Moons and Exomoons

Io, the third largest moon of Jupiter, as photographed by the Cassini spacecraft. Although Io is slightly larger and more massive than the Earth's moon, it is dwarfed by its enormous host planet. In this image, Io looks as if it could be swallowed by an eddy in Jupiter's clouds. This is no illusion, since Jupiter is more than 20,000 times as massive as its moon. Io's terrain is constantly torn by volcanic eruptions, whose lava flows give the surface its characteristic yellow-orange hue. Image courtesy NASA/JPL-Caltech.

Multiple moons have long been a signature of otherworldly scenes. The image on the left (courtesy NASA/JPL-Caltech) was created to accompany recent news releases to the effect that vegetation on otherwise Earth-like planets might not be green. The image on the right (courtesy European Space Agency & D. Neate) appears on the ESA Web site to illustrate the theories of Alain Leger and colleagues regarding "Ocean Planets" (Leger et al. 2004). In this case, the watery world seems to be a large moon orbiting the ringed gas giant visible above the horizon. The giant host planet is also accompanied by an additional moon on an interior orbit. Neither image seems primarily concerned with satellites; the hint of moons is merely an added extraterrestrial flourish. Each image implies that the host planet harbors at least two.

The tradition of double or multiple moons may have entered science fiction with the work of Edgar Rice Burroughs, whose first publication was a serialized novel entitled Under the Moons of Mars (1911). When the narrative was later published in book form, it became A Princess of Mars, the first in a series of 11 popular novels with the same exotic setting. Above is a detail of a drawing created by Burroughs' primary illustrator, J. Allen St. John, to accompany the fourth installment of the series, Thuvia, Maid of Mars (1920). A green barbarian has just abducted Thuvia, Princess of Ptarth, and now gallops down an avenue in one of the ruined cities that punctuate the Martian desert. Overhead are the obligatory moons, Cluros and Thuria (or as we say on Earth, Deimos and Phobos).

Deimos (left) and Phobos (right), the tiny moons of Mars, photographed against the stars from the Martian surface. These objects are far smaller and less dramatic than earlier generations imagined, but they still partake of the beauty inherent in most astronomical phenomena. Image courtesy NASA/JPL-Caltech.

Titan, the largest moon of Saturn and the second-largest moon in the Solar System, photographed in infrared light by the Cassini spacecraft. In ordinary light Titan presents a featureless sphere covered by impenetrable clouds, much like the planet Venus (see Terrestrial Extraterrestrials). Radar and infrared scans reveal a complex topography, with mountains of ice, and rivers and lakes of liquid methane. Internally, Titan is about 50% rock (concentrated at the core) and 50% water ice. Its nitrogen atmosphere is much thicker than the oxygen-nitrogen blend surrounding the Earth, while its orange cloud layer creates an "anti-greenhouse" effect that keeps Titan extremely cold. Image courtesy NASA/JPL/SSI.

This false-color image of Titan's surface, captured by the Cassini spacecraft, illustrates a small portion of the satellite's "Lake District." The lakes of Titan are actually pools of liquid methane in a landscape of grainy ice reminiscent of sand and gravel. Image courtesy NASA/JPL-Caltech/USGS.