Saturn

Saturn is the sixth planet from the Sun. It is the second largest planet in the Solar System, after Jupiter. Saturn is a gas giant, and its average radius is about 9 times that of Earth. Even though it is almost as big as Jupiter, it has less than a third of Jupiter's mass. Saturn is also much lighter for its size. It has about an eighth of Earth's average density, but it is over 95 times more massive than Earth.

Saturn is far from the Sun. It orbits the Sun at a distance of 9.59 AU, which is about 1,434 million km. One trip around the Sun takes it an orbital period of 29.45 years.

Scientists think Saturn's inside has a rocky core. This core is surrounded by a deep layer of metallic hydrogen, a middle layer of liquid hydrogen and liquid helium, and an outer layer of gas. Saturn has a pale yellow color because of ammonia crystals high in its atmosphere. An electrical current in the metallic hydrogen layer is thought to create Saturn's magnetic field. This field is weaker than Earth's, but its magnetic moment is 580 times that of Earth because Saturn is so much bigger. Saturn's magnetic field is about a twentieth as strong as Jupiter's. The outer atmosphere is usually plain and has little contrast, though some long-lasting features can appear. Wind speeds on Saturn can reach 1,800 kilometers per hour (1,100 miles per hour).

Saturn has a bright and wide system of rings. They are made mostly of ice particles, with a smaller amount of rocky pieces and dust. At least 293 moons orbit the planet. Of these, 63 have official names. This count does not include the hundreds of moonlets found in the rings.

The largest moon is Titan. It is the second largest moon in the Solar System. Titan is bigger than the planet Mercury, though it has less mass. It is also the only moon in the Solar System with a thick atmosphere.

Name and symbol

Saturn is named after the Roman god of wealth and agriculture, who was the father of the god Jupiter. Its astronomical symbol has been traced back to the Greek Oxyrhynchus Papyri. There, it can be seen to be a Greek kappa-rho joined symbol with a horizontal stroke, as a short form of Κρονος (Cronus), the Greek name for the planet. Later, it came to look like a lower-case Greek eta, with a cross added at the top in the 16th century to Christianize this pagan symbol.

The Romans named the seventh day of the week Saturday, Sāturni diēs, "Saturn's Day", for the planet Saturn.

Physical characteristics

Saturn is a gas giant, made mostly of hydrogen and helium. It does not have a solid surface. Saturn is the only planet in the Solar System that is less dense than water, about 30% less. Its average specific density is 0.69 g/cm³, partly because of its atmosphere. Jupiter's mass is 318 times Earth's, while Saturn is only 95 times Earth's. Inside Saturn, most of the mass is not in gas form, because hydrogen becomes a non-ideal liquid when it is dense enough. Deeper down, hydrogen acts like a metal. Saturn's core is similar in composition to Earth but is more dense, and it is surrounded by a thick, liquid metallic hydrogen layer. The interior is very hot, reaching 11,700 °C (21,100 °F) at the core.

The visible features on Saturn rotate at different rates depending on latitude. Because the planet spins, it is an oblate spheroid. It is flattened at the poles and bulging at the equator. Its outer atmosphere is mostly hydrogen, with some helium. Saturn has a simple magnetic dipole field, which is much weaker than Jupiter's. For this reason, its magnetosphere is much smaller than Jupiter's. Like Earth's, Saturn's magnetosphere produces aurorae.

Main article: Magnetosphere of Saturn

Orbit and observation

Saturn is on average more than 1.4 billion kilometers (9 AU) from the Sun. It travels around the Sun at an average speed of 9.68 km/s, so one trip takes 10,759 Earth days, or about 29½ years. Its orbit is tilted 2.48° compared to Earth's orbital plane, and it forms a near 5:2 mean-motion resonance with Jupiter. Saturn has only one known trojan asteroid, 2019 UO14, which was announced in September 2024. It orbits at the stable L₄ Lagrange point, 60° ahead of the planet. This leaves only Mercury without any known trojans. Orbital resonance mechanisms, including secular resonance, are believed to explain why so few Saturnian trojans are known.

Saturn is the most distant of the five planets that can be seen with the unaided eye from Earth. The others are Mercury, Venus, Mars, and Jupiter. Uranus, and sometimes 4 Vesta, can also be seen in dark skies. Saturn looks like a bright, yellowish point of light, with an average apparent magnitude of 0.46. Its brightness changes mostly because the tilt of its rings changes. It is brightest when the rings are tilted most, and faintest when they are tilted least. It takes about 29.4 years for Saturn to travel around the ecliptic through the background stars of the zodiac. Most people need a telescope or very large binoculars that magnify at least 30 times to see the rings clearly. About every 15 Earth-years, Earth passes through the plane of the rings, and the rings briefly disappear from view because they are so thin. This last happened in 2025, but Saturn was too close to the Sun to observe.

Saturn is best seen at or near opposition, when it is at an elongation of 180° and appears opposite the Sun in the sky. Opposition happens about every 378 days, when Saturn looks brightest. Saturn can also look brighter when its rings are angled so they are easier to see. From time to time, the Moon passes in front of Saturn and occults it. These events happen in "seasons," with monthly occultations for about 12 months, followed by about five years with none. Occultations only happen when Saturn is near the points where the Moon's orbit crosses its path, a pattern shaped by Saturn's year and the nodal precession of the Moon's orbit.

Natural satellites

Main article: Moons of Saturn

Saturn has 293 known moons, and 63 of them have formal names. Dozens to hundreds of moonlets, 40 to 500 meters across, are believed to be in Saturn's rings, but they are not considered true moons. Titan, the largest moon, makes up more than 90% of the mass in orbit around Saturn, including the rings. Saturn's second-largest moon, Rhea, may have a tenuous ring system of its own and a tenuous atmosphere. Many of the other moons are small, with 143 less than 50 km across. Most of Saturn's moons have traditionally been named after Titans of Greek mythology. Titan is the only satellite in the Solar System with a major atmosphere, where a complex organic chemistry occurs, and it is the only satellite with hydrocarbon lakes.

In June 2013, scientists at the IAA-CSIC reported finding polycyclic aromatic hydrocarbons in the upper atmosphere of Titan, a possible precursor for life. In June 2014, NASA claimed to have strong evidence that the nitrogen in Titan's atmosphere came from materials in the Oort cloud, which is linked to comets, rather than from the materials that formed Saturn long ago. Saturn's moon Enceladus has a chemical makeup similar to comets and has often been seen as a possible habitat for microbial life. Its salt-rich particles have an "ocean-like" makeup, which suggests that most of the ice it releases comes from liquid salt water evaporating. A 2015 flyby by Cassini through a plume on Enceladus found most of the ingredients needed to sustain life forms that live by methanogenesis. In April 2014, NASA scientists reported the possible start of a new moon within the A Ring, which Cassini had imaged on 15 April 2013.

Planetary rings

Main article: Rings of Saturn

Saturn is probably best known for its planetary rings, which make it visually unique. The rings stretch from 6,630 to 120,700 kilometers outward from Saturn's equator and are about 20 meters thick on average. They are mostly made of water ice, with small amounts of tholin and about 7% carbon. Their particles range in size from specks of dust to about 10 meters. Other gas giants also have rings, but Saturn's are the largest and most visible.

Scientists disagree about how old the rings are. One side believes they are ancient and were created at the same time as Saturn from the original nebular material (around 4.6 billion years ago), or shortly after the LHB (around 4.1 to 3.8 billion years ago). The other side believes they are much younger, formed around 100 million years ago. An MIT research team supporting the younger idea proposed that the rings are what remains of a destroyed moon of Saturn called "Chrysalis". Some of Saturn's moons, including Pandora and Prometheus, act as shepherd moons that keep the rings from spreading out.

History of observation and exploration

Scientists have studied Saturn in three phases: first by watching it with the naked eye, then with telescopes from Earth starting in the 1600s, and then with space probes that either orbit the planet or make a quick flyby. Today, telescopes on Earth still watch Saturn, including Earth-orbiting observatories such as the Hubble Space Telescope. The Cassini orbiter also studied Saturn until its 2017 retirement.

Saturn has been known since prehistoric times and played a major role in many early myths. Babylonian astronomers carefully watched and recorded its movements. The ancient Greeks called it Φαίνων Phainon, and the Romans called it the "star of Saturn" or the "star of the Sun (i.e. Helios)". In ancient Roman mythology, Phainon was sacred to an farming god, which is where the planet's modern name comes from. The Romans saw the god Saturnus as the equal of the Greek god Cronus, and in modern Greek the planet still keeps the name Cronus—Κρόνος: Kronos. The Greek scientist Ptolemy based his calculations of Saturn's orbit on observations he made while it was in opposition.

In Hindu astrology, there are nine astrological objects, known as Navagrahas. Saturn is known as "Shani" and judges people by the good and bad deeds they perform in life. Ancient Chinese and Japanese culture called Saturn the "earth star" or "soil star" (土星), based on the Five Elements that were used to classify natural things. In Hebrew, Saturn is called Shabbathai. Its angel is Cassiel. Its good spirit is 'Agȋȇl (Hebrew: אגיאל, romanized: ʿAgyal), and its darker spirit (demon) is Zȃzȇl (Hebrew: זאזל, romanized: Zazl). Zazel has been described as a great angel, invoked in Solomonic magic, who is "effective in love conjurations". In Ottoman Turkish, Urdu, and Malay, the name of Zazel is 'Zuhal', derived from the Arabic language (Arabic: زحل, romanized: Zuhal).

Saturn's rings need a telescope of at least 15 mm in diameter to be seen, and they were not known to exist until Christiaan Huygens saw them in 1655 and published his observations in 1659. Galileo, using a primitive telescope in 1610, wrongly thought Saturn's not-quite-round look meant it had two moons on its sides. When Huygens used stronger magnification, this idea was disproved and the rings were truly seen for the first time. Huygens discovered Saturn's moon Titan. Giovanni Domenico Cassini later found four more moons: Iapetus, Rhea, Tethys, and Dione. In 1675, Cassini found the gap now known as the Cassini Division. In 1789, William Herschel found two more moons, Mimas and Enceladus. The irregular moon Hyperion, which has a resonance with Titan, was found in 1848 by a British team. In 1899, William Henry Pickering found Phoebe, a highly irregular satellite that takes more than a year to orbit Saturn in a retrograde orbit. In 1944, research on Titan confirmed that it has a thick atmosphere, a feature unique among the Solar System's moons.

Main article: Exploration of Saturn

Pioneer 11 made the first flyby of Saturn in September 1979, passing within 20,000 km (12,000 mi) of the cloud tops. It took pictures of the planet and a few moons, but they were too low in resolution to show surface detail. It also studied the rings, revealing the thin F-ring and showing that dark gaps in the rings look bright when viewed at a high phase angle toward the Sun, which means they contain fine light-scattering material. It also measured the temperature of Titan.

In November 1980, the Voyager 1 probe visited Saturn and sent back the first high-resolution images of the planet, its rings, and its moons. Surface features of several moons were seen for the first time. Voyager 1 flew close to Titan and learned more about its atmosphere, which is impenetrable at visible wavelengths, so no surface details could be seen. The flyby changed the spacecraft's path out of the plane of the Solar System. In August 1981, Voyager 2 continued the study, taking more close-up images of the moons and finding evidence of changes in the atmosphere and rings. Its camera platform got stuck for a couple of days, so some planned images were lost. Saturn's gravity was used to send it toward Uranus. Together, the probes found and confirmed several new moons near or within the rings, as well as the small Maxwell Gap (a gap within the C Ring) and Keeler gap (a 42 km-wide gap in the A Ring).

Main article: Cassini–Huygens

The Cassini–Huygens space probe entered orbit around Saturn on 1 July 2004. In June 2004, it made a close flyby of Phoebe, sending back high-resolution images and data. Cassini's radar images of Titan showed large lakes and their coastlines, with many islands and mountains. After two Titan flybys, the orbiter released the Huygens probe on 25 December 2004. Huygens landed on the surface of Titan on 14 January 2005. Starting in early 2005, scientists used Cassini to track lightning on Saturn. The power of this lightning is about 1,000 times that of lightning on Earth.

In 2006, NASA reported that Cassini had found evidence of liquid water reservoirs no more than tens of meters below the surface of Saturn's moon Enceladus, which erupt in geysers. More than 100 geysers have been identified. In May 2011, NASA scientists reported that Enceladus "is emerging as the most habitable spot beyond Earth in the Solar System for life as we know it".

Cassini photographs revealed a previously unknown planetary ring, outside the brighter main rings and inside the G and E rings. Its source is thought to be a meteoroid crashing off Janus and Epimetheus. In July 2006, images showed hydrocarbon lakes near Titan's north pole, and their presence was confirmed in January 2007. In March 2007, hydrocarbon seas were found near the north pole, the largest of which is almost the size of the Caspian Sea. In October 2006, the probe detected an 8,000 km (5,000 mi) cyclone-like storm with an eyewall at Saturn's south pole. From 2004 to 2 November 2009, the probe discovered and confirmed eight new moons. In April 2013, Cassini sent back images of a hurricane at the north pole, 20 times larger than those on Earth, with winds faster than 530 km/h (330 mph). On 15 September 2017, the spacecraft performed the "Grand Finale" of its mission, making a number of passes through gaps between Saturn and its inner rings. The atmospheric entry of Cassini ended the mission.

NASA's Dragonfly mission, the fourth in the New Frontiers program, is a rotorcraft probe designed to explore Titan and search for signs of prebiotic chemical processes. NASA selected the mission in 2019. As of 2025, it is scheduled for launch in July 2028, with a total lifecycle cost of $3.35 billion. It would arrive at Titan in 2034. By April 2025, the mission had passed its Critical Design Review. It is planned to launch on a SpaceX Falcon Heavy rocket from Kennedy Space Center.

In fiction

Saturn has often appeared in stories since at least 1752, when Voltaire published his novel Micromégas. Early stories usually showed it as a solid planet, but Saturn is later correctly described as a gas planet.

Saturn's moons also appear in stories, especially Titan.