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Galilean moons



The three inner Galilean moons revolve in a 4:2:1 resonance.

Io is the innermost of the four Galilean moons moons of Jupiter and, with a diameter of 3,642 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 mid-twentieth century.4

With over 400 active volcanoes, Io is the most geologically active object in the Solar System.8 Its surface is dotted with more than 100 mountains some which are taller than Earth's Mount Everest.9 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 indicates that Io might have its own magnetic field.10 Io has an extremely thin atmosphere made up mostly of sulfur dioxide (SO2).11 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.12


The relative masses of the Galilean moons. Io and Callisto together are 50%, as are Europa and Ganymede. The Galileans so dominate the system that all the other Jovian moons put together are not visible at this scale.

Europa, 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 Earth's Moon. The name, Europa was after a mythical Phoenician noblewoman, Europa, who was courted by Zeus and became the queen of Crete, but did not become widely used until the mid-twentieth century.4

It is one of the smoothest objects in the solar system,13 with a layer of water surrounding the mantle of the planet, thought to be 100 kilometers thick.14 The smooth surface includes a layer of ice, while the bottom of the ice is theorized to be liquid water.15 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.16 Heat energy from tidal flexing ensures that the ocean remains liquid and drives geological activity.17 Life may exist in Europa's under-ice ocean, perhaps subsisting in an environment similar to Earth's deep-ocean hydrothermal vents or the Antarctic Lake Vostok.18 Life in such an ocean could possibly be similar to microbial life on Earth in the deep ocean.19 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.20

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.21 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. Europa is primarily made of silicate rock and likely has an iron core. It has a tenuous atmosphere composed primarily of oxygen.


Ganymede, the third Galilean is named the mythological Ganymede, cupbearer of the Greek gods and Zeus's beloved.22 Ganymede is 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 mass.23 It is the only satellite in the Solar System known to possess a magnetosphere, likely created through convection within the liquid iron core.24

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. 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.2526


Callisto 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. It does not form part of the orbital resonance that affects three inner Galilean satellites and thus does not experience appreciable tidal heating.27 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 dioxide28 and probably molecular oxygen.29 Investigation revealed that Callisto may have possibly a subsurface ocean of liquid water at depths greater than 100 kilometers.30 The likely presence of an ocean within Callisto indicates that it can or could harbor life. However, this is less likely than on nearby Europa.31 Callisto has long been considered the most suitable place for a human base for future exploration of the system of Jupiter.32


The Galilean moons seen with an amateur telescope.

All four Galilean moons are bright enough that they could, if they were farther away from Jupiter, be sighted from Earth without a telescope. They have apparent magnitudes between 4.6 and 5.6 when Jupiter is in opposition with the Sun,33 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. The maximum angular separations of the moons are between 2 and 10 minutes of arc from Jupiter,34 close to the limit of human visual acuity. Ganymede and Callisto, at their maximum separation, are the likeliest targets for potential naked-eye observation. The easiest way to observe them is to cover Jupiter with an object, for example, atree limb or a power line that is perpendicular to the plane of moons' orbits.

Moons of Jupiter Listed in increasing distance from Jupiter. Temporary names in italics.Inner moonsMetis · Adrastea · Amalthea · ThebeGalilean moonsIo · Europa · Ganymede · Callisto ThemistoHimalia groupLeda · Himalia · Lysithea · Elara · S/2000 J 11 Carpo · S/2003 J 12Ananke group
coreAnanke · Praxidike · Harpalyke · Iocaste · Euanthe · Thyoneperipheral Euporie · S/2003 J 3 · S/2003 J 18 · Thelxinoe · Helike · Orthosie · S/2003 J 16 · Hermippe · Mneme · S/2003 J 15
Carme groupS/2003 J 17 · S/2003 J 10 · Pasithee · Chaldene · Arche · Isonoe · Erinome · Kale · Aitne · Taygete · S/2003 J 9 · Carme · S/2003 J 5 · S/2003 J 19 · Kalyke · Eukelade · KallichorePasiphaë groupEurydome · S/2003 J 23 · Hegemone · Pasiphaë · Sponde · Cyllene · Megaclite · S/2003 J 4 · Callirrhoe · Sinope · Autonoe · Aoede · Kore S/2003 J 2 Rings of JupiterNatural satellites of the Solar SystemPlanetary satellitesTerrestrial · Martian · Jovian · Saturnian · Uranian · Neptunian Other satellite systemsPlutonian · Eridian · Haumean · Asteroid satellitesLargest satellitesGanymede · Titan · Callisto · Io · Moon · Europa · Triton
Titania · Rhea · Oberon · Iapetus · Charon · Umbriel · Ariel · Dione · Tethys · Enceladus · Miranda · Proteus · Mimas Inner satellites · Trojans · Irregulars · List · List by diameter · Timeline of discovery · NamingJupiterMoonsMoons of Jupiter
Galilean moons: Io · Europa · Ganymede · Callisto CharacteristicsAtmosphere · Rings · Moons · TrojansExplorationPioneer program · Voyager program · Galileo (spacecraft) · Juno (spacecraft) · Europa OrbiterOther topicsJupiter-crosser asteroid · Earthly Branches · Colonization · Comet Shoemaker-Levy 9The Solar System The Sun · Mercury · Venus · Earth · Mars · Ceres · Jupiter · Saturn · Uranus · Neptune · Pluto · Haumea · Makemake · Eris Planets · Dwarf planets · Moons: Terrestrial · Martian · Jovian · Saturnian · Uranian · Neptunian · Plutonian · Haumean · Eridian Small bodies: Meteoroids · Asteroids/Asteroid moons (Asteroid belt, Centaurs, TNOs: Kuiper belt/Scattered disc) · Comets (Oort cloud) See also astronomical objects, the Solar System's list of objects, sorted by radius or mass, and the Solar System Portal

See also

  • Copernicus
  • Galileo Galilei
  • Jupiter
  • Natural satellite
  • Ptolemy
  • Solar System
  • Telescope


  1. ↑ Albert Van Helden, The Telescope in the Seventeenth Century, Isis 65(1): 38-58.
  2. 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 Galilei and Van Helden (1989), 14-16.
  3. ↑ Xi Zezong, The Discovery of Jupiter's Satellite Made by Gan De 2000 years Before Galileo, Chinese Physics 2(3): 664-67.
  4. 4.0 4.1 4.2 4.3 4.4 C. Marazzini, The names of the satellites of Jupiter: from Galileo to Simon Marius. Lettere Italiana. 57(3):391-407.
  5. ↑ Harvard University, µ value, IAU-MPC Satellites Ephemeris Service. Retrieved January 10, 2009.
  6. ↑ NASA, Jupiter: Facts & Figures, JPL/NASA. Retrieved January 10, 2009.
  7. ↑ IAG Travaux, Report of the IAU/IAG working group on cartographic coordinates and rotational elements of the planets and satellites: 2000. Retrieved January 10, 2009.
  8. ↑ R.M.C. Lopes, Lucas W. Kamp, William D. Smythe, Peter Mouginis-Mark, Jeff Kargel, Jani Radebaugh, Elizabeth P. Turtle, Jason Perry, David A. Williams, R.W. Carlson, and S. Douté, Lava Lakes on Io: Observations of Io's Volcanic Activity from Galileo NIMS During the 2001 Fly-bys, Icarus 169(1):140-174. Retrieved January 10, 2009.
  9. ↑ P. Schenk, Henrik Hargitai, Ronda Wilson, Alfred McEwen, and Peter Thomas, 2001, The Mountains of Io: Global and Geological Perspectives from Voyager and Galileo, Journal of Geophysical Research 106(E12): 33201-33222. Retrieved January 10, 2009.
  10. ↑ Porco, C.C., et al. 2003. Cassini imaging of Jupiter's atmosphere, satellites, and rings. Science. 299:1541-1547.
  11. ↑ McEwen, A.S., et al. 1998. High-temperature silicate volcanism on Jupiter's moon Io, Science 281: 87-90.
  12. ↑ F.P. Fanale, T.V. Johnson, and D.L. Matson, 1974, Io: A Surface Evaporite Deposit? Science 186 (4167): 922-925. Retrieved January 10, 2009.
  13. ↑ NASA, Europa: Another Water World? Jet Propulsion Laboratory. Retrieved January 10, 2009.
  14. ↑ Schenk, Chapman, Zahnle, and Moore (2004).
  15. ↑ C.J. Hamilton, Jupiter's Moon Europa, Solar Views. Retrieved January 10, 2009.
  16. ↑ Charles S. Tritt, Possibility of Life on Europa, Milwaukee School of Engineering. Retrieved January 10, 2009.
  17. ↑ ASU, Tidal Heating. Retrieved January 10, 2009.
  18. ↑ NASA, Exotic Microbes Discovered near Lake Vostok. Retrieved January 10, 2009.
  19. ↑ N. Jones, Bacterial explanation for Europa's rosy glow, NewScientist.com. Retrieved January 10, 2009.
  20. ↑ C. Phillips, Time for Europa. Retrieved January 10, 2009.
  21. ↑ B. Arnett, Europa. Retrieved January 10, 2009.
  22. ↑ The Galileo Project, Satellites of Jupiter. Retrieved January 10, 2009.
  23. ↑ Nine Planets.org, Ganymede. Retrieved January 10, 2009.
  24. ↑ M.G. Kivelson, K.K. Khurana and M. Volwerk, 2002, The Permanent and Inductive Magnetic Moments of Ganymede, Icarus 157: 507-522. Retrieved January 10, 2009.
  25. ↑ D.T. Hall, P.D. Feldman, M.A. McGrath, and D.F. Strobel, 1998, The Far-Ultraviolet Oxygen Airglow of Europa and Ganymede, The Astrophysical Journal 499: 475-481. Retrieved January 10, 2009.
  26. ↑ Aharon Eviatar, Vytenis M. Vasyliunas, Donald A. Gurnett, et al., The ionosphere of Ganymede, Plan. Space Sci. 49: 327-336.
  27. ↑ Susanna Musotto, Ferenc Varadi, William Moore, and Gerald Schubert, 2002, Numerical Simulations of the Orbits of the Galilean Satellites, Icarus 159:500-504. Retrieved January 10, 2009.
  28. ↑ R.W. Carlson, A Tenuous Carbon Dioxide Atmosphere on Jupiter's Moon Callisto, Science 283: 820-821. Retrieved January 10, 2009.
  29. ↑ M.C. Liang, B.F. Lane, R.T. Pappalardo, Mark Allen, and Yuk L. Yung, 2005, Atmosphere of Callisto, Journal of Geophysics 110: E02003. Retrieved January 10, 2009.
  30. ↑ Adam P. Showman and Renu Malhotra, 1999, The Galilean Satellites, Science 286: 77-84. Retrieved January 10, 2009.
  31. ↑ Jere H. Lipps, Gregory Delory, Joe Pitman, and Sarah Rieboldta, 2004, Astrobiology of Jupiter's Icy Moons, Proc. SPIE. 5555: 10. Retrieved January 10, 2009.
  32. ↑ Pat Trautman and Kristen Bethke, 2003, Revolutionary Concepts for Human Outer Planet Exploration (HOPE), NASA. Retrieved January 10, 2009.
  33. ↑ Donald K. Yeomans, 2006, Planetary Satellite Physical Parameters, JPL Solar System Dynamics. Retrieved January 10, 2009.
  34. ↑ Jupiter near perihelion 2010-Sep-19: 656.7 (Callisto angular separation arcsec) - 24.9 (jup angular radius arcsec) = 631 arcsec = 10 arcmin


  • Galilei, Galileo, Albert Van Helden (trans.). 1989. Sidereus Nuncius. Chicago, IL: University of Chicago Press. ISBN 9780226279039.
  • Leutwyler, Kristin, and John R. Casani. 2003. The Moons of Jupiter. New York, NY: W.W. Norton. ISBN 0393050602.
  • Schenk, P.M., C.R. Chapman, K. Zahnle, and J.M. Moore. "Chapter 18: Ages and Interiors: the Cratering Record of the Galilean Satellites." In Bagenal, Fran, Timothy E. Dowling, William B. McKinnon (eds.), 2004. Jupiter: The Planet, Satellites and Magnetosphere. New York, NY: Cambridge University Press. ISBN 9780521818087.

External links

All links retrieved May 18, 2017.

  • Animation of Galileo's observation, March 1613.