The Solar System:


Untitled painting known as "Saturno devorando a su hijo" ("Saturn Devouring His Son") by Francisco de Goya (1746-1828 CE; Spanish), painted between 1819-1823.

According to the traditional interpretation, it depicts the Greek myth of the Titan Cronus (in the title Romanized to Saturn), who, fearing that he would be overthrown by one of his children, ate each one upon their birth. The work is one of the 14 Black Paintings that Goya painted directly onto the walls of his house sometime between 1819 and 1823. It was transferred to canvas after Goya's death and has since been held in the Museo del Prado in Madrid.

Credit: Francisco Goya, between 1819-1823; Museo de Prado, Madrid [link]

Saturn in 2016 from NASA's Cassini spacecraft. In its almost 30-year orbit, Saturn is shown here approaching its northern hemisphere summer solstice in May 2017.

Credit (image and some text): NASA/JPL-Caltech/Space Science Institute [link]

Saturn is the most oblate (flattened) of the planets

This is a result of:

  • Rapid spin (10 hours 33 minutes), and

  • Low average density (0.7 g/cm3) is less than water (1.0 g/cm3) - Saturn would float in water!

Saturn in 2008 from the Cassini spacecraft. The blue circle overlay shows Saturn's significant deviation from being spherical.

Credit: NASA/JPL/Space Science Institute [link]

The great storm of 2011

This series of images from NASA's Cassini spacecraft shows the development of the largest storm seen on the planet since 1990. These true-color and composite near-true-color views chronicle the storm from its start in late 2010 through mid-2011, showing how the distinct head of the storm quickly grew large but eventually became engulfed by the storm's tail as it wrapped all the way around Saturn.

Credit (image and some text): NASA/JPL-Caltech/Space Science Institute [link]

This global storm recurs every 20-30 years (it has been observed 6 times since the late 1800s).

  1. A deep liquid water layer stops convection (circulation that transfer heat) in Saturn's atmosphere.

  2. Energy (heat) is not transferred from the bottom to the top of the atmosphere.

  3. The upper atmosphere cools until it is dense enough to sink.

  4. The cool upper layers of the atmosphere sink down.

  5. Warm, wet layers of the atmosphere swirl up to the surface.

  6. This is the recipe for a thunderstorm!

On 24 December 2010, the storm has a north-to-south extent of about 6,000 miles (10,000 kilometers) three weeks after the storm started. The main part of the storm has an east-to-west extent of about 11,000 miles (17,000 kilometers). The tail of the storm extends almost one-third of the way around the planet - a distance of 62,000 miles (100,000 kilometers).

Credit (image and some text): NASA/JPL-Caltech/Space Science Institute [link]

This image from 25 February 2011 was obtained about 12 weeks after the storm began, and the clouds by this time had formed a tail that wrapped all the way around the planet. This tail, which appears as slightly blue clouds south and west (left) of the storm head, can be seen encountering the storm head in this image.

Credit: NASA/JPL-Caltech/Space Science Institute [link]

The clouds around Saturn’s north pole are shaped like a hexagon

This is believed to be a standing wave pattern in the jet stream wind currents. If the strong wind around the pole was going straight, then it would wiggle back and forth exactly 3 times in the distance it travels around the pole. When traveling in a circle, the wiggles are synchronized, resulting in a fixed pattern.

The hexagon-shaped jet-stream around Saturn's north pole is fully illuminated in this image from the Cassini spacecraft. The atmosphere appears darker in regions where the cloud deck is lower, such the region interior to the hexagon.

Credit: NASA/JPL-Caltech/Space Science Institute [link]

1,250 miles (2,000 km) across with cloud speeds as fast as 330 mph (150 meters per second)

One ring to rule them all…

Actually, Saturn has 7 main ring groups and thousands of individual rings.

Historically, understanding the structure of Saturn's rings was not a trivial task!

  • Galileo first commented on Saturn's odd shape in 1610, and thought it might be a "triple planet."

  • But it was not until 1655 that Christiaan Huygens (1629-1695 CE; Dutch) concluded that Saturn was surrounded by a ring. Huygens believed the ring to be a solid disk.

  • In 1675, Giovanni Cassini (1625-1712 CE; French-Italian) found gaps in the ring structure (the largest of these is now called the Cassini Division), demonstrating that the rings were not a single solid structure.

  • In 1787, the mathematician Pierre-Simon Laplace (1749-1827 CE; French) showed that large solid rings would be unstable and suggested that the rings were composed of a large number of solid ringlets.

  • In 1859, James Clerk Maxwell (1831-1879 CE; Scottish) proved that any ring configuration would be unstable except for rings composed of numerous small particles, each independently orbiting Saturn.

Christiaan Huygens (1629-1695 CE; Dutch), in a 1671 painting by Caspar Netscher (c. 1639-1684 CE; Dutch).

Credit: Caspar Netscher, 1671; Haags Historisch Museum [link]

Detail from a letter written by Galileo in 1610 showing a sketch of his observation of Saturn as a "triple planet".

Credit: Galileo Galilei, 1610; held in the Museo Galileo Galilei, Florence, Italy [link]

Illustration of historic drawings of Saturn from “Systema Saturnium” by Christian Huygens (1659).

Credit: Christiaan Huygens, 1659; Linda Hall Library [link]


Based on his telescope observations in 1655, Huygens proposed that Saturn was surrounded by a single solid ring. He correctly observed that the orientation of the ring changed over time (due to the axis tilt of Saturn). Huygens also discovered Titan, the first (and largest) moon of Saturn.

In order to secure credit as the discoverer of both Saturn’s moon and ring, Huygens announced each discovery in a published pamphlet. But because he was not yet certain, he made the announcements in an anagram code that only he could solve. Later, when he was certain he was right, he published the solutions to the anagrams, thereby proving that he had made the discoveries before anyone else.

In the page from his 1659 book Systema Saturnium shown here, Huygens reprints the original ring anagram:


Followed by its unscrambled solution:

Annulo cingitur, tenui, plano, nusquam cohaerente, ad eclipticam inclinato.

Translated from Latin, this reads:

It is surrounded by a ring, thin, flat, nowhere touching, and inclined to the ecliptic.

Huygen's solid disk ring interpretation for Saturn, from his book “Systema Saturnium” (1659). His encoded announcement of the ring discovery is reprinted near the top of the page.

Credit: Christiaan Huygens, 1659; Linda Hall Library [link]

Wide but thin

Saturn’s rings have an outer diameter of 21 Earth diameters from the center of Saturn.

But they are only about 10 meters (33 feet) thick!

Image of Saturn from the Cassini spacecraft, showing the rings viewed edge-on. The shadows of the rings cast on Saturn are visible in the upper half of the image. Saturn's small moon Enceladus (505 kilometers, or 314 miles across) is visible in the plane of the rings (on the right).

Credit: NASA/JPL/Space Science Institute [link]

“Shepherd Moons” of Saturn orbit within and around the ring system.

  • They clear out the gaps in the rings, and help keep the ring system stable through orbital resonances with the ring particles.

  • Saturn's rings are delicate, young structures (about 100 Myr old).

  • The moons’ gravity can also create 3-D structures in the rings.

Looming vertical structures created by the tiny moon Daphnis cast long shadows across the rings in this startling image obtained as Saturn approached its mid-August 2009 equinox.

Daphnis, 8 kilometers ( 5 miles) across, occupies an inclined orbit within the 42-kilometer (26-mile) wide Keeler Gap in Saturn's outer A ring. The moon's gravitational pull perturbs the orbits of the particles forming the gap's edge and sculpts the edge into waves having both horizontal and vertical components.

Measurements of the shadows in this and other images indicate that the vertical structures range between 0.5 to 1.5 kilometers tall (about one-third to one mile), making them as much as 150 times as high as the ring is thick. Daphnis itself can be seen casting a shadow onto the nearby ring.

Credit (image and some text): NASA/JPL/Space Science Institute [link]

Though Daphnis is small, its gravitational pull produces waves at the edges of the Keeler gap. The waves are within the plane of the ring and also out of the plane, due to Daphnis’ slight (0.0036°) orbital inclination. Out of the plane, ring particles are attracted toward the moon and then fall back into the ring plane as Daphnis moves away. Within the plane, the waves made by Daphnis in the inner edge of the gap precede it in orbit, while those on the outer edge lag behind it, due to the differences in relative orbital speed.

Credit (image and some text): NASA/Cassini; Planetary Science Institute [link]

A “mountain range” at the edge of a ring

Ring material is “piled up” at the edge of the ring by the gravity of small (1 km) “moonlets” orbiting just inside the ring.

Vertical structures, among the tallest seen in Saturn's main rings, rise abruptly from the edge of Saturn's B ring to cast long shadows on the ring in this image obtained by NASA's Cassini spacecraft in 2009.

Part of the Cassini Division, between the B and A rings, appears at the top of the image, showing ringlets in the inner division. This image shows a 1,200-kilometer-long (750-mile-long) section arcing along the outer edge of the B ring. Here, vertical structures tower as high as 2.5 kilometers (1.6 miles) above the plane of the rings - a significant deviation from the vertical thickness of the main A, B and C rings, which is generally only about 10 meters (about 30 feet).

Credit: NASA/JPL/Space Science Institute [link]

This montage of views from NASA's Cassini spacecraft shows three of Saturn's small ring moons: Atlas, Daphnis, and Pan, at the same scale for ease of comparison.

The moons all have equatorial ridges caused by the accumulation of ring material swept up during their orbits around Saturn.

Credit (image and some text): NASA/JPL-Caltech/Space Science Institute [link]

Many (all?) of Saturn's small moons, especially the ones orbiting in or near the rings, are probably just big snow and ice balls built up out of ring material.

The disturbance visible at the outer edge of Saturn's A ring (bottom center) in this image from NASA's Cassini spacecraft could be caused by a new moon in the process of forming. Estimated to be no more than 1 km across, this small moon could be in the process of migrating out of the ring.

Credit (image and some text): NASA/JPL-Caltech/Space Science Institute [link]

Pandora and Prometheus

This movie sequence from Cassini, obtained in 2005, shows dark spokes in the inner strands of Saturn's F ring caused by the gravitational influence of the shepherd moon Prometheus (102 kilometers, or 63 miles across). Prometheus appears first in the sequence, interior to the F ring, and Pandora (84 kilometers, or 52 miles across) follows along outside of the ring. Radial structure in the bright core of the ring is visible throughout the movie.

Credit: NASA/JPL/Space Science Institute [link]

Saturn's moon Prometheus, having perturbed the planet's thin F ring, moves away as it continues in its orbit in this image from Cassini in 2010.

Credit: NASA/JPL/Space Science Institute [link]

Saturn and six of its moons, with diameters labeled, imaged by the Cassini spacecraft.

Saturn has:

    • 62 moons with determined orbits

    • 53 moons with names

    • 13 moons larger than 50 km across

Credit: D. W. Hoard (2018), using a public domain image from the NASA Cassini mission [link]