National Aeronautics and Administration

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Galilean of The Jupiter’s four largest moons, or , are called the same face towards Jupiter as they , meaning that each SIGNIFICANT DATES the , after Italian Galilei, who turns once on its axis for every orbit around Jupiter. 1610 — and independently discover observed them in 1610. The German astronomer Simon Marius Voyagers 1 and 2 offered striking color views and global per- four moons orbiting Jupiter. This discovery, among others by apparently discovered them around the same . The names spectives from their flybys of the Jupiter in 1979. From Galileo, helped change the way people thought about the heav- Marius proposed for the moons in 1614 (suggested to him by a 1995 to 2003, the Galileo made observations from ens. The prevailing idea of the time was that all heavenly bodies fellow astronomer, ) are the ones we use today repeated elliptical around Jupiter, making numerous close orbit the : a planet with its own small orbiting bodies did — , , , and . approaches over the of the Galilean moons and pro- not conform to this . Io is the most volcanically active body in the . Its ducing images with unprecedented detail of selected portions of 1979 — photographs an erupting on Io: the is covered by and in many colorful forms. As the surfaces. first ever seen anywhere other than Earth. Io travels in its slightly elliptical orbit, Jupiter’s immense 1979–2000 — Using data from the Voyager and Galileo space- Close-up images taken by the Galileo spacecraft of portions causes “” in Io’s surface 100 meters (300 feet) high, craft, scientists gather strong evidence of an beneath the of Europa’s surface show places where has broken up and generating enough to give rise to the volcanic activity and icy of Europa; Galileo data indicate within Gany- moved apart, and where may have come from below and drive off most . Io’s volcanoes are driven by hot mede and Callisto. frozen on the surface. The low number of craters on Europa . 2003 — The Galileo mission ends with the spacecraft deliber- leads scientists to believe that a subsurface ocean has been ately descending into Jupiter’s and being vaporized. Europa’s surface is mostly water ice, and the icy crust is believed present in recent geologic history and may still exist today. The Mission controllers purposely put Galileo on a course to cover a global water ocean. Europa is thought to have twice heat needed to melt the ice in a place so far from the is with Jupiter to eliminate any chance that the spacecraft would as much water as does Earth. This moon intrigues astrobiolo- thought to come from inside Europa, resulting primarily from the crash into Europa and contaminate that moon with terrestrial gists because of its potential for having a “habitable zone.” same type of tidal that drive Io’s volcanoes. The possibil- microbes. forms have been found thriving near underwater volcanoes on ity of life existing on Europa in a subsurface ocean is so compel- Earth and in other extreme locations that are possible analogs to ling that scientists plan to send another spacecraft in 2020, the ABOUT THE IMAGES what may exist at Europa. Jupiter , to study this intriguing moon. 1 2 1 A comparison “portrait” of Jupiter’s Ganymede is the largest moon in the solar system (larger than FAST FACTS the planet ), and is the only moon known to have its 3 four Galilean moons own internally generated magnetic . Callisto’s surface is ex- Distance from Jupiter Io, Europa, Gany- tremely heavily cratered and ancient — a record of events from Io 422,000 km (262,200 mi) 4 5 mede, and Callisto, the early history of the solar system. However, at a small scale, Europa 671,000 km (417,000 mi) each with different Callisto shows very few craters, suggesting that are Ganymede 1,070,000 km (665,000 mi) characteristics. (In this composite, Jupiter is not at the active today. Callisto 1,883,000 km (1,170,000 mi) same scale as the satellites.) 2 During one of Io in 2000, the Galileo spacecraft photo- The interiors of Io, Europa, and Ganymede have a layered Satellite Mean Radius graphed Tvashtar Catena, a chain of giant erupting volcanoes. structure (as does Earth). Io has a core, and a of at least Io 1821.6 km (1,131.9 mi) and at the left of the image show newly erupted partially molten , topped by a crust of solid rock coated with Europa 1,560.8 km (969.8 mi) hot lava, seen in the false-color image because of sulfur compounds. Europa and Ganymede each have an - Ganymede 2,631 km (1,635 mi) emission. rich core; a rock envelope around the core; a thick, soft ice layer; Callisto 2,410 km (1,498 mi) 3 This false-color image of Europa shows the icy crust broken and a thin crust of impure water ice. Layering at Callisto is less Satellite Orbital (Earth Days) up into blocks that appear to have “rafted” into new positions. well defined and appears to be mainly a mixture of ice and rock. Io 1.769 Like Europa, Ganymede and Callisto have oceans, but they are Europa 3.551 4 Fresh, bright material was thrown out of an impact on deeper and less accessible than Europa’s, and sandwiched be- Ganymede 7.155 Ganymede. tween ice layers rather than in contact with their mantles. Callisto 16.689 5 Ice on Callisto excavated by younger impact craters con- Three of the moons influence each other in an interesting way. Satellite (g/cm3) trasts with darker, redder coatings on older surfaces. Io is in a tug-of-war with Ganymede and Europa, and Europa’s Io 3.528 FOR MORE INFORMATION (time to go around Jupiter once) is twice Io’s Europa 3.013 period, and Ganymede’s period is twice that of Europa. In other Ganymede 1.942 words, every time Ganymede goes around Jupiter once, Europa Callisto 1.834 makes two orbits and Io makes four orbits. The moons all keep Jupiter&Display=Moons

LG-2009-09-574-HQ — JPL 400-1344M 09/09