Solar System Moon Facts - Titan, Io, Europa, Ganymede, Callisto, Triton, Charon Check out our amazing space and astronomy facts for kids. Learn about different space objects and enjoy a range of cool trivia. Solar System Moon Facts Enjoy a range of interesting solar system moon facts. Learn about the variety of moons that orbit around planets such as Mars, Jupiter, Saturn and Neptune. What makes moons like Titan, Io, Europa, Ganymede, Callisto, Triton, Charon, Phobos and Deimos unique? Read on and find out! While we usually think of the Earth’s Moon there are many other moons in the Solar System . As of 2009, there were 336 moons in the Solar System. 168 of these orbit planets, 6 orbit dwarf planets, while the rest orbit asteroids and other Solar System objects (many yet to be classified). Jupiter’s four main moons are named the Galilean moons (after Galileo Galilei). Their names are Io, Europa, Ganymede and Callisto. Io features over 400 active volcanoes. Some scientists believe that water could exist below the surface of Europa.

Ganymede is the largest moon in the Solar System. Callisto has a similar diameter to the planet Mercury but only one third of its mass. Saturn’s largest moon is named Titan, it is the only moon known to have a dense atmosphere. Neptune’s largest moon, Triton, is similar in size to Earth’s moon. Discovered in 1846 it is the seventh largest moon in the Solar System. The largest moon of the dwarf planet Pluto is named Charon, it was discovered in 1978 and has a diameter that is around half as wide as Pluto’s. Mars has two moons named Phobos and Deimos, both were discovered in 1877.

Life in our Solar System – Europa, Ganymede & Callisto | Astrobioloblog Life in our Solar System – Europa, Ganymede & Callisto Tags: Alien life , Alvin , Black smoker , Callisto , ESA , Europa , Galilean moons , Ganymede , Io , Jupiter , Life , NASA , Tidal Heating Astrobiology is all about finding alien life, and many astrobiologists believe life could be found in our Solar System, on Mars, on moons like Titan, Europa or Enceladus, and potentially on other bodies, like Kuiper Belt Objects . In this post I’ll look at why some scientists think life may exist on Europa, and maybe even Ganymede and Callisto too, three of Jupiter’s largest moons. So what’s so special about theses ice worlds? Europa, Ganymede and Callisto are moons of Jupiter. Jupiter has 63 known moons, and potentially more undiscovered. Most are small and irregular shaped objects, likely captured bodies like asteroids and comets. However Jupiter also has four large moons, comparable in size to the planet Mercury called Io, Europa, Ganymede and Callisto (in fact Ganymede is bigger than Mercury).

They’re collectively known as the Galilean Moons as Galileo discovered them all in 1609 and 1610, as he improved his telescopes. The four Galilean Moons The Galilean Moons are outside of our Solar System’s habitable zone, the region of space around our Sun that orbiting planets would be at the right temperature to have surface liquid water, a vital condition for life. Obviously Earth is in the habitable zone; we have surface liquid water and life. Venus is just about in it, but doesn’t have liquid water as it has an enormous greenhouse effect due to the huge amounts of CO2 in it’s atmosphere and is far too hot (you could melt lead on its surface). Mars is on the outer edge of the habitable zone, but has no atmosphere, and no greenhouse effect, so water on its surface is mostly cold and frozen, and sublimes directly to water vapour when its heated, due to the low atmospheric pressure. The Galilean Moons are way outside of our Sun’s habitable zone though, even with an optimistic interpretation of our habitable zone they’re around two to three times too far out.

So why would we expect to find life on them? Surely they’re so far away from the Sun that any water they have should be frozen solid? This was the scientific consensus until 1979, when the NASA spacecrafts Voyager 1 and 2 arrived at Jupiter, and made an exciting discovery on Io. Io is the other Galilean moon, and the one that orbits closest to Jupiter, but it’s not a likely home for life as the Voyager probes revealed that it’s the most volcanically active body in our Solar System. Voyager 1 discovered that Io’s surface was covered in fresh lava flows, spotted with large volcanic craters and even saw volcanic plumes from ongoing eruptions. Although one team of scientists had predicted that Io would be volcanically active, most planetary scientists were surprised, as a planetary body as small as Io should have lost its internal heat by now, and should be geologically dead. Io (the orange halo and dark spots are volcanic ejecta) And this is true; Io has lost its original internal heat, but is heated instead by Jupiter’s gravity.

When our Moon orbits the Earth its gravity has an effect on the Earth, causing the Earth’s surface to bulge slightly in the direction of the Moon, partly causing the tides on Earth, and the same happens on the Moon, it bulges slightly towards the Earth. The same happens on Io, but it’s subject to a much greater degree of bulging as Jupiter is such a massive planet and Io orbits relatively close to it. Jupiter’s attraction causes Io’s rocky mantle and crust to bulge, and as Io orbits around Jupiter this bulge moves as the moon rotates, causing Io’s crust and mantle to be constantly flexed and stretched, creating large amounts of internal heat due to friction as the layers of rock scrape past each other, this is called tidal heating, and provides the heat that drives Io’s volcanoes. A volcanic eruption on Io (see the plume at the top of the photo) Scientists realised that if Io is heated internally by tidal heating then so must the other Galilean Moons, although to a lesser extent, as they orbit further away from Jupiter than Io. But on these moons we don’t see volcanoes, the tidal heating has a different effect.

Europa, Ganymede and Callisto all have large amounts of water, primarily frozen as ice. Europa likely has a small metallic core, an inner mantle of silicate rock, and an outer mantle and crust of ice, likely a 100km thick. Ganymede has a similar internal structure, but with a much thicker layer of ice, up to 800 to 1000km thick, and Callisto doesn’t appear to have differentiated internal layers, is likely made of a mixture of ice and rock, but with more rock towards the centre and more ice towards the surface. An example with Ganymede, showing a metallic core, a rocky inner mantle, an ice outer mantle, and a dirty-ice crust And here’s where the fun starts, many scientists believe that each of these moons may have a layer of liquid water beneath the surface, in which life could possibly be found.

Theoretical models suggest that Jupiter should supply enough tidal heat energy to melt regions of ice within each of the three moons, particularly if ammonia is present in the water, as this will lower its freezing point, and some observational data appears to provide evidence for this, as the bodies each have interesting magnetic effects that could be caused by a layer of liquid salty water. Europa is thought best bet for a sub-surface ocean, as its closer to Jupiter than Ganymede and Callisto, and thus experiences more tidal heating, but Ganymede and Callisto could also have sub-surface water bodies, beneath many kilometres of ice. A possible model of Europa’s interior However this sounds like a pretty strange place for life, as the sub-surface oceans would be cut-off from the Sun, as the Sun would be much weaker at Jupiter’s distance and sunlight cannot penetrate through more than a few meters of ice. Yet most life on Earth is supported by the Sun, energy is introduced into our ecosystems primarily through plants converting sunlight into chemical energy through photosynthesis. This energy is then passed up the food chain as herbivores eat the plants and carnivores eat the herbivores. So could we really expect to find alien life in these cold and dark oceans? Maybe.

Because life has been found on Earth in similar conditions. In 1977 the famous submarine Alvin was conducting a survey of a volcanically active region of the Pacific Ocean floor and make a surprise discovery when it came across an unknown ecosystem on the seabed. The geologists controlling Alvin found shrimp, crabs, clams, giant tubeworms and many other forms of life all on the ocean floor at a depth at which no sunlight could penetrate. Scientists quickly realised that this ecosystem wasn’t powered by sunlight, but by hot water and chemical reactions, as the organisms were found clustered around volcanic vents on the seabed. Volcanic vents, or hydrothermal vents, are created when hot magma beneath the sea floor heats water in the crust, causing it to expand and rush towards the surface of the seabed where it streams out of vents, like a geyser does on land. This hot water can carry a rich mix of dissolved minerals and elements in it as hot water can dissolve many substances pretty well (think about sugar in hot tea), but when the water reaches the surface of the sea-floor it quickly cools and the minerals and elements solidify and drop out of the water around the vent.

Sometimes this creates a kind of hydrothermal vent called a black smoker, when large amounts of minerals solidify out of the streaming water in clouds of black particles. Black smoker hydrothermal vents (like geysers on the ocean floor) The ecosystem discovered in 1977 was clustered around black smokers, with the food chain based on bacteria that created energy by reacting the solidified minerals and elements with oxygen, rather than plants capturing sunlight. Larger organisms then exploited and fed off these bacteria. An example hydrothermal vent ecosystem on Earth So similar life could also exist on Europa, and maybe even on Ganymede and Callisto, with silicate rock melted by tidal heating driving hydrothermal vents on the floor of the sub-surface oceans, around which ecosystems could thrive. Its likely that such environments wouldn’t be able to support ecosystems as large and as complex as those on Earth, so we may never talk to intelligent creatures from Europa, but still, it would be mind-blowing to find life on a different planetary body. This hydrothermal model is the ‘traditional view’ for life on Europa, but there are alternatives. The planetary scientist Richard Greenberg has proposed that life could live nearer Europa’s surface, and even swim in its oceans.

Europa’s icy crust shows evidence that it’s regularly cracked open by tidal forces, and many cracks may be repeatedly pulled open and pushed together as Europa orbits Jupiter, pumping water from the sub-surface ocean up into the cracks. Greenberg proposed that ecosystems of bacteria and even multicellular life could live in these surface cracks, having just enough shelter from UV radiation and being supplied with heat and nutrients in the water pumped into the cracks. He even proposed that ultraviolet radiation striking Europa’s surface could free oxygen atoms from the ice, which could be absorbed into the sub-surface ocean via these cracks, and could oxygenate Europa’s ocean, allowing more complex life-forms to be supported, that could feed off the simpler life; there may even be something similar to fish on Europa. An example of Greenberg’s idea (click to enlarge) So when are we going to the Galilean Moons to find out? NASA and ESA (the European Space Agency) are currently planning a joint mission to Europa in 2020, called the Europa Jupiter System Mission, but the project has yet to secure funding and may well be cancelled, as have other previously planned missions to Europa.

If it does go ahead, then the plan is to place at least one orbiter in the Jupiter system, to analyse Europa and the other Galilean moons from space, not to actually land on Europa. An example of an orbiter passing over Europa Scientists have speculated about sending a lander craft to Europa that would be able to drill through the outer ice layer and explore the sub-surface ocean. Yet there are two big hurdles to overcome before we can do this. One is that it would have to drill through tens of kilometres of ice, something that we can’t do on Earth yet, let alone on a distant moon, and the other is that the probe would have to be sterilized to an extreme degree to stop us accidentally infecting Europa with life from Earth, that could potentially wipe-out any existing indigenous life (I’ve talked more about this in a previous post here ). So in summary, Europa offers the best chance of life on Jupiter’s moons due to the possible presence of liquid water, and a number of different models of life have been proposed, with the most popular being ecosystems clustered around hydrothermal vents on the ocean floor.

And hopefully within our lifetime we’ll explore Europa, although it’ll be a tough nut to crack, particularly for us to safely explore its possible ocean. In the next post we’ll jump to Jupiter itself and explore the possibility that it could harbour life of its own. Share this: I read a Book called ” I Visited Ganymede ” by Yosip Ibrahim . First Edition came on Oct. 1972. My suggestion is to read it and find your own conclussion. The Jupiter System The Jupiter System Jupiter, Callisto, Ganymede, Europa, Io, Amalthea, Metis, Adrastea, Thebe Spacecraft: 2950 x 2340 4M The best of the Jupiter system is pictured in this collage of images acquired by the Voyager and Galileo spacecraft. Jupiter is the largest planet in our solar system. The four largest moons of Jupiter are known as the Galilean moons and are named Callisto, Ganymede, Europa, and Io. Inside the orbits of the Galilean moons are Thebe, Amalthea, Adrastea, and Metis. At the lower right is shown the Valhalla region of Callisto.

Ganymede is toward the bottom middle. Europa is a little above and to the right of Ganymede. Io is the top, left-most moon. Between Io and Jupiter are four little moons. The top-most little moon is Amalthea. Below and to the right of Amalthea are Metis and Adrastea. To the left of Adrastea is Thebe. 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. 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 Jupiter's Moons | StarDate Online 7.2 days 16.7 days Other than the Sun, portions of Io offer the hottest surface in the solar system. Several hundred volcanoes dot the surface, and they belch sulfur-rich lava that is hundreds of degrees hotter than the hottest lava on Earth. Io's interior is heated by a tug-of-war between Jupiter and the planet's other big moons. Io is "locked" so that the same hemisphere always faces the planet, just as the same hemisphere of our own moon always faces Earth. But as the other moons move past Io, their gravity tugs at it, too. That heats Io's interior enough to melt some of its rock, which "bubbles" to the surface. Europa: Water World? For decades, Mars was considered the most likely home for life in the solar system.

As observations continued to show a sterile, desolate world, though, scientists began turning their attention to Europa. Europa's icy crust appears to cover a large ocean of liquid water, where life may have gained a foothold. Tidal gravity may have created the ocean by heating Europa enough to melt some of its ice. There is abundant evidence of an ocean. The Galileo spacecraft found two types of terrain that may be related to water. "Chaotic" terrain looks like icebergs breaking off glaciers on Earth. The other terrain consists of smooth plains marked by ridges that are hundreds of miles long. The ridges may form as Jupiter's gravity rips apart the thin ice sheet. In addition, the motions of salty water below the surface may generate Europa's weak magnetic field. Ganymede and Callisto: Ice Worlds Ganymede is the solar system's largest moon -- larger than the planet Mercury. It consists of about half ice and half rock and metal. Grooves and ridges that crisscross its surface indicate that it has undergone great changes over the eons. During several passes, the Galileo spacecraft saw mountains of ice, plus sheets of ice that erupted from volcanoes.

It also saw deep canyons and broad, smooth plains created by the motions of Ganymede's crust. Like Europa, Callisto's icy surface may conceal an ocean. The case for an ocean is more tentative, but it is bolstered by a huge basin on one side of the moon. It was created by a powerful impact billions of years ago. But there is no jumble of rocks and mountains on the opposite side of Callisto, as there is with big impact basins on our own moon. A deep ocean could have cushioned the impact, preventing shock waves from piling up rocks half a world away. 1. Metis APOD Search Results for "Ganymede" Astronomy Picture of the Day Search Results for "Ganymede" APOD: 2016 July 10 - Moon Meets Jupiter Explanation: What's that next to the Moon? Jupiter -- and its four largest moons. Skygazers around planet Earth enjoyed the close encounter of planets and Moon in 2012 July 15's predawn skies .

And while many saw bright Jupiter next to the slender, waning crescent, Europeans also had the opportunity to watch the ruling gas giant pass behind the lunar disk, occulted by the Moon as it slid through the night. Clouds threaten in this telescopic view from Montecassiano , Italy , but the frame still captures Jupiter after it emerged from the occultation along with all four of its large Galilean moons. The sunlit crescent is overexposed with the Moon's night side faintly illuminated by Earthshine . Lined up left to right beyond the dark lunar limb are Callisto, Ganymede, Jupiter, Io, and Europa. In fact , Callisto, Ganymede, and Io are larger than Earth's Moon, while Europa is only slightly smaller. Last week, NASA's Juno became the second spacecraft ever to orbit Jupiter. APOD: 2016 March 3 - Moons and Jupiter Explanation: Some of the Solar System's largest moons rose together on February 23. On that night, a twilight pairing of a waning gibbous Moon and Jupiter was captured in this sharp telescopic field of view .

The composite of short and long exposures reveals the familiar face of our fair planet's own large natural satellite, along with a line up of the ruling gas giant's four Galilean moons. Left to right, the tiny pinpricks of light are Callisto , Io , Ganymede , [Jupiter], and Europa . Closer and brighter, our own natural satellite appears to loom large. But Callisto, Io, and Ganymede are actually larger than Earth's Moon, while water world Europa is only slightly smaller. In fact, of the Solar System's six largest planetary satellites , only Saturn's moon Titan is missing from the scene. (Editor's note: Composite corrected for orientation and field of view posted on March 7.) APOD: 2015 May 15 - Jupiter, Ganymede, Great Red Spot Explanation: In this sharp snapshot, the Solar System's largest moon Ganymede poses next to Jupiter, the largest planet. Captured on March 10 with a small telescope from our fair planet Earth, the scene also includes Jupiter's Great Red Spot, the Solar System's largest storm. In fact, Ganymede is about 5,260 kilometers in diameter.

That beats out all three of its other fellow Galilean satellites, along with Saturn's Moon Titan at 5,150 kilometers and Earth's own Moon at 3,480 kilometers. Though its been shrinking lately , the Great Red Spot's diameter is still around 16,500 kilometers. Jupiter, the Solar System's ruling gas giant, is about 143,000 kilometers in diameter at its equator . That's nearly 10 percent the diameter of the Sun. APOD: 2014 September 19 - Potentially Habitable Moons Explanation: For astrobiologists , these may be the four most tantalizing moons in our Solar System. Shown at the same scale, their exploration by interplanetary spacecraft has launched the idea that moons, not just planets, could have environments supporting life. The Galileo mission to Jupiter discovered Europa's global subsurface ocean of liquid water and indications of Ganymede's interior seas.

At Saturn, the Cassini probe detected erupting fountains of water ice from Enceladus indicating warmer subsurface water on even that small moon, while finding surface lakes of frigid but still liquid hydrocarbons beneath the dense atmosphere of large moon Titan . Now looking beyond the Solar System, new research suggests that sizable exomoons, could actually outnumber exoplanets in stellar habitable zones . That would make moons the most common type of habitable world in the Universe. APOD: 2013 February 15 - Shadows Across Jupiter Explanation: Two dark shadows loom across the banded and mottled cloud tops of Jupiter in this sharp telescopic view. In fact, captured on January 3rd, about a month after the ruling gas giant appeared at opposition in planet Earth's sky , the scene includes the shadow casters. Visible in remarkable detail at the left are the large Galilean moons Ganymede (top) and Io. With the two moon shadows still in transit, Jupiter's rapid rotation has almost carried its famous Great Red Spot (GRS) around the planet's limb from the right. The pale GRS was preceded by the smaller but similar hued Oval BA , dubbed Red Spot Jr., near top center.

North is down in the inverted image. APOD: 2012 July 20 - Moon Meets Jupiter Explanation: Skygazers around planet Earth enjoyed the close encounter of planets and Moon in July 15's predawn skies . And while many saw bright Jupiter next to the slender, waning crescent, Europeans also had the opportunity to watch the ruling gas giant pass behind the lunar disk, occulted by the Moon as it slid through the night. Clouds threaten in this telescopic view from Montecassiano, Italy, but the frame still captures Jupiter after it emerged from the occultation along with all four of its large Galilean moons. The sunlit crescent is overexposed with the Moon's night side faintly illuminated by Earthshine . Lined up left to right beyond the dark lunar limb are Callisto, Ganymede, Jupiter, Io, and Europa. In fact , Callisto, Ganymede, and Io are larger than Earth's Moon, while Europa is only slightly smaller. APOD: 2012 March 2 - Jupiter Unplugged Explanation: Five hand drawn sketches of Jupiter were used to create this beautifully detailed flat map of the ruling gas giant's turbulent cloud tops .

Made with colored pencils at the eyepiece of a 16 inch diameter telescope, the original drawings are about 5 inches (12.5 cm) in diameter. The drawn planisphere map dimensions are 16x8 inches (40x20 cm). Observing on different dates in November and December of 2011, astronomical artist Fred Burgeot has relied on Jupiter's rotation to cover the planet's complete circumference. Digital animator Pascal Chauvet has also translated Burgeot's drawings into an intriguing video (vimeo) , synthesizing a telescopic view of the rotating planet with a tilt and phase appropriate for the observing dates. The video includes the Galilean moons moving along their orbits, beginning with Ganymede and Io casting shadows as they glide in front of Jupiter, followed by Europa and Callisto passing behind the planet's banded disk. APOD: 2011 October 22 - Jupiter Near Opposition Explanation: On October 29 (UT) , Jupiter, the solar system's largest planet , will be at opposition, opposite the Sun in planet Earth's sky, shining brightly and rising as the Sun sets.

That configuration results in Jupiter's almost annual closest approach to planet Earth, so near opposition the gas giant offers earthbound telescopes stunning views of its stormy, banded atmosphere and large Galilean moons . This sharp snapshot of Jupiter was captured on October 13 with the 1 meter telescope at the Pic Du Midi mountain top observatory in the French Pyrenees . North is up in the image that shows off oval shaped vortices and planet girdling dark belts and light zones . Also seen in remarkable detail, Jupiter's icy Ganymede , the solar system's largest moon, is emerging from behind the planet (top) while volcanic Io enters the frame near the lower left edge. APOD: 2010 October 8 - Two Planet Opposition Explanation: In late September, two planets were opposite the Sun in Earth's sky, Jupiter and Uranus. Consequently closest to Earth, at a distance of only 33 light-minutes and 2.65 light-hours respectively, both were good targets for telescopic observers. Recorded on September 27, this well-planned composite of consecutive multiple exposures captured both gas giants in their remarkable celestial line-up accompanied by their brighter moons.

The faint greenish disk of distant planet Uranus is near the upper left corner. Of the tilted planet's 5 larger moons , two can be spotted just above and left of the planet's disk. Both discovered by 18th century British astronomer Sir William Herschel and later named for characters in Shakespeare's A Midsummer Night's Dream, Oberon is farthest left, with Titania closer in. At the right side of the frame is ruling gas giant Jupiter, flanked along a line by all four of its Galilean satellites . Farthest from Jupiter is Callisto , with Europa and Io all left of the planet's disk, while Ganymede stands alone at the right. APOD: 2009 September 20 - Ganymede Enhanced Explanation: What does the largest moon in the Solar System look like? Ganymede , larger than even Mercury and Pluto , has a surface speckled with bright young craters overlying a mixture of older, darker, more cratered terrain laced with grooves and ridges . Like Earth's Moon, Ganymede keeps the same face towards its central planet, in this case Jupiter.

In this historic and detailed image mosaic taken by the Galileo spacecraft that orbited Jupiter from 1995 to 2003, the colors of this planet-sized moon have been enhanced to increase surface contrasts. The violet shades extending from the top and bottom are likely due to frost particles in Ganymede 's polar regions. Possible future mission s to Jupiter are being proposed that can search Europa and Ganymede for deep oceans that may harbor elements thought important for supporting life . APOD: 2009 January 6 - Jupiter Eclipsing Ganymede Explanation: How hazy is Jupiter's upper atmosphere? To help find out, astronomers deployed the Hubble Space Telescope to watch Jupiter eclipse its moon Ganymede . Although Ganymede circles Jupiter once a week, a particularly useful occultation occurs more rarely. Such an occultation was captured in great visual detail in April 2007. When near Jupiter's limb, Ganymede reflects sunlight though Jupiter's upper atmosphere, allowing astronomers to search for haze by noting a slight dimming at different colors.

One result of this investigation was the above spectacular image , where bands of clouds that circle Jupiter are clearly visible, as well as magnificent swirling storm systems such as the Great Red Spot . Ganymede, at the image bottom, also shows noticeable detail on its dark icy surface . Since Jupiter and Ganymede are so bright, many eclipses can be seen right here on Earth with a small telescope. APOD: 2007 August 24 - Astronomer's Moon Explanation: Jupiter is an astronomer's planet -- its large size and contrasting global cloud belts and zones allow detailed studies with a range of earthbound telescopes . On the other hand, most telescopic views of Jupiter's moons usually show only featureless, tantalizing points of light hovering near the ruling gas giant. But this impressive picture from a small, ground-based telescope reveals a stunning amount of detail on Ganymede, a jovian moon about the same size as Earth's moon but at least 1,500 times farther away. The image was carefully constructed by combining and processing only the 409 sharpest frames from a total of 10,000 recorded at 30 frames per second by a digital camera.

Ganymede's radius is about 2,600 kilometers indicating that the surface markings visible are as small as around 900 kilometers across. APOD: 2007 March 29 - Jupiter Moon Movie Explanation: South is toward the top in this frame from a stunning movie featuring Jupiter and moons recorded last Thursday from the Central Coast of New South Wales, Australia. In fact, three jovian moons and two red spots are ultimately seen in the full video as they glide around the solar system's ruling gas giant. In the early frame above, Ganymede , the largest moon in the solar system, is off the lower right limb of the planet, while intriguing Europa is visible against Jupiter's cloud tops, also near the lower right. Jupiter's new red spot junior is just above the broad white band in the planet's southern (upper) hemisphere. In later frames, as planet and moons rotate (right to left), red spot junior moves behind Jupiter's left edge while the Great Red Spot itself comes into view from the right. Also finally erupting into view at the right, is Jupiter's volcanic moon, Io . To download the full 2 megabyte movie as an animated gif file, click on the picture.

APOD: 2004 December 9 - Jupiter and the Moon's Shadowed Horizon Explanation: Early Tuesday morning, December 7th, June Croft thought the southeastern sky above Atmore, Alabama, USA was beautiful. Watching the Moon rise through gossamer clouds, she noted, " ... the crescent Moon looked like it was held in the sky by a star just off its shadowed horizon." What was that star? Bright Jupiter of course, and some watched as the Moon actually occulted or passed in front of the Solar System's reigning gas giant planet. For astronomer Jimmy Westlake in Colorado, Jupiter was already hidden at moonrise that morning, but later he was able to record this lovely image, not unlike the view that inspired Croft. Seen through gossamer clouds, Jupiter along with large Jovian satellites Ganymede and Callisto (bottom to top) has emerged from behind the crescent Moon's shadowed horizon . APOD: 2004 November 11 - Pastel Planet, Triple Eclipse Explanation: This false-color image of banded gas giant Jupiter shows a triple eclipse in progress on March 28 - a relatively rare event, even for a large planet with many moons.

Captured by the Hubble Space Telescope's near-infrared camera are shadows of Jupiter's moons Ganymede (left edge), Callisto (right edge) and Io, three black spots crossing the sunlit Jovian cloud tops. In fact, Io itself is visible as a white spot near picture center with a bluish Ganymede above and to the right, but Callisto is off the right hand edge of the scene. Viewed from Jupiter's perspective, these shadow crossings would be seen as solar eclipses, analogous to the Moon's shadow crossing the sunlit face of planet Earth. Historically , timing the eclipses of Jupiter's moons allowed astronomer Ole Roemer to make the first accurate measurement of the speed of light in 1676. APOD: 2003 February 27 - When Moons and Shadows Dance Explanation: It's no wonder Jupiter is a favorite target for even modest earthbound telescopes. The most massive planet in the solar system with four of the largest moons also boasts the famous Great Red Spot , a giant hurricane-like storm system over three hundred years old.

Recorded on December 15, 2002 between 7:19 and 8:40 UT, over a thousand digital images were processed and stacked to create this spectacular 21 frame animation of the Jovian system . South is up and as the Great Red Spot tracks across the face of Jupiter, innermost Galilean moon Io enters the scene at the far right. Io occults (passes in front of) the edge of the more sedately orbiting Ganymede with Io's shadow moving quickly across the gas giant's cloud tops, just below the Red Spot. While the moon Callisto is outside the field of view, its large, dark shadow is also visible crossing the Jovian disk at the upper left. Viewed from Earth, the orbits of the Galilean moons presently lie nearly edge-on, offering many chances to observe similar dances of Jupiter's moons . APOD: 2003 February 16 - Southwest Mercury Explanation: The planet Mercury resembles a moon. Mercury 's old surface is heavily cratered like many moons. Mercury is larger than most moons but smaller than Jupiter 's moon Ganymede and Saturn 's moon Titan .

Mercury is much denser and more massive than any moon, though, because it is made mostly of iron. In fact, the Earth is the only planet more dense. A visitor to Mercury's surface would see some strange sights. Because Mercury rotates exactly three times every two orbits around the Sun , and because Mercury 's orbit is so elliptical, a visitor to Mercury might see the Sun rise, stop in the sky, go back toward the rising horizon , stop again, and then set quickly over the other horizon . From Earth , Mercury's proximity to the Sun causes it to be visible only for a short time just after sunset or just before sunrise. APOD: 2001 December 15 - Ganymede: Torn Comet Crater Chain Explanation: This striking line of 13 closely spaced craters on Jupiter's moon Ganymede was photographed by the Galileo spacecraft in 1997. The picture covers an area about 120 miles wide and the chain of craters cuts across a sharp boundary between dark and light terrain. What caused this crater chain?

Remarkably, the exploration of the Solar System , has shown that crater chains like this one are not unique , though they were considered mysterious until a dramatic object lesson was offered by comet Shoemaker-Levy 9. In 1994 many denizens of planet Earth watched as huge pieces of this torn comet slammed into Jupiter itself in a spectacular series of sequential impacts . It is very likely that similar torn comets from the early history of the Solar System are responsible for this and other crater chains . APOD: 2001 July 31 - Oceans Under Jupiters Callisto Explanation: Why does Jupiter 's moon Callisto alter the magnetic field of Jupiter in its vicinity? Callisto itself does not have a strong magnetic field. One possible answer is that Callisto harbors sub-surface oceans of electrically conducting salt-water . This hypothesis was bolstered recently by a new analysis of how Callisto creates and dissipates heat. Callisto is thought to create heat by the radioactive decay of internal rock -- a process that keeps the Earth's mantle molten. Callisto may not be able to dissipate this heat very efficiently, however, as it has thick layers of ice and rock on its surface.

Perhaps this heat is enough to keep sub-surface water from freezing into ice. With this hypothesis , Callisto joins two other of Jupiter's moons, Europa and Ganymede , in candidates for sub-surface oceans. Callisto's oceans, however, might prove too hostile to support Earth-like life . APOD: 2001 January 16 - Europa Rotating Explanation: Evidence has been mounting that beneath the vast planes of ice that cover Europa lies water -- liquid oceans that might be home to alien life . The smallest of Jupiter 's Galilean Moons (which include Io , Ganymede , and Callisto ), Europa's deep interior is composed of mostly of silicate rock . Upon close inspection, many surface cracks stop abruptly only to continue on somewhere else -- indicating surface plates that might be sliding . The above time-lapse sequence is a composite of images taken during the Voyager spacecraft flyby of the moon twenty years ago. Not all regions are resolved in high detail. The movie shows Europa during a complete rotation, which corresponds to a complete revolution around Jupiter since Europa always keeps the same face toward the giant planet. The cause of many of the surface colors on Europa also remains a topic of research .

APOD: 2001 January 2 - Jupiter, Europa, and Callisto Explanation: As the robot Cassini spacecraft rounds Jupiter on its way toward Saturn , it has taken a sequence of images of the gas giant with its four largest moons . Previously released images have highlighted Ganymede and Io . Pictured above are the two remaining Galilean satellites : Europa and Callisto . Europa is the bright moon superposed near Jupiter's Great Red Spot , while Callisto is the dark moon near the frame edge. Callisto is so dark that it would be hard to see here if its brightness was not digitally enhanced. Recent evidence indicates that both moons hold salt-water seas under surface ice that might be home to extra-terrestrial life. By noting the times that moons disappeared and reappeared behind Jupiter in 1676, Ole Roemer was able to make the first accurate estimation of the speed of light . APOD: 2000 December 19 - A Close Up of Aurora on Jupiter Explanation: Jupiter has aurorae . Like Earth , the magnetic field of the gas giant funnels charged particles released from the Sun onto the poles. As these particles strike the atmosphere, electrons are temporarily knocked away from existing gas molecules .

Electric force attracts these electrons back. As the electrons recombine to remake neutral molecules, auroral light is emitted. In the above recently released photograph by the Hubble Space Telescope taken in ultraviolet light , the aurorae appear as annular sheets around the pole. Unlike Earth's aurorae , Jupiter's aurorae include several bright streaks and dots . These marks are caused by magnetic flux tubes connecting Jupiter to its largest moons . Specifically, Io caused the bright streak on the far left, Ganymede caused the bright dot below center, and Europa caused the dot to its right. APOD: 2000 December 18 - Oceans Under Jupiter's Ganymede Explanation: The search for extraterrestrial life came back into our own Solar System last week with the announcement that there may be liquid oceans under the surface of Jupiter 's moon Ganymede . Ganymede now joins Callisto and Europa as moons of Jupiter that may harbor seas of liquid water under layers of surface ice . The ocean hypothesis surfaced as an explanation for Ganymede's unusually strong magnetic field . Ganymede, the largest moon in the Solar System, also has the largest measured magnetic field of any moon.

Some exobiologists hypothesize that life may be able to emerge in such an ocean, much as it did in the oceans of ancient Earth . Above , a frame from a computer simulation shows what it would look like to fly over the surface of Ganymede, as extrapolated from photographs of the grooved moon taken by the robot spacecraft Galileo currently orbiting Jupiter. APOD: 2000 December 12 - Jupiter Eyes Ganymede Explanation: Who keeps an eye on the largest moon in the Solar System ? This moon, visible on the lower right, is Ganymede , and the planet it orbits, Jupiter , seems to be keeping a watchful eye, as its Great Red Spot appears serendipitously nearby. This recently released enhanced-contrast image from the robot spacecraft Cassini captures new details of the incredible intricacies of Jupiter's complex cloud patterns. Features as small as 250 kilometers can be seen. Counter-clockwise rotating high-pressure white ovals that are similar to the Great Red Spot appear in the red band below the spot. Between these spots are darker low-pressure systems that rotate clockwise.

The hydrogen and helium that compose most of Jupiter's clouds is nearly invisible - the trace chemicals that give Jupiter these colors remain unknown . The Cassini spacecraft is using Jupiter to pull it toward Saturn , where it is scheduled to arrive in 2004. APOD: 2000 November 18 - Jupiter And Family Explanation: This composite image features classic portraits of members of one of the Solar System's most prominent families - Jupiter and its four large "Galilean" moons. Starting from the top the moons are Io , Europa , Ganymede , and Callisto . The top-to-bottom order is also the order of increasing distance from Jupiter. These are big moons indeed which attend the largest planet . The smallest of the lot, Europa, is the size of Earth's moon while Ganymede is the largest moon in the Solar System. In fact, Ganymede with a diameter of 3,100 miles, is larger than the planets Mercury and Pluto . The swirling Great Red Spot appears at the edge of Jupiter. A hurricane-like storm system that has persisted for over 300 years, two to three earths could fit inside it.

Battered Callisto's image was recorded during the 1979 flyby of Voyager . The other portraits were taken by the Galileo spacecraft which began exploring the Jovian system in 1995. APOD: 2000 October 24 - Io Rotating Explanation: The surface of Io is continually changing. Jupiter's moon is the home to many powerful volcanoes so active they are effectively turning the moon inside out. The above time-lapse sequence is a composite of images taken during two space missions that approached the violent moon: Voyager and Galileo . The sequence shows Io during a complete rotation, which corresponds to a complete revolution around Jupiter since Io always keeps the same face toward the giant planet. The rampant volcanism is thought to be caused by Jupiter's more distant Galilean Moons ( Europa , Ganymede , and Callisto ) pulling on Io and continually distorting its shape, causing internal frictional heating. Io is composed mostly of rock, with the yellow color originating from sulfur . The causes of many of the other colors remain a topic of research .

APOD: 2000 June 20 - Ganymede: The Largest Moon in the Solar System Explanation: If Ganymede orbited the Sun, it would be considered a planet. The reason is that Jupiter 's moon Ganymede is not only the largest moon in the Solar System , it is larger than planets Mercury and Pluto . The robot spacecraft Galileo currently orbiting Jupiter has been able to zoom by Ganymede several times and snap many close-up pictures. Ganymede, shown above in its natural colors, sports a large oval dark region known as Galileo Regio . In general, the dark regions on Ganymede are heavily cratered, implying they are very old, while the light regions are younger and dominated by unusual grooves . The origin of the grooves is still under investigation . APOD: 2000 May 8 - Jupiters Moons Thebe, Amalthea, and Metis Explanation: The robot spacecraft Galileo in orbit around Jupiter has recently photographed the inner moons of Jupiter in greater detail than ever before. These pictures of Thebe , Amalthea , and Metis are shown to scale, and reveal details as small as three kilometers across.

Amalthea , by contrast, has a total length of about 200 kilometers. The moons are composed mostly of ice, are much smaller than Jupiter's more famous Galilean satellites ( Io , Europa , Ganymede , and Callisto ), and orbit between Io and Jupiter's rings . Thebe appears dominated by a huge impact crater 40 kilometers across. Astronomers are uncertain of the origin of the unusual white gash at the bottom of Amalthea . APOD: 2000 February 13 - Southwest Mercury Explanation: The planet Mercury resembles a moon. Mercury 's old surface is heavily cratered like many moons. Mercury is larger than most moons but smaller than Jupiter 's moon Ganymede and Saturn 's moon Titan . Mercury is much denser and more massive than any moon, though, because it is made mostly of iron. In fact, the Earth is the only planet more dense. A visitor to Mercury's surface would see some strange sights.

Because Mercury rotates exactly three times every two orbits around the Sun , and because Mercury 's orbit is so elliptical, a visitor to Mercury might see the Sun rise, stop in the sky, go back toward the rising horizon , stop again, and then set quickly over the other horizon. From Earth , Mercury's proximity to the Sun cause it to be visible only for a short time just after sunset or just before sunrise. APOD: March 4, 1999 - Ganymede Mosaic Explanation: Ganymede , one of the four Galilean moons of Jupiter, is the largest moon in the Solar System. With a diameter of 5,260 kilometers it is even larger than planets Mercury and Pluto and just over three quarters the size of Mars. Ganymede is locked in synchronous rotation with Jupiter. This detailed mosaic of images from the Galileo spacecraft shows the trailing hemisphere of this planet-sized moon. Speckled with bright young craters, Ganymede's surface shows a mixture of old, dark, cratered terrain and lighter regions laced with grooves and ridges .

Ganymede's true colors tend toward subtle browns and grays, but this mosaic's colors have been enhanced to increase surface contrasts. The violet shades extending from the top and bottom are likely due to frost particles in Ganymede's polar regions. APOD: August 5, 1998 - Ganymede: Torn Comet Crater Chain Explanation: This remarkable line of 13 closely spaced craters on Jupiter's moon Ganymede was photographed by the Galileo spacecraft in 1997. The picture covers an area about 120 miles wide and the chain of craters cuts across a sharp boundary between dark and light terrain. What caused this crater chain? During the exploration of the Solar System , crater chains like this one have been discovered in several places and were considered mysterious until a dramatic object lesson was offered by comet Shoemaker-Levy 9. In 1994 many denizens of planet Earth watched as huge pieces of this torn comet slammed into Jupiter itself in a spectacular series of sequential impacts . It is very likely that similar torn comets from the early history of the Solar System are responsible for this and other crater chains .

APOD: July 22, 1998 - Dark Craters on Ganymede Explanation: Ganymede has craters within craters within craters . The old surface of the largest moon in the Solar System shows its age by the large amount of these impact features. The above picture released last week shows two old craters with dark floors located in a relatively bright region known as Memphis Facula , a region itself thought created by an ancient collision. The strange dark floors of these craters were themselves created long ago and now house craters of their own. Crater Chrysor, on the left, spans about 6000 meters, about half that of crater Aleyn on the right. The robot spacecraft Galileo took the above photograph during a flyby of this moon of Jupiter in June 1996. APOD: May 13, 1998 - Occultations and Rising Moons Explanation: On April 23, the rising crescent Moon occulted (passed in front of) Venus and Jupiter. The double occultation was a rare event and only visible from certain locations tracing a path across Earth's surface.

This dramatic telephoto picture was taken at one such location, Ascension Island in the South Atlantic. The sunlit crescent is over-exposed revealing the rest of the lunar surface illuminated by faint earthshine . Venus is emerging just beyond the crescent's tip and Jupiter is trailing above the dark lunar edge with a spot of light, Jupiter's moon Ganymede, between the lunar limb and the planet's disk. Look closely at Jupiter and you can see yet another Jovian moon , Io, just visible against Jupiter's glare! APOD: May 12, 1998 - Callisto Enhanced Explanation: Callisto is half rock and half ice. This moon of Jupiter is approximately the size of the planet Mercury , making it the third largest moon in the Solar System , after Ganymede and Titan . Callisto 's icy surface is billions of years old, lacks any sign of volcanic activity , and is densely covered with rifts and craters. These features are particularly apparent in this contrast-enhanced image taken by the Galileo spacecraft , and released last week. Visible near the image center is Valhalla , one of the largest impact craters in the Solar System, measuring about 4,000 kilometers across.

The rings and size of Valhalla make its appearance similar to the Caloris Impact Basin on Mercury. APOD: February 2, 1998 - A Triple Eclipse on Jupiter Explanation: Part of Jupiter is missing. Actually, three parts appear to be missing. In reality though, the three dark spots seen in the above photograph are only shadows. The unusual alignment of three of Jupiter 's moons between the Jovian giant and the Sun was imaged last November 10th. The shadows of Io , Callisto , and Ganymede move across Jupiter as these moons progress in their orbits. It was by noting the times of eclipse of Jupiter's moons in 1675 that Ole Roemer became the first person to measure the speed of light . When a shadow from Earth's Moon crosses the Earth's surface, the people inside the shadow see an eclipse of the Sun. APOD: December 9, 1997 - Mysterious Features on Ganymede Explanation: Where is the rest of the circle? Jupiter 's largest moon Ganymede has some truly unusual terrain, including the pictured half circle above cut by nearly parallel curves.

Full circles can be easily explained by impact craters , but partial circles imply that some resurfacing has occurred since the original impact. The diameter of the half circle is about 32 kilometers. Also of interest is a dense linear crater chain that cuts across the top of the half-circle. Mysteries like these are typically solved by the hard work of reconstructing a sequence of natural occurrences, which in this case may yield a better understanding of Ganymede 's interesting past. APOD: September 29, 1997 - Jupiter And Family Explanation: This composite image features classic portraits of members of one of the Solar System's most prominent families - Jupiter and its four large "Galilean" moons. Starting from the top the moons are Io , Europa , Ganymede , and Callisto . The top-to-bottom order is also the order of increasing distance from Jupiter. These are big moons indeed which attend the largest planet . The smallest of the lot, Europa, is the size of Earth's moon while Ganymede is the largest moon in the Solar System. In fact, Ganymede with a diameter of 3,100 miles, is larger than the planets Mercury and Pluto .

The swirling Great Red Spot appears at the edge of Jupiter. A hurricane-like storm system that has persisted for over 300 years, two to three earths could fit inside it. Battered Callisto's image was recorded during the 1979 flyby of Voyager . The other portraits were taken by the Galileo spacecraft which began exploring the Jovian system in 1995. APOD: June 12, 1997 - Jupiter's Dry Spots Explanation: Known for its spectacular images of Jupiter's moons, Io , Ganymede , Callisto , and Europa , the robot spacecraft Galileo has also aggressively explored the Jovian atmosphere. In December of 1995 , Galileo's atmospheric probe descended into Jupiter's clouds and reported a surprising absence of water . It is now believed that the probe entered through one of Jupiter's dry spots , similar to the dark region in this image of the swirling Jovian cloud deck . The smallest features visible here are tens of miles in size. These dry regions appear to correspond to locations where winds converge creating downdrafts. The downdrafts generate local cloudless clearings through which Jupiter's deeper warmer layers can be glimpsed.

Just as the dark areas are extremely dry , the surroundings are full of moisture. The contrast is analogous to the desert and tropics of Earth . APOD: December 23, 1996 - The Hills of Ganymede Explanation: This computer generated 3D close-up view of Jupiter's large moon Ganymede was created using image data from NASA's Galileo spacecraft. Simulating stereo vision by combining two recent images recorded from different angles, 3 dimensional information was reconstructed for a section of Ganymede's surface. The result shows the furrows, craters, and hills in the region appropriately known as "Galileo Regio" with a resolution of about 250 feet. Currently exploring the Jovian System , the Galileo spacecraft just completed a flyby of Europa. It is scheduled to return to Ganymede in April next year. APOD: November 7, 1996 - Fields of Minerals on Ganymede Explanation: What treasures lie on the surface of Ganymede ? Last week, NASA released a map of Jupiter's largest moon made by the Galileo Orbiter highlighting ice and minerals deposits.

The leftmost photograph by Voyager shows surface features in visible light, but the rightmost photograph , taken in infrared light by Galileo, shows the locations of minerals in red and ice grains in blue. The NIMS team is working to identify these minerals with preliminary hopes of finding emission indicative of montmorillonite and alunite APOD: October 25, 1996 - A Flyby View of Ganymede Explanation: This is what it would look like to fly over the surface of Jupiter's moon Ganymede. NASA's robot spacecraft Galileo recently approached only 6000 miles from this frozen ice-ball of a moon. The above image is a digital reconstruction from two images taken during this flyby. The blue color of the sky is artificial. Deep furrows cover Ganymede's surface , and several impact craters are also present. Galileo continues to orbit Jupiter and send back breathtaking photos of Ganymede , Io , Europa , Callisto , and, of course, Jupiter itself. APOD: October 22, 1996 - The Cracked Ice Plains of Europa Explanation: What caused the cracks in this giant ice-ball?

Jupiter 's moon Europa has smoothest surface in the solar system and is composed mostly of cracked water-ice. In the above false-colored picture released last week by the NASA team in charge of the Galileo mission , blue hues represent ice plains divided by dirty red and brown bands of mottled terrain. As the robot Galileo spacecraft orbits Jupiter, it sends back revealing pictures of Jupiter and its large moons including Europa , Io , Ganymede , and Callisto . The region of Europa highlighted above is known as Minos Linea. The cause for many of the cracks remains unknown but may involve shifting stresses from gravity and temperature variations. The new Galileo pictures have increased evidence that liquid oceans may indeed exist under these giant ice-sheets, a place possibly ripe for the development of life . APOD: September 13, 1996 - Southwest Mercury Explanation: The planet Mercury resembles a moon. Mercury 's old surface is heavily cratered like many moons. Mercury is larger than most moons but smaller than Jupiter 's moon Ganymede and Saturn 's moon Titan . Mercury is much denser and more massive than any moon, though, because it is made mostly of iron.

In fact, the Earth is the only planet more dense. A visitor to Mercury's surface would see some strange sights. Because Mercury rotates exactly three times every two orbits around the Sun , and because Mercury 's orbit is so elliptical, a visitor to Mercury might see the Sun rise, stop in the sky, go back toward the rising horizon , stop again, and then set quickly over the other horizon. From Earth , Mercury's proximity to the Sun cause it to be visible only for a short time just after sunset or just before sunrise. APOD: August 5, 1996 - Erupting Volcanoes on Io Explanation: Jupiter's moon Io has active volcanoes. The Voyager spacecraft caught several erupting when they passed the energetic moon in 1979. In the above picture, several of Io's volcanoes are visible and one is seen actually erupting. Debris from this explosive event can be seen on the upper left of the photo, just beyond Io's edge. Io 's volcanism is thought to be caused by the large tidal distortions raised by Jupiter , Europa , and Ganymede . These tides stretch Io , cause internal friction, and thus heat the interior.

The hot interior then expands and forces its way out through volcanoes. Currently, the spacecraft Galileo is orbiting Jupiter and photographing Jupiter's Galilean moons . APOD: July 19, 1996 - Galileo's First Color Image of Io Explanation: Above is the first color image of Jupiter's volcanic moon Io released by the Galileo Project. (Io sounds like "eye-oh".) The image was made on June 25 when the Galileo spacecraft approached within 1.4 million miles. It reveals features as small as 14 miles across - comparable to the resolution of the best 1979 vintage Voyager images. The Voyager flybys discovered active volcanos on Io's mottled surface and this image indicates that dramatic changes have occurred since , notably in the region of the Masubi volcano located in Io's southern hemisphere. This region, apparently covered with new deposits of sulfur and sulfur dioxide frost deposited by volcanic eruption, is seen as the pronounced white area at the bottom of the picture. While scientists continue to analyze this image and other recent Galileo data the robot spacecraft will continue to explore Jupiter's moons .

Its next scheduled close encounter is set for September 6th with the moon Ganymede . Higher resolution images of Io are also expected during the ongoing mission. APOD: July 12, 1996 - Ancient Cratered Plains on Ganymede Explanation: The largest moon in the Solar System shows regions that are ancient and battered. The high density of craters demonstrate that patches of Ganymede are indeed billions of years old. This photo is one of a series released by NASA two days ago from the Galileo spacecraft orbiting Jupiter . The large impact crater on the left is 19 kilometers across, while dark indentations in Ganymede 's crusty surface-ice run diagonally. Ganymede is composed of half rock and half water-ice. The Galileo spacecraft will continue to orbit Jupiter over the next 16 months and send back data about Jupiter and its four largest moons: Io , Europa , Ganymede , and Callisto . APOD: July 11, 1996 - Ganymede: A Really Groovy Moon Explanation: Ganymede's surface is a wrinkled mess.

As large ice-sheets shift on the moon's surface , parts of the surface buckle causing high ridges, deep furrows, and parallel grooves. This photo , taken by the Galileo spacecraft currently orbiting Jupiter , was released yesterday . The large circular feature near the picture bottom is a large impact crater. The impact that caused this large crater also caused the strange dark ejecta seen to the crater's right. The Sun illuminates the scene from the lower left. The Galileo spacecraft has also just discovered that Ganymede has a region of orbiting charged particles called a magnetosphere - a first for any moon. How Ganymede is able to generate a magnetosphere is a mystery. APOD: July 10, 1996 - Galileo Photographs Ganymede Explanation: Ganymede's surface is slowly being pulled apart. This photo of Ganymede was released earlier today by the Galileo team at NASA. The Galileo Spacecraft arrived at Jupiter in December 1995. In late June, the spacecraft passed within 10,000 kilometers of Ganymede 's icy surface, and took pictures showing complex surface details for the first time.

The line-like features in this photo are sunlit ridges rising above Ganymede 's ice-plains. The circular features are impact craters. Ganymede is the largest moon of Jupiter and hence the largest of the four Galilean satellites: Io , Europa , Ganymede , and Callisto . APOD: June 27, 1996 - Voyager's Preview of Galileo at Ganymede Explanation: NASA's robot spacecraft Galileo began its long voyage to Jupiter in October of 1989. In December of last year it arrived in the Jovian system, beginning its unprecedented, detailed exploration by dropping a probe into the gas giant's atmosphere. By early this morning it will have accomplished another milestone in its ambitious mission. Now in orbit around Jupiter , Galileo will make its first close flyby of Ganymede, Jupiter's (and the solar system's) largest moon at 2:29 a.m. EDT. As planned , approaching to within 524 miles, it will make a series of high resolution images of the surface which will reveal features as small as 33 feet across.

This close-up color image from the Voyager 2 flyby in 1979 previews sights Galileo will see in greater detail. Showing features as small as 3 miles across, it reveals a variety of terrain on Ganymede's icy surface, including impact craters with bright rays and long strips of light grooved structures suggesting large scale motions of the frozen crust. Galileo's flyby images will be stored onboard for playback and should be available during the week of July 10. APOD: December 24, 1995 - Uranus' Moon Miranda Explanation: NASA's robot spacecraft Voyager 2 passed the planet Uranus and its moons in 1986. While the cloud tops of Uranus proved to be rather featureless, the surface of Miranda , the innermost of Uranus ' large moons, showed several interesting features. Voyager 2 passed closer to Miranda than to any Solar System body and hence photographed it with the clearest resolution. Miranda's heavily cratered terrain shows grooves like Jupiter 's moon Ganymede and several valleys and cliffs. Miranda is made of a roughly equal mix of ice and rock. Miranda was discovered by Gerard Kuiper in 1948.

APOD: November 25, 1995 - Saturn's Cleanest Moon: Enceladus Explanation: Enceladus orbits Saturn between the smaller Mimas and the larger Tethys . Enceladus is composed mostly of water ice and has the cleanest and purest ice surface in the Solar System . It's surface therefore appears nearly white. The surface also has many unusual groves and relatively few craters, like Jupiter 's moon Ganymede . This indicates that the surface is young and/or newly reformed. To explain this, some astronomers speculate that Enceladus is susceptible to some sort of volcanic activity. Enceladus was originally discovered in 1789 by William Herschel . APOD: November 3, 1995 - Jupiter's Moon Amalthea Explanation: Amalthea is Jupiter 's fifth largest moon, much smaller than the four Galilean satellites Io , Europa , Ganymede , and Callisto . The orbit of Amalthea is inside of these moons, and with its long axis always pointing toward Jupiter . It's dark surface color is probably due to sulfur being expelled from Io .

Amalthea is not massive enough for its internal gravity to make it spherical. Amalthea was discovered in 1892 by Edward Barnard. APOD: October 13, 1995 - Jupiter, Io, and Ganymede's Shadow Explanation: Jupiter , the solar system 's largest planet, is seen here next to Io , its closest Galilean moon. On the cloud tops of Jupiter near the left edge of the picture can be seen a dark circular spot which is caused by the shadow of Jupiter 's largest moon Ganymede . Jupiter 's cloud tops show light bands and dark belts. The clouds are primarily composed of hydrogen and helium , but their intense colors are probably caused by very small amounts of heavier elements such as sulfur or organic (carbon-containing) compounds. APOD: September 6, 1995 - Callisto: Dark Smashed Iceball Explanation: Callisto is a dirty battered world, showing the most beaten surface of Jupiter 's major moons. Made of a rocky core covered by fractured ice, Callisto 's past collisions with large meteors are evident as large craters surrounded by concentric rings.

The four largest moons of Jupiter: Io , Europa , Ganymede , and Callisto were all discovered by Galileo and Marius in 1610 with early telescopes and are now known as the Galilean satellites. The NASA spacecraft Galileo is scheduled to arrive at Jupiter is December of 1995. APOD: September 4, 1995 - Ganymede: Moonquake World Explanation: Ganymede probably undergoes frequent ground shaking events not unlike terrestrial earthquakes. Ganymede , the largest moon of Jupiter and the Solar System , has a thick outer coating of water ice. Passing Voyager spacecraft found a large number of cracks and grooves in the ice so it is thought that Ganymede, like the Earth , has large shifting surface masses called tectonic plates. Ganymede was discovered by Galileo and Marius in 1610, and is larger than the planets Mercury and Pluto. The NASA spacecraft Galileo is scheduled to arrive at Jupiter is December of 1995.

Question: Io, Europa, Ganymede and Callisto are all moons of which planet in our solar system?
Answer: Jupiter