Rings of Saturn

• 1. Introduction

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  The rings of Saturn stretch from 7,000 km to 80,000 km above the planet's equator, yet in places they are only 10 meters thick. That is the ratio of a sheet of paper to a football field, scaled to a structure wider than six Earths side by side. Galileo Galilei became the first person to see them in 1610, through a new instrument barely a year old, and promptly lost his mind trying to explain what he was looking at. He wrote to the Duke of Tuscany that Saturn appeared to be "composed of three" bodies that "almost touch one another and never move." He called them the planet's "ears." Then, in 1612, they vanished. He asked, in genuine bewilderment, whether Saturn had "swallowed his children."

  The questions that have haunted astronomers ever since are still not fully answered. How old are these rings? Where did they come from? Why are they almost pure water ice when the rest of Saturn is not? And what are the ghostly radial streaks, the spokes, that ignore the laws of gravity? Those questions will shape this documentary.

• 2. From Ears to a Flat Ring

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  Galileo encoded his observation in a Latin anagram: the scrambled letters, once decoded, read "I have observed the most distant planet to have a triple form." That trick was common among early astronomers who wanted to stake a claim without revealing their findings before publication.

  In 1657, Christopher Wren became Professor of Astronomy at Gresham College, London, and had been working since around 1652 on a hypothesis to explain Saturn's strange appearance. He wrote it up in a manuscript called De corpore saturni, coming close to suggesting a ring but unable to say whether it was attached to the planet or free of it. Before he could publish, Christiaan Huygens stepped in.

  Huygens had been grinding lenses with his father Constantijn since 1655. Using a 43x power refracting telescope of his own design, he was the first to argue that Saturn was surrounded by a flat ring detached from the planet. He hid his conclusion in a letter string, then three years later revealed the key: the letters decoded to a Latin sentence stating that Saturn "is surrounded by a thin, flat, ring, nowhere touching" the planet, inclined to the ecliptic. He published the full hypothesis in Systema Saturnium in 1659, the same work in which he announced the discovery of Saturn's moon Titan.

  Wren read Huygens' work and immediately recognized it as the better explanation. De corpore saturni was never published. Giovanni Domenico Cassini went further in 1675, determining that the ring was actually multiple smaller rings with gaps between them. The largest gap, later named the Cassini Division, is a region 4,800 km wide separating the A Ring from the B Ring.

• 3. Mathematics Takes Over

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  Pierre-Simon Laplace proved in 1787 that a solid ring could not be stable; it would break apart. His solution was that the rings must consist of a large number of solid ringlets.

  James Clerk Maxwell pushed further in 1859. He showed that even non-uniform solid ringlets and continuous fluid rings would also be unstable. The only stable configuration, he argued, was a swarm of countless independent particles, each orbiting Saturn on its own. Sofia Kovalevskaya separately confirmed that the rings could not be liquid.

  The proof came from spectroscopy. In 1895, James Keeler at the Allegheny Observatory and Aristarkh Belopolsky at the Pulkovo Observatory independently measured the Doppler shift of light reflected from different parts of the rings. The inner edge was moving faster than the outer edge, exactly as independent orbiting particles would behave, not as a solid disk would. Maxwell had been right.

  By then, the naming convention for the rings was already alphabetical by order of discovery. The A and B Rings were identified by Galileo in 1610, with their structure clarified by Huygens. The C Ring was found in 1850 by William Cranch Bond and his son George Phillips Bond. The D Ring followed in 1933, E in 1967, F in 1979, and G in 1980.

• 4. Four Robotic Visitors

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  Pioneer 11 made its closest approach to Saturn in September 1979 at a distance of 20,900 km. That mission yielded the discovery of the F Ring.

  Voyager 1 arrived in November 1980, passing within 64,200 km. A failed photopolarimeter prevented the kind of detailed ring profile scientists had planned, but images from the spacecraft still revealed the existence of the G Ring and showed the F Ring as three narrow strands that appeared to be braided. It is now understood that the outer two strands consist of knobs and kinks that create the illusion of braiding, while a fainter third strand lies inside them.

  Voyager 2 followed in August 1981 at a distance of 41,000 km. Its photopolarimeter was functioning, which allowed it to observe the rings at higher resolution and discover many previously unseen ringlets.

  The Cassini spacecraft entered orbit around Saturn in July 2004 and transformed understanding of the rings. Its images are the most detailed yet obtained. Among Cassini's direct measurements was the total mass of the ring system, determined by observing the gravitational effect of the rings during the spacecraft's final orbits, which passed between the rings and the cloud tops. The measured value was 1.54 (plus or minus 0.49) times 10 to the 19th power kilograms, equivalent to about 0.41 Mimas masses. That is roughly two-thirds the mass of Earth's entire Antarctic ice sheet, spread across a surface area 80 times larger than Earth itself.

• 5. Spokes and Moonlets

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  In 1980, Voyager images revealed radial features in the B Ring called spokes. They were dark in backscattered light and bright in forward-scattered light, and they rotated around the rings almost in step with Saturn's magnetosphere rather than following the orbital speed expected of ring particles at their distances. Gravity alone cannot explain this behavior.

  The leading hypothesis is that the spokes consist of microscopic dust grains suspended above the main ring plane by electrostatic repulsion. What triggers that charge is still unknown; proposals include lightning in Saturn's atmosphere and micrometeoroid impacts on the rings. An alternative view links them to electric fields across the boundary between illuminated and unlit ring surfaces, similar to a phenomenon called lunar horizon glow.

  The spokes disappeared from view when Cassini arrived at Saturn in early 2004. Some scientists predicted they would not return until 2007. They reappeared in images taken on the 5th of September 2005. Their seasonal pattern became clear over the following years: they fade around Saturn's midwinter and midsummer, returning as the planet approaches equinox. Saturn's orbit takes 29.7 years, and the spokes appear to follow that cycle.

  In 2009, during Saturn's equinox, a moonlet embedded in the B Ring was detected by the shadow it cast. Estimated at 400 meters in diameter, it was given the provisional designation S/2009 S 1. In the A Ring, four tiny moonlets were found in Cassini images in 2006, each only about a hundred meters across. What Cassini sees is not the moonlets themselves but the propeller-shaped disturbances they create in the ring material, each several kilometers wide. By 2008, over 150 such propeller moonlets had been detected, and one tracked over several years was nicknamed Bleriot.

• 6. The F Ring's Restless Edge

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  The F Ring sits 3,000 km beyond the outer edge of the A Ring and is described by researchers as perhaps the most active ring in the Solar System, with features changing on a timescale of hours.

  Prometheus, the moon that orbits inside the F Ring, pulls on it at each encounter at its apoapsis. The gravitational tug creates kinks and knots in the ring and leaves a dark channel in its inner part as Prometheus effectively steals material away. Because Prometheus orbits faster than the F Ring material, each new channel forms about 3.2 degrees ahead of the previous one, carving a repeating pattern around the ring over time.

  By 2023, researchers described the ring's clumpy structure as driven by thousands of small parent bodies, between 0.1 and 1.0 km in size, that collide and produce dense strands of micrometer-to-centimeter particles. Those particles re-accrete back onto the parent bodies over a period of months in a kind of steady-state churn.

  The Cassini UVIS team, led by Larry Esposito, used stellar occultation to detect 13 objects within the F Ring ranging from 27 meters to 10 km across. They are translucent, suggesting they are temporary clumps of ice boulders. Esposito regards this clumping and breaking process as the fundamental behavior of the Saturnian ring system as a whole.

• 7. The Outermost Ring

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  In October 2009, a ring of a completely different character was announced: a faint disk of material just inside the orbit of the moon Phoebe. It was found using NASA's infrared Spitzer Space Telescope and extends from roughly 128 to 207 Saturn radii in the observations, with calculations suggesting it may reach outward to 300 radii and inward to the orbit of Iapetus at 59 radii.

  The ring's particles are believed to have originated from impacts on Phoebe. Because they share Phoebe's retrograde orbit, they travel in the opposite direction to the next moon inward, Iapetus. The ring lies in the plane of Saturn's orbit rather than above Saturn's equator, and is tilted 27 degrees from the other rings as a result of Saturn's axial tilt.

  Material from this ring migrates inward due to the reemission of solar radiation. A 3 cm particle would take roughly the age of the Solar System to migrate from the vicinity of Phoebe to the vicinity of Iapetus. When that material strikes Iapetus, it darkens and reddens the moon's leading hemisphere. However, it does not directly create the dramatic two-tone appearance of Iapetus. Instead, the infalling material initiates a thermal feedback process in which ice sublimates from warmer areas and recondenses on cooler ones, leaving a dark residue across the equatorial region of the leading hemisphere.

  The existence of this ring had been proposed in the 1970s by Steven Soter. The discovery was made by Anne J. Verbiscer and Michael F. Skrutskie of the University of Virginia, and Douglas P. Hamilton of the University of Maryland, College Park. All three had studied together as graduate students at Cornell University.

• 8. How Old Are the Rings

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  No consensus exists on when Saturn's rings formed. Earlier theoretical models pointed toward an origin early in the Solar System's history, perhaps as old as Saturn itself. Data from Cassini now points toward a much more recent formation, likely within the last 100 million years, possibly as recently as 10 million years ago.

  The evidence centers on mass, dust influx, and ring rain. The rings are almost pure water ice, at 99.9% purity. If they were ancient, the steady infall of interplanetary dust should have darkened them significantly over billions of years. Cassini directly measured a process called ring rain, in which gravity pulls electrically charged ice grains down from the rings along planetary magnetic field lines into Saturn's atmosphere. Ground-based observations using the Keck telescope measured that flow rate at between 432 and 2,870 kg per second. Adding a second measured flux, a charge-neutral equatorial inflow of 4,800 to 44,000 kg per second detected by Cassini in September 2017, implies the rings could be entirely gone within 100 million years. Based on current depletion rates alone, the estimate is they may disappear within 292 million years.

  Where did they come from? The most discussed hypothesis traces to Edouard Roche in the 19th century: a moon orbited too close and was pulled apart by tidal forces. Numerical simulations in 2022 supported this, and the authors proposed the name "Chrysalis" for the destroyed moon. In 2023, further simulations showed that a collision between two icy moons would explain the near-total absence of rock in the rings.

  A hypothesis by R. M. Canup proposes an origin in the icy mantle of a much larger, Titan-sized differentiated moon that spiraled into Saturn during the period when Saturn was still surrounded by a gaseous nebula. The rocky core would have been stripped away in the process, leaving only ice, which explains the rings' unusual purity. The rings would have been roughly 1,000 times more massive at the start, with the outer portions gradually coalescing into the innermost moons of Saturn, out to Tethys.