Saturn is the sixth planet in the Solar System in order of distance from the Sun, and the second largest in size and mass after Jupiter, which is like it a gas giant planet. Its average radius of 58,232 km is about nine and a half times that of the Earth and its mass of 568.46 × 1024 kg is 95 times larger. Orbiting on average about 1.4 billion kilometers from the Sun (9.5 astronomical units), its period of revolution is worth just under 30 years while its rotation period is estimated at 10 h 33 min.
| Orbital characteristics | |
|---|---|
| Semi-major axis | 1,426,700,000 km (9,536 7 au) |
| Aphelion | 1,503,500,000 km (10.05 au) |
| Perihelion | 1,349,800,000 km (9.023 au) |
| Orbital circumference | 8,957,500,000 km (59,877 au) |
| Eccentricity | 0,053 9 |
| Period of revolution | 10 754 days (≈ 29.44 a) |
| Synodic period | 378.039 days |
| Average orbital speed | 9,640 7 km/s |
| Maximum orbital speed | 10.182 km/s |
| Minimum orbital speed | 9.141 km/s |
| Tilt on the ecliptic | 2,486° |
| Ascending node | 113,7° |
| Perihelion argument | 338,94° |
| Known satellites | 82 confirmed (of which 53 have been named) and about 150 minor moons. |
| Known rings | 7 main, finely divided |
| Physical characteristics | |
| Equatorial radius | 60,268 km (9,449 2 Earths) |
| Polar radius | 54,359 km (8,552 1 Earths) |
| Volumetric mean radius |
58,232 km (9,014 Earths) |
| Flattening | 0,097 96 |
| Equatorial perimeter | 378,675 km |
| Area | 4,346 6 × 1010 km2 (83,703 Earths) |
| Volume | 8,271 3 × 1014 km3 (763 Earths) |
| Mass | 5,684 6 × 1026 kg (95,152 Earths) |
| Global density | 687.3 kg/m3 |
| Surface gravity | 10.44 m/s2 (1.064 g) |
| Release speed | 35.5 km/s |
| Rotation period (sidereal day) |
0.448 days (10 h 33 min) |
| Rotational speed (at the equator) |
34,821 kph |
| Axis tilt | 26,73° |
| Right ascension of the North Pole | 40,60° |
| Declination of the North Pole | 83,54° |
| Visual geometric albedo | 0,47 |
| Bond’s Albedo | 0,342 |
| Solar irradiance | 14.90 W/m2 (0.011 Earth) |
| Blackbody equilibrium temperature |
81.1 K (−191.9 °C) |
| Surface temperature | |
| • Temperature at 10 kPa | 84 K (−189 °C) |
| • Temperature at 100 kPa | 134 K (−139 °C) |
| Characteristics of the atmosphere | |
| Density at 100 kPa |
0.19 kg/m3 |
| Ladder height | 59.5 km |
| Average molar mass | 2.07 g/mol |
| Dihydrogen H2 | > 93% |
| Helium He | > 5% |
| Methane CH4 | 0,2 % |
| Water vapor H2O | 0,1 % |
| Ammonia NH3 | 0,01 % |
| Ethane C2H6 | 0,0005 % |
| Phosphorus hydride PH3 | 0,0001 % |
| History | |
| Babylonian deity | Ninurta (Ninib) |
| Greek deity | Κρόνος |
| Chinese Name(Related Item) |
Tǔxīng 土星 (land) |
The most famous feature of the planet is its prominent ring system. Composed mainly of ice particles and dust, they were first observed in 1610 by Galileo and would have formed less than 100 million years ago. Saturn is the planet with the largest number of natural satellites with 82 confirmed and hundreds of minor satellites in its procession. Its largest moon, Titan, is the second largest in the Solar System (behind Jupiter’s moon Ganymede, both with a diameter larger than Mercury’s) and it is the only known moon to have a substantial atmosphere. Another remarkable moon, Enceladus, emits powerful ice geysers and is thought to be a potential habitat for microbial life.
The interior of Saturn is most likely composed of a rocky core of silicates and iron surrounded by layers consisting in volume of 96% hydrogen which is successively metallic then liquid then gaseous, mixed with helium. Thus, it has no solid surface and is the planet with the lowest average density at 0.69 g/cm3 — 70% of that of water. An electric current in the metallic hydrogen layer gives rise to its magnetosphere, the second largest in the Solar System but much smaller than Jupiter’s. Saturn’s atmosphere is generally dull and lacks contrast, although long-lasting features may appear as a hexagon at its north pole. Winds on Saturn can reach speeds of 1,800 km/h, the second fastest in the Solar System after those of Neptune. It has been explored by four space probes: Pioneer 11, Voyager 1 and 2 and Cassini-Huygens (named after two astronomers who greatly advanced knowledge of the Saturnian system in the seventeenth century).
Observable with the naked eye in the night sky thanks to its average apparent magnitude of 0.46 – although having a fainter brightness than other planets – it has been known since prehistoric times and has long been the furthest planet from the Sun known. Also, its observation has inspired myths and it bears the name of the Roman god of agriculture Saturn (Cronus in Greek mythology), its astronomical ♄ symbol representing the sickle of the god.
Physical characteristics of Saturn
Mass and dimensions
Saturn has the shape of an ellipsoid of revolution: the planet is flattened at the poles and bulged at the equator, a consequence of its rapid rotation on itself and an extremely fluid internal composition. By convention, the surface of the planet is defined as where atmospheric pressure is equal to 1 bar (100,000 Pa) and is used as a reference point for altitudes. Its equatorial and polar radii differ by nearly 10% with 60,268 km against 54,364 km, giving a volumetric average radius of 58,232 km — 9.5 times larger than the Earth’s radius. This amounts to a flattening of 0.098, the largest of the giant planets — and of the planets in the Solar System in general.
Saturn is the second most massive planet in the Solar System, with a mass 3.3 times less than Jupiter, but 5.5 times that of Neptune and 6.5 times that of Uranus. Jupiter and Saturn represent respectively 318 times and 95 times the Earth’s mass, the two planets possess 92% of the total planetary mass of the Solar System.
The gravity of the surface along the equator, 8.96 m/s2, is 90% of that on the surface of the Earth’s equator. However, the escape velocity at the equator is 35.5 km/s, about three times more than on Earth.
Saturn is the least dense planet in the Solar System with 0.69 g/cm3, or about 70% of the density of water. Indeed, although Saturn’s core is considerably denser than water, the average density is lowered due to its large atmosphere. To illustrate this, it is sometimes mentioned that if there were an ocean large enough to contain it, it would float. In reality, it would obviously be impossible to have a planet with a sufficiently deep ocean — it would be of the order of magnitude of the Sun and would thus not be stable — and Saturn’s cohesion would not be maintained because it is gaseous, so its very dense core would sink accordingly.
Internal structure of Saturn
Saturn is classified as a gas giant because it is mainly composed of hydrogen and helium. Thus, standard planetary models suggest that Saturn’s interior is similar to that of Jupiter, with a rocky core surrounded by hydrogen and helium as well as traces of volatile substances — also called “ice”.
The rocky core would be of a similar composition to the Earth, consisting of silicates and iron, but denser. t is estimated from the planet’s gravitational field and geophysical models of gaseous planets that the core must have a mass ranging from 9 to 22 Earth masses reaching a diameter of about 25,000 km. This is surrounded by a layer of thicker liquid metallic hydrogen, followed by a liquid layer of molecular hydrogen and helium that gradually turns into gas as altitude increases. The outermost layer extends for 1,000 km and consists of gas. Also, most of Saturn’s mass is not in the gas phase because hydrogen becomes liquid when the density is greater than 0.01g/cm3, this boundary being reached on the surface of a sphere corresponding to 99.9% of Saturn’s mass.
Saturn has a very high internal temperature, reaching 12,000 K (11,727 °C) at its core and radiating, like Jupiter, more energy into space than it receives from the Sun — about 1.78 times. Jupiter’s thermal energy is generated by the Kelvin-Helmholtz mechanism of slow gravitational compression, but such a process alone is not sufficient to explain Saturn’s heat production because it is less massive. An alternative or additional mechanism would be the generation of heat by the ” rain” of helium droplets in the depths of Saturn. As the droplets descend through the lower-density hydrogen, the process would release heat through friction and leave Saturn’s outer layers depleted of helium. These descending droplets may have accumulated in a helium shell surrounding the nucleus. This immiscibility of hydrogen and helium, theoretically predicted since the 1970s, was verified experimentally in 2021. It is also suggested that diamond showers occur inside Saturn, as well as within Jupiter and the ice giants Uranus and Neptune.
However, given its distance from the Sun, Saturn’s temperature drops rapidly to 134 K (−139 °C) at 1 bar and then 84 K (−189 °C) at 0.1 bar, for an effective temperature of 95 K (−178 °C).
Atmosphere
composition
Saturn’s upper atmosphere is 96.3% hydrogen and 3.25% helium by volume. This proportion of helium is significantly lower than the abundance of this element in the Sun. The amount of elements heavier than helium (called metallicity) is not precisely known, but the proportions are assumed to correspond to the primordial abundances resulting from the formation of the Solar System; the total mass of these elements is estimated to be 19 to 31 times that of Earth, with a significant fraction located in the region of Saturn’s core. Traces of methane CH4, ethane C2H6, ammonia NH 3, acetylene C2H2 and phosphine PH3 were also detected.
Ultraviolet radiation from the Sun causes photolysis of methane in the upper atmosphere, leading to the production of hydrocarbons, with the resulting products being transported downwards by turbulence vortices and diffusion. This photochemical cycle is modulated by Saturn’s seasonal cycle.
Cloud layers
Similar to Jupiter, Saturn’s atmosphere is organized in parallel bands, although these bands are less contrasted and wider near the equator. These bands are caused by the presence of methane in the global atmosphere, which are darker the higher the concentration.
Saturn’s cloud system was first observed during Voyager missions in the 1980s. Since then, ground-based telescopes have progressed and make it possible to follow the evolution of the Saturnian atmosphere. Thus, common features on Jupiter, such as long-lived oval storms, are found on Saturn; moreover, the nomenclature used to describe these bands is the same as on Jupiter. In 1990, the Hubble Space Telescope observed a very large white cloud near Saturn’s equator that was not present during the passage of the Voyager probes, and in 1994 another storm of a smaller size was observed.
The composition of Saturn’s clouds varies with increasing depth and pressure. In the highest regions, where temperatures range from 100 K (−173 °C) to 160 K (−113 °C) and pressure between 0.5 and 2 bar, clouds consist of ammonia crystals. Between 2.5 and 9 bar is water ice H2W at temperatures of 185 K (−88 °C) to 270 K (−3 °C). These clouds intermingle with clouds of ammonium hydrosulfide NH4SH ice between 3 and 6 bar, with temperatures ranging from 190 K (−83 °C) to 235 K (−38 °C). Finally, the lower layers, where pressures are between 10 and 20 bar and temperatures from 270 K (−3 °C) to 330 K (57 °C), contain a region of water droplets with ammonia (ammonia in aqueous solution).
In the images transmitted in 2007 by the Cassini probe, the atmosphere of the northern hemisphere appears blue, similar to that of Uranus. This color is probably caused by Rayleigh scattering.
Storms
Saturn’s winds are the second fastest among the planets of the Solar System, after those of Neptune. Voyager data indicate easterly winds of up to 500 m/s (1,800km/h).
The 1990 storm is an example of the Great White Spot, a unique but short-lived phenomenon occurring once a Saturnian year, or every 30 Earth years, around the time of the northern hemisphere summer solstice. Large white spots were previously observed in 1876, 1903, 1933 and 1960. The last Great White Spot was observed by Cassini in 2010 and 2011. Releasing large quantities of water periodically, these storms indicate that the lower Saturnian atmosphere would contain more water than that of Jupiter.
A persistent hexagonal wave system around the north polar vortex at a latitude of about +78° — called Saturn’s hexagon — is noted for the first time thanks to Voyager images. The sides of the hexagon are each about 13,800 km long, more than the diameter of the Earth. The entire structure rotates with a period of just over 10:39: 24, which corresponds to the period of radio emissions from the planet and is assumed to be the rotation period of Saturn’s interior. This system does not shift in longitude like other cloud structures in the visible atmosphere. The origin of the pattern is not certain but most scientists believe that it is a set of standing waves in the atmosphere. Indeed, similar polygonal shapes have been reproduced in the laboratory by differential rotation of fluids.
At the South Pole, images taken by the Hubble Space Telescope from 1997 to 2002 indicate the presence of a jet stream, but not a polar vortex or an analogous hexagonal system. However, NASA reported in November 2006 that Cassini had observed a cyclone-like storm, hovering at the south pole and possessing a clearly defined eye. It is the only eye ever observed on a planet other than Earth; for example, images from the Galileo spacecraft do not show an eye in Jupiter’s Great Red Spot. Also, thermography reveals that this polar vortex is hot, the only known example of such a phenomenon in the Solar System. While the effective temperature on Saturn is 95 K (−178 °C), temperatures on the vortex reach up to 151 K (−122 °C), making it probably Saturn’s hottest spot. It would be nearly 8,000 km wide, a size comparable to that of the Earth, and would experience winds of 550km/h. It could be billions of years old.
From 2004 to 2009, the Cassini probe observed the formation, development and end of violent storms, including the Dragon Storm or gaps in the cloud structure forming “pearl chains”. Saturn’s storms are particularly long; For example, a thunderstorm lasted from November 2007 to July 2008. Similarly, a very violent storm began in January 2009 and lasted more than eight months. These are the longest storms observed so far in the Solar System. They can extend more than 3,000 km in diameter around the region called “storm alley” located 35° south of the equator. The electrical discharges caused by Saturn’s storms emit radio waves ten thousand times stronger than those of terrestrial storms.
Magnetosphere
Saturn has an intrinsic magnetic field that has a simple shape and behaves like a magnetic dipole, almost aligned with the planet’s axis of rotation and whose magnetic north pole corresponds to the geographic south pole. It was discovered in 1979 by the Pioneer 11 probe when it measured its intensity: its strength at the equator is about 0.2 Gauss (20 μT), one-twentieth of Jupiter’s field and slightly weaker than the Earth’s magnetic field. As a result, Saturn’s magnetosphere — a cavity created in the solar wind by the planet’s magnetic field — is the second largest in the Solar System but remains much smaller than Jupiter’s. The magnetopause, the boundary between Saturn’s magnetosphere and the solar wind, lies only about twenty times Saturn’s radius (1,200,000 km) from the center of the planet, while the magnetic tail stretches behind it hundreds of times the Saturnian radius.
Most likely, the magnetic field is generated in the same way as that of Jupiter with convection currents in the liquid metallic hydrogen layer creating a dynamo effect. This magnetosphere is effective in diverting particles from the solar wind. The interaction of Saturn’s magnetosphere and solar winds, as in the case of Earth, produces aurora borealis on the planet’s poles in the visible, infrared, and ultraviolet range.
Saturn’s magnetosphere is filled with plasma originating from the planet and its natural satellites, including Enceladus which ejects up to 600 kg/s of water vapor through its geysers located at its south pole or Titan’s atmosphere whose ionized particles interact with the magnetosphere. In addition, there is inside the magnetosphere a radiation belt, similar to the Van Allen belt for the Earth, which contains energy particles up to ten megaelectronvolts.
Training
The most commonly adopted formation mechanism for planet formation is the core accretion model from the accretion disk. Giant planets, like Saturn, form beyond the ice line, an area beyond Mars’ orbit where matter is cool enough for different types of ice to remain solid. They grow until they become massive enough to begin accumulating helium-hydrogen gas from the disk, the lightest but also the most abundant elements. As this phenomenon accelerated, it is estimated that Jupiter and Saturn would have accumulated most of their mass in only 10,000 years. The significantly smaller mass of Saturn compared to Jupiter would be explained by the fact that it would have formed a few million years after Jupiter, while there was less gas available in its environment.
Two characteristics of Saturn are surprising in the context of classical patterns of planet formation: its obliquity of about 26.7°, too high to be explained by an impact, and the presence of imposing rings about 100 million years old. The rapid distance from Titan, still observed today, could have initially increased the obliquity up to 36° when passing through a precession resonance with Neptune. A sudden event would then have shifted the resonance, the distance from Titan then having the consequence of reducing the obliquity to its current value. This event could be the disappearance of a satellite called Chrysalis, whose destabilization of the orbit would also explain the formation of the rings by a grazing encounter with Saturn.
Orbital characteristics
Saturn orbit
The semi-major axis of Saturn’s orbit around the Sun is 1.427 billion kilometers (or 9 astronomical units). With an average orbital velocity of 9.68km/s, its period of revolution is about 29 and a half years (10,759 Earth days). Saturn’s elliptical orbit is inclined 2.48° with respect to the Earth’s orbital plane, the ecliptic. The perihelion and aphelion distances are 9.195 and 9.957 AU, respectively, on average, due to its orbital eccentricity of 0.054.
Rotation
Similar to Jupiter, the features visible on Saturn rotate at different speeds depending on latitude—a differential rotation—and thus all have their own rotation periods. By convention, several systems are defined, each with its own rotation period.
The first, with a period of 10 h 14 min 0 s, corresponds to the equatorial zone extending between the northern edge of the southern equatorial belt and the southern edge of the boreal equatorial belt. The north and south polar regions are also attached to the first system.
The second concerns all other latitudes and has by convention a rotation period of 10 h 39 min 24 s.
Finally, a third system relies on the rotation of Saturn’s radio emissions, notably detected by Voyager 1 and Voyager 2 because the waves emitted by Saturn are at low frequencies blocked by the Earth’s atmosphere, and has a rotation period of 10 h 39 min 22 s. This value was then considered equal to the internal rotation period of the planet, even if it remained unknown. Approaching Saturn in 2004, however, Cassini found that Saturn’s radio rotation period had increased significantly since previous flybys, at about 10 h 45 min 45 s without the exact cause of the change being known.
In March 2007, it was then observed that the variation in the period of radio emissions from the planet did not actually correspond to the rotation of Saturn but was caused by convection movements of the plasma disk surrounding Saturn, which are independent of rotation. These could be the consequence of the presence of the geysers of the moon Enceladus. Indeed, the water vapor emitted into Saturn’s orbit by this activity is electrically charged and induces drag on Saturn’s magnetic field, slightly slowing its rotation relative to that of the planet.
In 2019, a study suggests that seasonal variations may be a confounding variable when it comes to measuring the rotation period. Indeed, unlike Jupiter whose rotation period has long been known thanks to radio measurements and which has an axis inclination of 3 °, Saturn has an inclination of 27 ° – more than the 23 ° of the Earth – and therefore knows seasons. This variation in the solar energy received would affect the plasma around Saturn and therefore its rotation period by creating a trail. The same year, NASA advanced that the rotation period of Saturn, according to the latest data captured by the Cassini probe, is 10 h 33 min 38 s. This value was obtained by observing disturbances in its rings. However, in 2020, the planet’s NASA Fact Sheet still indicates as a rotation period the value of the third system returned by Voyager, namely 10.656 hours or 10 h 39 min 22 s.
Procession of Saturn
Moons
As of 2020, 82 natural satellites of Saturn are known, 53 of them being named and the other 29 having a provisional designation. In addition, there is evidence of tens to hundreds of minor satellites with diameters ranging from 40 to 500 meters present in Saturn’s rings, which, however, cannot be considered moons. Most moons are small: 34 have a diameter of less than 10 km and 14 others have a diameter between 10 and 50 km. Only seven are massive enough to have taken a spheroidal form under their own gravity: Titan, Rhea, Iapetus, Dione, Tethys, Enceladus, and Mimas (by decreasing mass). Along with Hyperion, which has an irregular shape, these eight moons are said to be “major”.
Traditionally, Saturn’s 24 regular moons—that is, those with a prograde, nearly circular, and slightly inclined orbit—are named after Titans from Greek mythology or characters associated with the god Saturn. The others are all irregular satellites with an orbit much farther away and strongly inclined relative to the planet’s equatorial plane — suggesting that they are objects captured by Saturn — as well as a size of less than thirty kilometers, with the exception of Phoebe and Siarnaq. They are named after giants from Inuit, Norse and Celtic mythologies.
Titan is Saturn’s largest satellite, accounting for about 96% of the mass orbiting the planet, including the rings. Discovered by Christian Huygens in 1655, it was the first moon observed. It is the second largest natural satellite in the Solar System after Ganymede — its diameter is larger than that of Mercury or Pluto, for example — and the only one with a major atmosphere consisting mainly of dinitrogen in which complex organic chemistry occurs. It is also the only satellite with hydrocarbon seas and lakes.
The satellite, composed mainly of rock and water ice, sees its climate shape its surface in a similar way to what happens on Earth, so that it is sometimes compared to a “primitive Earth”. In June 2013, scientists at the Instituto de Astrofísica de Andalucía reported the detection of polycyclic aromatic hydrocarbons in the mesosphere of Titan, a possible precursor to life. Thus, it is a possible host of microbial extraterrestrial life and a possible subterranean ocean could serve as a favorable environment for life. In June 2014, NASA claimed to have strong evidence that nitrogen in Titan’s atmosphere came from materials in the Oort cloud, associated with comets, and not from the materials that formed Saturn.
Saturn’s second-largest moon, Rhea, has its own ring system and a tenuous atmosphere Iapetus, on the other hand, is remarkable for its coloration — one of its hemispheres being particularly bright while the other is very dark — and for its long equatorial crest. With Dione and Tethys, these four moons were discovered by Jean-Dominique Cassini between 1671 and 1684.
William Herschel then discovered Enceladus and Mimas in 1789. The first, whose chemical composition seems similar to comets, is notable because it emits powerful geysers of gas and dust and may contain liquid water beneath its south pole. Thus, it is also considered a potential habitat for microbial life. Evidence for this possibility includes, for example, salt-rich particles with an “ocean-like” composition that indicates that most of the ice expelled from Enceladus comes from the evaporation of liquid salt water. A flyby of Cassini in 2015 through a plume on Enceladus notes the presence of most of the ingredients necessary to support life forms practicing methanogenesis. Mimas, meanwhile, is responsible for the formation of Cassini’s division, and its appearance — with a crater one-third of its diameter — means that it is regularly compared to the Death Star from the Star Wars saga.
In October 2019, a team of astronomers from the Carnegie Institution for Science observed 20 new satellites, making Saturn the planet in the Solar System with the most known natural satellites with 82 confirmed, ahead of Jupiter and its 79 moons.
Planetary rings
One of Saturn’s best-known features is its planetary ring system which makes it visually unique. The rings form a disk whose diameter is nearly 360,000 km — a little less than the Earth-Moon distance — with the main rings — named A, B and C — extending from about 75,000 to 137,000 km from the planet’s equator and having a thickness of only a few tens of meters. Also, they always maintain the same inclination as the equator of the planet. They are mainly composed of water ice (95 to 99% pure water ice according to spectroscopic analyses), with traces of tholin impurities and an amorphous carbon coating. Although they appear continuous when viewed from Earth, they are actually made up of countless particles ranging in size from a few micrometers to about ten meters, each with a different orbit and orbital speed. While the other giant planets — Jupiter, Uranus and Neptune — also have ring systems, Saturn’s is the largest and most visible in the Solar System with an albedo of 0.2 to 0.6, even observable from Earth using binoculars.
They were first seen on 25 July 1610 by the Italian scientist Galileo thanks to an astronomical telescope of his own making. He interprets what he sees as two mysterious appendages on either side of Saturn, disappearing and reappearing during the orbit of the planet as seen from Earth. Benefiting from a better telescope than Galileo, the Dutchman Christian Huygens was the first to suggest in 1655 that it was in fact a ring surrounding Saturn, thus explaining the disappearances observed by the fact that the Earth passes in the plane of it. In 1675, Jean-Dominique Cassini discovered that there were actually several rings in a division between them; as such, the observed separation, located between rings A and B, is called “Cassini division” in his honor. A century later, James Clerk Maxwell demonstrated that rings are not solid but actually composed of a very large number of particles.
The rings are named alphabetically in the order of their discovery. They are relatively close to each other, spaced by often narrow “divisions” – with the exception of the Cassini division, which is nearly 5 thousand kilometers wide – where the particle density decreases greatly. These divisions are mostly caused by the gravitational interaction of Saturn’s moons, including shepherd satellites. For example, Pan is in the division of Encke and Daphnis is in the division of Keeler, which they would have respectively created by their effects — this also makes it possible to calculate precisely the mass of these satellites. The Cassini division, on the other hand, seems to be formed by the gravitational pull of Mimas.
The water abundance of the rings varies radially, with the outermost ring A being the purest in icy water; This variance in abundance can be explained by meteorite bombardment. The A, B and C rings are the most visible — the B ring is the brightest among them — and thus considered “main”. The D, E, F and G rings, on the other hand, are more tenuous and were discovered later. Some of the ice in the E ring comes from the geysers of the moon Enceladus.
In 2009, a much more distant ring was highlighted by the Spitzer satellite in infrared. This new ring, called the Ring of Phoebe, is very tenuous and is aligned with one of Saturn’s moons: Phoebe. It is thus assumed that the moon would be the origin and shares its retrograde orbit.
| Characteristics of Saturn’s rings and divisions | |||||||
|---|---|---|---|---|---|---|---|
| Name | Internal radius | External radius | width (km) |
thickness (m) |
Named after | ||
| Miles | RS | Miles | RS | ||||
| Ring D | 66 900 | 1,110 | 74 510 | 1,236 | 7 610 | ? | |
| C ring | 74 658 | 1,239 | 92 000 | 1,527 | 17 342 | 5 | |
| Ring B | 92 000 | 1,527 | 117 580 | 1,951 | 25 580 | 5-10 | |
| Division de Cassini | 117 500 | 1.95 | 122 200 | 2,03 | 4 700 | — | Jean-Dominique Cassini |
| Ring A | 122 170 | 2,027 | 136 775 | 2,269 | 14 605 | 20-40 | |
| Encke Division | 133 589 | 2,216 | — | — | 325 | — | Johann Franz Encke |
| Division de Keeler | 136 530 | 2,265 | — | — | 35 | — | James Edward Keeler |
| Division de Roche | 136 775 | 2,284 | 139 380 | 2.313 | 2 600 | ? | Edward Roche |
| Ring F | 140 180 | 2,326 | — | — | 30-500 | ? | |
| G ring | 170 000 | 2,82 | 175 000 | 2,90 | 5 000 | 1 × 105 | |
| E ring | 181 000 | 3 | 483 000 | 8 | 302 000 | 1 × 107 | |
| Ring of Phoebe | ~ 4,000,000 | 66 | > 13,000,000 | 216 | — | — | Phoebe |
There is no consensus as to the mechanism of their formation, but two main hypotheses are mainly proposed concerning the origin of the rings. One hypothesis is that the rings are the remains of a destroyed moon of Saturn and the second is that the rings remained from the original nebular material from which Saturn formed. While these theoretical models assume that the rings appeared early in the history of the Solar System, data from the Cassini probe indicate that they may have formed much later and their age is estimated at about 100 million years in 2019. In addition, they could disappear within 100 million years. As a result of these discoveries, the preferred mechanism to explain the appearance of rings is that an icy moon or a very large comet would have penetrated Saturn’s Roche limit.
Other entourage of Saturn
A Trojan asteroid of a planet is an asteroid located around one of the two stable Lagrangian points (L4 or L5) of the Sun-planet system, that is, they are located 60° ahead or behind the orbit of the planet. However, Saturn has no known Trojan asteroids unlike Earth, Mars, Jupiter, Uranus and Neptune. It is hypothesized that orbital resonance mechanisms, including secular resonance, are responsible for the absence of a Trojan for Saturn.
Saturn observation
If Uranus is visible to the naked eye in very good conditions — especially when it is in opposition — and in a very dark sky, Saturn is often considered the furthest planet from the Sun and Earth visible to the naked eye in general. In the night sky, the planet appears as a bright, yellowish bright spot with its mean apparent magnitude of 0.46 — a standard deviation of 0.34. Most of the magnitude variation is due to the tilt of the ring system relative to the Sun and Earth. Indeed, the brightest magnitude -0.55 occurs at about the time when the plane of the rings is most inclined, and the faintest magnitude 1.17 occurs when it is least inclined.
In addition, Saturn and its rings are best visible when the planet is close to the opposition, at an elongation of 180° relative to the Sun. A Saturnian opposition occurs almost every year because Saturn’s synodic period is 378 days but has less impact than the position of the rings on its visibility. For example, when opposing the 17 December 2002, Saturn appeared at its brightest due to a favorable orientation of its rings relative to Earth, even though the planet was closer during the next opposition in late 2003.
In order to obtain a clear image of Saturn’s rings, it is necessary to use powerful binoculars or a small telescope. When the Earth crosses the plane of the rings, which happens twice a Saturnian year (approximately every 15 Earth years), the rings briefly disappear from view because of their thickness of a few hundred meters on average. Such a “disappearance” will occur for the next time in 2025, but Saturn will be too close to the Sun to observe it. In addition, it is also possible to observe major features using an amateur telescope, such as the large white spots that appear near the summer solstice of the northern hemisphere.
It takes Saturn about 29.5 years to complete a full orbit and complete an entire circuit of the ecliptic in front of the zodiac background constellations. From time to time, Saturn is occulted by the Moon — that is, the Moon covers Saturn in the sky. As with all planets in the Solar System, occultations of Saturn occur in “seasons”. Saturnian occultations occur monthly for about 12 months, followed by about five years during which no such activity is recorded. Since the Moon’s orbit is inclined several degrees relative to Saturn’s, occultations will only occur when Saturn is near one of the points in the sky where the two planes intersect—both Saturn’s year length and the 18.6-Earth year nodal precession period of the Moon’s orbit influence the periodicity.
History of observations
Before telescopes
Saturn has been known since prehistoric times and is at the beginning of history recorded as a major figure in various mythologies. Since ancient times and before the discovery of Uranus in 1781, it is the furthest planet from the Sun known and thus marks the extreme limit of the Solar System in the minds of astronomers. In ancient Egypt, it symbolized the deity Horus under the name Hor-ka-pet (“celestial bull”) while the Sumerians called it Lubat-saguš (“star of the sun”). Babylonian astronomers have been systematically observing and recording Saturn’s motions since at least the ninth century BC, calling it Kajamanu.
In ancient Greek, the planet was known as Φαίνων Phainon, then in Roman times as “the star of Saturn”, the god of agriculture, from which the planet takes its modern name. The Romans considered the god Saturn to be the equivalent of the Titan Cronus; in modern Greek, the planet retains the name Kronos (Modern Greek: Κρόνος). In addition, the Greek name remains used in adjectival form, especially for the asteroids kronocroiseur. The Greek astronomer Claudius Ptolemy based his calculations of Saturn’s orbit on observations he made while it was in opposition and assumed that it was very cold because of its distance from the Sun, which he then placed between Venus and Mars.
In Hindu astrology, Saturn is known as “Shani” and judges men according to their actions. Ancient Chinese and Japanese culture refers to Saturn as the “star of the earth” (土星) in the Wuxing cosmology of the five elements. In ancient Hebrew, Saturn is called “Shabbathai” and his angel is Cassiel.
The Star of the Three Kings, or Star of Bethlehem, is sometimes referred to as having been a nova, supernova or Halley’s Comet, these hypotheses having finally been set aside because none of these phenomena took place during the reign of Herod. Thus, the current explanation is that the intense light was produced by a conjunction between Jupiter and Saturn during the year 7 BC.
Telescope research from the seventeenth century
In 1610, Galileo, after discovering four moons of Jupiter — the Galilean satellites — thanks to a telescope of his design, decided to use his new instrument to observe Saturn. By pointing it at the planet, he observes for the first time its rings but does not understand their nature because of the too-low resolution of his telescope (magnification of 20): he sees them and draws them as two very large moons surrounding Saturn. In a letter, he describes the planet as “not a single star, but a composition of three that almost touch, never moving relative to each other, and that are aligned along the zodiac, the middle one being three times larger than the two lateral ones”.
In 1612, Earth passing through the plane of the rings – which happens about once every 15 years – they disappear from his sight: this surprises him but allows him to understand that Saturn is actually a single body; He is also the first in history to have observed this astronomical event. However, he does not understand the origin of this disappearance, and even writes, in reference to the mythological origin of the name of the star, that Saturn would have “devoured his own children”. Then, in 1613, they reappeared without Galileo being able to hypothesize what he observed.
In 1616, he drew the rings again, this time as handles around the planet. He wrote: “The two companions are no longer small globes but are now much larger and no longer round… they are half-ellipses with small black triangles in the middle and figure and contiguous to the globe of Saturn, which is always seen as round”.
In 1655, Christian Huygens, with a telescope with a magnification of 50, discovered near Saturn a celestial body that would later be named Titan. In addition, he postulated for the first time that Saturn would be surrounded by a solid ring, formed by “arms”. Three years later, in his book Systema Saturnium, he explained the phenomenon of the disappearance of rings previously observed by Galileo. In 1660, Jean Chapelain speculated that these rings would be composed of a very large number of small satellites, which went unnoticed because the majority of astronomers thought that the ring was solid.
In 1671 and 1672, during a phenomenon of the disappearance of the rings, Jean Dominique Cassini discovered Iapetus and Rhea, the two largest moons of Saturn after Titan. Later, in 1675 and 1676, he determined that the ring was composed of several rings, separated by at least one division the largest of these — and the one he probably observed, separating the A and B rings — would later be named Cassini’s division after him. Finally, in 1684, he discovered two new moons: Tethys and Dione. He then named the four moons discovered Sidera Lodoicea (“the stars of Louis”) in honor of the king of France Louis XIV.
No other major discoveries were made for a century until the work of William Herschel — also the discoverer of the planet Uranus. In 1780, he reported a black line on the B ring, a division that was probably the same as that observed by Johann Franz Encke in 1837 and which took the latter’s name as the division of Encke. In 1789, when the rings disappeared, he identified two other moons: Enceladus and Mimas. This observation also allowed him to confirm that the planet was flattened at the poles, which was only previously suspected, and to make the first estimate of the thickness of the rings, at about 500 kilometers. Finally, in 1790, he determined the rotation period of the rings as 10 h 32 min, a value very close to reality. Pierre-Simon de Laplace, with Kepler’s laws, then provides a first estimate of the distance of the planet from the Sun at 1.4 billion kilometers. Also, from its apparent size, he estimates the diameter of the planet at 100,000 km and the diameter of the rings at 270,000 km.
In 1848, William Cranch Bond and his son George Phillips Bond observed for the first time Hyperion, a satellite in an orbital resonance with Titan, also discovered independently two days later by William Lassell — discoverer two years earlier of Neptune’s largest moon, Triton. The following year, Edouard Roche suggested that the rings would have formed when a satellite would have approached Saturn and that it would have decomposed because of tidal forces; a concept that will later take the name of Roche limit.
In the 1850s, several observations were made through the C ring, just discovered by a father and son Bond, undermining the theory of solid rings. In 1859, James Clerk Maxwell published his book On the Stability of the Motion of Saturn’s Rings in which he argued that the rings are actually composed of an “indefinite number of unconnected particles”, all orbiting Saturn independently; this work earned him the Adams Prize. This theory was proved correct in 1895 by spectroscopic studies conducted by James Keeler and William Campbell at Lick Observatory, in which they observed that the inner parts of the rings orbited faster than the outer parts.
In 1872, Daniel Kirkwood managed to define that the Cassini and Encke divisions resonate with the four inner moons then known: Mimas, Enceladus, Tethys and Dione.
During the second half of the nineteenth century, photography developed and Saturn was then a prime target: many astrophotographers ranging from Warren de la Rue to John Rogers Commons via the brothers Paul-Pierre and Prosper-Mathieu Henry then took it in image, the merit of the first successful photograph being shared between Commons and the Henry brothers.
In 1899, William Henry Pickering discovered Phoebe, an irregular satellite that was not in synchronous rotation and had a retrograde orbit. It is the first of its kind found and, moreover, it is the only moon of Saturn discovered from a terrestrial observation without taking advantage of a disappearance of the rings.
In the twentieth century and then in the twenty-first century, most of the information about the planet is then known thanks to the various space exploration missions. Events where the Earth crosses the plane of the rings, however, remain used for Earth observation. For example, in 1966, the Allegheny Observatory photographed what would later be called the E ring and the moons Janus and Epimetheus were discovered. then, in 1979 and 1980, three new ones were created by separate teams: Télesto, Calypso and Hélène. The Hubble Space Telescope also tracks the activity of the Saturnian system continuously, sometimes returning remarkable images such as a quadruple transit observed in 2009.
Saturn exploration
Overflights
In the last quarter of the twentieth century, Saturn was visited by three NASA space probes that flew by it: Pioneer 11 in 1979, Voyager 1 in 1980 and Voyager 2 in 1981.
After using Jupiter’s gravitational assist, Pioneer 11 made the first flyby of Saturn in September 1979 and passed about 21,000 km from the top of the planet’s clouds, slipping between the inner ring and the upper layers of the atmosphere. The spacecraft takes low-resolution photographs of the planet and some of its satellites, although their resolution is too low to discern details of their surface. The spacecraft also studies the planet’s rings, revealing the thin F ring and confirming the existence of the E ring; Also, the fact that the divisions in the rings are shown as bright when viewed with a high phase angle by the probe reveals the presence of a thin light-scattering material and are therefore not empty. In addition, Pioneer 11 provides extensive data on Saturn’s magnetosphere and atmosphere, as well as the first measurement of Titan’s temperature at 80 K (−193 °C).
A year later, in November 1980, Voyager 1 visited the Saturnian system. The probe returns the first high-resolution images of the planet, its rings and moons, including Dione, Mimas and Rhea. Voyager 1 also flew by Titan, increasing knowledge of the moon’s atmosphere, including that it is impenetrable in visible wavelengths—preventing imaging of surface details—and the presence of traces of ethylene and other hydrocarbons. This last flyby has the consequence of profoundly changing the trajectory of the probe and ejecting it out of the plane of the ecliptic.
Almost a year later, in August 1981, Voyager 2 continued the study. Passing 161,000 km from the center of the planet on August 26, 1981, it takes close-ups of the moons and provides evidence of the evolution of the atmosphere and rings thanks to its more sensitive cameras than previous probes. Unfortunately, during the flyby, the steerable camera platform remained stuck for several days, implying that some photographs could not be taken at the intended angle and resulting in the loss of some of the data taken. Saturn’s gravitational assist was eventually used to direct the probe to Uranus and then to Neptune, making it the first and only probe to have visited both planets.
The Voyager program allowed many discoveries such as that of several new satellites orbiting near or in the rings of the planet, including Atlas and the Shepherd satellites Prometheus and Pandora (the first ever discovered), or three new divisions in the rings, then respectively called Maxwell, Huygens and Keeler. In addition, the G ring was discovered and “spokes” — dark spots — were observed on the B ring.
| Summary of Overviews | ||||
|---|---|---|---|---|
| Probe | Date | Space Agency | Distance (km) | Main achievements |
| Pioneer 11 | September 1, 1979 | Nasa | 79 000 | First successful flyby of Saturn. Discovery of the F ring. |
| Voyager 1 | November 12, 1980 | Nasa | 184 300 | First images in high resolution. |
| Voyager 2 | August 25, 1981 | Nasa | 161 000 | Using Saturn’s gravitational assist to get to Uranus and then Neptune. |
Cassini–Huygens
Cassini-Huygens is a mission to explore NASA’s Saturnian system in collaboration with the European Space Agency and the Italian Space Agency, part of the Flagship program. Launched on 15 October 1997, the space probe is composed of the Cassini orbiter developed by NASA and the Huygens lander developed by ESA — respectively named after Jean-Dominique Cassini and Christian Huygens, two scientists who greatly advanced knowledge about the planet in the seventeenth century. It was placed in orbit around Saturn in July 2004, with the lander landing on Titan in January 2005 and the orbiter continuing its study — after two mission extensions in addition to the originally planned four-year duration — until September 15, 2017, where it decays in Saturn’s atmosphere to avoid any risk of contamination of natural satellites.
Huygens collected information and took a flood of photographs during the descent and after landing. Despite design problems and the loss of a communication channel, the lander managed to land near a hydrocarbon lake to make measurements.
Cassini continues to orbit Saturn and continues the scientific study of Saturn’s magnetosphere and rings, taking advantage of its close passages from satellites to collect detailed data on them and obtain quality images of the Saturnian system.
Regarding Saturn’s moons, Cassini makes it possible to refine the knowledge of the surface of Titan – with its large hydrocarbon lakes and its many islands and mountains – and on the composition of its atmosphere, to discover the geysers of Enceladus making it a place conducive to the appearance of life, to obtain the first detailed images of Phoebe — which he flew by in June 2004 — and to discover six new named moons, including Methone and Pollux for example.
The orbiter analyzed the structure of Saturn’s rings in detail, even photographing a new, previously unknown one inside the E and G rings, and observed amazing formations of the giant planet’s atmosphere at its poles — such as Saturn’s hexagon. In addition, the data collected on Saturn’s rings during the last orbits make it possible to estimate their age: they would have appeared less than 100 million years ago and should disappear within 100 million years.
In short, the Cassini space probe made 293 orbits around Saturn during its mission and made 127 flybys of Titan, 23 of Enceladus and 162 of other moons of the planet in conditions that allowed for extensive investigations. 653 gigabytes of scientific data are collected and more than 450,000 photographs are taken. The Cassini-Huygens mission fulfills all its scientific objectives and is thus considered a great success thanks to the many quality of data produced.
Future missions
Space probe exploration of a planet as far away as Saturn is very expensive because of the high speed required for a spacecraft to do so, the length of the mission, and the need for energy sources that can compensate for the lower solar radiation, such as very large solar panels or a thermoelectric generator. radioisotope.
In 2008, NASA and the European Space Agency studied the Titan Saturn System Mission (TSSM), consisting of an orbiter as well as a lander and hot air balloon to study Titan, but this project was abandoned the following year. A less expensive mission as part of the Discovery program is also considered, Titan Mare Explorer (2011), but is ultimately not selected.
However, given the scientific interest in Saturn and its moons (including Titan and Enceladus which could harbor life), successors to Cassini-Huygens are proposed as part of NASA’s New Frontiers program. Thus, in 2017, five missions are being evaluated: a spacecraft that would conduct a sounding by plunging into Saturn’s atmosphere (SPRITE), two missions that would accurately analyze the material ejected by Enceladus’ geysers by flying over this moon several times and determine the possible presence of shape clues of life (ELSAH and ELF) and finally two missions to study Titan in depth, the first in orbit (Oceanus) and the second, more daring on the technical level, by means of a drone making flights of several tens of kilometers on the surface of the moon by exploiting its low gravity and the high density of its atmosphere (Dragonfly). Finally, only the Dragonfly mission is selected in 2019 for a departure scheduled for 2026 and an arrival on Titan in 2034.
Saturn in culture
Science fiction
Saturn is present in many works of science fiction and its representation has evolved according to knowledge about the planet. Among the first works touching on science fiction evoking Saturn are notably Micromegas (1752) by Voltaire. At the time, it was the furthest known planet from the Sun — Uranus was discovered in 1781 and Neptune in 1846 — and its gaseous structure was unknown. Thus, the planet is described as solid and inhabited by giants two kilometers tall, having 72 senses and a life expectancy of 15,000 years; the secretary of the “Academy of Saturn” then accompanies the main character Micromegas to Earth. A century later, in Hector Servadac (1877), Jules Verne brings adventurers close to Saturn by riding a comet. The author describes and draws it as rocky with a deserted solid surface and having 8 satellites and 3 rings.
After modern science revealed that the planet has no solid surface and that its atmosphere and temperature are hostile to human life, its representation evolves accordingly. Also, its planetary rings and vast system of moons become a more common setting for science fiction, for example in The Martian Way (1952) by Isaac Asimov or in The Zone of the Outside (2007) by Alain Damasio. Floating cities in Saturn’s atmosphere are also considered, as in Charles Stross’s Accelerando (2005).
In cinema, it is notably represented in Beetlejuice (1988) by Tim Burton, where it is populated by gigantic sandworms, or serves as a setting in Interstellar (2014) by Christopher Nolan, NASA having sent four astronauts near the planet in order to reach a wormhole.
Music
“Saturn, the one who brings old age” is the 5th movement of the work for the large orchestra Les Planètes, composed and written by Gustav Holst between 1914 and 1916. “Saturn “ is a song by the American rock band Sleeping at Last.
Symbolism of Saturn
Its symbol “”, of ancient origin would represent the sickle of the god Saturn or would be derived from the Greek letter kappa ♄ lowercase, the initial of the ancient Greek Κρόνος (Krónos). Nevertheless, the International Astronomical Union recommends replacing the symbol “” with the abbreviation “♄ S”, corresponding to the Latin letter S capital S, the initial of the English Saturn.
References (sources)
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