Galilean moons

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Galilean moons
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Montage of Jupiter's four Galilean moons, in a composite image depicting part of Jupiter and their relative sizes (positions are illustrative, not actual). From top to bottom: Io, Europa, Ganymede, Callisto.
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upright=1.2|Two Hubble Space Telescope views of a rare triple transit of Jupiter by Europa, Callisto and Io (24 January 2015)
The Galilean moons (or Galilean satellites) are the four largest moons of Jupiter—Io, Europa, Ganymede, and Callisto. They were first seen by Galileo Galilei in December 1609 or January 1610, and recognized by him as satellites of Jupiter in March 1610.Drake, Stillman (1978). Galileo At Work. Chicago: University of Chicago Press. {{ISBN|0-226-16226-5}}. They were the first objects found to orbit another planet. They are among the largest objects in the Solar System with the exception of the Sun and the eight planets, with a radius larger than any of the dwarf planets. Ganymede is the largest moon in the Solar System, and is even bigger than the planet Mercury, though only around half as massive. The three inner moons—Io, Europa, and Ganymede—are in a 4:2:1 orbital resonance with each other. Because of their much smaller size, and therefore weaker self-gravitation, all of Jupiter's remaining moons have irregular forms rather than a spherical shape.The Galilean moons were observed in either 1609 or 1610 when Galileo made improvements to his telescope, which enabled him to observe celestial bodies more distinctly than ever. Galileo's observations showed the importance of the telescope as a tool for astronomers by proving that there were objects in space that cannot be seen by the naked eye. The discovery of celestial bodies orbiting something other than Earth dealt a serious blow to the then-accepted Ptolemaic world system, a geocentric theory in which everything orbits around Earth.Galileo initially named his discovery the Cosmica Sidera ("Cosimo's stars"), but the names that eventually prevailed were chosen by Simon Marius. Marius discovered the moons independently at nearly the same time as Galileo, 8 January 1610, and gave them their present names, derived from the lovers of Zeus, which were suggested by Johannes Kepler, in his Mundus Jovialis, published in 1614.JOURNAL
, Pasachoff, Jay M.
, Simon Marius's Mundus Iovialis: 400th Anniversary in Galileo's Shadow
, Journal for the History of Astronomy, 2015, 46
, 2, 218–234
, 2015AAS...22521505P, 10.1177/0021828615585493



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upright|Galileo Galilei, the discoverer of the four moons
As a result of improvements Galileo Galilei made to the telescope, with a magnifying capability of 20×,JOURNAL, Albert, Van Helden, The Telescope in the Seventeenth Century, Isis, 65, 1, March 1974, 38–58, 228880, 10.1086/351216, he was able to see celestial bodies more distinctly than was ever possible before. This allowed Galilei to observe in either December 1609 or January 1610 what came to be known as the Galilean moons.Galilei, Galileo, Sidereus Nuncius. Translated and prefaced by Albert Van Helden. Chicago & London: University of Chicago Press 1989, 14–16BOOK, The Starry Messenger, Galilei, Galileo, 1610, Venice,weblink On the seventh day of January in this present date 1610...., 978-0-374-37191-3, live, registration, On January 7, 1610, Galileo wrote a letter containing the first mention of Jupiter's moons. At the time, he saw only three of them, and he believed them to be fixed stars near Jupiter. He continued to observe these celestial orbs from January 8 to March 2, 1610. In these observations, he discovered a fourth body, and also observed that the four were not fixed stars, but rather were orbiting Jupiter.Galileo's discovery proved the importance of the telescope as a tool for astronomers by showing that there were objects in space to be discovered that until then had remained unseen by the naked eye. More importantly, the discovery of celestial bodies orbiting something other than Earth dealt a blow to the then-accepted Ptolemaic world system, which held that Earth was at the center of the universe and all other celestial bodies revolved around it.WEB,weblink Satellites of Jupiter, 9 August 2007, The Galileo Project, Rice University, 1995, live,weblink" title="">weblink 11 February 2012, Galileo's Sidereus Nuncius (Starry Messenger), which announced celestial observations through his telescope, does not explicitly mention Copernican heliocentrism, a theory that placed the Sun at the center of the universe. Nevertheless, Galileo accepted the Copernican theory.A Chinese historian of astronomy, Xi Zezong, has claimed that a "small reddish star" observed near Jupiter in 362 BCE by Chinese astronomer Gan De may have been Ganymede, predating Galileo's discovery by around two millennia.Zezong, Xi, "The Discovery of Jupiter's Satellite Made by Gan De 2000 years Before Galileo", Chinese Physics 2 (3) (1982): 664–67.

Dedication to the Medicis

File:Sidereus Nuncius Medicean Stars.jpg|thumb|right|upright|The Medician stars in the Sidereus NunciusSidereus NunciusIn 1605, Galileo had been employed as a mathematics tutor for Cosimo de' Medici. In 1609, Cosimo became Grand Duke Cosimo II of Tuscany. Galileo, seeking patronage from his now-wealthy former student and his powerful family, used the discovery of Jupiter's moons to gain it. On February 13, 1610, Galileo wrote to the Grand Duke's secretary:"God graced me with being able, through such a singular sign, to reveal to my Lord my devotion and the desire I have that his glorious name live as equal among the stars, and since it is up to me, the first discoverer, to name these new planets, I wish, in imitation of the great sages who placed the most excellent heroes of that age among the stars, to inscribe these with the name of the Most Serene Grand Duke."Galileo asked whether he should name the moons the "Cosmian Stars", after Cosimo alone, or the "Medician Stars", which would honor all four brothers in the Medici clan. The secretary replied that the latter name would be best.On March 12, 1610, Galileo wrote his dedicatory letter to the Duke of Tuscany, and the next day sent a copy to the Grand Duke, hoping to obtain the Grand Duke's support as quickly as possible. On March 19, he sent the telescope he had used to first view Jupiter's moons to the Grand Duke, along with an official copy of Sidereus Nuncius (The Starry Messenger) that, following the secretary's advice, named the four moons the Medician Stars. In his dedicatory introduction, Galileo wrote:Scarcely have the immortal graces of your soul begun to shine forth on earth than bright stars offer themselves in the heavens which, like tongues, will speak of and celebrate your most excellent virtues for all time. Behold, therefore, four stars reserved for your illustrious name ... which ... make their journeys and orbits with a marvelous speed around the star of Jupiter ... like children of the same family ... Indeed, it appears the Maker of the Stars himself, by clear arguments, admonished me to call these new planets by the illustrious name of Your Highness before all others.


File:Apparatus to demonstrate the motion of Jupiter's satellites in Putnam Gallery, 2009-11-24.jpg|thumb|A Jovilabe:WEB,weblink Jovilabe, (Museo Galileo]], 15 April 2015, live,weblink 16 April 2015, an apparatus from the mid-18th century for demonstrating the orbits of Jupiter's satellites)Galileo initially called his discovery the Cosmica Sidera ("Cosimo's stars"), in honour of Cosimo II de' Medici (1590–1621).(wikt:Cosimo|Cosimo) is the Italian form of the Greek name (wikt:Cosmas|Cosmas) itself deriving from (wikt:κόσμος#Ancient Greek|cosmos) (whence the neuter plural adjective cosmica). Sidera is the plural form of the Latin noun (wikt:sidus#Latin|sidus) "star, constellation". At Cosimo's suggestion, Galileo changed the name to Medicea Sidera ("the Medician stars"), honouring all four Medici brothers (Cosimo, Francesco, Carlo, and Lorenzo). The discovery was announced in the Sidereus Nuncius ("Starry Messenger"), published in Venice in March 1610, less than two months after the first observations.Other names put forward include: The names that eventually prevailed were chosen by Simon Marius, who discovered the moons independently at the same time as Galileo: he named them at the suggestion of Johannes Kepler after lovers of the god Zeus (the Greek equivalent of Jupiter): Io, Europa, Ganymede and Callisto, in his Mundus Jovialis, published in 1614.Galileo steadfastly refused to use Marius' names and invented as a result the numbering scheme that is still used nowadays, in parallel with proper moon names. The numbers run from Jupiter outward, thus I, II, III and IV for Io, Europa, Ganymede, and Callisto respectively. Galileo used this system in his notebooks but never actually published it. The numbered names (Jupiter x) were used until the mid-20th century when other inner moons were discovered, and Marius' names became widely used.JOURNAL, Marazzini, C., 2005, The names of the satellites of Jupiter: from Galileo to Simon Marius, Lettere Italiana, 57, 3, 391–407,

Determination of longitude

{{see also|History of longitude#Galileo.27s_proposal_.E2.80.94_Jovian_moons|l1=History of longitude}}Galileo was able to develop a method of determining longitude based on the timing of the orbits of the Galilean moons.Howse, Derek. Greenwich Time and the Discovery of the Longitude. Oxford: Oxford University Press, 1980, 12. The times of the eclipses of the moons could be precisely calculated in advance, and compared with local observations on land or on ship to determine the local time and hence longitude. The main problem with the technique was that it was difficult to observe the Galilean moons through a telescope on a moving ship; a problem that Galileo tried to solve with the invention of the celatone. The method was used by Cassini and Picard to re-map France.BOOK, Howse, Derek, Greenwich Time and the Longitude, Philip Wilson, 1997, 26, 31,


{{further|Moons of Jupiter}}Some models predict that there may have been several generations of Galilean satellites in Jupiter's early history. Each generation of moons to have formed would have spiraled into Jupiter and been destroyed, due to tidal interactions with Jupiter's proto-satellite disk, with new moons forming from the remaining debris. By the time the present generation formed, the gas in the proto-satellite disk had thinned out to the point that it no longer greatly interfered with the moons' orbits.WEB,weblink Cannibalistic Jupiter ate its early moons, Chown, Marcus, 7 March 2009, New Scientist, 18 March 2009, live,weblink" title="">weblink 23 March 2009, Other models suggest that Galilean satellites formed in a proto-satellite disk, in which formation timescales were comparable to or shorter than orbital migration timescales.JOURNAL, 10.1088/0004-637X/806/2/203, 1504.04364, 2015ApJ...806..203D, Capture and Evolution of Planetesimals in Circumjovian Disks, The Astrophysical Journal, 806, 2, 203, 2015, d'Angelo, Gennaro, Podolak, Morris, Io is anhydrous and likely has an interior of rock and metal.BOOK, Canup, Robin M., Ward, William R., Origin of Europa and the Galilean Satellites, University of Arizona Press, 2008-12-30, 0812.4995,, 59, 978-0-8165-2844-8, Europa is thought to contain 8% ice and water by mass with the remainder rock. These moons are, in increasing order of distance from Jupiter:{| class="sortable wikitable"|+! Name! class="unsortable" |Image! class="unsortable" |Model of interior ! Diameter(km)! Mass(kg)! Density(g/cm3)! Semi-major axis(km)Computed using the weblink" title="">IAU-MPC Satellites Ephemeris Service µ value! Orbital period (days)Moons of Jupiter {{webarchive|url= |date=2017-06-08 }} NASA (relative to Io)! Inclination(°)Computed from IAG Travaux 2001 {{webarchive|url= |date=2011-08-07 }}.! Eccentricity
Io Jupiter I(File:Io highest resolution true color.jpg|121x121px)
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{{valp=× p=× |3630.6}} }} {{val|3.528}} {{val|421800}} 1.769 (1) 0.050 0.0041
Europa Jupiter II
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{{val|3121.6}} }} 3.014 {{val|671100}} 3.551 (2.0) 0.471 0.0094
Ganymede Jupiter III(File:Moon Ganymede by NOAA - cropped.jpg|175x175px)(File:PIA18005-NASA-InsideGanymede-20140501a.png|175px) {{val|5268.2}} }} 1.942 {{val|1070400}} 7.155 (4.0) 0.204 0.0011
Callisto Jupiter IV(File:Callisto (cropped).jpg|160x160px)
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- PIA01478 Interior of Callisto.jpg -
{{val|4820.6}} }} 1.834 {{val|1882700}} 16.69 (9.4) 0.205 0.0074


File:Io Tupan Patera.jpg|thumb|left|Tupan PateraTupan PateraIo (Jupiter I) is the innermost of the four Galilean moons of Jupiter and, with a diameter of 3642 kilometers, the fourth-largest moon in the Solar System. It was named after Io, a priestess of Hera who became one of the lovers of Zeus. Nevertheless, it was simply referred to as "Jupiter I", or "The first satellite of Jupiter", until the mid-20th century.With over 400 active volcanos, Io is the most geologically active object in the Solar System.JOURNAL, 10.1016/j.icarus.2003.11.013, Lava lakes on Io: Observations of Io's volcanic activity from Galileo NIMS during the 2001 fly-bys, Icarus, 169, 1, 140–74, 2004, Lopes, Rosaly M.C, Kamp, Lucas W, Smythe, William D, Mouginis-Mark, Peter, Kargel, Jeff, Radebaugh, Jani, Turtle, Elizabeth P, Perry, Jason, Williams, David A, Carlson, R.W, Douté, S., 2004Icar..169..140L, the Galileo NIMS, SSI Teams, Its surface is dotted with more than 100 mountains, some of which are taller than Earth's Mount Everest.JOURNAL, 10.1029/2000JE001408, The mountains of Io: Global and geological perspectives from Voyager and Galileo, Journal of Geophysical Research: Planets, 106, E12, 33201–22, 2001, Schenk, Paul, Hargitai, Henrik, Wilson, Ronda, McEwen, Alfred, Thomas, Peter, 2001JGR...10633201S, Unlike most satellites in the outer Solar System (which have a thick coating of ice), Io is primarily composed of silicate rock surrounding a molten iron or iron sulfide core.Although not proven, recent data from the Galileo orbiter indicate that Io might have its own magnetic field.JOURNAL, 10.1126/science.1079462, 12624258, Cassini Imaging of Jupiter's Atmosphere, Satellites, and Rings, Science, 299, 5612, 1541–7, 2003, Porco, C. C., West, Robert A., McEwen, Alfred, Del Genio, Anthony D., Ingersoll, Andrew P., Thomas, Peter, Squyres, Steve, Dones, Luke, Murray, Carl D., Johnson, Torrence V., Burns, Joseph A., Brahic, Andre, Neukum, Gerhard, Veverka, Joseph, Barbara, John M., Denk, Tilmann, Evans, Michael, Ferrier, Joseph J., Geissler, Paul, Helfenstein, Paul, Roatsch, Thomas, Throop, Henry, Tiscareno, Matthew, Vasavada, Ashwin R., 2003Sci...299.1541P,weblink live,weblink" title="">weblink 2017-09-22, Io has an extremely thin atmosphere made up mostly of sulfur dioxide (SO2).JOURNAL, 10.1126/science.281.5373.87, 9651251, High-Temperature Silicate Volcanism on Jupiter's Moon Io, Science, 281, 5373, 87–90, 1998, McEwen, A. S., Keszthelyi, L., Spencer, J. R., Schubert, G., Matson, D. L., Lopes-Gautier, R., Klaasen, K. P., Johnson, T. V., Head, J. W., Geissler, P., Fagents, S., Davies, A. G., Carr, M. H., Breneman, H. H., Belton, M. J. S., 1998Sci...281...87M, If a surface data or collection vessel were to land on Io in the future, it would have to be extremely tough (similar to the tank-like bodies of the Soviet Venera landers) to survive the radiation and magnetic fields that originate from Jupiter.JOURNAL, 10.1126/science.186.4167.922, 17730914, Io: A Surface Evaporite Deposit?, Science, 186, 4167, 922–5, 1974, Fanale, F. P., Johnson, T. V., Matson, D. L., 1974Sci...186..922F,


(File:PIA19048 realistic color Europa mosaic (original).jpg|thumb|Europa)Europa (Jupiter II), the second of the four Galilean moons, is the second closest to Jupiter and the smallest at 3121.6 kilometers in diameter, which is slightly smaller than the Moon. The name comes from a mythical Phoenician noblewoman, Europa, who was courted by Zeus and became the queen of Crete, though the name did not become widely used until the mid-20th century.It has a smooth and bright surface,WEB,weblink Europa: In Depth, Hefler, Michael, 2001, NASA, Jet Propulsion Laboratory, 9 August 2007, NASA, Solar system Exploration, live,weblink" title="">weblink 14 November 2015, with a layer of water surrounding the mantle of the planet, thought to be 100 kilometers thick.Schenk, P. M.; Chapman, C. R.; Zahnle, K.; Moore, J. M.; Chapter 18: Ages and Interiors: the Cratering Record of the Galilean Satellites, in Jupiter: The Planet, Satellites and Magnetosphere, Cambridge University Press, 2004 The smooth surface includes a layer of ice, while the bottom of the ice is theorized to be liquid water.WEB,weblink Hamilton, C. J., Jupiter's Moon Europa, live,weblink" title="">weblink 2012-01-24, The apparent youth and smoothness of the surface have led to the hypothesis that a water ocean exists beneath it, which could conceivably serve as an abode for extraterrestrial life.WEB,weblink Possibility of Life on Europa, Tritt, Charles S., 10 August 2007, Milwaukee School of Engineering, 2002, dead,weblink" title="">weblink 9 June 2007, Heat energy from tidal flexing ensures that the ocean remains liquid and drives geological activity.WEB,weblink Tidal Heating, 2007-10-20,,weblink" title="">weblink 2006-03-29, Life may exist in Europa's under-ice ocean. So far, there is no evidence that life exists on Europa, but the likely presence of liquid water has spurred calls to send a probe there.WEB, Phillips, Cynthia,weblink Time for Europa,, 28 September 2006, 5 January 2014, live,weblink" title="">weblink 11 December 2011, (File:Hubble sees recurring plume erupting from Europa.jpg|thumb|Recurring plume erupting from Europa.WEB, Hubble sees recurring plume erupting from Europa,weblink, 24 April 2017, live,weblink 25 April 2017, )The prominent markings that criss-cross the moon seem to be mainly albedo features, which emphasize low topography. There are few craters on Europa because its surface is tectonically active and young.JOURNAL, 10.1006/icar.1998.5986, Tectonic Processes on Europa: Tidal Stresses, Mechanical Response, and Visible Features, Icarus, 135, 1, 64–78, 1998, Greenberg, Richard, Geissler, Paul, Hoppa, Gregory, Tufts, B.Randall, Durda, Daniel D., Pappalardo, Robert, Head, James W., Greeley, Ronald, Sullivan, Robert, Carr, Michael H., 1998Icar..135...64G, Some theories suggest that Jupiter's gravity is causing these markings, as one side of Europa is constantly facing Jupiter. Also, volcanic water eruptions splitting the surface of Europa, and even geysers have been considered as a cause. The color of the markings, reddish-brown, is theorized to be caused by sulfur, but scientists cannot confirm that, because no data collection devices have been sent to Europa.JOURNAL, Distribution of hydrate on Europa: Further evidence for sulfuric acid hydrate, 10.1016/j.icarus.2005.03.026, R.W., Carlson, M.S. Anderson, 2005, 2005Icar..177..461C, 177, 2, Icarus, 461–471, Europa is primarily made of silicate rock and likely has an iron core. It has a tenuous atmosphere composed primarily of oxygen.


(File:Ganymede g1 true-edit1.jpg|thumb|Ganymede)Ganymede (Jupiter III), the third Galilean moon is named after the mythological Ganymede, cupbearer of the Greek gods and Zeus's beloved.WEB,weblink Satellites of Jupiter, The Galileo Project, 2007-11-24, live,weblink" title="">weblink 2012-02-11, Ganymede is the largest natural satellite in the Solar System at 5262.4 kilometers in diameter, which makes it larger than the planet Mercury – although only at about half of its massWEB,, Ganymede, October 31, 1997,weblink 2008-02-27, live,weblink" title="">weblink February 8, 2012, since Ganymede is an icy world. It is the only satellite in the Solar System known to possess a magnetosphere, likely created through convection within the liquid iron core.JOURNAL, 10.1006/icar.2002.6834, The Permanent and Inductive Magnetic Moments of Ganymede, Icarus, 157, 2, 507–22, 2002, Kivelson, M.G., Khurana, K.K., Volwerk, M., 2002Icar..157..507K, 2060/20020044825, Ganymede is composed primarily of silicate rock and water ice, and a salt-water ocean is believed to exist nearly 200 km below Ganymede's surface, sandwiched between layers of ice.WEB,weblink Solar System's largest moon likely has a hidden ocean, 2008-01-11, 2000-12-16, Jet Propulsion Laboratory, NASA, live,weblink" title="">weblink 2012-01-17, The metallic core of Ganymede suggests a greater heat at some time in its past than had previously been proposed. The surface is a mix of two types of terrain—highly cratered dark regions and younger, but still ancient, regions with a large array of grooves and ridges. Ganymede has a high number of craters, but many are gone or barely visible due to its icy crust forming over them. The satellite has a thin oxygen atmosphere that includes O, O2, and possibly O3 (ozone), and some atomic hydrogen.JOURNAL, 10.1086/305604, The Far‐Ultraviolet Oxygen Airglow of Europa and Ganymede, The Astrophysical Journal, 499, 1, 475–481, 1998, Hall, D. T., Feldman, P. D., McGrath, M. A., Strobel, D. F., 1998ApJ...499..475H, JOURNAL, 10.1016/S0032-0633(00)00154-9, The ionosphere of Ganymede, Planetary and Space Science, 49, 3–4, 327–36, 2001, Eviatar, Aharon, m. VasyliÅ«nas, Vytenis, a. Gurnett, Donald, 2001P&SS...49..327E,


(File:Valhalla crater on Callisto.jpg|thumb|left|Callisto's Valhalla impact crater as seen by Voyager)Callisto (Jupiter IV) is the fourth and last Galilean moon, and is the second largest of the four, and at 4820.6 kilometers in diameter, it is the third largest moon in the Solar System, and barely smaller than Mercury, though only a third of the latter's mass. It is named after the Greek mythological nymph Callisto, a lover of Zeus who was a daughter of the Arkadian King Lykaon and a hunting companion of the goddess Artemis. The moon does not form part of the orbital resonance that affects three inner Galilean satellites and thus does not experience appreciable tidal heating.JOURNAL, 10.1006/icar.2002.6939, Numerical Simulations of the Orbits of the Galilean Satellites, Icarus, 159, 2, 500–4, 2002, Musotto, S, Varadi, Ferenc, Moore, William, Schubert, Gerald, 2002Icar..159..500M, Callisto is composed of approximately equal amounts of rock and ices, which makes it the least dense of the Galilean moons. It is one of the most heavily cratered satellites in the Solar System, and one major feature is a basin around 3000 km wide called Valhalla.Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxideJOURNAL, 10.1126/science.283.5403.820, 9933159, A Tenuous Carbon Dioxide Atmosphere on Jupiter's Moon Callisto, Science, 283, 5403, 820–1, 1999, Carlson, R. W., 1999Sci...283..820C, 2014/16785,weblink live,weblink" title="">weblink 2008-10-03,, and probably molecular oxygen.JOURNAL, 10.1029/2004JE002322, Atmosphere of Callisto, Journal of Geophysical Research, 110, E2, E02003, 2005, Liang, Mao-Chang, Lane, Benjamin F., Pappalardo, Robert T., Allen, Mark, Yung, Yuk L., 2005JGRE..110.2003L, Investigation revealed that Callisto may possibly have a subsurface ocean of liquid water at depths less than 300 kilometres.JOURNAL, 10.1006/icar.2000.6456, Subsurface Oceans on Europa and Callisto: Constraints from Galileo Magnetometer Observations, Icarus, 147, 2, 329–47, 2000, Zimmer, C, Khurana, Krishan K., Kivelson, Margaret G., 2000Icar..147..329Z,weblink live,weblink" title="">weblink 2009-03-27,, The likely presence of an ocean within Callisto indicates that it can or could harbour life. However, this is less likely than on nearby Europa.BOOK, 10.1117/12.560356, Astrobiology of Jupiter's icy moons, Instruments, Methods, and Missions for Astrobiology VIII, Instruments, 5555, 78, Instruments, Methods, and Missions for Astrobiology VIII, 2004, Lipps, Jere H., Delory, Gregory, Pitman, Joseph T., Rieboldt, Sarah, 2004SPIE.5555...78L, Callisto has long been considered the most suitable place for a human base for future exploration of the Jupiter system since it is furthest from the intense radiation of Jupiter.WEB, Revolutionary Concepts for Human Outer Planet Exploration(HOPE), Trautman, Pat, Bethke, Kristen, NASA, 2003,weblink dead,weblink" title="">weblink 2012-01-19,

Comparative structure

File:Jupiter.moons2.jpg|left|thumb|Comparison of (a part of) Jupiter and its four largest natural satellites]]{| class=wikitable style="text-align:center; font-size:11px"|+ Jovian Radiation! Moon !! rem/day
| 540
| 8
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Fluctuations in the orbits of the moons indicate that their mean density decreases with distance from Jupiter. Callisto, the outermost and least dense of the four, has a density intermediate between ice and rock whereas Io, the innermost and densest moon, has a density intermediate between rock and iron. Callisto has an ancient, heavily cratered and unaltered ice surface and the way it rotates indicates that its density is equally distributed, suggesting that it has no rocky or metallic core but consists of a homogeneous mix of rock and ice. This may well have been the original structure of all the moons. The rotation of the three inner moons, in contrast, indicates differentiation of their interiors with denser matter at the core and lighter matter above. They also reveal significant alteration of the surface. Ganymede reveals past tectonic movement of the ice surface which required partial melting of subsurface layers. Europa reveals more dynamic and recent movement of this nature, suggesting a thinner ice crust. Finally, Io, the innermost moon, has a sulfur surface, active volcanism and no sign of ice. All this evidence suggests that the nearer a moon is to Jupiter the hotter its interior. The current model is that the moons experience tidal heating as a result of the gravitational field of Jupiter in inverse proportion to the square of their distance from the giant planet. In all but Callisto this will have melted the interior ice, allowing rock and iron to sink to the interior and water to cover the surface. In Ganymede a thick and solid ice crust then formed. In warmer Europa a thinner more easily broken crust formed. In Io the heating is so extreme that all the rock has melted and water has long ago boiled out into space.
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center|Surface features of the four members at different levels of zoom in each row


(File:Moons of solar system v7.jpg|thumb|900px|center|Galilean moons compared with moons of other planets (and with Earth; the scale is changed to 1 pixel = 94 km at this resolution).)

Latest flyby

{{multiple image| width = 100| footer = Jupiter and Galilean moons circa 2007, imaged by New Horizons during flyby. (greyscale colour)| align = center| image1 = NhJupiterIo010807.svg| alt1 =Jupiter]] and w:Io (moon)|Io]]| image2 = NewHorizonsIo.svg| alt2 =Io]]| image3 = NewHorizonsEuropa.svg| alt3 =Europa]]| image4 = NewHorizonsGanymede.svg| alt4 =Ganymede]]| image5 = NewHorizonsCallisto.svg| alt5 =Callisto]]}}{{clear}}

Origin and evolution

(File:Relative Masses of Jovian Satellites.png|thumb|right|300px|The relative masses of the Jovian moons. Those smaller than Europa are not visible at this scale, and combined would only be visible at 100× magnification.)Jupiter's regular satellites are believed to have formed from a circumplanetary disk, a ring of accreting gas and solid debris analogous to a protoplanetary disk.BOOK, Canup, Robert M., Ward, William R., Europa, University of Arizona Press, 2009, Origin of Europa and the Galilean Satellites, {{Google books, Jpcz2UoXejgC, 59, yes, |pages=59–83 |isbn=978-0-8165-2844-8}}JOURNAL, Alibert, Y., Mousis, O., Benz, W., Modeling the Jovian subnebula I. Thermodynamic conditions and migration of proto-satellites, 2005, Astronomy & Astrophysics, 439, 3, 1205–13, 2005A&A...439.1205A, 10.1051/0004-6361:20052841, astro-ph/0505367, They may be the remnants of a score of Galilean-mass satellites that formed early in Jupiter's history.Simulations suggest that, while the disk had a relatively high mass at any given moment, over time a substantial fraction (several tenths of a percent) of the mass of Jupiter captured from the Solar nebula was processed through it. However, the disk mass of only 2% that of Jupiter is required to explain the existing satellites. Thus there may have been several generations of Galilean-mass satellites in Jupiter's early history. Each generation of moons would have spiraled into Jupiter, due to drag from the disk, with new moons then forming from the new debris captured from the Solar nebula. By the time the present (possibly fifth) generation formed, the disk had thinned out to the point that it no longer greatly interfered with the moons' orbits. The current Galilean moons were still affected, falling into and being partially protected by an orbital resonance which still exists for Io, Europa, and Ganymede. Ganymede's larger mass means that it would have migrated inward at a faster rate than Europa or Io.


(File:Thomas Bresson - Jupiter(2) (by).jpg|thumb|250px|Jupiter and its four Galilean moons as seen with an amateur telescope)|thumb|Jupiter with the Galilean moons – Io, Ganymede, Europa, and Callisto (near maximum elongation), respectively – and the full Moon as seen around conjunction on 10 April 2017]]All four Galilean moons are bright enough to be viewed from Earth without a telescope, if only they could appear farther away from Jupiter. (They are, however, easily distinguished with even low-powered binoculars.) They have apparent magnitudes between 4.6 and 5.6 when Jupiter is in opposition with the Sun,WEB, Yeomans, Donald K., 2006-07-13, Planetary Satellite Physical Parameters, JPL Solar System Dynamics,weblink 2008-08-23, live,weblink" title="">weblink 2010-05-27, and are about one unit of magnitude dimmer when Jupiter is in conjunction. The main difficulty in observing the moons from Earth is their proximity to Jupiter, since they are obscured by its brightness.Jupiter is about 750 times brighter than Ganymede and about 2000 times brighter than Callisto.Ganymede: (5th root of 100)^(4.4 Ganymede APmag − (−2.8 Jup APmag)) = 758Callisto: (5th root of 100)^(5.5 Callisto APmag − (−2.8 Jup APmag)) = 2089 The maximum angular separations of the moons are between 2 and 10 arcminutes from Jupiter,Jupiter near perihelion 2010-Sep-19: 656.7 (Callisto angular separation arcsec) − 24.9 (jup angular radius arcsec) = 631 arcsec = 10 arcmin which is close to the limit of human visual acuity. Ganymede and Callisto, at their maximum separation, are the likeliest targets for potential naked-eye observation.

Orbit animation

GIF animation of the resonance of Io, Europa, and Ganymede
missing image!
- Galilean moon Laplace resonance animation.gif -
upright=1.2|The three inner Galilean moons revolve in a 1:2:4 resonance.

See also




External links

{{commons category|Moons of Jupiter}}{{Moons of Jupiter}}{{Solar System moons (compact)}}{{Jupiter}}{{Solar System}}{{Galileo Galilei}}{{Good article}}

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