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Top Ten Largest Moons

This is the newest series to come to I Read Encyclopedias for Fun, the weekly Top Ten. I will be publishing top ten lists of various topics scientific, geographic, and anything related to books, TV shows, and movies. They are not merely lists, however. They also include some interesting information. Some of the lists will be purely factual, while others will be my opinion. So, let’s get started.

Top Ten Largest Moons

10. Oberon

Voyager_2_picture_of_OberonWith a mean radius of 761.4 km, Uranus’ moon Oberon is the tenth largest moon in the Solar System. It has a mass of (3.014±0.075)×1021 kg , and a mean density of 1.63±0.05 g/cm³. It’s the second largest moon of Uranus, and was discovered by William Herschel in 1787. It’s estimated to be about 50% water ice, and differentiated into a rocky core and icy mantle. The surface is slightly reddish and is covered with craters and chasmata. It has only been visited once, by Voyager 2, which mapped 40% of its surface.

9. Rhea

PIA07763_Rhea_full_globe5With a mean radius of 763.8 km, Rhea is the ninth largest satellite in the Solar System. It has a mass of (2.306518±0.000353)×1021 kg, and a mean density of 1.236±0.005 g/cm³. It’s the second largest moon of Saturn, and was discovered by Giovanni Domenico Cassini in 1672. Due to its low density, it’s estimated to be about 75% water ice, and only about 25% rock. It appears it has a homogeneous interior, meaning there is no core. It is heavily cratered and has only a few chasmata or fractures on the trailing hemisphere. It’s been extensively photographed and mapped by the Cassini orbiter currently at Saturn.

8. Titania

Titania_(moon)_color_croppedWith a mean radius of 788.4±0.6 km, Titania is the eighth largest moon in the Solar System, and Uranus’ largest. It has a mass of (3.527±0.09)×1021 kg, and a mean density of 1.711±0.005 g/cm³. It was discovered by William Herschel in 1787. It’s estimated to be around 50% water ice, and may have a rocky core. The surface is cratered fairly heavily, but not as much as other large Uranian moons, meaning it has a younger surface. It features several large chasmata and scarps, pointing towards tectonic activities in its past. It may have a tenuous carbon dioxide atmosphere. About 40% of the surface has been photographed by Voyager 2.

7. Triton

480px-Triton_moon_mosaic_Voyager_2_(large)With a mean radius of 1353.4±0.9 km, Triton is the seventh largest moon in the Solar System, and Neptune’s largest. It has a mass of 2.14×1022 kg, and a mean density of 2.061 g/cm3. it was discovered by William Lassell in 1846. Triton is unique in that it’s the only large moon that orbits its planet in a retrograde direction. Because of this, the tidal forces from Neptune will cause its orbit to degrade and crash into the planet or create a new ring system in about 3.6 billion years. Triton is also very active geologically. It has several nitrogen geysers that have been observed to be erupting. The surface is relatively young, having been resurfaced in the past, leaving it smoother and more reflective. It’s likely to be differentiated, having a core, mantle, and crust. It’s guessed it may have a liquid water ocean beneath the surface. The core likely generates heat to help keep the water liquid. The atmosphere is a thin nitrogen atmosphere, and clouds have been observed. It’s theorised that Triton is a captured moon, likely a Kuiper Belt object.

6. Europa

Europa-moonWith a mean radius of 1560.8±0.5 km, Europa is the sixth largest moon, and the fourth largest of Jupiter’s Galilean satellites. It has a mass of (4.799844±0.000013)×1022 kg and a mean density of 3.013±0.005 g/cm3. It was discovered by Galileo Galilei in 1610. Europa is one of the most exciting satellites in the Solar System for one major reason, it has a large subsurface ocean of liquid water. It’s so large that it has more water than Earth. Because of tidal heating, it is quite possible that it has hydrothermal vents, which are teeming with life in Earth’s oceans. The surface is incredibly smooth and made of water ice. It has features similar to the Arctic polar ice cap and very few craters. The surface is young and constantly being resurfaced. Beneath about 10-30 km of ice, there’s a thick ocean of liquid water, around 100 km thick. And then beneath that is a rocky interior and a metallic core. One of the most prominent features is the lineae, or the extensive lines that cover the surface. These are cracks where fresh water may be coming to the surface, and plates of icy crust move against each other.

5. Moon

LRO_WAC_Nearside_MosaicWith a mean radius of 1737.1 km, the Earth’s Moon is the fifth largest moon in the Solar System. Its mass is 7.342×1022 kg and mean density is 3.344 g/cm3. It’s the second densest moon in the Solar System. It’s the most familiar moon, as we can see it very easily from Earth. Its discovery is prehistoric, since we’ve always known it’s there. The surface is heavily cratered, except in the Maria, which are basins flooded with lava. There are also volcanoes, some of which are relatively young. This suggests a warmer interior than previously thought. The interior is differentiated, with distinct crust, mantle, and core. The surface is dominated by silica, alumina, lime, and iron oxide. It’s tidally locked, so we only see one side of the Moon. The far side is very different, lacking the maria we see on the near side. It’s also the only moon that has been landed on by humans, and one of only two moons that has been landed on by robotic probes.

4. Io

Io_highest_resolution_true_colorWith a mean radius of 1821.6±0.5 km, Io is the fourth largest satellite, and the third largest of Jupiter’s Galilean satellites. It has a mass of (8.931938±0.000018)×1022 kg and a mean density of 3.528±0.006 g/cm3. It is the most dense natural satellite in the solar system, and is also the most geologically active. It has more than 400 active volcanoes, which makes it the most active object in the Solar System, more than Earth. It was discovered by Galileo Galilei in 1610. When Io was first viewed up close, it was a surprise. The surface is a very colourful mixture of red, orange, yellow, white, and black. Several volcanoes can be erupting at the same time, and lava lakes have been observed. The surface composition is mostly silicates, sulfur, and sulfur dioxide. Due to tidal interactions with Jupiter, the interior is constantly being flexed. With an internal structure more similar to terrestrial planets than other moons, it has almost no water ice. It has a crust, mantle, and hot core.

3. Callisto

CallistoWith a mean radius of 2410.3±1.5 km, we’re moving into the top three, and the true heavyweights of the Solar System’s moons. It’s the third largest in the Solar System, and the second largest of Jupiter’s moons. It has a mass of (1.075938±0.000137)×1023 kg and a mean density of 1.8344±0.0034 g/cm3. With a relatively low density, it is about half ice and half rock. It was discovered by Galileo Galilei in 1610. The outermost Galilean satellite, it has far less tidal influence from Jupiter, and therefore has an old, relatively inactive surface. It is covered by craters, and is considered the oldest surface in the Solar System. Unlike other large moons, it’s not very differentiated, and is mostly a mixture of rock and ice covered by a possible liquid water ocean and an icy crust. It has a thin carbon dioxide atmosphere.

2. Titan

Titan_in_true_colorWith a mean radius of 2575.5±2.0 km, Titan is the second largest moon in the Solar System, and Saturn’s largest. It’s even larger than Mercury, but has a smaller mass. It has a mass of (1.3452±0.0002)×1023 kg and a mean density of 1.8798±0.0044 g/cm3. It was discovered by Christiaan Huygens in 1655. It is the only satellite outside the Earth-Moon system that has been landed on by a space probe, the Huygens lander. It has a very thick atmosphere, thicker than Earth’s and with a 45% higher air pressure at the surface than Earth’s. The composition is mostly nitrogen with some methane and hydrogen. What makes Titan incredibly interesting is the surface. It is very Earth-like, with mountains, rivers, and lakes. However, the lakes are not water, but liquid hydrocarbons, mainly ethane and methane. There are some craters, but the surface appears to be quite young, with possible cryovolcanism. Basically, the surface is like Earth’s but ice replaces rock and hydrocarbons replace water. It’s a differentiated body with a silicate core, an ice layer, then a possible liquid water ocean, and the ice crust.

1. Ganymede

Ganymede_g1_true-edit1With a mean radius of 2634.1±0.3 km, Ganymede is the largest moon in the Solar System, and Jupiter’s largest. It has a mass of 1.4819×1023 kg and a mean density of 1.936 g/cm3. It was discovered by Galileo Galilei in 1610. The surface is quite complex with older cratered regions and a younger grooved terrain. It’s mainly water ice with some carbon dioxide and sulfur dioxide. Some salts hint at a salty ocean beneath the surface. It’s fully differentiated with a hot iron-nickel core and silicate mantle. It’s believed there’s a thick liquid water ocean between two layers of ice. There’s an extremely tenuous oxygen atmosphere. Ganymede has its own magnetic field, and has even been observed to have aurorae. This also helps strengthen the theory that there’s a salty ocean. Tidal stresses from Jupiter aid in heating the interior, which makes Ganymede another possible candidate for life.

If you have any questions about these or any other moons of the Solar System, I’ll do my best to answer them. As this was one of my biggest interests in university, I’ve been paying attention to news related to the Solar System and its moons. Ask your questions or leave comments in the comments section below.

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Astronomy Quick Facts – Arche

Arche is a very small moon of Jupiter’s, and is a member of the Carme group. It’s also known as Jupiter XLIII. It was discovered by a team of astronomers led by Scott S. Sheppard at the University of Hawaii in 2002. Very little is known about it.

Arche

Data

  • Mean radius: 1.5 km
  • Mass: Unknown
  • Density: Unknown
  • Surface gravity: Unknown
  • Albedo: Unknown
  • Temperature: Unknown
  • Mean orbital radius: 23,717,000 km (semi-major axis)
  • Orbital period: 746.185 d (retrograde)
  • Inclination: 162° to Saturn’s equator
  • Eccentricity: 0.149

Name Origin

Arche was the fourth of the muses of origin. She was one of the five Boeotian muses. She is one of the daughters of Zeus and Mnemosyne.

5 Interesting Facts

1. Arche is a member of the Carme group, which orbits retrograde and at a highly inclined and eccentric orbit. Probably a captured asteroid.

2. Its original temporary designation was S/2002 J 1.

3. It’s pronounced ar-kee.

4. Sheppard has been involved in the discovery of 75 moons.

5. There’s very little known about Arche. I can’t do five interesting facts.

With such a highly inclined and eccentric orbit, and such a small size, it’s unlikely we’ll see any kind of exploration of this moon any time soon.

Astronomy Quick Facts – Aoede

Aoede is a very small moon of Jupiter’s, and is a member of the Pasiphae group. It’s also known as Jupiter XLI. It was discovered by a team of astronomers led by Scott S. Sheppard at the University of Hawaii in 2003. Very little is known about it.

Data

  • Mean radius: 2 km
  • Mass: Unknown
  • Density: Unknown
  • Surface gravity: Unknown
  • Albedo: Unknown
  • Temperature: Unknown
  • Mean orbital radius: 23,980,000 km (semi-major axis)
  • Orbital period: 761.5 d (retrograde)
  • Inclination: 162° to Saturn’s equator
  • Eccentricity: 0.4311

Name Origin

Aoede is one of the original three muses. Her father is Zeus and mother is Mnemosyne.  She was the muse of voice and song. Her sisters are Melete and Mneme.

5 Interesting Facts

1. Aoede is a member of the Pasiphae group, which orbits retrograde and at a highly inclined and eccentric orbit. Probably a captured asteroid.

2. Its original temporary designation was S/2003 J 7.

3. It’s pronounced ay-ee-dee.

4. Sheppard has been involved in the discovery of 75 moons. Okay, so this isn’t about Aoede.

5. Really, there’s very little known about Aoede. I can’t do five interesting facts.

With such a highly inclined and eccentric orbit, and such a small size, it’s unlikely we’ll see any kind of exploration of this moon any time soon.

Astronomy Quick Facts – Anthe

Anthe is a very small moon of Saturn, and a member of the Alkyonides. It’s also known as Saturn XLIX. It was discovered by the Cassini Imaging Team on May 30, 2007. Very little is known about it.

AthneData

  • Mean radius: 1 km (estimated)
  • Mass: 5×1012 kg
  • Density: Unknown
  • Surface gravity: Unknown
  • Albedo: Unknown
  • Temperature: Unknown
  • Mean orbital radius: 197,700 km (semi-major axis)
  • Orbital period: 1.03650 d
  • Inclination: 0.1° to Saturn’s equator
  • Eccentricity: 0.001

Name Origin

Anthe is one of the Alkyonides. She was one of the seven daughters of Alcyoneus, who was killed by Heracles. When he died, they threw themselves into the sea and were changed into halcyons by Amphitrite.

5 Interesting Facts

1. It orbits between Mimas and Enceladus.

2. Its 10:11 mean-longitude resonance with Mimas causes its semi-major axis to vary by 20 km every 2 years.

3. The other Alkyonides, Methone and Pallene are in similar orbits, and they may have a similar origin, possibly a larger moon broken apart by a large impact.

4. The Anthe Ring Arc may be caused by micrometeoroid impacts on Anthe. This partial ring appears in the vicinity of Anthe.

5. It was the sixtieth confirmed moon of Saturn.

Not much is known about this moon, but there are several images available. Here’s another one with the ring arc.

Anthe-ring

Astronomy Quick Facts – Ananke

Ananke is a small retrograde moon of Jupiter, and is a member of the Ananke group of moons. This group consists of retrograde irregular moons that orbit at a high inclination. Ananke’s also known as Jupiter XII. It was discovered in 1951 by Seth Barnes Nicholson at Mount Wilson Observatory.

Ananké
Image copyright OHP/CNRS/IMCCE. Used under Creative Commons license from Wikimedia.

Data

  • Mean radius: 14 km
  • Mass: 3.0×1016 kg
  • Density: 2.6 g/cm3 (estimated)
  • Surface gravity: 0.010 m/s2 (0.001 g)
  • Albedo: 0.04 (estimated)
  • Temperature: 124 K (-149°C)
  • Mean orbital radius: 21,280,000 km
  • Orbital period: 610.45 d
  • Inclination: 149.9° (to Jupiter’s equator)
  • Eccentricity: 0.24

Name Origin

Ananke was the personification of destiny, necessity, and fate. She is the mother of the Fates, whose father is Zeus. She’s a rather powerful figure in Greek mythology, being in total control of fate.

5 Interesting Facts

1. Ananke got its name in 1975. However, it was known as Adrastea from 1955 to 1975, which is the name of another moon of Jupiter.

2. Ananke is the largest member of the Ananke group, which may have been formed from the breakup of a larger moon.

3. It appears to be light red in the visible spectrum.

4. The infrared spectrum indicates it possibly has water (ice).

5. With a surface area of approximately 2,500 km2, it’s about the same size as Luxembourg.

This is a rather small satellite, and Galileo wasn’t able to get a picture of it. It’s a very minor target anyway, and orbits quite far from Jupiter.

Encyclopedia Entry #10 – Amalthea

Amalthea is a moderate sized moon of Jupiter. Although it is relatively small, it’s the fifth largest natural satellite of Jupiter. It’s also the largest of the inner satellites of Jupiter. It was discovered in 1892, and is also known as Jupiter V. It’s tidally locked with Jupiter, as most moons are, especially those that are close to the planet.

Amalthea

Data

  • Dimensions: 250 × 146 × 128 km
  • Mass: (2.08±0.15)×1018 kg
  • Density: 0.857±0.099 g/cm³
  • Surface gravity: ≈ 0.020 m/s² (≈ 0.002 g)
  • Albedo: 0.090±0.005
  • Temperature: 120 K (-153°C)
  • Mean orbital radius: 181,365.84±0.02 km
  • Orbital period: 0.498 d (11 h, 57 min, 23 s)
  • Inclination: 0.374°±0.002° (to Jupiter’s equator)
  • Eccentricity: 0.00319±0.00004

Name Origin

Amalthea is named after the nymph from Greek mythology who nursed the infant Zeus. She is sometimes represented as a goat who suckled Zeus in a cave on Crete, or as a goat-tending nymph who fed him the milk of her goat.

5 Interesting Facts

1. Amalthea was discovered by Edward Emerson Barnard on September 9, 1892, at Lick Observatory.

2. The colour of Amalthea is red, which is thought to either come from Io or some other non-ice material.

3. The largest crater, Pan, is 100 km across, covering a huge portion of one side of the moon.

4. Due to the size of Amalthea, its gravity should have pulled it into a more spheroid shape. However, it’s thought that the moon is made of a more rigid material, allowing it to retain its highly irregular shape.

5. If you were to view Jupiter from the surface of Amalthea, it would be 46 degrees across, or about one quarter of the sky from horizon to horizon.

Due to the closeness of the satellite to Jupiter, and the high amount of radiation in that region, a clear image of Amalthea has never been taken. The image provided is from the Galileo probe, and is the clearest available.