Photos: The Galilean Moons of Jupiter Photos: The Galilean Moons of Jupiter By SPACE.com | June 14, 2013 06:49am ET MORE Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute Introduction 67 moons orbit the great gas giant Jupiter; of these, the four largest are known as the Galilean moons, having been discovered by Galileo Galilei using his telescope in 1610. The four moons are Io, Europa, Ganymede, and Callisto, in order of distance from Jupiter. (Their names derive from lovers of Zeus.) These moons provided evidence that not all celestial bodies orbit the Earth, a powerful revelation as, up until that time, astronomers considered Earth the center of the universe. [See Jupiter’s Moons reference page for more information.] First: Io Credit: NASA/JPL/University of Arizona Io Io orbits closest to Jupiter, out of the four Galilean moons.

Io has been studied by spacecraft from Pioneer 10 and 11 in 1973 and 1974, all the way up to New Horizons in 2007, augmented by Earth- and space-based telescopes. This satellite possesses volcanoes, the only celestial body in the solar system other than Earth known to harbor volcanic activity. In fact, Io represents the most geologically active object in the solar system. Io contains an iron or iron sulfide core and a brown silicate outer layer, producing colorful orange, yellow, black, red, and white patches on the surface. Io also possesses sulfur dioxide snowfields, covering much of its surface. [See more information about Io .] Next: Europa Credit: NASA/JPL/University of Arizona Europa Europa, slightly smaller than Earth’s moon, represents one of the largest bodies in the solar system, though smaller than the other Galilean satellites of Jupiter. Cracks and streaks mark the entirety of the icy surface, which contains few craters, as the shiny, smooth skin of the moon only dates back 20 to 180 million years, a youthful age. Researchers speculate that a liquid water ocean lies beneath the ice.

[See more information about Europa .] Next: Ganymede Credit: NASA Ganymede Ganymede, the third Gallilean moon, looms larger than the others. It compares in size to Mercury, but possesses about half the mass of that planet. Ganymede distinguishes itself from the other Galilean moons by possessing its own magnetic field. A thick crust of mostly ice wraps around the satellite’s iron core. Highly cratered dark regions cover 40 percent of Ganymede’s surface, while the remaining 60 percent hosts a light grooved terrain, forming intricate patterns on that moon. [See more information about Ganymede .] Next: Callisto Credit: NASA Callisto Callisto, the fourth Galilean moon, farthest from Jupiter, represents the most heavily cratered object in the solar system. The moon’s landscape remains largely unchanged since the time of its formation, attracting much interest from researchers. Callisto roughly approximates Mercury in size, but the moon possesses a lower density. It also experiences the least amount of effect from Jupiter’s magnetic field, as it orbits farther away from the planet, beyond Jupiter’s primary radiation belt.

[See more information about Callisto .] 6 of 6 The Galilean Moons of Jupiter The Galilean Moons of Jupiter Summary: Jupiter has more than 60 known moons, but understanding the geology of its four largest will hopefully lead to some groundbreaking discoveries. Sections: The Galilean Moons Each of the Jovian planets has a number of moons, although Jupiter has the most with more than 60 catalogued to date. Jupiter's 4 largest moons exhibit some of the most interesting geology in the solar system. They were discovered by Galileo Galilei and are known as the Galilean moons. Jupiter's largest moon, Ganymede, is larger than Mercury while the other three are larger than Pluto. Outward from Jupiter: Io: Volcanoes and All Jupiter's Moon Io The Voyager spacecraft took the first close-up images of Io more than 300 years after the moon's discovery. The images showed a surface with no signs of craters from past impacts. What we saw instead was a surface almost entirely covered with large volcanoes. Cameras on Voyager actually captured volcanic eruptions in progress.

The frequency of these sulfuric eruptions has filled in almost all of the impact craters and left Io with one of the youngest looking surfaces in the solar system. Io Close-Up (click to enlarge) Close-up photos of eruptions in progress show powerfully hot lava glowing orange and red. Photos taken on the night side of Io show not only the hot volcanic vents, but also a thin sulfur dioxide atmosphere produced by constant outgassing. Io's unusual red and orange colors come primarily from sulfur, which condenses on the surface after being outgassed by the volcanoes. Although there is no direct evidence of tectonic activity on Io, scientists feel confident it exists since the processes that fuel volcanism also fuel tectonics. The volcanic eruptions are so frequent and cover the surface so thoroughly that any clear evidence of tectonic activity is likely to be buried. Europa: What Lies Beneath? Europa Europa's surface and crust are made almost entirely of water ice, and its bizarre, fractured appearance is proof enough that tidal heating has acted there. The icy surface is nearly devoid of impact craters and may be only a few million years old.

Europa's Interior Observations made by the Galileo spacecraft show that Europa has a metallic core and a rocky mantle. Surrounding the rocky interior appears to be an icy layer 100 kilometers thick, the top few kilometers of which seem to be frozen solid. The stretching and squeezing of tidal friction should provide enough heat to melt some of this into liquid water beneath a thin ice shell. If it does, then Europa may have an ocean with more than twice as much liquid water as all of Earth's oceans combined. Analyzing Europa's Cracked Surface Close-up photos of the surface of Europa support the idea of a liquid ocean beneath the surface. These photos, taken by the Galileo spacecraft, show what appears to be icebergs stuck in a layer of ice. Other evidence comes from double-ridged cracks on the surface. Tidal flexing that allows water to well up and build ridges may create these cracks. Ganymede: Largest Moon in the Solar System Two Regions of Ganymede's Surface (click to enlarge) The surface of Ganymede shares many similarities with Europa. Ganymede's surface is also made of water ice, but unlike Europa's surface, it shows signs of varying age.

The darker regions are heavily cratered, suggesting they are billions of years old. The lighter regions show no signs of craters and it is thought that eruptions of water covered the surface before freezing over. These areas are geologically younger than the darker regions. If liquid water occasionally makes its way to the surface to fill in craters, could that suggest a liquid ocean similar to the one that might exist on Europa? Callisto: The Outermost Galilean Moon Callisto Callisto is the stereotypical outer solar system satellite. It is one of the largest and most heavily cratered satellites in the solar system. The surface is very icy and dates back four billion years. Beneath the icy crust is possibly a salty ocean supported by a deeper rocky interior. Callisto's Surface Callisto doesn't have any large mountains, show evidence of volcanic or tectonic activity or have any appreciable level of internal heat. Nonetheless, observations of Callisto's magnetic field may cause scientists to add the large moon to the list of possible worlds with subsurface salty oceans.

Jupiter - The Galilean satellites | planet | Britannica.com The Galilean satellites Europa Galileo proposed that the four Jovian moons he discovered in 1610 be named the Medicean stars, in honour of his patron, Cosimo II de’ Medici , but they soon came to be known as the Galilean satellites in honour of their discoverer. Galileo regarded their existence as a fundamental argument in favour of the Copernican model of the solar system, in which the planets orbit the Sun. Their orbits around Jupiter were in flagrant violation of the Ptolemaic system , in which all celestial objects must move around Earth. In order of increasing distance from the planet, these satellites are called Io, Europa, Ganymede, and Callisto, for figures closely associated with Jupiter in Greek mythology . The names were assigned by the German astronomer Simon Marius , Galileo’s contemporary and rival, who likely discovered the satellites independently. There proved to be a particular aptness in the choice of Io’s name: Io—“the wanderer” (Greek iōn, “going”)—has an indirect influence on the ionosphere of Jupiter, as discussed above.

Although approximate diameters and spectroscopic characteristics of the Galilean moons had been determined from Earth-based observations, it was the Voyager missions that indelibly established these four bodies as worlds in their own right. The Galileo mission provided a wealth of additional data. Before Voyager it was known that Callisto and Ganymede are both as large as or larger than the planet Mercury ; that they and Europa have surfaces covered with water ice; that Io’s orbit is surrounded by a torus of atoms and ions that include sodium, potassium, and sulfur; and that the inner two Galilean moons have mean densities much greater than those of the outer two. This density gradient from Io to Callisto resembles that found in the solar system itself and seems to result from the same cause (see below Origin of the Jovian system ). The density values suggest that Io and Europa have a rocky composition similar to that of the Moon, whereas roughly 50 percent of Ganymede and Callisto must be made of a much less dense substance, water ice being the obvious candidate.

Montage of Jupiter’s four Galilean moons—(left to right) Io, Europa, Ganymede, and … NASA/JPL/DLR Saturn The icy surface of this satellite is so dominated by impact craters that there are no smooth plains like the dark maria observed on the Moon. In other words, there seem to be no areas on Callisto where upwelling of material from subsequent internal activity has obliterated any of the record of early bombardment. This record was formed by impacting debris (comet nuclei and asteroidal material) primarily during the first 500 million years after the formation of the solar system in much the same way that the craters on the Moon were produced. The unmodified appearance of the surface is consistent with the absence of a differentiated interior. Evidently no tidally induced global heating and consequent melting occurred on Callisto, unlike the other three Galilean moons. The Galileo spacecraft revealed that craters smaller than 10 km (6 miles) are hidden by drifts of fine, dark material resembling a mixture of clay minerals.

A heavily cratered region near Callisto’s equator, in an image taken by the Galileo spacecraft on … NASA/JPL In addition to the predominant water ice, solid carbon dioxide is present on the surface, and an extremely tenuous carbon dioxide atmosphere is slowly escaping into space. Other trace surface constituents are hydrogen peroxide, probably produced from the ice by photochemical reactions driven by solar ultraviolet radiation; sulfur and sulfur compounds, probably coming from Io; and organic compounds that may have been delivered by cometary impacts. Callisto has a weak magnetic field induced by Jupiter’s field that may imply the existence of a layer of liquid water below its icy crust. Ganymede Planets: Fact or Fiction? Unlike Callisto, Ganymede , an equally icy satellite, reveals distinct patches of dark and light terrain. This contrast is reminiscent of the Moon’s surface, but the answer to which terrain came first—dark or light—is exactly reversed. In contrast to the Moon, the dark regions on Ganymede are the older areas, showing the heaviest concentration of craters. The light regions are younger, revealing a complex pattern of parallel and intersecting ridges and grooves in addition to unusually bright impact craters typically surrounded by systems of rays.

This manifestation of active crustal movement and resurfacing is accompanied by clear evidence of internal differentiation. Unlike Callisto, Ganymede has an iron-rich core and a permanent magnetic field that is strong enough to create its own magnetosphere and auroras. Hubble Space Telescope observations of how Ganymede’s auroras change when interacting with Jupiter’s magnetic field reveal the likely existence of a subsurface ocean about 100 km (60 miles) thick. The trace components identified in Ganymede’s icy surface include a smaller amount of the same claylike dust found on Callisto and the same traces of solid carbon dioxide, hydrogen peroxide, and sulfur compounds, plus evidence for molecular oxygen and ozone trapped in the ice. Portion of Ganymede’s icy surface showing characteristic dark and light grooved terrain, as … JPL/NASA/Brown University Ringling Bros. Folds Its Tent The surface of Europa is totally different from that of Ganymede or Callisto, despite the fact that the infrared spectrum of this object indicates that it, too, is covered with ice. There are few impact craters on Europa—the number per unit area is comparable to that on the continental regions of Earth, indicating that the surface is relatively recent.

Some scientists think the surface is so young that significant resurfacing is still taking place on the satellite. This resurfacing evidently consists of the outflow of water from the interior to form an instant frozen ocean. An overview of Jupiter’s moon Europa, including a discussion about the possibility of … © Open University (A Britannica Publishing Partner) An elaborately patterned area of disrupted ice crust on Europa’s surface, shown in an image made … NASA/JPL Models for the differentiated interior suggest the presence of an iron-rich core surrounded by a silicate mantle surmounted by an icy crust some 150 km (90 miles) thick. This moon possesses both induced and intrinsic magnetic fields. Slightly mottled regions on the surface have been found to contain salt deposits, suggesting evaporation of water from a reservoir below the crust. Europa’s frozen surface is crisscrossed with dark and bright stripes and curvilinear ridges and grooves. Spatter cones along some of the grooves again suggest fluid eruptions from below. The relief is extremely low, with ridge heights perhaps a few hundred metres at most. Europa thus has the smoothest surface of any solid body examined in the solar system thus far.

Traces of sulfur, sulfur compounds, hydrogen peroxide, and organic compounds have been identified on the surface. Connect with Britannica Facebook Twitter YouTube Instagram Pinterest The major open question is whether there is a global ocean of liquid water beneath Europa’s ice, warmed by the release of tidal energy in Europa’s interior. The possibility of such an ocean arose from Voyager data, and high-resolution Galileo images suggested fluid activity near the surface. In addition, explanation of Europa’s induced magnetic field appears to require an interior, electrically conducting fluid medium, implying a salt-containing liquid water layer at some depth beneath the surface ice. If this ocean and its required source of heat exist, the possible presence of at least microbial life-forms must be admitted (see the article extraterrestrial life ). Io Seen through a telescope from Earth, Io appears reddish orange, while the other moons are neutral in tint. Io’s infrared spectrum shows no evidence of the absorption characteristics of water ice. Scientists expected Io’s surface to look different from those of Jupiter’s other moons, but the Voyager images revealed a landscape even more unusual than anticipated. An overview of Io, a moon of Jupiter with many active volcanoes.

© Open University (A Britannica Publishing Partner) Jupiter’s moon Io with Jupiter in the background, photographed by the Voyager 1 spacecraft on March … Photo NASA/JPL/Caltech (NASA photo # PIA00378) Volcanic fissures , instead of impact craters, dot the surface of Io. Nine volcanoes were observed in eruption when the two Voyager spacecraft flew by in 1979, while the closer encounters by Galileo indicated that as many as 300 volcanic vents may be active at a given time. The silicate lava emerging from the vents is extremely hot (about 1,900 K [3,000 °F, 1,630 °C]), resembling primitive lavas on early Earth. This unprecedented level of activity makes Io the most tectonically active object in the solar system. The surface of the satellite is continually and completely replaced by this volcanism in just a few thousand years. Various forms (allotropes) of sulfur appear to be responsible for the black, orange, and red areas on the moon’s surface, while solid sulfur dioxide is probably the main constituent of the white areas.

Sulfur dioxide was detected as a gas near one of the active volcanic plumes by Voyager’s infrared spectrometer and was identified as a solid in ultraviolet and infrared spectra obtained from Earth-orbital and ground-based observations. These identifications provide sources for the sulfur and oxygen ions observed in the Jovian magnetosphere and prove that Io’s volcanic activity is the source of its torus of particles. Before-and-after images of a volcanic eruption on Jupiter’s moon Io, made by the Galileo spacecraft … Photo NASA/JPL/Caltech (NASA photo # PIA00744) Britannica Lists & Quizzes Origin of the jovian ring and the galilean satellites Nature 280, 300 - 302 (26 July 1979); doi:10.1038/280300a0 Origin of the jovian ring and the galilean satellites A. J. R. PRENTICE† & D.

TER HAAR*‡ †Department of Mathematics, Monash University, Clayton Victoria 3168, Australia ‡Instituut voor Theoretische Fysica, Princetonplein 5, Utrecht, 3508TA, The Netherlands *Permanent address: Department of Theoretical Physics, 1 Keble Road, Oxford, UK. THE discovery by Voyager I of a thin flat stream of rocks orbiting Jupiter at a radius of some 130,000 km, or ~2R P (R p = 6.677 × 109 cm = present polar radius), in its equatorial plane opens a new avenue of information in our understanding of the formation of the Solar System. Each of the major planets which possesses a regular satellite system also possesses a ring system. Theories for the formation of the major planets should therefore address themselves to this phenomenon of the co-appearance of rings and regular satellite systems, especially in considering the question whether Neptune, which has no regular satellites, may also not have a ring. We suggest here that Jupiter's rocky belt formed through condensation from a gaseous disk which was shed by the rotating gaseous envelope that originally contracted to form that planet.

The four galilean satellites were formed from gaseous rings which were shed by the same envelope at an earlier and more turbulent stage in its evolution. It is possible that Jupiter may possess another rocky satellite belt at orbital radius 4R P 1. References Untitled Document Planets and their moons Having formed in a single rotating disk, all of the planets orbit the Sun in the same direction (anticlockwise when viewed from above) and pretty much the same plane. Most, including most of any moons they may have, all rotate the same way too (also anticlockwise when viewed from above). Exceptions are generally thought due to the high number of collisions which took place between the planets and their moons and other objects during the Solar Systems early history. The Sun is the only object in the Solar System to produce its own light. The planets and their moons are seen because they reflect the Suns light. Distances within the Solar System are measured in Astronomical Units, the mean distance between the Sun and the Earth (1AU = 150 million kilometres). Our knowledge and understanding of the planets and their moons continues to increase dramatically as Earth and space-based observations and remote probes provide us with more and more information.

Mercury Mercury is the nearest planet to the Sun. At a mean distance of 58 million miles (0.4AU), and travelling at about 48km/s, it takes Mercury about 88 days to orbit the Sun once. Mercury rotates on its axis once every 59 days. With a diameter of about 4,878km (no polar flattening), Mercury is 0.4 times the size of the Earth. 60% of Mercurys surface is heavily cratered, the remaining 40% consists of relatively flat lava plains. The largest surface feature observed on Mercury is known as the Caloris Basin, a lava filled impact crater about 1,300km across. Mercurys surface is also covered in wrinkles or rupes, formed as the planet contracted as it cooled shortly after formation. With daytime highs of 430C and nightime lows of -170C, Mercury experiences the greatest temperature range of all the planets (though Venus is the hottest). Mercury possesses a very thin and temporary atmosphere of helium, sodium and oxygen with traces of argon, potassium and hydrogen. Mercury has no moons. Venus Venus is the second planet from the Sun.

At a mean distance of 108 million kilometres (0.7AU), and travelling at about 35km/s, it takes Venus about 224.7 days to orbit the Sun once. Venus exhibits retrograde rotation (it rotates the opposite way to other planets), spinning on its axis once every 243 days. With a diameter of about 12,102km (no polar flattening), Venus is about 0.9 times the size of the Earth. Venus possesses a dense, cloudy atmosphere dominated by carbon dioxide with minor amounts of nitrogen, water vapour, sulphur dioxide and sulphuric acid. Atmospheric pressure is about 90 times that on Earth. The surface of Venus is known to be cratered and mountainous with evidence of much volcanic activity. With highs of about 480ï¿½C, Venus is the hottest planet in the Solar system (runaway Greenhouse Effect). Venus has no moons. Earth The Earth is the third planet from the Sun.

At a mean distance of 150 million kilometres (1.0AU), and travelling at about 30km/s, it takes the Earth about 365.25 days to orbit the Sun once. The Earth rotates on its axis once every 23 hours 56 minutes. The Earth is not perfectly spherical. Its equatorial diameter of 12,756km is slightly greater than its diameter measured from pole to pole (0.3% polar flattening). The Earth possesses a protective atmosphere dominated by nitrogen and oxygen with traces of carbon dioxide, water vapour and other gases. About 70% of the Earthï¿½s surface is covered by water making it unique in the Solar System. Surface features on its land masses are wide and varied. The Earths crust and upper mantle are divided into about 12 mobile tectonic plates, the edges of which are defined by earthquake and volcanic activity. Surface temperatures range from about 60ï¿½C to -90ï¿½C. The Earth has one moon, the Moon. Many astronomers consider the Earth and the Moon to be a binary system. The Earth is the only planet known to support life for sure.

Mars Mars is the fourth planet from the Sun. At a mean distance of 228 million kilometres (1.5AU), and travelling at about 24km/s, it takes Mars about 687 days to orbit the Sun once. Mars rotates on its axis once every 24 hours 37 minutes. With an equatorial diameter of about 6,786km (0.5% polar flattening), Mars is about 0.5 times the size of the Earth. Mars possesses a thin atmosphere dominated by carbon dioxide with minor amounts of nitrogen, argon and other gases. Surface features on Mars are wide and varied and include mountains, deserts, canyons, volcanoes, craters and polar caps of frozen carbon dioxide. Many features were once produced by flowing water which has since ï¿½disappearedï¿½. Two impressive features on Mars are Olympus Mons, an extinct volcano about 600km across and 25km high (the largest volcanic feature in the Solar System), and Valles Marineris, a canyon system over 4,000km long and up to 7km deep.

Surface temperatures range from about 20C to -140C. Mars has two irregularly shaped moons, Phobos (22km across) and Deimos (13km across). The controversial discovery of microfossils in a meteorite thought to have come from Mars has recently rekindled the life on Mars debate. Jupiter Jupiter is the fifth planet from the Sun and the first of the gas giants. At a mean distance of 778 million kilometres (5.2AU), and travelling at about 13km/s, it takes Jupiter about 11.86 years to orbit the Sun once. Jupiter rotates on its axis once every 9 hours 48 minutes. With an equatorial diameter of about 142,980km (6.7% polar flattening), Jupiter is about 11.2 times the size of the Earth and the largest planet in the Solar System. As a gas giant, Jupiter has no real surface as such. Its atmospheric cloud layers, dominated by hydrogen and helium with traces of methane, ammonia and water vapour, give Jupiter a banded and colourful yet turbulent appearance.

The temperature at Jupiterï¿½s cloud tops is about -110C. Jupiters most prominent feature is its phosphorous-rich Great Red Spot, a long lived, rotating, anticyclonic storm reaching up to 40,000km across. Jupiter has a narrow and faint ring system of very fine grained dust particles and 16 known moons, the largest of which, the Galilean moons, are Io, Europa, Ganymede and Calisto. Io is known to be volcanically active. Europas icy surface layers are thought by some astronomers to hide underlying oceans of water which might support life Saturn Saturn is the sixth planet from the Sun and the second of the gas giants. At a mean distance of 1,427 million kilometres (9.5AU), and travelling at about 10km/s, it takes Saturn about 29.46 years to orbit the Sun once. Saturn rotates on its axis once every 10 hours 15 minutes. With an equatorial diameter of about 120,540km (10.4% polar flattening), Saturn is about 9.5 times the size of the Earth.

Like Jupiter, Saturn has no real surface. Its atmospheric cloud layers are dominated by hydrogen and helium with traces of methane, ammonia and water vapour. The cloud layers are thick and show only faint banding. The temperature at Saturnï¿½s cloud tops is about - 180C. Saturn is best known for its prominent and spectacular ice and dust dominated ring system (gravitationally trapped debris or a failed moon). Saturnï¿½s rings vary in thickness up to only 200m but extend into space around the planet for about 1 million kilometres (diameter). Saturn has at least 18 confirmed moons (possibly as many as 24), the largest of which is Titan (about 5,000km across) with a thick, nitrogen dominated atmosphere. Many moons are co-orbital. Uranus Discovered in 1781, Uranus is the seventh planet from the Sun and the third of the gas giants. At a mean distance of 2,871 million kilometres (19.2AU), and travelling at about 7km/s, it takes Uranus about 84.01 years to orbit the Sun once.

Uranus rotates on its ï¿½sideï¿½ once every 17 hours 14 minutes. With an equatorial diameter of about 51,118km (2.3% polar flattening), Uranus is about 4.0 times the size of the Earth. Like the other gas giants, Uranus has no real surface. Its almost featureless atmospheric cloud layers of hydrogen, helium and methane are bluish in colour. The temperature at Uranusï¿½s cloud tops is about -216C. Uranus possesses a faint ring system made out of the darkest material known in the Solar System and 15 known moons, all named after Shakespearean characters. The most unusual moon is Miranda with its patchwork surface features. Neptune Discovered in 1846, Neptune is normally the eighth planet from the Sun and the last of the gas giants. At a mean distance of 4,497 million kilometres (30.0AU), and travelling at a little over 5km/s, it takes Neptune about 164.80 years to orbit the Sun once. Neptune rotates on its axis once every 16 hours 6 minutes.

With an equatorial diameter of about 49,528km (1.8% polar flattening), Neptune is about 3.9 times the size of the Earth. Like Uranus, its twin, Neptune has no real surface. Its atmospheric cloud layers, dominated by hydrogen, helium and methane, are blue in colour and faintly banded. The temperature at Neptunes cloud tops is about -216C. Neptunes one time most prominent feature, the Great Dark Spot, a rotating, anticyclonic storm over 20,000km across, no longer exists. Winds sweep around Neptunes equator at up to 2,000km/h making it the windiest planet in the Solar System. Neptune possesses a faint ring system and 8 known moons, the largest of which is Triton (2,700km across, retrograde orbit). Triton is consistently the coldest place in the Solar System with estimated surface temperatures as low as -235C Pluto Discovered as recently as 1930, Pluto is normally the furthest and ninth planet from the Sun.

At a mean distance of 5914 million miles (39.5AU), and travelling at almost 5km/s, it takes Pluto about 248.5 years to orbit the Sun once. Plutoï¿½s orbit around the Sun is so elliptical that for about 20 years of its orbital period it is closer to the Sun than Neptune (e.g. between 1979 and 1999). Pluto rotates on its axis once every 6.4 days. With a diameter of only 2,300km (no polar flattening), Pluto, at 0.2 times the size of the Earth, is the smallest planet in the Solar System. Pluto may possess a very thin, temporary atmosphere of methane and nitrogen. Pluto probably has a surface temperature of about -220C. Pluto has one moon, Charon. Many astronomers consider Pluto together with its single moon to be a binary system.

Jupiter's Moons: Facts About the Largest Jovian Moons Jupiter's Moons: Facts About the Largest Jovian Moons By Kim Ann Zimmermann | July 5, 2012 05:33pm ET MORE This montage shows the best views of Jupiter's four large and diverse "Galilean" satellites as seen by the Long Range Reconnaissance Imager (LORRI) on the New Horizons spacecraft during its flyby of Jupiter in late February 2007. The four moons are, from left to right: Io, Europa, Ganymede and Callisto. The images have been scaled to represent the true relative sizes of the four moons and are arranged in their order from Jupiter. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute With a total of 67 known moons — including four large moons known as the Galilean satellites — Jupiter almost qualifies as a solar system unto itself. Most of the moons of Jupiter are small, with about 50 of the satellites being less than 6.2 miles in diameter.

The number of moons changes fairly frequently, with 66 and 67 being confirmed at the end of 2011. Most of the moons were discovered in the late 1970s and later as a result of several explorations by automated spacecraft, including NASA’s Voyager in 1979 and Galileo in 1995. Not only is Jupiter the largest planet in the solar system , it is also the most massive at more than 300 times the mass of Earth. Its size plays a role in the number of moons orbiting Jupiter because there is a large area of gravitational stability around it to support many moons. Jupiter also has the strongest magnetic field of any planet, so anything passing near it, such as an asteroid, is either destroyed by gravitational tides or captured into its orbit. Earth only has one moon because it lacks the strong gravitational field and mass necessary to hold another satellite in orbit. The moons of Jupiter have orbital periods ranging from seven hours to almost three Earth years. Some of the orbits are nearly circular, while the moons farthest from Jupiter have more irregular orbits.

The outer 33 moons orbit in the opposite direction in which Jupiter spins, which is unusual and indicates the moons were asteroids that were sucked into Jupiter’s orbit after the initial system was formed. A translation of the key passages of Galileo Galilei's journal detailing his discovery of four moons orbiting Jupiter. Galileo made these sketches after discovering four moons orbiting Jupiter in January 1610. The moons, later named Io, Europa, Callisto and Ganymede, were the first discovered beyond Earth. Credit: NASA Galilean moons In January 1610, Italian astronomer Galileo Galilei discovered four of Jupiter’s moons — now called Io, Europa, Ganymede, and Callisto. He originally referred to the individual moons numerically as I, II, III, and IV. The numerical system for naming the moons lasted for a few centuries until scientists determined that simply using numbers as a naming device would be confusing and impractical as more moons were discovered. Galileo’s discovery was pivotal point in the history of astronomy as his observation revealed that not all celestial bodies revolved around the Earth. Until that time, Earth was thought to be the center of the universe.

Eight satellites — the four Galilean and four smaller moons — are closer to the planet and provide the dust that make up Jupiter's rings. The closest of the Galilean moons to Jupiter is Io, the first moon to be discovered by Galileo . This satellite’s distinctive feature is its volcanoes, making it the only celestial body in the solar system besides Earth to have volcanic activity. This moon also has sulfur dioxide snowfields, leading to its characterization as a moon of fire and ice. Io has an iron or iron sulfide core and a brown silicate outer layer, which gives it a splotchy orange, yellow, black, red, and white appearance. Moving outward from Jupiter is Europa . While slightly smaller than Earth’s moon, it is still one of the largest bodies in the solar system but the smallest of the Galilean satellites. Cracks and streaks crisscross the entire icy surface, which is marked with very few craters. Europa has a high degree of reflectivity, making it among the brightest moons in the solar system. At 20 to 180 million years old, the surface is fairly young. It is possible that an extensive ocean beneath the surface harbors life.

Ganymede is the third Galilean moon from Jupiter and the largest of the four. This low-density moon is about the size of Mercury but has about half the mass. Its outstanding characteristic is that it is the only moon to have its own magnetic field. The satellite’s iron core is topped off by a thick crust that is mostly ice. Forty percent of the surface of Ganymede is covered by highly cratered dark regions, and the remaining sixty percent is covered by a light grooved terrain, which forms intricate patterns across Ganymede. Callisto , the fourth and farthest of the Galilean moons from Jupiter, is the most heavily cratered object in the solar system. The moon’s landscape has essentially remained unchanged since its formation, which has garnered much interest among scientists. It is about the size of Mercury but very low in density. It is also experiences the least impact of Jupiter’s magnetic field as its orbit is the farthest from the planet and beyond Jupiter’s primary radiation belt. — Kim Ann Zimmermann, SPACE.com Contributor Related: Galilean Satellites Galilean Satellites: The satellites discovered by Galileo with his small telescope form a small, `mini' solar system around Jupiter.

They each have special characteristics related to their formation process, but have the following traits in common: 1) all orbit Jupiter 2) they all are tidally locked to Jupiter 3) they all have radii larger than our Moon 4) the inner moons have densities higher than outer moons (implies that Jupiter was much warmer in the past, such that the moons formed near Jupiter have less of the volatile elements such as CO2 and H2O) Io: Io is the innermost world, closest to Jupiter and can be classified as one of the most unusual moons in our solar system. Its unique properties include: surface of Io is a mottled yellow, reds and blackish-browns = sulfur compounds, where different colors correspond to regions with different temperatures. The mean temperature on Io's surface is about -143 C (-230 F). A hot spots associated with a volcanic features measure about 17 C (60 F). Scientists believe the hot spots may be lava lakes, although the temperature indicates the surface is not molten. thin patchy atmosphere of SO2 active plumes (not volcanos since there is no mantle), geysers of molten sulfur and SO2.

The mechanism for heating Io's core is tidal friction from Jupiter. Jupiter's gravitational field is much stronger than, say, the Moon's on the Earth. The tides pull and squeeze the core of the moons around Jupiter producing mechanical friction. This friction is turned into heat, which makes the inner moons much more active than the outer moons. When liquid sulfur on surface flows into vents and makes contact with the hot interior, there is an explosive conversion into gas producing the geyers. Io is composed primarily of rocky material with very little iron. Io has the ``youngest surface'' in the Solar System, constantly changing, as the above two images demonstrate. The one on the left is from Voyager, the one of the right is from the Galileo probe, 20 years later. no impact craters, which means that the surface of Io is very young, less than a million years old = lots of erosion Europa: Europa is the next world out from Io and is considered a strong candidate to find primitive life.

Its characteristics are: icy world with high albedo, smooth surface with only a few hills/mountains and craters, meaning the surface is young and renewing itself large long cracks on the surface, due to ice plate tectonics floating on water mantle visible surface is actually H2O ice mixed with rock (dirty icefield) about 100 km thick Ganymede: Ganymede is much less dense than Europa or Io.

Its interesting characteristics are: very diversified surface with dark regions, valleys, mountains, evidence of past tectonic activity and lots of vertical relief low mean density implies a very deep ice mantle, about 1000 km thick white craters and rims = impact exposes mantle ice ghost craters = smoothed by ice flow thin oxygen atmosphere, possible aurora at its poles (magnetic field?) Callisto: Callisto is the outermost of the 4 primary satellites: similar surface to Ganymede but with 10 times more cratering, which means less tectonic activity, cooler interior little vertical relief = craters destroyed landscape = weak surface material (low density) large impact basins Summary of Galilean Moons: mini-Solar System model where the closer moons are rocky, high density and the outer moons are icy, low density geological activity decreases with distance from Jupiter, due to the decreasing tidal friction from Jupiter on interior of moons - this has a marked influence on the appearance of their surfaces little or no atmospheres high icy component to crust of all moons Titan: Titan is the largest satellite of Saturn, unique in its methane atmosphere: detected to have spectral lines of methane in 1944 heavy atmosphere (1.4 times the Earth's

) with structure in the form of haze - reddish color from organic molecules formed by breakdown of CH4 (methane) by cosmic rays composition of atmosphere is 90% N2, 7% CH4, 2% Ar and traces of ethane, acetylene, ethylene, propane and hydrogen cyanide (i.e.

smog) Titan's surface temperature averages around -178C (-289F) the temperature and pressures are near the triple point of methane (CH4) -> methane rain, snow, ice, clouds, rivers, etc. Earth's environment is near the triple point of water - water serves as the solvent for organic chemicals to interact = life. Titan is an excellent place to search for new forms of life based on methane as a solvent Recent images from the Cassini-Huygens mission: Huygens descent movie Above is a moasic of three frames taken by the lander as it entered Titans atmoshere. It shows detail of a high ridge area including the flow down a major river channel (liquid methane river) into what appears to be a "wet" plain. The above image shows the boundary between lighter-colored uplifted terrain and drainage channels into the darker lower areas. This picture was taken from an altitude of 8 kilometers. Above is a color image from the surface of Titan. Rocks near the bottom are pebble-sized, objects in the middle of the frame are meters in size. The surface is darker than expected, consisting of a mixture of water and hydrocarbon ice.

There is evidence of erosion indicating fluid activity in the recent past. Solar System Exploration: : Galileo Legacy Site Education Galileo plunged into Jupiter's crushing atmosphere on Sept. 21, 2003. The spacecraft was deliberately destroyed to protect one of its own discoveries - a possible ocean beneath the icy crust of the moon Europa. Galileo changed the way we look at our solar system. The spacecraft was the first to fly past an asteroid and the first to discover a moon of an asteroid. It provided the only direct observations of a comet colliding with a planet. Galileo was the first to measure Jupiter's atmosphere with a descent probe and the first to conduct long-term observations of the Jovian system from orbit. It found evidence of subsurface saltwater on Europa, Ganymede and Callisto and revealed the intensity of volcanic activity on Io. Read on to learn more about the historic legacy of the Galileo mission. Quick Facts The Solar System: Jupiter | The Institute for Creation Research The Solar System: Jupiter Resources › Physical Sciences Resources › Astronomy The planet Jupiter is a wonderful example of the creativity of the Lord. It is remarkably different from the worlds we have examined previously in this Impact series.

Jupiter does not possess a solid surface but is an enormous spheroid of gas—eleven times the diameter of Earth—held together by its own gravity. The gas pressure increases with depth, eventually becoming liquid. Properties of Jupiter The fifth planet away from the sun, Jupiter is composed primarily of hydrogen and helium, the same gases that comprise the sun. However, the much cooler temperature of Jupiter allows the formation of molecules such as ammonia, water, and methane from various trace elements. These molecular compounds create Jupiter’s colorful cloud formations. Jupiter is massive—the equivalent of 318 Earths! If we could put it on a scale, it would weigh more than twice as much as all the other planets combined. Jupiter is so massive that its gravity slightly, but noticeably, affects the motions of the other planets. Astronomers must factor this into their computations when predicting the precise positions of planets. Due to its large size, the disk of Jupiter is easily visible with virtually any small telescope, or even through good binoculars, despite being 500 million miles away from Earth.

A number of Jupiter’s properties are consistent with its biblical age of about 6,000 years but are difficult to explain via frameworks that adhere to a multi-billion-year-old solar system. One of these is its strong magnetic field. Since magnetic fields naturally decay with time, it is hard to understand how Jupiter could maintain such a powerful field over billions of years. Another indication of youth is Jupiter’s internal heat. Jupiter emits nearly twice the amount of energy that it receives from the sun. Like popcorn fresh out of the microwave, planets gradually cool as they radiate heat into space. Jupiter is large enough to do this for thousands of years. But if it were really billions of years old, why hasn’t it cooled off by now? Views of Jupiter Jupiter is an ideal target for a small backyard telescope. The planet is easy to find because it is extremely bright—only Venus is brighter. Jupiter shines through the most light-polluted city skies and always puts on a good show, revealing detailed cloud features to viewers. Powerful winds stretch the clouds into colorful “belts” and “zones” that encircle Jupiter parallel to its equator.

Belts are the dark brownish-orange features and zones are lighter in color.1 Like Earth’s jet streams, these clouds mark the differential rotation of Jupiter’s atmosphere. Typically, the Northern and Southern Equatorial Belts that sandwich Jupiter’s equator are the most visible. But with larger telescopes under good viewing conditions, it is often possible to see several thinner belts and perhaps even small disturbances taking place within them. Unsurprisingly, these cloud formations are dynamic, changing from year to year, growing or shrinking slightly, and experiencing subtle changes in color. Occasionally, a belt disappears completely for some time. This happened to the Southern Equatorial Belt in 2010, radically changing the appearance of Jupiter until the belt returned in 2011. Since it never looks exactly the same way twice, Jupiter is always fun to watch. The Great Red Spot, a large, somewhat permanent feature just below Jupiter’s Southern Equatorial Belt, is of particular interest. The Red Spot is a storm vortex, essentially a hurricane, roughly twice the width of Earth.2 Hurricanes on Earth are powered by warm ocean water and dissipate when they move over land. However, since Jupiter has no land, its Great Red Spot continues indefinitely.

At least 349 years old, this magnificent storm was first discovered by Robert Hooke in 1664. Before this time, telescopes were not optically sufficient to discern such features, so the Red Spot may actually be much older. Despite its name, the Great Red Spot is currently more of an orange hue and its color changes with time. In the 1990s it had turned a light pink color, reducing its contrast and making it very difficult to see through backyard telescopes. Though it has darkened since then, the spot is not always easy to see. Since Jupiter rotates, the Great Red Spot is on the other side of the planet and impossible to see 50 percent of the time. But even when on the Earth-facing side of Jupiter, the Great Red Spot is hard to see near the limb (the edge of the disk). The best view is when the spot is said to “transit” or pass near the horizontal center of the disk, an occurrence predictable up to a month or so in advance. Viewers can find upcoming transit times posted on various astronomy websites. Jupiter rotates in less than 10 hours—faster than any other planet.

This rapid rotation, combined with the extraordinary size of Jupiter, causes the planet to bulge at the equator. Even through a decent backyard telescope it is clear that Jupiter is not exactly round. Such rapid rotation also means that if you view Jupiter twice in one evening separated by a couple of hours, you will get a different view on the second pass since Jupiter will have undergone one fifth of its rotation. The effect is particularly noticeable when the Red Spot is near Jupiter’s center. The Galilean Satellites Jupiter itself is wonderful to view in a small telescope, but the observer will also be struck by the presence of four beautiful, bright “stars” that are always near Jupiter and aligned with its equator. These are moons—natural satellites that orbit Jupiter. They always appear in a nearly straight line because they orbit in the plane of Jupiter’s equator, and we see the Jupiter system from nearly edge-on; it is the same reason a Frisbee looks like a straight line when viewed from the side. These four moons are even bright enough to be seen in binoculars. They were the first moons discovered orbiting another planet, a discovery that changed our view of the universe.

Before Galileo constructed his own telescope in 1609 and began peering into the heavens, no one had considered the possibility that other planets could have moons. As far as anyone knew, there was simply the moon that orbits Earth. At that time, most astronomers believed that everything orbited Earth, including the other planets and the sun—this is known as a geocentric view. When Galileo first pointed his telescope at Jupiter, he was delighted to discover four new worlds never before known by anyone other than God. What a moment that must have been!3 And by observing these moons night after night, he could clearly see that they were orbiting Jupiter. This strongly refuted the geocentric claim that everything must orbit around Earth. Galileo had found a clear counterexample that paved the way for increased acceptance of the heliocentric solar system—the notion that planets, including Earth, orbit the sun.4 In honor of their discoverer, these four moons are now called the Galilean satellites. In order of increasing orbital distance from Jupiter, the Galilean satellites are named Io, Europa, Ganymede, and Callisto. Unlike gaseous Jupiter, these moons are solid bodies composed of rock and ice.

Jupiter and its moons are like a miniature solar system with the inner moons orbiting faster than the others. Io takes only 1.77 days to orbit Jupiter, and the orbital periods of Europa, Ganymede, and Callisto are 3.55 days, 7.15 days, and 16.7 days respectively. Notice the pattern: Europa’s period is twice that of Io, and Ganymede’s period is twice that of Europa. This is an example of orbital resonance.5 Callisto alone is not in resonance. Using a small telescope, it is possible to actually see these moons orbiting over time. If you made a careful mental note of the positions of the Galilean moons at a given moment and then looked again an hour later, you might notice that their locations appreciably changed. To see celestial objects change so drastically in such a short time is a rare delight. The Galilean moons are comparable in size to Earth’s moon. Io is 2,264 miles in diameter—only five percent larger than the moon. Europa is 10 percent smaller than our moon, and Callisto is 39 percent larger.

Ganymede is the largest moon in the solar system at 50 percent larger than Earth’s moon. Ganymede is even larger than the planet Mercury! So, if this moon orbited the sun directly instead of Jupiter, it would certainly be classified as a planet. Due to its larger surface area, Ganymede appears noticeably brighter than Io and Europa and slightly brighter than Callisto. Each of the Galilean satellites is an amazing world, but Io is the clear winner in terms of its unique qualities. High-resolution images of Io taken by the Voyager spacecrafts in 1979 revealed that, in contrast to all other known moons, this moon has absolutely no impact craters. Instead, it is covered with sulfuric compounds that are responsible for its colorful surface. Volcanoes pervade the surface of Io. Some are even taller than Mount Everest. Eruptions are almost constant, repeatedly covering the surface with volcanic material and erasing any previous record of impact craters. Io is the most volcanically active world in the solar system. When Voyager 1 flew past Io, it detected nine volcanoes erupting simultaneously!

The “gravitational stretching” occurring every 3.55 days, when Io passes between Jupiter and Europa, seems to provide the internal energy required for Io’s volcanism. More Moons and Rings Galileo’s telescope was quite small and limited—comparable to modern binoculars. In the centuries that followed, astronomers discovered many additional fainter, smaller moons with the aid of higher-quality telescopes and spacecraft. At latest count, Jupiter has 67 known moons—more than any other planet!6 Aside from the Galilean satellites, all these moons are very small, generally only a few miles across. Based on their orbital properties, these moons fall into eight natural groups. One group consists of the only four moons that are closer than the Galilean satellites to Jupiter. The orbits of these inner moons and the orbits of the Galilean moons are nearly circular, are in the same plane as Jupiter’s equator, and are prograde, meaning they orbit in the same direction that Jupiter rotates.7 The next seven beyond Callisto are also prograde, but their orbits tend to be tilted substantially, relative to Jupiter’s equator, and are more elliptical than those of the inner moons.

The remaining 52 moons also have tilted, elliptical orbits, but, amazingly, they are all retrograde—orbiting in the opposite direction of Jupiter’s rotation. So, the region where moons orbit in one direction is separate from the region where they orbit the other way. This reduces the possibility of collision or close gravitational perturbations.8 Jupiter also has a system of rings like Saturn that were first detected in 1979 by the Voyager 1 spacecraft. These rings are much less substantial than Saturn’s, however, and too faint to be seen with a backyard telescope. The main rings consist of dust particles orbiting mainly around and inside the orbit of Jupiter’s innermost moons, Metis and Adrastea. An even thinner “gossamer ring” extends farther out. Conclusions Most of what we now know about this fascinating planet was hidden from humanity for thousands of years. To our distant ancestors, Jupiter was a bright point of light in our night sky. Who would have imagined that this bright “star” would have so many remarkable characteristics? And what other celestial secrets has the Lord hidden for us to find and cherish? References Belts are regions where we are able to see deeper into Jupiter’s atmosphere.

Zones are lighter because they are clouds of ammonia that are higher in the atmosphere and block the brownish-orange color of the lower atmosphere. Hurricanes on Earth are low-pressure systems. The Red Spot in Jupiter’s southern hemisphere is a high-pressure system. For this reason, it rotates counterclockwise—in the opposite direction of hurricanes in Earth’s southern hemisphere (which rotate clockwise). Galileo published his discoveries in 1610 in a small, enjoyable book called The Starry Messenger. Galileo’s sense of wonder and excitement are obvious throughout. He knew that he was seeing something that no human being had ever seen—something that would forever change our perceptions of the solar system. Galileo challenged a number of the prevailing scientific views of the day. Curiously, the Roman Catholic Church had largely adopted the mainstream scientific view of the day (geocentrism) and incorporated it into their theology. They even attempted to use poetic passages in Scripture, such as Psalm 93:1 , as evidence for geocentrism. However, such passages were ripped out of context. The psalmist is not addressing motion in the sense of Newtonian physics but rather the inescapable stability of the universe caused by the power of God.

Many people think that the Galileo incident was an example of science prevailing over Scripture when in fact it was one view of science prevailing over another view that the Roman Catholic Church had endorsed. Perhaps the lesson to take away is that we should not tie the Bible to any specific scientific theory but instead let Scripture interpret Scripture. A resonance is when the ratio of two periods can be expressed as a simple fraction or integer, such as 1-to-1 or 3-to-4. In this case, the inner three Galilean moons have an orbital resonance of 4:2:1. That is, Io orbits four times for every two orbits of Europa and every one orbit of Ganymede. In this Impact series, we first explored the concept of resonance in the article on Mercury in the August 2013 issue of Acts & Facts. Planets: Jupiter. Solar System Exploration. NASA. Posted on solarsystem.nasa.gov, accessed October, 2, 2013. Jupiter rotates counterclockwise as viewed from above its North Pole. When one massive object passes closely by another, their mutual gravity will affect their orbits. In some cases, this can result in one of the objects being ejected from the system. * Dr.

Lisle is Director of Physical Sciences at the Institute for Creation Research and received his Ph.D. in astrophysics from the University of Colorado. Cite this article: Jason Lisle, Ph.D. 2013. The Solar System: Jupiter . Acts & Facts. 42 (12).

Question: Which planet possesses the Galilean satellites?
Answer: Jupiter