The Moons of Uranus

The Moons of Uranus from closest to farthest starting from the left

Image Credit:

NASA/Voyager

Uranus has 27 known moons which are mostly icy bodies similar to comets. Most of them might be captured comets from the Kuiper Belt or the Oort Cloud. However, there are a few that are considered planetary mass bodies, which means they have enough mass to be differentiated and spherical in shape. Most likely, these five major moons were probably formed at the same time as Uranus.

Titania

Image Credit:

NOAA

Titania has the largest diameter of all of the moons of Uranus, but it is still smaller than our Moon. Its diameter is 1600 km, compared to 3500 km for the Earth. Even though it has the largest diameter, it is not the most massive. That honor goes to Oberon which will be discussed later. Of the five major moons, it is the fourth out from Uranus and has an inclination of 0.34°, meaning it orbits along the equatorial plane of Uranus. William Herschel discovered Titania along with Oberon in January of 1787. Observations of Titania with telescopes and with the Voyager probes tell us a lot of Titania. We know that the moon has two main layers, and icy crust/mantle and a rocky core. The surface is heavily cratered, though not as much as Oberon which also tells us that it was more active in the past than Oberon. Titania was named after the Queen of the Fairies in William Shakespeare's A Midsummer's Night Dream.

Oberon

Image Credit:

NOAA

Oberon has the largest mass, and is second to Titania in diameter. It is also the farthest of the major moons. Discovered at the same time as Titania, Oberon was named after the King of the Fairies in A Midsummer's Night Dream. It is the most heavily cratered moon orbiting Uranus and much like Titania, it has a icy crust/mantle with a rocky core. With an inclination of only 0.058°, it is almost completely aligned with Uranus' equator.

Umbriel

Image Credit:

NOAA

Umbriel is the third largest in mass and diameter of the moons, but was not discovered until 1851 by William Lassell. Like Titania and Oberon, it is an icy and rocky body with an inclination of 0.128°. It is the second most cratered body but went under internal geologic processes in its past to give it the surface it has now. It was named after a character in Alexander Pope's The Rape of the Lock.

Ariel

Image Credit:

NOAA

Ariel is the fourth largest moon and second closest. It was discovered with Umbriel by William Lassell. Again, it is an icy and rocky body that formed in the accretion disk around Uranus based on its inclination of 0.260°. It is also tidally locked in the same way the Moon is tidally locked to Earth. Its name comes from two sources: Pope's The Rape of the Lock and Shakespeare's The Tempest.

Miranda
Image Credit:
NOAA
The last and smallest major moon is Miranda, which is also the closest to Uranus. It is also tidally locked to Uranus and was the last one discovered. Gerard Kuiper (after whom the Kuiper Belt is named - we'll discuss the Kuiper Belt later) found Miranda in 1948. Voyager imaged Miranda as it passed by the Uranus system and discovered that it is the most geologically active because it is the closest and tidal forces keep the interior warm enough. It has the highest inclination at 4.232° and was named after a character in Shakespeare's The Tempest.

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.

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 Moons of Jupiter

"Jupiter family" by NASA/JPL - PIA01481This image or video was catalogued by Jet Propulsion Laboratory of the United States National Aeronautics and Space Administration (NASA) under Photo ID: PIA01481. Licensed under Public domain via Wikimedia Commons

 

Jupiter has 67 known natural satellites, with 51 of them having diameters of 20 km or less. Many of these moons are probably captured asteroids that got caught in Jupiter's massive gravitational field. Most of these moons have only been discovered since 1975 with improvements in telescopes and the Pioneer and Voyager missions.

Jupiter's moons are generally divided into groups based on proximity to Jupiter, composition of the moon, and other orbital characteristics.

These groups are:

• The Inner Group: as the name implies, these are the inner most moons of Jupiter. There are four of these moons are they are all less than 200 km in diameter with semi-major axes less than 200,000 km from Jupiter. They have generally low eccentricities and have inclinations close to 0°, i.e. their orbits are almost directly above the equator of Jupiter.
• The Galilean moons: These are probably the most well-known of all of Jupiter's moons. These are the four moons discovered by Galileo in 1609 or 1610 after Galileo pointed his telescope towards Jupiter. He noticed these small objects moving along with Jupiter as Jupiter went around the Sun. These four moons will be discussed in more detail later on.
• Themisto is a single moon that does not belong to any group. It is farther away from Jupiter than the Galilean moons but closer to Jupiter than the next group, the Himalia group. Themisto is located approximately halfway between Callisto and the innermost Himalia moon, Leda. It has a semi-major axis of 7.39 million km, an eccentricity of 0.2006, and an inclination to Jupiter's rotation of 47.48°.
• The Himalia group: This group is named after the largest member, Himalia. There are five known satellites in this group with orbits ranging from 11.15 million km to 11.75 million km. They orbit at an inclination of 26.6° to 28.3° and have eccentricities of 0.11 to 0.25. These moons have compositions similar to C-type asteroids, which lead astronomers to believe that this group is made up of a captured asteroid that was ripped apart by tidal forces from Jupiter's gravity. Any moons that are found in this group will have a name ending with "-a".
• Carpo is another group made up of a single moon. It has an orbit at the inner edge of the next group, the Ananke group, but its orbital parameters are different. It has an inclination of 55°, a semi-major axis of 17.15 million km, and an eccentricity of 0.4316.
• The Ananke group: This group is named after the largest satellite in the group, Ananke. Theses asteroids range in eccentricities from 0.02 to 0.28 (much less than Carpo), semi-major axes from 19.3 million km to 22.7 million km, and inclinations from 145.7° to 154.8°. From the inclination, and if you remember from the post about a day on Venus, the inclinations of these satellites tell you that these satellites orbit retrograde. Looking down on Jupiter from the north, Jupiter rotates counter-clockwise (or for you Europeans, anti-clockwise), but these satellites orbit clockwise around Jupiter. Any satellites found in this group will have a name ending in "-e".
  
• The Carme group: Named after the largest satellite, Carme. They have semi-major axes of 22.9 million km to 24.1 million km and eccentricities between 0.23 to 0.27. The inclinations range from 164.9° to 165.5°, which means these also orbit retrograde around Jupiter. It is believed that these moons formed after a D-type asteroid was captured by Jupiter and broke up. Like the Ananke group, any future Carme group moons will end in an "e". Two exceptions to these moons are Kalyke is much redder than the other asteroids, and Taygete has a much higher eccentricity (e=0.3678).
• The Pasiphae group: These satellites share similar orbital distances but at a slightly lower inclination (144.5° to 158.3°). The largest moon in this group is Pasiphae.

The Satellites of Mars

Mars has two satellites, Phobos and Deimos. They are named after creatures summoned by Ares (the Greek equivalent of the Roman war god Mars) in the Iliad and their names mean Fear (Phobos - where the word Phobia comes from) and Fright (Deimos). Unlike the Moon, these satellites did not form in the same location in the solar nebula as Mars but rather formed elsewhere (likely, the asteroid belt between Mars and Jupiter) and wandered too close to Mars and were captured by its gravity.

AreaVoices.com

 

Phobos is an 11-km diameter, irregularly shaped object that only orbits 6000 km from the surface of Mars. Compare that to the Moon, which orbits 384,400 km from the surface of the Earth. If the Moon orbited only 6000 km from the Earth, not only would our tides be much higher (see post on tides) but the Moon would appear 64 times bigger in the sky making it about 32° across. At that apparent diameter, it would fill up a sixth of the sky!  Because Phobos is so close to Mars, it only takes about seven and a half hours to orbit Mars, which means that it crosses the Martian sky twice in one Martian day, taking only about four and a quarter hours to cross the sky.  It also orbits retrograde around Mars, meaning that it rises in the west and sets in the east.

 

Deimos is smaller than Phobos, being only 6.2 km in diameter, but orbits much farther away, at 23,500 km from the Martian surface.  At that distance, Deimos takes about 30.3 hours to complete one orbit around Mars, or about a Martian day and a quarter.

 

Both Phobos and Deimos were discovered in August of 1877 by Asaph Hall at the United States Naval Observatory in Washington, D.C. (Quick fact - the USNO is home to the official Master Clock for the US and is also the official residence of the Vice President.)  Despite being smaller, Deimos was actually discovered first on August 12th and Phobos was discovered on August 18th.  The names were suggested by Henry Madan from the Iliad.

 

The reason why Phobos and Deimos are believed to be captured asteroids is because they are similar in composition, albedo, densities of C- or D-type asteroids.

• C-type asteroids (carbonaceous asteroids)
• The most common type of asteroids (make up about 75% of all asteroids
• They have a low albedo which means they do not reflect a lot of light, almost appearing black
• Their compositions are similar to the early solar nebula except for the lack of volatile elements (gases, water, etc) but do contain hydrated minerals (water-containing minerals)
• D-type asteroids
• They have a lower albedo than C-type asteroids
• Their spectra are the strongest toward the red end of the electromagnetic spectrum
• They contain organic, carbon, and anhydrous (lacking-water) silicates
• However, they may have water ice cores