How We Came to Know the Cosmos: Space & Time

Discover How We Came to Know the Cosmos

Chapter 22. The planet Saturn

22.1 Characteristics of Saturn

Saturn is the sixth-closest planet to the Sun and the second-largest planet in the Solar System, after Jupiter. It takes almost 30 years to orbit the Sun and one day on Saturn is just under 11 hours long.[1]

A photograph of Saturn.

Figure 22.1
Image credit

Saturn, an image taken by Cassini.

Like Mercury, Venus, Mars, and Jupiter, Saturn is visible from the Earth without the use of a telescope, and like them, it is also named after a Roman god, the god of agriculture.[2] The rings of Saturn were first observed by Galileo Galilei[3] and first identified by Christiaan Huygens,[4] who discovered Saturn’s moon Titan in 1655.[5]

Saturn is almost entirely composed of hydrogen, with some helium and trace amounts of other elements. It has a small core of rock and frozen water, surrounded by liquid hydrogen and helium. Saturn’s rings are mostly composed of frozen water but they also contain rock and dust.[2] A much larger ring was discovered using NASA’s Spitzer Space Telescope in 2009. This is made of dust and it is visible in infrared.[6]

A photograph of Saturn showing its infrared ring.

Figure 22.2
Image credit

Saturn’s infrared ring.

22.2 Saturn’s moons

22.2.1 Titan

Saturn has at least 62 moons, and the largest of these is Titan.[7] Titan is the second-largest moon in the Solar System, after Jupiter’s Ganymede, and it’s about the same size as Mercury.[8]

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.[9] Titan is also thought to have an ocean of liquid water beneath its surface[10] like Jupiter’s moons Ganymede, Callisto, and Europa.

In 2005, the ESA’s Huygens probe found evidence of frozen water on Titan’s surface, and radar images showed that Titan has coastlines, islands,[11] sand dunes, and mountains.[9] In 2006, NASA, ESA, and ASI’s Cassini spacecraft found evidence of hydrocarbon lakes, the only stable bodies of surface liquid ever discovered outside of Earth.[12]

A photograph of Titan.

Figure 22.3
Image credit

Titan, a flattened view from the Huygens probe at three altitudes.

An artist’s impression of Titan.

Figure 22.4
Image credit

An artist’s impression of Titan, showing hydrocarbon lakes.

Painting of people in rowboats on the surface of Titan, Saturn is setting in the background.

Figure 22.5
Image credit

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

22.2.2 Enceladus

Enceladus is one of the brightest objects in the Solar System, despite being only about 500 km in diameter.[13] This is because it’s covered in frozen water, which reflects light from the Sun back into space. Enceladus frequently erupts in geysers that can extend for up to 1500 km.[14]

Cassini found evidence of liquid water on Enceladus in 2006,[15] and in 2015, Cassini found evidence of a global ocean beneath Enceladus’ surface[16] that “could contain environments suitable for living organisms”.[17]

A photograph showing plumes of water on Enceladus.

Figure 22.6
Image credit

Plumes of water on Enceladus, a mosaic of images taken by the Cassini spacecraft in 2009.

Saturn Fact Sheet[1]

Designation = Gas giant planet
Mass = 5.7×1026 kg (95 × the mass of Earth)
Radius = 60,268 km (9.4 × the radius of Earth)
Density = 687 kg/m3 (12.5% density of Earth)
Length of Day = 10.7 hours
Length of year = 10,747 Earth-days (29.4 Earth-years)
Days per year = 24,105 days on Saturn per year on Saturn
Distance from the Sun = 1.4×109 km (9.6 AU)
Orbital Velocity = 9.7 km/s
Orbital Eccentricity = 0.057
Obliquity (tilt) = 26.7°
Mean Temperature = -140 °C
Moons = 62 (including Titan and Enceladus)
Ring System = Yes

22.3 Missions to Saturn

NASA’s Pioneer 11 passed within 20,000 km of Saturn in 1979. NASA’s Voyager 1 came within 124,000 km in 1980, and Voyager 2 came within about 700,000 km of Saturn in 1981.

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 Cassini spacecraft has made flybys of many of Saturn’s moons, including Phoebe, Iapetus, Mimas, Tethys, Hyperion, Dione, Rhea, and Enceladus. Its final operations were in 2017.

NASA plans to launch Dragonfly in 2026, which will arrive at Titan in 2034 and land a probe on the surface to study its chemistry and suitability for life.

22.4 References

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