Space Today Online - Moons of the Solar System
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Moons of the Solar System
Massive planet Jupiter below the moon Io
click NASA image to enlarge
A moon is a natural satellite rotating around a planet. While moons vary in size, each moon is much smaller than its planet. Almost 140 moons are known in the Solar System.
Several moons are larger than the planet Pluto and two moons are larger than the planet Mercury. There also are many small moons that may be asteroids captured by their planets.
Only Mercury and Venus do not have any moons. By comparison, Earth has one moon and Mars has two. Jupiter has the most of any planet. Saturn is second.
Pictures taken by the Hubble Space Telescope suggest there may be even more moons around those outer planets.
earth's moon »
Largest. The largest moon is Ganymede with a diameter of 3,280 miles, even larger than either of the planets Mercury and Pluto.
Saturn's moon Titan is the second largest in the Solar System with a diameter of 3,200 miles, half again as large as Earth's Moon.
The Planets and Their Moons
PLANET
Jupiter
62
Io, Europa, Ganymede, Callisto, Amalthea, Himalia, Elara, Pasiphae, Sinope, Lysithea, Carme, Ananke, Leda, Metis, Adrastea, Thebe, Callirrhoe, Themisto, Kalyke, Iocaste, Erinome, Harpalyke, Isonoe, Praxidike, Megaclite, Taygete, Chaldene, Autonoe, Thyone, Hermippe, Eurydome, Sponde, Pasithee, Euanthe, Kale, Orthosie, Euporie, Aitne, plus others yet to receive names
Saturn
33
Titan, Rhea, Iapetus, Dione, Tethys, Enceladus, Mimas, Hyperion, Prometheus, Pandora, Phoebe, Janus, Epimetheus, Helene, Telesto, Calypso, Atlas, Pan, Ymir, Paaliaq, Siarnaq, Tarvos, Kiviuq, Ijiraq, Thrym, Skadi, Mundilfari, Erriapo, Albiorix, Suttung, plus others yet to receive names
Uranus
27
Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Belinda, Puck, Miranda, Ariel, Umbriel, Titania, Oberon, Caliban, Sycorax, Prospero, Setebos, Stephano, Trinculo, plus others yet to receive names
Neptune
Triton, Nereid, Naiad, Thalassa, Despina, Galatea, Larissa, Proteus, plus others yet to receive names
Pluto
139
 
Saturn. When the Cassini spacecraft from Earth arrived at Saturn in 2004, it promptly found two previously unseen moons. They turned out to be the smallest bodies seen until then around the ringed planet.
The tiny natural satellites are about 2 miles and 2.5 miles in diameter. That's smaller than the city of Boulder, Colorado. Previously, the smallest moons seen around Saturn were are about 12 miles across. The moons are 120,000 miles and 131,000 miles from the center of planet Saturn between the moons Mimas and Enceladus.
The newly discovered bodies were labeled S/2004 S1 and S/2004 S2. Later, they will be given names. The NASA JPL team wondered if S/2004 S1 might not be an object called S/1981 S14 that had turned up in a 1981 Voyager image.
Saturn moons »
Smallest. The smallest moon is Deimos, at Mars, only seven miles in diameter, although its size now is rivaled by the small shepherd moons discovered by Cassini at Saturn and by others yet to be counted and named in the rings around Jupiter, Saturn and other giant gas planets in the outer Solar System. There may be tiny moons as small as only around a mile across.
Pluto. Charon is the moon closest in size to its planet, Pluto. Earth's Moon is second in that comparison.
Neptune. The interplanetary probe Voyager 2 in 1989 found six previously-unknown moons orbiting Neptune. They ranged in diameter from 33 miles to 250 miles. In 1991, they were named Naiad, Thalassa, Despina, Galatea, Larissa, Proteus.
The names Galatea and Larissa were controversial since asteroids previously had been given those names. Names are assigned by the nomenclature committee of the International Astronomical Union (IAU).
Triton. Before Voyager 2, astronomers knew Neptune had two moons, Triton and Nereid. With a surface temperature of –391 degrees, Triton was found by Voyager 2 to have a thin veneer of methane and nitrogen on top of water ice on its surface.
Triton had been thought to have a diameter of 2,361 miles, close in size to Earth's Moon, but turned out to be smaller, around 1,690 miles. Nereid  is 210 miles in diameter.
Pan. A new moon only 12 miles in diameter was discovered in 1990 circling the planet Saturn. It was Saturn's 18th and most distant moon. Photos of the planet, moons and rings, left over from the 1981 Saturn flyby by Voyager 2, had been filed away at NASA's Ames Research Center in Mountain View, California, for a decade. A planetary scientist was tipped off to Pan's presence by a disturbance in a 200-mi.-wide gap — Encke's Gap — in Saturn's outermost A ring. Pan's gravity moved particles in the large A ring, creating a gap with waves along the edges like the wake of a motorboat.
Checking the wavy edges, he calculated the probable position and mass of the moon and compared them with Voyager 2 positions and camera angles in 1980-81. When finally uncovered, the moon stood out as a small bright spot in 11 pictures among 30,000 photos scanned by a computer. In 1991, the moon officially was named Pan.
It was the second time since Neptune was discovered in 1894 that a gravity disturbance had been used to pinpoint a previously-unknown Solar System body.
Inner Solar System. The inner Solar System includes the planets Mercury, Venus, Earth and Mars. Of all the known moons in our Solar System, only three are in the inner region. Mars has two. Earth has one. Mercury and Venus are the only planets without moons. The outer Solar System includes Jupiter, Saturn, Uranus, Neptune and Pluto and the rest of the moons.
More moons are discovered each year in the outer Solar System by astronomers using ever more powerful equipment.
Atmospheres. Most moons are airless, but Jupiter's Io, Saturn's Titan and Neptune's Triton seem to have atmospheres. Titan may be flooded with an ocean of liquid ethane. Triton may be covered by an ocean of liquid nitrogen. Io seems to have a thin sulphur dioxide atmosphere from volcanos.
Titan. Titan appears to have an organic chemistry in its atmosphere which may resemble the primitive Earth before the dawn of life. It is the only moon known to have a thick, organic-rich nitrogen atmosphere. Surface temperature is around –290 degrees Fahrenheit. Solar System's moons are listed below:
Exploring the Moons of the Outer Solar System
The moons of the outer planets are popular targets with planners of future spaceflights.
Jupiter has more moons than any other planet. Its best known moons are the four large planet-sized bodies Io, Europa, Ganymede and Callisto.
jupiter »
Callisto is the outermost of the four and the most heavily cratered body in the Solar System. It may have a vast water ocean indside.
Io is innermost of the four. This sulfurous moon is the most volcanic world in our Solar System. It is peppered with hundreds of volcanoes, several of which might be active at any given moment.
Europa has an icy surface that may hide a layer of warm slush or even liquid water. That would make it one of the few places in the Solar System with that life-sustaining liquid.
Ganymede is the largest moon of Jupiter and the largest moon in the Solar System. In fact, Ganymede is larger in diameter than the planet Mercury and much larger than the planet Pluto. Ganymede has its own magnetic field.
Saturn has some unique moons. Here are some examples:
Titan is Saturn's largest moon.
Mimas has an enormous crater – probably a hole caused by a meteorite.
Enceladus has a very bright surface. It may have active water volcanoes.
Tethys has a canyon running three-quarters of the way around its surface. That means it may once have been split almost in half.
Dione and Rhea have wispy white markings on one side.
Hyperion is oddly shaped and the length of its day is constantly changing.
Iapetus is half bright and half dark.
Pan, by comparison, is a very tiny moon. Its diameter is only 20 km, which on the sale above would be only half the size of a BB.
Uranus' moons are eye-openers. They have statuesque mountains towering more than ten miles high. Incredibly deep valleys. Vast plains, some with a mysterious dark surface. Moon diameters range from a bit fatter than 25 miles up to about 1,100 miles.