strongly differentiated composition, with a molten core of iron or iron alloys, an intermediate mantle of silicate rocks, and a surface layer of water ice about 500 miles (800 km) in thickness (Anderson et al. 1996). Ganymede has a trace atmosphere of oxygen but no observable weather patterns.

Like Earth, and unlike Mars, Ganymede sustains both a magnetic field and plate tectonics. Its terrain is distinguished by shallow, eroded impact craters; dark plains formed by ancient lava flows; and an extensive pattern of grooves that are interpreted as tectonic shears, formed when seismic activity caused differential movement and subsequent cracking in the icy mantle.


Callisto consists of a mixture of rock and ice in proportions not too different from those observed in Ganymede. However, measurements by the Galileo spacecraft establish that Callisto's interior is almost completely undifferentiated, in strong contrast to the layer-cake construction of Ganymede (Anderson et al. 1997). Callisto has no metallic core, no magnetic field, and no plate tectonics. It is also the least reflective of the Galilean moons, suggesting that its icy terrain includes substantial dust.

The surface of Callisto is saturated by cratering, meaning that every one of its craters either contains or intersects with another. These features were produced by meteor impacts in the early days of the Solar System, most likely during the period of Late Heavy Bombardment. Callisto is otherwise quite flat, without mountains, grooves, or ridges. Its topography has remained unchanged for billions of years, simply gathering dust. A trace atmosphere of carbon dioxide has been noted.










All text is copyright Raymond Harris 2006-2007. Credits for each image are listed in the accompanying caption.