Rings of Saturn

has the most extensive and complex ring system of any planet in the Solar System. The rings consist of particles in orbit around the planet and are made almost entirely of water ice, with a trace component of rocky material. Particles range from micrometers to meters in size. There is no consensus as to when the rings formed: while investigations using theoretical models suggested they formed early in the Solar System's existence, newer data from Cassini suggests a more recent date of formation.
Though light reflected from the rings increases Saturn's apparent brightness, they are not themselves visible from Earth with the naked eye. In 1610, the year after his first observations with a telescope, Galileo Galilei became the first person to observe Saturn's rings, though he could not see them well enough to discern their true nature. In 1655, Christiaan Huygens was the first person to describe them as a disk surrounding Saturn. The concept that Saturn's rings are made up of a series of tiny ringlets can be traced to Pierre-Simon Laplace.
The rings have numerous gaps where particle density drops sharply. Two of these are cleared by moons embedded within them, and many others are at locations of known destabilizing orbital resonances with the moons of Saturn. Other gaps remain unexplained. Stabilizing resonances, on the other hand, are responsible for the longevity of several rings, such as the Titan Ringlet and the G Ring. Well beyond the main rings is the Phoebe ring, which is believed to originate from Phoebe and thus shares its retrograde orbital motion. It is aligned with the plane of Saturn's orbit. Saturn has an axial tilt of 27 degrees, so this ring is tilted at an angle of 27 degrees to the more visible rings orbiting above Saturn's equator.

History

Early observations

was the first to observe the rings of Saturn in 1610 using his telescope, but was unable to identify them as such. He wrote to the Duke of Tuscany that "The planet Saturn is not alone, but is composed of three, which almost touch one another and never move nor change with respect to one another. They are arranged in a line parallel to the zodiac, and the middle one is about three times the size of the lateral ones." He also described the rings as Saturn's "ears". In 1612 the Earth passed through the plane of the rings and they became invisible. Mystified, Galileo remarked "I do not know what to say in a case so surprising, so unlooked for and so novel." He mused, "Has Saturn swallowed his children?" — referring to the myth of the Titan Saturn devouring his offspring to forestall the prophecy of them overthrowing him. He was further confused when the rings again became visible in 1613.
Early astronomers used anagrams as a form of commitment scheme to lay claim to new discoveries before their results were ready for publication. Galileo used the anagram "" for Altissimum planetam tergeminum observavi for discovering the rings of Saturn.
In 1657 Christopher Wren became Professor of Astronomy at Gresham College, London. He had been making observations of the planet Saturn from around 1652 with the aim of explaining its appearance. His hypothesis was written up in De corpore saturni, in which he came close to suggesting the planet had a ring. However, Wren was unsure whether the ring was independent of the planet, or physically attached to it. Before Wren's hypothesis was published Christiaan Huygens presented his hypothesis of the rings of Saturn. Immediately Wren recognised this as a better hypothesis than his own and De corpore saturni was never published. Robert Hooke was another early observer of the rings of Saturn, and noted the casting of shadows on the rings.

Huygens' ring hypothesis and later developments

began grinding lenses with his father Constantijn Huygens in 1655 and was able to observe Saturn with greater detail using a 43× power refracting telescope that he designed himself. He was the first to suggest that Saturn was surrounded by a ring detached from the planet, and published his findings in obfuscated form as the letter string "". Three years later, he revealed that the letters were those of the sentence Annulo cingitur, tenui, plano, nusquam coherente, ad eclipticam inclinato. He published his ring hypothesis in Systema Saturnium which also included his discovery of Saturn's moon, Titan, as well as the first clear outline of the dimensions of the Solar System.
In 1675, Giovanni Domenico Cassini determined that Saturn's ring was composed of multiple smaller rings with gaps between them; the largest of these gaps was later named the Cassini Division. This division is a region between the A ring and B Ring.
In 1787, Pierre-Simon Laplace proved that a uniform solid ring would be unstable and suggested that the rings were composed of a large number of solid ringlets.
In 1859, James Clerk Maxwell demonstrated that a nonuniform solid ring, solid ringlets or a continuous fluid ring would also not be stable, indicating that the ring must be composed of numerous small particles, all independently orbiting Saturn. Later, Sofia Kovalevskaya also found that Saturn's rings cannot be liquid ring-shaped bodies. Spectroscopic studies of the rings which were carried out independently in 1895 by James Keeler of the Allegheny Observatory and by Aristarkh Belopolsky of the Pulkovo Observatory showed that Maxwell's analysis was correct.
Four robotic spacecraft have observed Saturn's rings from the vicinity of the planet. Pioneer 11s closest approach to Saturn occurred in September 1979 at a distance of. Pioneer 11 was responsible for the discovery of the F ring. Voyager 1s closest approach occurred in November 1980 at a distance of. A failed photopolarimeter prevented Voyager 1 from observing Saturn's rings at the planned resolution; nevertheless, images from the spacecraft provided unprecedented detail of the ring system and revealed the existence of the G ring. Voyager 2s closest approach occurred in August 1981 at a distance of. Voyager 2s working photopolarimeter allowed it to observe the ring system at higher resolution than Voyager 1, and to thereby discover many previously unseen ringlets. Cassini spacecraft entered into orbit around Saturn in July 2004. Cassini images of the rings are the most detailed to-date, and are responsible for the discovery of yet more ringlets.
The rings are named alphabetically in the order they were discovered: A and B in 1675 by Giovanni Domenico Cassini, C in 1850 by William Cranch Bond and his son George Phillips Bond, D in 1933 by Nikolai Barabashov and, E in 1967 by Walter A. Feibelman, F in 1979 by Pioneer 11, and G in 1980 by Voyager 1. The main rings are, working outward from the planet, C, B and A, with the Cassini Division, the largest gap, separating Rings B and A. Several fainter rings were discovered more recently. The D Ring is exceedingly faint and closest to the planet. The narrow F Ring is just outside the A Ring. Beyond that are two far fainter rings named G and E. The rings show a tremendous amount of structure on all scales, some related to perturbations by Saturn's moons, but much unexplained.

Saturn's axial inclination

Saturn's axial tilt is 26.7°, meaning that widely varying views of the rings, of which the visible ones occupy its equatorial plane, are obtained from Earth at different times. Earth makes passes through the ring plane every 13 to 15 years, about every half Saturn year, and there are about equal chances of either a single or three crossings occurring in each such occasion. The most recent ring plane crossings were on 22 May 1995, 10 August 1995, 11 February 1996, 4 September 2009 and 23 March 2025; upcoming events will occur on 15 October 2038, 1 April 2039 and 9 July 2039. Favorable ring plane crossing viewing opportunities only come during triple crossings.
Saturn's equinoxes, when the Sun passes through the ring plane, are not evenly spaced. The sun passes south to north through the ring plane when Saturn's heliocentric longitude is 173.6 degrees, about the time Saturn crosses from Leo to Virgo. 15.7 years later Saturn's longitude reaches 353.6 degrees and the sun passes to the south side of the ring plane. On each orbit the Sun is north of the ring plane for 15.7 Earth years, then south of the plane for 13.7 years. Dates for north-to-south crossings include 19 November 1995 and 6 May 2025, with south-to-north crossings on 11 August 2009 and 23 January 2039. During the period around an equinox the illumination of most of the rings is greatly reduced, making possible unique observations highlighting features that depart from the ring plane.

Physical characteristics

The dense main rings extend from to away from Saturn's equator, whose radius is . With an estimated local thickness of as little as 10 meters and as much as 1 km, they are composed of 99.9% pure water ice with a smattering of impurities that may include tholins or silicates. The main rings are primarily composed of particles smaller than 10 m.
Cassini directly measured the mass of the ring system via their gravitational effect during its final set of orbits that passed between the rings and the cloud tops, yielding a value of 1.54 × 10¹⁹ kg, or 0.41 ± 0.13 Mimas masses. This is around two-thirds the mass of the Earth's entire Antarctic ice sheet, spread across a surface area 80 times larger than that of Earth. The estimate is close to the value of 0.40 Mimas masses derived from Cassini observations of density waves in the A, B and C rings. It is a small fraction of the total mass of Saturn. Earlier Voyager observations of density waves in the A and B rings and an optical depth profile had yielded a mass of about 0.75 Mimas masses, with later observations and computer modeling suggesting that was an underestimate.
Although the largest gaps in the rings, such as the Cassini Division and Encke Gap, can be seen from Earth, the Voyager spacecraft discovered that the rings have an intricate structure of thousands of thin gaps and ringlets. This structure is thought to arise, in several different ways, from the gravitational pull of Saturn's many moons. Some gaps are cleared out by the passage of tiny moonlets such as Pan, many more of which may yet be discovered, and some ringlets seem to be maintained by the gravitational effects of small shepherd satellites. Other gaps arise from resonances between the orbital period of particles in the gap and that of a more massive moon further out; Mimas maintains the Cassini Division in this manner. Still more structure in the rings consists of spiral waves raised by the inner moons' periodic gravitational perturbations at less disruptive resonances.
Data from the Cassini space probe indicate that the rings of Saturn possess their own atmosphere, independent of that of the planet itself. The atmosphere is composed of molecular oxygen gas produced when ultraviolet light from the Sun interacts with water ice in the rings. Chemical reactions between water molecule fragments and further ultraviolet stimulation create and eject, among other things, O₂. According to models of this atmosphere, H₂ is also present. The O₂ and H₂ atmospheres are so sparse that if the entire atmosphere were somehow condensed onto the rings, it would be about one atom thick. The rings also have a similarly sparse OH atmosphere. Like the O₂, this atmosphere is produced by the disintegration of water molecules, though in this case the disintegration is done by energetic ions that bombard water molecules ejected by Saturn's moon Enceladus. This atmosphere, despite being extremely sparse, was detected from Earth by the Hubble Space Telescope.
Saturn shows complex patterns in its brightness. Most of the variability is due to the changing aspect of the rings, and this goes through two cycles every orbit. However, superimposed on this is variability due to the eccentricity of the planet's orbit that causes the planet to display brighter oppositions in the northern hemisphere than it does in the southern.
In 1980, Voyager 1 made a fly-by of Saturn that showed the F ring to be composed of three narrow rings that appeared to be braided in a complex structure; it is now known that the outer two rings consist of knobs, kinks and lumps that give the illusion of braiding, with the less bright third ring lying inside them.
New images of the rings taken around the 11 August 2009 equinox of Saturn by NASA's Cassini spacecraft have shown that the rings extend significantly out of the nominal ring plane in a few places. This displacement reaches as much as at the border of the Keeler Gap, due to the out-of-plane orbit of Daphnis, the moon that creates the gap.