How We Came to Know the Cosmos: Space & Time

Discover How We Came to Know the Cosmos

Chapter 21. The planet Jupiter

21.1 Characteristics of Jupiter

Jupiter is the fifth-closest planet to the Sun. It takes almost 12 years to complete one orbit, and one day on Jupiter is just under 10 hours long.[1] Jupiter is over twice as massive as all the other planets in the Solar System combined. It can be seen from Earth without a telescope, and as with Mercury, Venus, and Mars, Jupiter is named after a Roman god, the god of the sky, known to the Greeks as Zeus.[2]

A photograph of Jupiter.

Figure 21.1
Image credit

Jupiter’s south pole, an image taken by Cassini on its way to Saturn.

Jupiter is thought to have a solid core that is surrounded by hydrogen and helium gas.[2] Jupiter’s Great Red Spot, which was first observed by Robert Hooke in 1664,[3] is now known to be a storm that is over three times the size of Earth.[4]

Jupiter Fact Sheet[1]

Designation = Gas giant planet
Mass = 1.9×1027 kg (318 × the mass of Earth)
Radius = 71,492 km (11.2 × the radius of Earth)
Density = 1326 kg/m3 (24% density of Earth)
Length of Day = 9.9 hours
Length of year = 4331 Earth-days (11.9 Earth-years)
Days per year = 10,499 days on Jupiter per year on Jupiter
Distance from the Sun = 7.8×108 km (5.2 AU)
Orbital Velocity = 13.1 km/s
Orbital Eccentricity = 0.049
Obliquity (tilt) = 3.1°
Mean Temperature = -110 °C
Moons = 67 (including Ganymede, Callisto, Io, and Europa)
Ring System = Yes

21.2 Jupiter’s moons

Jupiter has a faint ring system and at least 79 moons, most of which are less than 10 km in diameter.[5] The largest of Jupiter’s moons are the Galilean satellites - Ganymede, Callisto, Io, and Europa - these were discovered by Galileo Galilei in 1610[6] (discussed in Chapter 3).

Composite image showing photographs of Jupiter’s moons to scale.

Figure 21.2
Image credit

A composite image of Jupiter and the Galilean moons (to scale) from left to right: Io, Europa, Ganymede, and Callisto.

21.2.1 Io

Io is the closest Galilean moon to Jupiter and has the third-largest mass of the Galilean moons.[7] 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 and iron sulfide.[8]

The strong gravitational pull of Jupiter causes Io to be geologically active, and it’s thought to be the most geologically active object in the Solar System, with over 500 volcanic centres and over 100 mountains, some almost twice the height of Everest.[9] Volcanoes on Io emit plumes of sulfur dioxide, which can reach 500 km, over seven times the diameter of Io and five times the distance to space from the surface of the Earth.[10]

21.2.2 Europa

Europa is the second Galilean moon from Jupiter. It’s also the least massive of the Galilean moons and has a diameter similar to that of the Earth’s Moon.[7] It has a thin oxygen atmosphere and a surface composed of frozen water.[11] Europa’s interior is mostly composed of silicate rock[12] with an iron core.[13] Europa is also expected to contain a salt-water ocean, which extends for about 100 km below its frozen surface.[14]

An artist’s impression of Europa’s ocean.

Figure 21.3
Image credit

An artist’s impression of Europa with Jupiter and Io in the background.

21.2.3 Ganymede

Ganymede is the third Galilean moon from Jupiter. It’s the largest and most massive moon in the Solar System. It’s also larger than Mercury, although not as massive.[7] Ganymede is primarily composed of silicate rock and frozen water,[15] but a saltwater ocean is thought to exist nearly 200 km below the surface, between two layers of ice.[16]

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.[15] It’s the only moon in the Solar System known to have a magnetosphere.[7]

21.2.4 Callisto

Callisto is the furthest of the Galilean moons. It has the second-largest mass of the Galilean moons and is about the same size as Mercury.[7] The surface of Callisto is composed of silicate rock and frozen water. Callisto does not feel Jupiter’s tidal effects as strongly as the other moons, and so it has no active volcanoes. This means that there is nothing to periodically fill in old craters, and so Callisto is the most heavily cratered object in the Solar System.[17]

Callisto has a thin atmosphere composed of carbon dioxide.[18] It’s thought to have a small silicate core and a large icy mantle, with an ocean of liquid water between the mantle and the surface.[19]

21.3 Missions to Jupiter

The first spacecraft to observe Jupiter, as well as all the Galilean moons, was NASA’s Pioneer 10 probe in 1973. This was 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 (discussed in Chapter 27). 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. Cassini 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.

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 contaminating any moons that may harbour life.

NASA launched Juno in 2011 and it entered into orbit around Jupiter in 2016. Juno is currently observing Jupiter’s gravitational and magnetic fields and the composition and dynamics of its atmosphere to help explain how planets like Jupiter form.

The ESA plan to launch a mission to the Galilean moons in 2022, known as Jupiter Icy Moon Explorer (JUICE). The JUICE spacecraft should arrive at Jupiter in 2030. It will then fly by Callisto and Europa, before orbiting Ganymede.

NASA also plans 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 Clipper.

A painting showing people in balloons exploring the clouds of Jupiter.

Figure 21.4
Image credit

An artist’s impression of how people could explore Jupiter in the distant future.

21.4 References

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