Jupiter and Saturn


The planets, sizes approximately to scale (distances are not). Image credit: Lunar and Planetary Institute.


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 12 years, and one day on Jupiter is just under 10 hours long. Jupiter is visible from Earth, and sometimes appears as bright as Mars. 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 is thought to have a rocky outer core. Above this, about 75% of its mass is composed of hydrogen gas, and the rest is mostly helium. Jupiter's Great Red Spot, which was first observed by British physicist Robert Hooke in 1664, is now known to be a cyclone that is 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.


Jupiter's southern pole, image taken by Cassini on its way to Saturn. Image credit: NASA/JPL/Space Science Institute.

Io is the closest Galilean moon to Jupiter and has the third largest mass of the Galilean moons. The strong gravitational pull of Jupiter causes it to be geologically active, and it is thought to be 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 sulfur 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.

Jupiter and moons

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.

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 also expected to contain a salt-water ocean, which extends for about 100 km below its frozen surface.


Artist's impression showing the structure of Europa, where the blue is an ocean of water. Image credit: NASA.


Artist's impression of lakes on Europa. Image credit: Britney Schmidt/Dead Pixel VFX/Univ. of Texas at Austin.

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. The surface of Ganymede is covered in dark cratered regions and lighter regions, which are covered in ridges and grooves. Ganymede has a thin oxygen atmosphere and a liquid iron core. It is the only moon in the Solar System to have a magnetosphere.

Callisto is the furthest of the Galilean moons. This means that it does not feel Jupiter's tidal effects as strongly as the other moons, and so it 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 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. New Horizons flew within 2 million km of Jupiter, and passed all the Galilean moons, in 2007, while on its way to Pluto.

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.

In 2011, NASA launched a new mission to Jupiter, Juno, which should arrive in 2016. The ESA plan to launch a mission to the Galilean moons in 2022, known as Jupiter Icy Moon Explorer (JUICE). The JUICE spacecraft arrive at Jupiter in 2030. It will then fly by Callisto and Europa, before orbiting Ganymede. NASA also plan to launch a mission to the Galilean moons in the 2020s, which will focus on looking for evidence for life on Europa. This is known as the Europa Multiple-Flyby Mission.


The four outer planets to scale. Image credit: Lunar and Planetary Institute.


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 almost 30 years, and one day on Saturn is just under 11 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 first identified by Dutch mathematician Christiaan Huygens, who discovered Saturn's moon Titan in 1655.


Saturn. Image credit: NASA/JPL-Caltech/Space Science Institute/G. Ugarkovic.

Epimetheus and Titan

Epimetheus and Titan behind Saturn's rings (Titan is the larger planet). Image credit: NASA/JPL/Space Science Institute.

Infrared ring

Saturn's infrared ring. Image credit: NASA/JPL-Caltech/Keck.

Saturn is almost entirely composed of hydrogen, with some helium, and trace amounts of other elements. It has a small core of iron, nickel, silicon, and oxygen compounds, surrounded by liquid hydrogen and helium. Saturn's rings are mostly composed of frozen water, but also contain rock and dust. Another ring was discovered using NASA's Spitzer Space Telescope in 2009, this is made of dust, and is visible in infrared.

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.

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.


Titan, a flattened view from the Huygens probe at three altitudes. Image credit: ESA/NASA/JPL/University of Arizona.

The Cassini spacecraft was launched in 1997, and became the first spacecraft to enter into orbit around Saturn in 2004. The Huygens probe separated from Cassini later that year, and landed on Titan in 2005. The Huygens probe found evidence of frozen water on the surface, and radar images showed that Titan has sand dunes, coastlines, islands, and mountains. In 2006, Cassini found evidence of hydrocarbon lakes, the only surface liquid ever discovered outside of Earth.


Plumes of water on Enceladus, a mosaic of images taken from the Cassini spacecraft in 2009. Image credit: NASA/JPL/Space Science Institute.

The Cassini spacecraft has made flybys of many of Saturn's moons, including Phoebe, Iapetus, Mimas, Tethys, Hyperion, Dione, Rhea, and Enceladus. Cassini found evidence of liquid water on Enceladus in 2005. This erupts in geysers that can send water into orbit around Saturn. In 2015, Cassini found evidence of a global ocean beneath Enceladus' surface, which "could contain environments suitable for living organisms”. Cassini is currently still in operation, and a timeline of mission highlights is shown below.

Cassini Timeline

Cassini Mission Timeline. Image credit: NASA/ESA/ASI.

The ESA and NASA were considering sending a new probe to land on Titan as part of the Titan Saturn System Mission. However this has been postponed, and currently has no launch date.

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