Epimetheus and Titan behind Saturn's rings (Titan is the larger planet) & Saturn's infrared ring
Image credit: NASA/JPL/Space Science Institute & NASA/JPL-Caltech/Keck
Jupiter's south pole, image taken by Cassini on its way to Saturn
Image credit: NASA/JPL/Space Science Institute

Ganymede outside and in   Image credit: NASA & NASA

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The Star Garden
A composite image of Jupiter and the Galilean moons from left to right: Io, Europa, Ganymede, and Callisto (to scale)
Image credit: NASA/JPL/DLR
Ganymede is the third Galilean moon from Jupiter. It is the largest and most massive moon in the Solar System. It is also larger than Mercury, although not as massive. Ganymede is primarily composed of silicate rock and frozen water but a saltwater ocean is thought to exist nearly 200 km below the surface, between two layers of ice. Ganymede has a thin oxygen atmosphere and a liquid iron core. It is the only moon in the Solar System to have a magnetosphere. The surface of Ganymede is covered in dark cratered regions and lighter regions, which are covered in ridges and grooves.

Titan, a flattened view from the Huygens probe at three altitudes
Image credit: ESA/NASA/JPL/University of Arizona
Saturn has at least 60 moons, and the largest of these is Titan. Titan is the second largest moon in the Solar System, after Jupiter's Ganymede, and it is about twice the diameter of the Earth's Moon. Titan is the only moon in the Solar System to possess a significant atmosphere and this is mostly composed of nitrogen, with some methane, and ethane clouds. Like Jupiter's Ganymede, Callisto, and Europa, Titan is thought to have an ocean of liquid water beneath its surface.
Plumes of water on Enceladus, a mosaic of images taken from the Cassini spacecraft taken in 2009
Image credit: NASA/JPL/Space Science Institute
Jupiter and Saturn

Jupiter is thought to have a rocky outer core, about 75% of its mass is composed of hydrogen gas, and the rest is mostly helium. Its red spot is a cyclone, three times the size of Earth. Jupiter has a faint ring system and at least 60 moons, most of which are less than 10 km in diameter. The largest of Jupiter's moons are the Galilean satellites - Ganymede, Callisto, Io, and Europa - these were discovered by Italian physicist Galileo Galilei in 1610.
The first spacecraft to observe Jupiter, as well as all the Galilean moons, was NASA's Pioneer 10 probe in 1973, followed by NASA's Pioneer 11 in 1974, and Voyager 1 and Voyager 2, both in 1979. All of these probes were on their way out of the Solar System. Pioneer 11 came closest to Jupiter, flying within 34,000 km, and Voyager 2 was furthest coming within 570,000 km. In 1990, NASA and the European Space Agency (ESA) launched Ulysses, which came within 400,000 km of Jupiter in 1992, and within 120 million km in 2004. NASA, the ESA, and the Italian Space Agency (ASI) launched the Cassini spacecraft in 1997, which passed within 10 million km of Jupiter on its way to Saturn. In 2006, NASA launched the New Horizons spacecraft which flew within 2 million km of Jupiter and passed all the Galilean moons, in 2007, while on its way to Pluto.
In 2011, NASA launched a new mission to Jupiter, Juno, which should arrive in 2016. The ESA also plan to launch a mission to the Galilean moons in 2022, known as Jupiter Icy Moon Explorer (JUICE). The JUICE spacecraft will fly by Callisto and Europa, before orbiting Ganymede.
Cassini also found evidence of liquid water on Saturn's moon Enceladus. This erupts in geysers that can send water into orbit around Saturn. The Cassini mission is still in operation.
Europa is the second Galilean moon from Jupiter, it is also the least massive of the Galilean moons and has a diameter similar to that of the Earth's Moon. It has a thin oxygen atmosphere and, like Io, it is mostly composed of silicate rock with an iron core. Europa is expected to contain a salt-water ocean, which extends for about 100 km below its frozen surface.
Saturn is almost entirely composed of hydrogen, with some helium, and trace amounts of higher elements. It has a small core of iron, nickel, silicon, and oxygen compounds, surrounded by liquid hydrogen and helium, and produces a magnetosphere. Saturn's rings are mostly composed of frozen water, but also contain rock and dust. Another, virtually invisible, ring of dust was discovered using NASA's infrared Spitzer Space Telescope in 2009.
Jupiter is the fifth closest planet to the Sun, and it is over twice as massive as all the other planets in the Solar System combined. It takes over 4000 days to orbit the Sun, this is almost twelve years. One day on Jupiter is just under ten hours. Jupiter is sometimes as bright as Mars, making it visible from Earth. As with Mercury, Venus, and Mars, Jupiter is named after a Roman God, the god of the sky, known to the Greeks as Zeus. Jupiter's Great Red Spot was first observed by British physicist Robert Hooke in 1664.

Io is the closest Galilean moon to Jupiter and has the third largest mass of the Galilean moons. It is the most geologically active object in the Solar System, with over 400 active volcanoes and 100 mountains, some taller than Everest. These emit plumes of sulphur dioxide, which can reach 500 km, over five times the distance to space from the surface of the Earth. Io is mostly composed of silicate rock. Its surface is covered with sulfur, and sulfur dioxide frost, and it has a molten core made of iron, or iron sulfide.
Callisto is the furthest of the Galilean moons, this means that it does not feel Jupiter's tidal effects as strongly and has no active volcanoes. It has the second largest mass of the Galilean satellites and is about the same size as Mercury. Callisto has a thin atmosphere composed of carbon dioxide and oxygen. The surface is composed of silicate rock, frozen water, carbon dioxide, and other organic compounds. It is heavily cratered. Callisto is thought to have a small silicate core, and an ocean of liquid water about 100 km below its surface.
The only spacecraft to orbit Jupiter, so far, is NASA's Galileo spacecraft, launched in 1989. The Galileo orbiter went into orbit around Jupiter in 1995, after flying by all of the Galilean moons. It released an atmospheric probe, which lasted for just under an hour. The orbiter was deliberately steered into Jupiter and destroyed in 2003, to avoid it contaminating any moons that may harbour life.
Saturn is the sixth closest planet to the Sun and the second largest planet in the Solar System, after Jupiter. It takes over 10,000 days to orbit the Sun, this is about thirty years. One day on Saturn is just under eleven hours long. Saturn is visible from the Earth and, like Mercury, Venus, Mars, and Jupiter. It is named after a Roman God, the god of agriculture. The rings of Saturn were first observed by Galileo and identified by Dutch mathematician Christiaan Huygens, who discovered Saturn's moon Titan in 1655.
Pioneer 11 passed within 20,000 km of Saturn in 1979. Voyager 1 came within 124,000 km in 1980, and Voyager 2 came within about 700,000 km of Saturn in 1981. In 2004, the Cassini spacecraft entered into orbit around Saturn after flying by many of its moons. Cassini's radar images of Titan showed that it has lakes, sand dunes, coastlines, islands, mountains, and hydrocarbon "seas", the only surface liquid ever discovered outside of Earth. It completed two orbits before releasing a probe, the Huygens probe, which landed on Titan in 2005, and found evidence of frozen water on the surface.