General Astronomy/Planetary Moons

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Mercury has no moons.


Venus has no moons.


Earth has one moon, usually called "the Moon" and sometimes called Luna. It is the fifth largest moon in the solar system. Of all the moons of the eight planets, Earth's moon is by far the largest relative to its planet, with a diameter of 3476 km and hence a ratio to Earth's diameter of 0.2764. By comparison, the next largest moon relative to a major planet (Triton of Neptune) has a diameter ratio of just .0546. The Moon's diameter is over 50% greater than that of the dwarf planet Pluto.


Mars has two very small moons, both discovered by Asaph Hall. Deimos was discovered Deimos on August 12, 1877. Deimos, the outermost moon, was named for the Greek god of panic. Deimos has a diameter of just 16 km. The inner moon Phobos has a diameter of 26 km and is heavily cratered. Phobos was discovered on August 18, 1877. Phobos was named for the Greek god of fear. Deimos and has far fewer craters then Phobos and the craters are also much smaller.


Jupiter currently has 64 discovered satellites and 1 not identified object- like- satellite, but the four Galilean moons (Ganymede, Callisto, Io, and Europa) are by far the largest (in that order).


Ganymede is the largest planetary satellite in the solar system. With a diameter of 5268 km, it is about 8% larger than the planet Mercury. It is also the only moon in the Solar System with its own magnetosphere. Ganymede was discovered on January 7, 1610 by Galileo Galilei, along with Io, Europa, and Callisto.


Callisto is the second largest Jovian moon (that is, moon of Jupiter), and third in the Solar System (with a diameter of 4820 km). It is 98% of the size of Mercury. Callisto has the oldest surface in the Solar System (this means is the most heavily cratered body in the Solar System) even older than Mercury's. Callisto was discovered in 1610, along with Io, Europa, and Ganymede.


Io (pronounced EYE-oh) is the third largest Jovian moon and the fourth largest moon in the solar system. It is slightly bigger than Earth's moon, with a diameter of 3642 km.


Wikimedia Commons True Color Image

Io has had a long history since its discovery in 1610 by Galileo Galilei. Telescope technology limited observations of Io until the late 1800s when large scale features were able to be resolved on its surface. Observations show that Io has areas of red, brown, orange, and yellow which were later attributed to the abundance of sulfur and sodium on the surface and the unusually high amount of volcanic activity. Io is the most volcanically active body in the solar system. Its activity is due to a great deal of tidal heating from Jupiter and a resonant orbit with Jupiter’s other moons.

Origin of Internal Energy[edit]

Io has a similar density to our moon, but is made mostly of sulfur compounds[1]. Io experiences a large amount of tidal heating because it orbits Jupiter at a relatively close distance of 422,000 km and gravitational interactions with other Galilean moons. Jupiter’s large gravitational field and Io’s interactions with other satellites have resulted in an orbital resonance between Io, Europa, and Ganymede which means their orbital periods are integer multiples of each other[2].

Wikimedia Commons Io Eruption Plume Image

For each orbit of Ganymede, Europa orbits twice and Io orbits four times, this forces Io to have a more eccentric orbit. The eccentric orbit means that Io's orbital distance continuously varies, which causes expansion and contraction of its tidal bulge. This frictional energy is dissipated in the form of heat. Io’s internal heat melts the sulfur-rich core into liquid magma, which is contained in large subsurface chambers[3]. This process raises the internal pressure of Io and causes volcanic activity. It is believed that Io has a thin rigid lithosphere and a constant flowing liquid mantle due to its constant internal heating. It is estimated that the power dissipation from tidal interaction is two orders of magnitude greater than radioactive decay. For this reason, Io has a maximum observed temperature of ~1800K- the second highest in the solar system[3]. Another manifestation of the large power dissipation is a large heat flow, 30 times larger than found in Earth’s volcanoes.

Volcanic Features[edit]

It is not surprising that Io’s surface is dominated by volcanic features. The surface is littered with pit craters, calderas (volcanic craters), lava flows, shield volcanoes, ash plains, sulfur (S) deposits and other features.

Image of Active lava flows in the Tvashtar Paterae region. Images taken by Galileo in 1999 and 2000

There are over 500 volcanoes on the surface of Io. There are also over 200 calderas, some of which stretch up to 200km across; these calderas are surrounded by different patterns such as bright rings from condensing sulfur ejecta[2]. Volcanic eruptions cause these plumes of ejecta, which can be sprayed up to a height of 300km and 1000km in diameter[1]. The formation of the ejecta plumes is determined by the type of eruption and the characteristics of the material ejected (i.e. molecular composition or phase of matter). Large lava flows are common on the surface due to large volcanic plume velocities of ~1km/s. Lava flows create intervent planes which are areas between volcanoes where lava flows are frequently resurfacing, keeping them flat[4]. Unlike the Earth, there has been no evidence shown that Io has plate tectonics; there are also anomalous isolated mountains in the polar regions[2].

Plasma Torus[edit]

Recently, it has been discovered that Io has a plasma torus. The plasma torus is created by the volcanoes that eject sodium and sulfur gas and plasma into Io’s atmosphere. Since Io is not very massive it is not able to hold a substantial atmosphere, leaving behind a “tail” which follows Io’s orbit around Jupiter. Jupiter’s magnetic field strips off more than 1000kg of Io's material per second[5]. The plasma torus is held in place by Jupiter’s large magnetic field, which causes the charged particles (protons and electrons) in the field to orbit a center of rotation, which causes these particles to stay in a relatively small area. Some of these ions are drawn towards Jupiter’s atmosphere by its magnetic field lines, when these ions interact with Jupiter’s atmosphere, auroras are created. The excess stripped ions in the torus double the size of Jupiter’s magnetosphere compared with theoretical calculations[5]. Spectral observations show that Io’s torus and ionosphere are depleted by processes that produce a large nebula expanding out to ~500 Jupiter radii[6]. One of the latest methods of studying the plasma torus is with high resolution (R~120000) spectra of neutral oxygen emissions (630nm) as well as other emissions with the Wisconsin H-Alpha Mapper (WHAM) instrument in Madison, Wisconsin. These observations have shown that emissions show significant long and short-term variations of oxygen emissions. Since intensity variations exist along with a long-term intensity average, it has been concluded that the emissions cause an interaction between Io’s torus and its atmosphere[7]. The reason for studying emissions is to measure the composition as well as velocity structures. This spectra is being taken with a dual Fabry-Perot etalon system to filter out the multiple orders that are produced by etalons[8].

Io's Surface and Subsurface[edit]

Wikimedia Commons Io Interior Image

Research is also being directed towards Io’s surface and subsurface processes. Scientists are currently trying to map the anti-Jovian hemisphere as well as analyze volcanic, tectonic, and other processes affecting the region[9]. Observations show that Io’s entire surface is constantly changing because of subsurface processes, but most eruptions are found to be confined to isolated areas. Volcanic events only take place on about 17% of the surface which points back to the large lava flows that can extend up to 1000km[9]. Similar to Earth’s geysers, Io’s fissures release gases and liquids like sulfur vapor up to 225km and can last for days. Io’s surface has been found to be very active and volatile; it has been observed that changes in volcanic spray were detected in a 5.5 hour period. A New Frontiers class mission named Argus is a planned mission to send a spacecraft to Io that would orbit the moon 40 times at about 100km altitude. The goal of the mission is to attempt to answer basic questions about the possible habitation of planetary bodies like Europa and Enceladus, which are both under the influence of significant tidal heating[10].


Europa is the solar system's sixth largest moon. With a diameter of 3120 km, Europa is slightly smaller than Earth's moon, and is substantially larger than the dwarf planet Pluto. Europa has the smoothest and one of the youngest surface in the Solar System, Scientist think that Europa might have a liquid ocean under its icy crust and thus, scientist think it may harbor life. Europa was discovered in January 7 1610 by Galileo Galilei in 1610 at the same time that Io, Ganymede, and Callisto were found.

The remaining 60 moons[edit]

Jupiter has other remaining 60 moons (some may be captured Asteroids) are all far smaller than Ganymede, Callisto, Io and Europa even the fifth largest moon of Jupiter has a diameter under 200 km. The other moons names in order are in the Inner Group: (since Jupiter has a lot of moons they are divided into groups) Metis, Adrastea, Amathea and Thebe followed by the Themisto group with only one member which is Temistho. The Himalia Group: Leda Himalia, Lysithea, and Elara. The Carpo Group which only member is the moon Carpo. An unamed group which only member is an unnamed moon (S\2003 J 12). Anake Group: Anake, Praxidike, Harpalyke, Iocaste, Euanthe, Thyone, Euporie , S/2003 J 3 , S/2003 J 18 , Thelxinoe , Helike , Orthosie , S/2003 J 16 , Hermippe , Mneme , S/2003 J 15 and S/2010 J 2. The Carme Group: Herse , S/2003 J 10 , Pasithee , Chaldene , Arche , Isonoe , Erinome , Kale , Aitne , Taygete , S/2003 J 9 , Carme , S/2003 J 5 , S/2003 J S/2003 J 19, Kalyke , Eukelade and Kallichore. The Pasiphae Group: Eurydome , S/2003 J 23 , Hegemone , Pasiphaë , Sponde , Cyllene , Megaclite , S/2003 J 4 , Callirrhoe , Sinope , Autonoe , Aoede , Kore and S/2010 J 1 and the final group is another unnamed group with an only moon with a group all of its own (S\2003 J 2)


Saturn has 62 moons.


Titan is the second largest moon in the solar system; with a diameter of 5150 km, it is slightly smaller than Jupiter's Ganymede; it is larger than the planet Mercury. Titan is the only planetary moon having a dense atmosphere. Titan's atmosphere is mostly composed of nitrogen with a trace of methane. Titan's Atmosphere is by far the densest of any satellite in the Solar System, being 50-60% denser than Earth's atmosphere. Other moons, if they have atmospheres (which the majority don't), have less than 1/1000 the pressure of Earth's atmosphere at sea level. The Atmosphere on Titan is dense enough for there to be rain; however, the rain is liquid methane. The temperature on Titan is so low (-333 degrees Fahrenheit) that methane is a liquid.


Rhea is the second largest moon of Saturn and the ninth largest moon in the Solar System, with a diameter of 1528 km. The moon has a possible ring system. Rhea's low density of 1.23 suggests that it is composed of 75% water ice and 25% rock.


Iapetus is the third largest moon of Saturn and the 11th largest moon in the Solar System. Iapetus holds the record for the Farthest Round Major moon of any Planet\Dwarf Planet being 93 times farther from Saturn than the Moon from the Earth. Iapetus diameter is 1472 miles (735 km). Half its surface is extremely bright while the other is extremely dark.


Dione is the second densest moon of Saturn (after Titan) at a density of 1.47 this means that it has more rocky material within than the other moons. Dione has a face which it's heavily cratered and another that it's almost completely clean of craters. At a diameter of 1122 km, Dione is the fourth largest moon of Saturn and the 15th largest in the Solar System


Tethys is similar to Dione in many ways like size number of craters ext... .Tethys is the 16th largest moon of the Solar System and the 5th largest of Saturn. Investigation and Research indicates that in the past, Tethys was a giant ball of liquid water that with the temperatures of the outer Solar System froze eventually


Enceladus is the 17 largest moon of the Solar System at 500 km across. Enceladus is the only Moon of the Solar System that shoots tons of water from its inside to outer space. Enceladus is much smaller than Titan, Rhea, Iapetu, Dione and Tethys but much bigger than Mimas, the Smallest round moon of Saturn.


Mimas is the smallest round moon of Saturn and the 20th largest moon of the Solar System at a diameter of 396 km. Mimas has a huge crater being almost as half a big as Mimas itself which it would have been a little bigger and it would have destroyed Mimas.

Hyperion and Phoebe[edit]

Hyperion is the 8th largest moon of Saturn and 23rd in the Solar System. Hyperion is the only moon in the Solar System that rotates chaotically. The density of Hyperion is one of the lowest in the Solar System at 0.5 grams per cubic centimetres which is lower than Saturn's which is 0.7 grams per cubic centimetre. Phoebe is the ninth largest moon of Saturn and one of the largest irregular moons in the Solar Sytem at a diameter of 220 km. Like Hyperion, Phoebe's rotation is not the same as its year, however Phoebe's rotation can be determined while Hyperions's cannot. Phoebe rotates once every eleven hours while its year is 550 days.

The Remaining 53 Moons[edit]

The remaining 53 minor mons of Saturn are S/2009 S 1 ,Propeller moonlets ,Pan ,Daphnis ,Atlas ,Prometheus , ?(S/2004 S 6 · S/2004 S 4 · S/2004 S 3) ,Pandora Epimetheus ,Janus Aegaeon , (Telesto • Calypso) , (Helene • Polydeuces) Methone ,Anthe ,Pallene Kiviuq ,Ijiraq ,Paaliaq ,Siarnaq ,Tarqeq,Skathi , S/2007 S 2 , Skoll , S/2004 S 13 , Greip , Hyrrokkin , Mundilfari , Jarnsaxa , S/2006 S 1 , S/2004 S 17 , Narvi , Bergelmir , Aegir , Suttungr , S/2004 S 12 , Bestla , Farbauti , Hati , S/2004 S 7 , Thrymr , S/2007 S 3 , Surtur, Fenrir , Ymir , Loge , Fornjot ,Albiorix , Bebhionn , Erriapus , Tarvos and Kari


Uranus has 27 moons including Titania, Oberon,Umbriel and Ariel which are all over 1000 km

Titania and Oberon[edit]

Titania and Oberon are the largest moons of Uranus Titania being 1578 km in diameter make it also the 8th largest moon of the Solar System and Oberon the 10th largest moon of the Solar System at a diameter of 1522 km. Both moons are heavily cratered like most moons. Both moons were dicovered by William Hershell on January 11 1787

Umbriel and Ariel[edit]

Umbriel is the 13th largest moon of the Solar System at a diameter of 1168 Ariel is the 14th largest moon of the Solar System at a diameter of 1156 km. Both moons were discovered by William Lassell on October 24 1851


Miranda is the smallest round moon of Uranus and the 18th largest moon in the Solar System. Miranda is an icy moon which it is mostly composed of water ice and a small percent of rock. This moon was dicovered in February 15 1948 by Gerard P. Kuiper.

The Remaining 22 moons[edit]

The other moons of Uranus are Cordelia , Ophelia · Bianca , Cressida , Desdemona , Juliet , Portia , Rosalind , Cupid , Belinda , Perdita , Puck , Mab,Francisco , Caliban , Stephano , Trinculo , Sycorax , Margaret , Prospero , Setebos and Ferdinand


Neptune has 13 moons, the large one being Triton


Triton is the seventh largest moon in the solar system; its diameter of 2705 km is about 10% larger than the Dwarf Planet Pluto. Scientists believe that this moon is really an object that has been captured by Neptune's gravity. Triton is one of the coldest bodies in the Solar System being even colder than Pluto at -393 degrees Fahrenheit. In 360,000,000 years it is expected to pass the Neptune roche limit and disintegrate to form a planetary ring system


Nereid is the third largest moon of Neptune at 170 km in Diameter. Nereid is also one of the few moons in the Solar System that its year is not the same as its rotation


Proteus is the largest irregular moon of the Solar System and the second largest moon of neptune and the 19th largest moon of the Solar System being 420 km in Diameter.

The Remaining 10 Moons[edit]

The other 10 moons are Naiad , Thalassa , Despina , Galatea , Larissa Halimede , Sao , Laomedeia , Psamathe and Neso

Dwarf Planets[edit]


Ceres has no moons


Pluto has 4 moons. Their names are Charon, Hydra, Nix and the 4th unnamed moon of Pluto discovered in 2011


Charon is the Largest moon of Pluto with a diameter of 1207 km it is also the largest moon proportionally compared to its host planet. Being roughly half of Pluto's size Pluto and Charon orbit each other.

←==Hydra, Nix and P4== Hydra is the second largest moon of Pluto at a diameter of 61-150 km . Nix is the third largest moon of Pluto at a diameter of 46-


Haumea has two moons (Hi'aka and Namaka)


Hi'aka is probably one of the biggest irregular shaped moons of the Solar System at a diameter of 310 km. However its shape is currently unknown


Namaka is roughly the same size as the Neptunian moon Nereid at 170 km. Its shape is unknown.


Makemake has no Moon


Eris has one moon (Dysnomia)


Dysnomia is the only moon that the Dwarf planet Eris has. Its shape is not known but scientist expect that it may not be as large to rounded by self gravitation

Other Objects[edit]


Quaoar has one moon being 1\12 Quaoar's diameter the moon it's called Weyvot.


Orcus has a moon that it is 1\3 its size that moon it's called Vant and it's 380 km in diameter, probably large enough to be round


  1. a b Rothery, D. 1992, Satellites of Outer Planets, pp.138-142
  2. a b c Christiansen, E. 1995, Exploring the Planets, pp.331-333
  3. a b Weissman, Paul R. 1999, Encyclopedia of the Solar System, pp.359-363
  4. Hartmann, W. 1993, Moons & Planets, p.387
  5. a b Galilean Moons: Io, NASA/JPL 2008,
  6. Mendillo, M. 2007, The Source of Sodium Escaping from Io revealed by spectral high definition imaging, Nature Journal, Vol.448, pp.330-332
  7. Oliversen, R. 2001, Sunlit Io Atmospheric 6300A Emission and the Plasma Torus, Journal of Geophysical Research, Vol. 106, pp.183-193
  8. Oliversen, R. 2008, Observation of Io and the Plasma Torus During 2007 and 2008, Unpublished
  9. a b Williams, D. 2008, Volcanism on Io: Insights from Global Geologic Mapping, American Geophysical Union
  10. Chen, E. 2008, Argus: A New Frontiers Mission to Observe Io, Unpublished