Neptune's Rings

Image of Neptune's Rings taken from Voyager 2 in 1989.

Three rings are easily seen in this image: Adams (outermost), Le Verrier (middle) and Galle (inner ring). To the left of the image, in the Adams Ring, Galatea is visible. Between Adams and Le Verrier, two faint rings can be made out: Arago and Lassell.

Image Credit:

NASA/JPL/Voyager

 

Neptune has five main rings: Galle, Le Verrier, Lassell, Arago, and Adams, named after five important astronomers in Neptune's history. Much like the rings of Uranus, the rings of Neptune are made up of dust particles, and kept in place by shepherd moons, which include Galatea (Adams Ring) and Despina (Le Verrier Ring). It is believed that Neptune has other moons which help keep the rings narrow and stable (so to speak), but they have yet to be discovered.

The Adams Ring is unique in that it contains arcs, which are caused by gravity from Galatea. The arcs were discovered when Neptune occulted a star and where the rings should have been, the star shone through the rings. Close ups by Voyager confirmed that the Adams Ring contained arcs.

Much like the rings around Jupiter, Saturn, and Uranus, the material in the rings of Neptune are not permanent. They are continuously replenished by collisions with the moons of Neptune and are kept in orbit by shepherd satellites. The rings themselves are mostly dust with some ice particles and are covered in organic material (carbon compounds) that make them dark.

Image Credit:

Sky and Telescope

The Rings of Uranus

Image Credit:

NASA

 

 

Image Credit:

NASA

 

Uranus has a faint ring system around the planet. William Herschel thought that he had discovered rings around the planet when he first spotted Uranus, but based on how faint they are, it seems unlikely that what he saw were the rings.

The rings are oriented parallel to the equator of Uranus and much like those of Jupiter and Saturn, are continually replenished by collisions by objects colliding with the moons of Uranus. They are very dark, though they contain icy particles because they are covered with dust from the moons and the lack of sunlight reaching the region around Uranus.

The rings themselves are very narrow, made up of meter-sized particles and smaller. The majority of the rings are no more than 10km wide, with the widest ring only 100km (less than 0.2% the diameter of Uranus). Much like Pandora and Prometheus keeping the F-Ring around Saturn narrow, the rings of Uranus are kept narrow because of shepherd satellites, like Cordelia and Ophelia maintain the ε-Ring (epsilon ring). The ε-Ring is actually very eccentric because of the highly eccentric orbits of Cordelia and Ophelia.

The ring around Uranus were first indirectly discovered in 1977 when Uranus occulted a star, i.e. passed in front of the star. Just before the disk of Uranus passed in front of the star, the star dimmed a little, and just after the Uranus passed in front, the star did not return to its original brightness. Based on the dimming, astronomers were able to conclude, correctly, that Uranus possibly had a ring system. It wasn't unitl Voyager 2 passed Uranus that we were able to image the rings directly, and show that Uranus did have rings.

Uranus

Image Credit:

NASA/Voyager 2

 

No planet or celestial object causes as much snickering as the seventh planet from the Sun. Depending on whom you ask, it can be pronounced one of two ways. The first way is the one that causes the snickering, especially among high school and college students (I know, I taught college astronomy). The other way, the way that I prefer to use, does not make it sound so humorous. Of course, I am talking about the planet, Uranus.

I pronounce Uranus as if it sounds like "You're A Nus" or "You're a Nis". Trust me, it helps people from giggling when you say its name. The name Uranus itself comes from Greek mythology as Uranus was the father of the first Titans (including Cronus) and the grandfather of the Olympians, the Greek gods. It is the first planet (other than Earth) not named for a Roman god. If the tradition had held using Roman names, Uranus should have been called Caelus, the father of Saturn and in turn, the grandfather of Jupiter.

Uranus is also the first planet discovered with a telescope. Up to its discovery in 1781 by William Herschel, only the six planets were known (Mercury, Venus, Earth, Mars, Jupiter, and Saturn). Of course, before the heliocentric model of the Solar System, Earth was not considered a planet. There is evidence that Galileo saw Uranus, but mistook it for a star. Two other astronomers also observed Uranus, but did not identify it as a planet.

Like Jupiter and Saturn, Uranus is a Jovian planet, i.e. a gas giant planet. It does not have a solid surface, but contains hydrogen, helium, methane, ammonia, and water in the outer layers. Its "mantle" is a mixture of ice and rock and has a heavy element core. The density of Uranus is 1.29 g/cm³, making it the seventh densest planet, only ahead of Saturn.

Uranus is the third largest planet in terms of diameter, but the fourth largest in mass (Neptune is larger in mass, but has a smaller diameter). It orbits 19.18 AU from the Sun, taking just over 84 years to orbit the Sun. Since its discovery in 1781, it has only completed two orbits, its third orbit won't be complete until 2033.

Since it is a Jovian planet, it also exhibits two properties that Jupiter and Saturn display: a ring system and multiple moons. The rings are more similar to Jupiter's rings than Saturn's rings as they are very faint and were not confirmed until Voyager 2 imaged them directly. Uranus has 27 confirmed moons which are all icy bodies. Five are considered to have planetary mass which means that they are spherical. Titania and Oberon were actually first discovered by William Herschel in 1787, after his discovery of Uranus.

Image Credit:

W. M. Keck Observatory

Lastly, the most amazing thing about Uranus is its day. Recall that Venus has an inclination of almost 180°. Uranus' inclination is not as severe, but may considered stranger. Its inclination is 98° which means that its axis of rotation is almost parallel to Uranus' orbital plane.

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 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.

The Rings of Jupiter

A side view of Jupiter's Rings from Galileo spacecraft

Image Credit:

NASA, JPL, Galileo Project, (NOAO), J. Burns (Cornell) et al.



Saturn is not the only planet with rings. All the Jovian planets have some sort of ring system, though Saturn's rings are the most impressive. Jupiter's rings are so thinly distributed that they were not discovered until 1979 when Voyage I discovered them on its journey through the Solar System.

Jupiter's rings are not as bright as Saturn's rings, which are mostly composed of ice compounds. Jupiter's rings are dark and reddish which tells us that the material making up the rings are of a rocky origin. The ring is located within the Roche limit, the distance from Jupiter where gravity would rip apart a poorly consolidated moon, asteroid, or comet wandering inside that distance. The origin of the ring is probably a moon or asteroid that wandered too close to Jupiter.

 

The rings of Jupiter would have been dissipated long ago if the ring was not continuously replenished by the moons near the rings. The moons get hit by small meteorites, impacting the surface of the moon, and blowing dust out into orbit. This dust then gets incorporated into the rings. We know this by looking at the distribution of dust particles in the rings and see that the portions of the rings near moons are more densely packed with dust than those sections farther away. For example, near the moons Amalthea and Metis, the ring is densest between these moons. Amalthea orbits just outside the ring and Metis, just within the rings diameter.

 

Besides the main ring seen above, there is a much thinner ring outside Amalthea's orbit. This ring is called the gossamer ring because it is so thin. Again, by noting that this ring is densest near Amalthea and Thebe (orbiting farther out from Amalthea), we can conclude that micrometeorite impacts keep the rings intact. Also, Amalthea, Metis, and Thebe are also shepherd satellites of the rings, which will be explain more in detail when we talk about Saturn, its rings, and its moons.

Jupiter

Jupiter: Note the Great Red Spot to the lower right and the shadow of Europa on the lower left

Image is from the Cassini Spacecraft

 

Jupiter is by far the largest planet in the solar system in terms of mass and size. However, it is still less then 99% of the Sun's mass. It has a semi-major axis distance of 5.2 AU which gives it an orbital period of about 11 years. It rotates once on its axis every 10 hours, making it one of the fastest rotating bodies in the Solar System.

Previously, we had talked about Galileo and how he had discovered the first moons around a planet other than our Earth. This discovery helped prove the geocentric theory of the solar system was incorrect (Objects can orbit something other than the Earth). Galileo originally wanted to call them the Medician moons after his sponsors, the Medicis, but it was eventually agreed to name them the Galilean moons and to name them after four of the lovers of Zeus; Io, Europa, Ganymede, and Callisto.

Saturn is no the only planet with rings. In fact, all four Jovian planets have a ring system, though not quite as magnificent as Saturn's rings. Jupiter's rings are not as extensive as the rings around Saturn and were not discovered until 1979 by Voyager 1.

Jupiter is also home to the largest storm in the Solar System. The Great Red Spot is at least 183 years old and may be older. It may have been first observed in 1665, but it isn't clear if it was observed again until 1831. It is between 24,000 to 40,000 km east-west and 12,000 to 14,000 km north-south, making it capable of holding 2 to 3 Earths.

Jupiter also shares its orbit with a two groups of asteroids known as the Greek and Trojan asteroids. These asteroids were discussed previously here. These asteroids, much like the Amor asteroids, will never impact Jupiter. However, Jupiter's gravity is strong enough to capture both asteroids and comets. Many of Jupiter's moons are possibly captured asteroids, and back in 1994, Comet Shoemaker-Levy 9 entered Jupiter's gravitational field, broke up, and collided with Jupiter.

Jupiter has a very thick atmosphere, as the majority of its volume are the many layers of gas. There is a rocky core, approximately Earth-sized, but is only a small fraction of the entire diameter of Jupiter. The combination of its large volume and the majority of the composition of the planet give Jupiter the largest mass of all the planets, but also a low density, just a little bit above that of water.