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The Moon -

The familiar near side of Earth's moon. Orbiting Earth at an average distance of 384,400 km (238,900 mi), our moon is the largest natural satellite in the solar system relative to the size of its planet. Based on its mineral composition, scientists believe that the moon is around 4.43 billion years old. That's around the time Earth's core settled.

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Titan -

Titan is the largest moon of Saturn and the second-largest natural satellite in the solar system. Titan is the only moon wrapped in a thick atmosphere and has a surface of rock-hard water ice. Intriguingly, it also likely has a liquid water ocean beneath its surface. Image: NASA/JPL-Caltech/Space Science Institute image.

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Ganymede -

Jupiter's moon Ganymede is the largest satellite in the solar system (our own moon is only larger when set against the size of Earth). Otherwise, Ganymede is colossal, being larger than Mercury and Pluto, and only slightly smaller than Mars.  Ganymede also likely has a salty ocean underneath its icy surface, making it a potential location for life.

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Pan -

Pan, the innermost named moon of Saturn, appears like a lone envelope of ravioli. Named after the half-man, half-goat satyr from Greek mythology, this small moon was only discovered in 1990. Image: NASA, Matúš Motlo.

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Europa -

According to NASA, the Hubble Space Telescope has spotted water vapor on Europa—the sixth-largest moon in the solar system and Jupiter's fourth-largest satellite. It's hypothesized that a water ocean exists under Europa's icy surface. Image: NASA/JPL/DLR.

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Phobos -

Phobos, the inner and larger of Mars' two moons (the other being Deimos), is thought by scientists to be in a "death spiral" in that it's slowly orbiting toward the surface of Mars. The natural satellite is named after the Greek god Phobos, the personification of fear and panic, and the origin of the world phobia. Image: NASA/JPL-Caltech/University of Arizona.

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Enceladus -

Enceladus is an icy moon of Saturn, and the sixth-largest orbiting the ringed planet. It is an active moon that hides a global ocean of liquid salty water beneath its crust, which astrobiologists believe contains all the ingredients essential for life as we know it—water, organic compounds, and energy.

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Io -

With over 400 active volcanoes, Io is the most geologically active object in the solar system. Plumes of sulfur spew upward as high as 300 km (190 mi) from the surface of Jupiter's fifth moon, which even has lakes of molten silicate lava.

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Mimas -

Orbiting Saturn, Mimas is mostly made of ice water mixed with rock debris. The moon's enormous Herschel crater is named after the astronomer William Herschel (1738–1822), who discovered Mimas in 1789. After the release of the movie 'Star Wars' in 1977, Mimas was dubbed the "Death Star" for its eerie resemblance to the Empire's moon-sized space station.

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Callisto -

Callisto wins the prize for possibly the most highly cratered body in the solar system. Callisto is the second-largest moon of Jupiter, after Ganymede, and thought to be a long dead world, with hardly any geologic activity on its surface. It's one of the oldest landscapes in the solar system, dated back to at least four billion years. Image: NASA.

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Hyperion -

The largest known non-spherical moon in the solar system, Hyperion is probably a fragment of a much larger, ancient moon destroyed from an impact in the early solar system. It is a moon of Saturn and is distinguished by its porous, spongy appearance. Image: NASA/JPL/SSI/Gordan Ugarkovic.

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Atlas -

Saturn's flying saucer moon, Atlas is another solar system oddball. An inner satellite discovered in 1980, Atlas appears like a deep space UFO. Image: NASA/Jet Propulsion Lab-Caltech/Space Science Institute.

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Miranda -

The eleventh of Uranus' moons and the fifth largest, Miranda has a surface unlike anything in the solar system. Its broken, haphazard terrain features the Verona Rupes range—10,058 m (33,000 ft) tall and thought to be the highest cliffs in the solar system. They are visible at the bottom of the photograph. Image: NASA/JPL-Caltech.

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Triton -

Neptune's satellites are dominated by one massive moon: Triton. One of the most intriguing bodies in the solar system, Triton features a south pole composed of frozen nitrogen and methane. NASA believes this weird and remote world to be among the most promising places in the entire solar system to look for signs of life. Image: NASA/Jet Propulsion Lab/US Geological Survey.

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Lapetus -

One of the stranger moons of Saturn, Lapetus is distinguished by a half-light, half-dark surface. This two-toned anomaly has a bright hemisphere and a dark hemisphere. In fact, the bright hemisphere is 10-to-20 times more reflective than its counterpart.

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Charon -

Charon is the largest moon of the dwarf planet Pluto. It is technically the largest known moon relative to its parent planet in our solar system. But because Pluto is a dwarf planet, our own moon is still cited as the largest natural satellite. This mysterious and distant world was discovered in 1978. One of its defining geological features is Mordor Macula, a large reddish area near the moon's north pole named after the black land called Mordor in J.R.R. Tolkien's 'The Lord of the Rings.' Image: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker.

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Nereid -

Nereid, imaged here by Voyager 2 in 1989, is one of the outermost of Neptune's known moons. It is the third largest of the planet's satellites and is unique because it has one of the most eccentric orbits of any moon in our solar system: it is so far from Neptune that it requires 360 Earth days to make one orbit. Image: NASA.

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Epimetheus -

Epimetheus, a moon of Saturn, fascinates scientists because it shares an orbit with another moon of the planet, called Janus. Image: NASA/JPL-Caltech/Space Science Institute.

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Janus -

Janus and Epimetheus are both irregularly-shaped moons that orbit Saturn so close together that every four years the two satellites exchange orbital positions. Image: NASA/Jet Propulsion Laboratory/Space Science Institute.

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Epimetheus and Janus -

This complex interplay between Epimetheus and Janus takes around 100 days to complete and is akin to an orbital dance. Epimetheus (lower left) and Janus (right) are pictured about 40,000 km (24,854 mi) apart while circling Saturn. Image: NASA/JPL/Space Science Institute. 

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Methone -

A small egg-shaped moon of Saturn, Methone is remarkably smooth and completely devoid of craters. The tiny moonlet orbits between the much larger moons of Mimas and Enceladus. Image: NASA/JPL-Caltech/Space Science Institute.

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Rhea -

Rhea is the second-largest moon of Saturn after Titan. A huge icy body with a rocky core, Rhea is heavily cratered with bright wispy markings. Research suggests that it could be capable of sustaining an internal liquid-water ocean through heating by radioactive decay. Image: NASA/JPL/Space Science Institute.

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Dione -

Dione is the densest of Saturn's moons with the exception of Titan. Its icy surface includes heavily cratered terrain, with many craters larger than 35 km (21 mi) in diameter. Image: NASA.

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Umbriel -

Umbriel is the darkest of Uranus' moons, a twilight world that reflects only 16% of the light that strikes its surface. Its one luminous spot is the Wunda crater, located on the north pole of the moon. Its walls enclose a ring of bright material as yet unexplained by science. Image: NASA / Jet Propulsion Lab.

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Oberon -

Little is known about Oberon, the outermost major moon of the planet Uranus. Consisting of approximately equal amounts ice and rock, Oberon appears to have been primarily shaped by asteroid and comet impacts. Image: NASA. 

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Deimos -

The smaller of Mars' two moons, Deimos while still lumpy in appearance is less irregular in shape than its companion, Phobos. It's named after Deimos, the ancient Greek god and personification of dread and terror. Image: NASA/JPL-caltech/University of Arizona.

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Thebe -

Thebe bears the scars of a massive asteroid impact, the result of which is named the Zethus crater. This small, irregular shaped moon of Jupiter has had a hand in shaping the planet: dust ejected from its surface formed the Thebe Gossamer Ring around its gargantuan neighbor. Image: NASA/JPL.

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Proteus -

The second biggest moon orbiting Neptune after Triton, Proteus is one of the darkest objects in the solar system. Scientists theorize that Proteus might have been formed from debris as Triton was captured in Neptune's orbit. Image: Voyager 2, NASA.

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Ariel -

According to NASA, Ariel's surface appears to be the youngest of all the moons of Uranus. Its surface shows signs of more recent geological activity than other Uranian moons, most likely due to tidal heating—a process whereby orbital and rotational energy is dissipated as heat in either (or both) the surface ocean or interior of a planet or satellite. Image: NASA/JPL.

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Dactyl -

Rounding off this menagerie of moons is an alternative moon— Dactyl. It differs because it's the natural satellite to the asteroid Ida, located in the belt between Mars and Jupiter. Before its discovery, scientists had no evidence that asteroids could have moons.

Sources: (NASA Solar System Exploration) (CNN) (Space) (European Space Agency) (NASA Space Place) (Mary Ann Liebert, Inc.) (Johns Hopkins University Applied Physics Laboratory) (Icarus) (Go Astronomy) 

See also: Probing deep space: Journeys into darkness

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