The Composition of Saturn

Like Jupiter, Saturn contains hydrogen, helium, ammonia, and methane. However, 88% of the mass of Saturn is composed of hydrogen with 11% helium, the two lightest elements on the periodic table.

Saturn has a diameter of 9.42 times that of Earth and a mass that is 95.15 times that of Earth. Despite being so much larger than Earth, these properties lead to a strange phenomena. Saturn's density is only 0.69 g/cm³. Water has a density of 1.0 g/cm³. Yes, Saturn has a lower density than water. What this means that if you could construct a large enough tank and filled it with water, Saturn would float in that tank. No other planet has a density of less than 1.0 g/cm³, though there are some satellites that have low densities.

Saturn is also banded, but not to the extent as Jupiter. Since it is farther away, the clouds are not as brightly illuminated as they are on Jupiter. Also, the ammonia ice crystals in the atmosphere of Saturn are above the cloud layers, preventing light from reaching the clouds, and help make the clouds darker than otherwise. Of course, Saturn's gorgeous rings make up for the blandness of its atmosphere.

The Hexagonal Storm on Saturn

Saturn's Hexago

Image Credit:

NASA/Cassini

North Polar Hexagon on Saturn

Image Credit:

NASA/JPL-Caltech/Space Science Institute

 

One of the strangest features on Saturn is the hexagon-shaped storm in its northern hemisphere. The storm was first discovered by Voyager in 1981-1982 and has been persistent since. It is located at 78° N latitude, but does not change longitude over time, unlike other storms on Saturn. The sides of the storm are about 13,800 km, longer than the Earth's diameter of abotu 13,500 km.

The Cassini mission also has taken images of the storm since its arrival in 2006. And if the conditions are right, i.e. the storm is in daylight, fuzzy images of the storm can be seen from Earth-based telescopes, even by amateur astronomers.

Oxford University astronomers proposed a hypothesis for the formation of the storm. In the lab, regular shpaes were created in a circular tank of liquid that had different rotation rates at the center and at the edges. Squares, hexagons, and octagons were all created, with hexagons being the most common shape. These latitudinal gradients in the rotation are one probable cause for the hexagon on Saturn.

Other Saturn Moons

Moons of Saturn 2007

From top left to right: Mimas, Enceladus, Tethys, Dione, Rhea, Titan in the background, Hyperion in bottom right, Iapetus, and Phoebe

 NASA/JPL/Space Science

Wikipedia 

 

Saturn has more moons than any other planet, including Jupiter. Besides Titan, it has many satellites that are strange in there own way.

 

The second largest moon of Saturn is Rhea, which is less than half the size of the Earth's Moon. It was discovered by Giovanni Cassini in 1672. Of all bodies in the Solar System in hydrostatic equilibrium, it is the smallest, i.e. gravity and fluid pressure are in balance. It is mainly ice, and much like Titan and the Moon, it is tidally locked to Saturn. It is also the largest moon without an atmosphere.

Rhea

Image Credit:

NOAA

The third largest moon of Saturn is Iapetus. It was also discovered by Giovanni Cassini in 1671 and is the largest body in the Solar System not in hydrostatic equilibrium. It is two-toned with the leading face of the moon dark with a low albedo and the back end brighter, like the color of dingy snow. It almost resembles the yin-yang symbol. It is believed that the dark face comes from Iapetus colliding with dark carbon-rich or silicate-rich materials. It is also tidally locked with Saturn.

Iapetus

Image Credit:

NASA/Voyager

Dione is the fourth largest moon of Saturn, discovered by Cassini in 1684. Like Rhea and Iapetus (and all the moons in this post), it is an icy body. Again, it is tidally locked to Saturn and shares an orbital resonance with Enceladus of 1:2. It also has two co-orbiting satellites, Heleneand Polydeuces, located at the L4 and L5 Lagrange points for Dione. Though it is mainly water ice, its density leads us to believe that it has a rocky interior.

Dione

Image Credit:

NASA/Cassini

Tethys is the fifth largest moon of Saturn, and was discovered by Cassini in 1684. It is spherical and has a density very nearly that of water. We also know that Tethys is not very active as it is heavily crated and contains a long, deep ice crack about 500 km long and 3 km deep.

Tethys with Odysseus crater

Image Credit:

NASA/Cassini

Enceladus is the sixth largest moon of Saturn, but was not discovered until 1789 by William Herschel. As an icy body, it also has a very high albedo, making it very reflective. Not only is it an icy body, it is also an active body. The interior of Enceladus is kept hot by tidal forces between itself, Saturn, and Dione. Waper vapor geysers have been discovered erupting from the surface from the Cassini spacecraft. Some of the water vapor does fall back down as snow, but most escapes due to low gravity on Enceladus. Material in the E Ring is replenished by the erupting geysers. Under the surface ice, Enceladus is believed to have a liquid ocean which may contain organic molecules (molecules with carbon).

Enceladus

Image Credit:

NASA/Cassini

Mimas is the seventh largest of Saturn's moon, and the one that many people might recognize. It was discovered by William Herschel in 1789 and much like the other moons listed here, it is a small icy body, with many craters. It has a deep 2 km crack that was probably created early in its history. Mimas is the smallest body in the Solar System that is spherical due to self-gravity. It has a 2:1 resonance with the Cassini Division, which helps keep it clear of particles. Mimas has one really distinguishing feature, and that is a large crater in the northern hemisphere.

 

Mimas with Herschel Crater in upper left

Image Credit:

Solar Views

Death Star from Star Wars. I wonder where George Lucas got his inspiration?

 

Hyperion is the eighth largest of the major moons of Saturn, and was discovered by William Lassell, William Cranch Bond, and George Phillips Bond, all in 1848. It is the largest of the irregularly shaped moons and as seen below, looks like a potato. Its shape is best determined by the diameters along its three axes, 410 km by 260 km by 220 km.

Irregularly shaped Hyperion

Image Credit:

NASA/Cassini

Titan

Cassini Image of Titan showing the atmosphere of the moon

Image Credit:

NASA/Cassini

 

Titan is the largest Saturnian moon and the second largest in the Solar System behind Jupiter's moon Ganymede. Like Ganymede, it is also larger than Mercury. Christian Huygens, who discerned the rings of Saturn, also discovered Titan in 1655, making it the fifth satellite discovered with the telescope.

 

It is the only moon known to have a dense atmosphere, where atmospheric pressure is measurable on the surface. Though Europa, Ganymede, and Callisto may have liquid oceans below their outer crusts, Titan is the only body in the Solar System to have surface liquid besides the Earth. However, you wouldn't want to swim in those oceans as they are bodies of methane.

 

Because it has liquid methane oceans and lakes, much like Earth has a water cycle, Titan experiences a methane cycle. Surface methane evaporates and forms clouds in the Titan sky. The rain Titan experiences is methane.

 

Although Titan is smaller than Earth, its atmosphere is dense enough to create higher surface pressure than on Earth, at about 1.45 atmospheres (146.7 kPa). One atmosphere on Earth is the normal pressure at sea level which is 101.5 kPa (kilopascals). As shown in the above image, the atmosphere is extended with a composition of 98.4% Nitrogren (N₂), 1.4% methane (CH₄), and trace other molecules including water in the stratosphere (higher levels) and 95% N₂, 4.9% CH₄, and other molecules in the troposphere. Because of the methane clouds, the sky is very hazy on Titan so would have poor visibility on the surface when we make our first crewed mission to Titan sometime in the future.

 

The gravity on Titan is only 85% of that on the Moon even though it is larger because of its smaller density. If you were to stand on Titan, your weight would only be 15% of that on Earth.

 

Much like the Moon and Earth are tidally locked, Titan is tidally locked to Saturn, so for every orbit around Saturn, which is almost 16 Earth days, it only rotates once on its axis.



F Ring and Shepherd Satellites

The F-Ring, Prometheus (inner moon), and Pandora (outer moon). The A-Ring fills up the bottom half of the image with the Keepler Gap easily visible

Image Credit:

NASA/JPL/Voyager

 

Saturn's F-Ring is the outermost of the discrete rings discovered in 1979 by Pioneer 11. Compared to the other rings, it is very active with features changing in the structure of the ring on a timescale of hours. The F-Ring is 3,000 km from the A-Ring and is separated from the A-Ring by the Roche Division. Compared to the other discrete rings, the F-Ring is very narrow, only a few hundred kilometers thick. So how exactly does the F-Ring maintain its shape?

The ring is between the orbits of two satellites, Prometheus and Pandora. Prometheus orbits just inside the inner edge of the F-Ring and Pandor just outside the outer edge. These two satellites are able to use their gravitational influence on the ring to keep it stationary and in place. If these moons were not there, the F-Ring would have dissipated long ago.

Prometheus also creates kinks and knots in the ring from its orbit which show up in the ring when Prometheus is at apoapis (farthest distance from Saturn). Because Prometheus does have an elliptical orbit, at each successive apoapis, the knots and kinks are 3.2° ahead of the previous section.

Prometheus creating knots and streamers in the inner F-Ring

Image Credit:

Cassini/ISS/JPL/EPA/NASA

Saturn's Roche Division

Close up of the Roche Division. Visible from the bottom left to the upper right, the A Ring with the Encke Gap and the Keeler Gap, Atlas in the center, and the F Ring

Image Credit:

NASA

 

The Roche Division is the space between the A Ring and the F Ring in the ring system of Saturn.  Despite sharing the name with the Roche limit of Saturn and being near it, it is not named for the limit, but actually is named for Edouard Roche.

Like the Cassini Division, it is not empty, but contains material similar to the D Ring, E Ring, and F Ring, but very sparsely distributed.

The Cassini spacecraft discovered to small ringlets in the division, both near the orbits of a moon. One ringlet shares an orbit with the moon Atlas and the other is close to the orbit of Prometheus, which will be discussed in the next blog post.

The A Ring of Saturn

A close up of the A Ring. The Cassini Division lies between the A Ring and the B Ring, the Encke Gap (though the image says division) is within the A Ring, and the Roche Division separates the A Ring from the F Ring

Image Credit:

 Sky and Telescope

 

The A Ring of Saturn is the farthest of the main rings and the first one discovered. It is composed of material similar to that of the B Ring and therefore is bright. It is separated from the B Ring by the Cassini Division, and has similar structure as the B ring.

One of the main features of the A Ring is the Encke Gap, discovered by James Keeler when he was working at Lick Observatory near San Jose, California. (Keeler was working at Allegheny Observatory when he discovered the rings were not solid). The gap was named in honor of Johann Encke who had discovered that the A Ring was not uniformly bright. The gap is about 325 km wide and centered at 133,590 km from Saturn's center. It is kept clear by the orbit of a small moon, Pan, and contains at least three thin ringlets which are knotted due to the gravitational influence of passing moons.

Encke Gap (PIA06534)

Image Credit:

NASA/JPL/Space Science Institute

The Keeler Gap, named for James Keeler, was discovered by the Voyager probe and is about 42 km wide and 250 km from the outer edge of the A Ring. Daphnis orbits with the gap and keeps it clear, much like Pan with the Encke Gap. Daphnis actually is inclined with respect to the rings and actually causes waves at the edges of the gap.

Keeler Gap with Daphnis within. Notice the waves at the edges of the gap.

Image Credit:

NASA/JPL/Space Science Institute

 

Besides the gaps, there are many moonlets that orbit within the ring and helps create waves and spokes within the ring structure. They were first discovered in Cassini images and by 2008, over 150 moonlets have been identified. They were only discovered because of the influence their gravity has on the A ring. Based on evidence, there are possibly thousands of these small moonlets, no more than several kilometers in diameter. They orbit in a path that is only 3000 km wide at a distance of 130,000 km.

As you may have noticed, the rings themselves contain both divisions and gaps. The IAU (International Astronomical Union) defines a division as a separation between two distinct rings and a gap as a small opening in a ring itself. Hence the Cassini division divides the B Ring from the A Ring and the Encke Gap and Keeler Gap are gaps in the structure of the A Ring.

The Cassini Division

Image Credit: NASA

The Cassini Division is a gap in the ring system of Saturn between the B Ring and the A Ring. It was discovered in 1675 by Giovanni Cassini, hence its name. From Earth, it appears dark and therefore, it was believed that this part of the ring system did not contain any particles. However, when Voyager passed by Saturn, it discovered that there were particles in the division, similar to those in the C Ring. However, they are much more dispersed than in the C Ring, so the division looks empty from Earth.

There are ringlets in the Cassini Division which is caused by resonance with two moons: Mimas which shares a 2:1 resonance with the division (for every one orbit of Mimas, the ringlet makes two) and Enceladus which shares a 3:1 resonance. Mimas and Enceladus tug on the particles in the Cassinin Division, keeping it relatively clear of any particles.

Also, there is a gap in the division called the Huygens Gap. The Huygens Gap contains the ringlet created by the resonance with Mimas.

The B Ring of Saturn

Saturn's rings dark side mosaic

Image Credit:

NASA/JPL/Space Science Institute

Saturn's Ring Plane

Image Credit:

NASA/JPL/Space Science Institute

 

The B Ring is the second ring of Saturn discovered, and the third ring from Saturn. It is composed mostly of golf ball-sized and smaller ice particles, making it very reflective and bright, compared to all the other rings. It is much brighter than the C Ring and the A Ring, and is by far the widest of all the main rings. Because of its width and its depth of 5 to 15 km, it is also the heaviest ring.

 

Unlike the C Ring, which is very transparent, the B Ring blocks 91% of all light incident on it, reflecting most of it, allowing us to see it very easily.

 

Also unlike the C Ring, which has small scale structures inside the ring itself, the B Ring does not contain gaps, but only small ringlets within its structure. The unique feature of the B Ring, however, is the radial lines evident in the rings. These spokes, as they are referred to, are not from gravity, but from Saturn's magnetic field. If the spokes were due to gravity, they would remain in place, even as the planet rotated. But instead, they have a period the same as the magnetic field of Saturn, so we know that they are created by magnetism.

Dark Spokes in the B Ring

Image Credit:

NASA/JPL/Space Science Institute

Rotation of the Spokes

Image Credit:

NASA/JPL/Space Science Institute

 

 

The C Ring of Saturn

Saturn's rings dark side mosaic

Image Credit:

NASA/JPL/Space Science Institute

Saturn's Ring Plane

Image Credit:

NASA/JPL/Space Science Institute

 

The C Ring was the third ring of Saturn's ring system to be discovered and as shown in the above photos, is the closest of the three main rings. It is fainter than both the B Ring and the A Ring, and is between the B ring (25,500 km) and the A ring (14,600 km) in width at 17,500 km. It was first discovered by William and George Bond in 1850, though William R. Dawes and Johann Galle also independently saw it. William Lassell nicknamed the ring, "Crepe Ring" as it is darker than the A Ring and the B Ring, and to him, resembled the black cloth associated with funerals.

 

It is only five meters thick from top to bottom, and even though it is dark, it is relatively transparent. Between 5% to 12% of light incident on it well be blocked, so it is very easy to see though. It is composed of boulder-sized ice chunks, while the A Ring and B Ring are golf ball sized and smaller. Even though it is made up of ice, it is still darker, meaning that the ice may be covered with a crust of dust, prevently light from being reflected efficiently.

 

The C Ring has smaller parts to it, including the Colombo Gap and Titan Ringlet, the Maxwell Gap and Maxwell Ringlet, the Bond Gap, 1.470 Rs Ringlet, 1.495 Rs Ringlet, and the Dawes Gap. The Titan Ringlet is unique in that it shares a resonance with Titan, so that Titan somewhat controls the rotation of the Ringlet. The Maxwell Gap and Ringlet are named after James Clerk Maxwell, who had mathematically calculated that the rings could not be solid disks, the Bond Gap named after William and George Bond, and the Dawes Gap is named after William R. Dawes.

The Rings of Saturn

PIA17172 Saturn And Its Rings with Earth, Mars, and Venus

Image Credit:

NASA / JPL-Caltech / Space Science Institute

 

The first image that pops in people's heads when they think of Saturn is probably a planet with rings. And they wouldn't be wrong. Saturn's most famous feature are its impressive rings and in my opinion, the most striking feature of any planet in our Solar System. So what exactly are the rings and where did they come from?

The rings were first discovered by Galileo in 1609 when he saw these objects on the sides of Saturn that he called "ears". His telescope was not good enough to resolve the ears into a disk and to see the rings clearly. It wasn't until 1659 when Christian Huygens was able to resolve the rings into a disk and see that the rings were not attached to Saturn physically.

 For the next few centuries, it was believed that the rings were a solid torus around Saturn. No evidence was observed to make anyone think differently. In 1859, James Clerk Maxwell, famous for his four equations of electromagnetism (which you can learn about here), proved mathematically that a solid ring would be unstable and not be able to orbit around Saturn. It wasn't until 1895 that two astronomers, James Keeler at Allegheny Observatory outside of Pittsburgh, PA (where I used to work) and Aristarkh Belopolsky of Pulkovo Observatory near Saint Petersburg, Russia, independently spectroscopically determined that the rings were not solid, but made up of many particles. Using the Doppler effect, which is that effect that causes waves to change wavelengths based on the speed of the observer, the source of the wave, or both, they both were able to show that the outer rings travel slower than the inner rings. This would not be possible if the rings were solid. If the rings were solid, the angular velocity of the inner parts of the rings and the outer parts of the ring would have to be the same, and they showed that this was not true. Maxwell's mathematical prediction was true.

The rings themselves are made up of both rocky dust and ice particles, depending on where in the ring structure the particles are located. The rings orbit (in general) above Saturn's equator. Much like the rings of Jupiter, the particles must be continually replenished by micrometeorite collisions with the moons of Saturn, adding particles to the rings while parts of the rings are dissipated by Saturn's gravity or the gravity of the nearby moons. The rings are believed to have been first formed when Saturn was formed when small planetessimals were within the Roche limit of Saturn and could not consolidate into moons.

The rings were originally named in the order in which they were discovered, starting with A. But as more were found, the newer ones were given proper names. Starting with the innermost ring, the ring system is broken down in this manner:

• The D Ring: the fourth ring discovered in 1980 by Voyager 1 is a very faint ring system. Its distance from Saturn ranges from 66,900 km to 74,510 km
• The C Ring: the third ring discovered in 1850 by George and William Bond. Its distance ranges from 74,658 km to 92,000 km and will be discussed in more detail in its own post.
• The B Ring: the second ring discovered and the most massive of the rings. Its distance ranges from 92,000 km to 117,580 km and will be discussed in more detail in its own post
• The Cassini Division: a space between the B ring and the A ring discovered by Giovanni Cassini in 1675. Its range is from 117,580 km to 122,170 km and again, will be discussed further in its own post.
• The A Ring: the first ring to be discovered when Huygens first detected the rings as rings. It ranges from 122,170 km to 136,775 km and will be its own post
• The Roche Division: the gap between the A Ring and the fainter F Ring. There is material in this division, but is so thinly populated that we do not see it very well. The moon Atlas orbits in this division. Its range is 136,775 km to 139,380 km.
• The F Ring: thin ring orbiting outside the Roche Division. It has a small range or 30 to 500 km but orbits around 140,180 km from Saturn. It is kept in place by two small moons, Pandora and Prometheus and will be discussed in detail when talking about those two moons.
• The Janus/Ephimetheus Ring: a ring that is maintained by the moons Janus and Ephimetheus. It was discovered by the Cassini spacecraft in 2006. It ranges from 149,000 km to 154,000 km.
• The G Ring: faint ring with a bright inner edge. Halfway between the F Ring and the E Ring, it has the moonlet Aegaeon orbiting nearby. It ranges from 166,000 km to 175,000 km.
• The Methone Ring Arc: not a full ring, but a 10° arc orbiting around Saturn. It shares an orbit with Methone and was detected for the first time in September of 2006. It orbits abour 194,230 km from Saturn
• The Anthe Ring Arc: not a full ring, but a 20° arc orbiting around Saturn, much like the Methone Ring Arc. It shares an orbit with Anthe and was detected for the first time in June of 2007. It orbits abour 197,665 km from Saturn
• The Pallene Ring: shares an orbit with the moon Pallene at around 211,000 km to 213,500 km. It was discovered by Cassini in 2006.
• The E Ring: the last of the lettered rings, though the fifth discovered. The second outermost ring, but the outermost orbiting equatorially with Saturn. It is very wide and is between the orbits of Mimas and Titan. There are moons that orbit within the ring and they are tinted by particles from the ring. It orbits between 180,000 km to 480,000 km, by far the widest of the rings.
• The Phoebe Ring: the outermost ring orbiting just to the interior of the moon Phoebe. It was discovered in October of 2009 by NASA's infra-red Spitzer Space Telescope and orbits at an angle of 175° to the equator of Saturn, so it also orbits retrograde. It orbits between 4 million and 13 million km from Saturn and will be discussed more in its own post.

Saturn

Image Credit:

NASA/Voyager 2

 

Saturn is the second largest planet in the Solar System, about 95 times the mass of the Earth. However, compared to Jupiter, Saturn is tiny. It is only 30% the mass of Jupiter. Its radius at the equator is 9.44 times that of Earth and its polar radius is only 8.5 times Earth's. Despite this, if you could stand on Saturn, you would feel the same gravity as you do on Earth. Its day is just a little longer than Jupiter's at 10.57 Earth hours. Its average distance from the Sun is 9.5 AU which gives it an orbital period of 29.46 Earth years. It has an inclination of 26.5° with respect to its orbit and its orbit is only tilted at 2.5° to the ecliptic (the orbit of the Earth). With respect to the Sun's equator, it is tilted at 5.51°.

 

The thing Saturn is most known for is seen in the above image from Voyager 2. Its ring system is the most extensive of all the Jovian planets and Saturn has been known to have rings since the 1600s. The rings are a fascinating aspect of the most beautiful planet (in my opinion) in our Solar System, so that I can't talk about all rings in only one post. Stay tuned to learn a lot about Saturn's rings.

 

Saturn also has many moons, almost three times as many as Jupiter with 150 known, though only 51 have formal names. The largest satellite in the Solar System, Titan, belongs to Saturn and could be considered a mini-world in its own right. It also has moons that keep some of Saturn's rings in line, called shepherd satellites. It has a moon that doesn't look like a moon and moons that share an orbit.

 

Saturn's composition is similar to Jupiter, containing the same gases, but in different concentrations. This difference in concentrations, the thickness of its atmosphere, and the size of its heavy element core give rise to a strange phenomenon when looking at Saturn's density.