Tag Archives: Neptune

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.


The Great Shrinking Red Spot

In the past, the planets weren’t really thought of as being dynamic.  It wasn’t until we had telescopes good enough and spacecraft traveling the solar system that we found how amazingly changeable the planets and moons really are.

Well, the Great Red Spot on Jupiter has been shrinking at a very fast rate lately.  It’s not really known why it’s happening, but it has shrunk by half in the past century.

You can see the changes in Jupiter's Great Red Spot in this sequence of pictures.
You can see the changes in Jupiter’s Great Red Spot in this sequence of pictures.

It’ll be another two years until Juno arrives at Jupiter to study its atmosphere.  I think there’ll be some intense observations of the Great Red Spot.  Read here for some more information.

But Jupiter isn’t the only planet with a spot.  Neptune’s Great Dark Spot has actually disappeared.

The Great Dark Spot on Neptune may be a hole in the clouds, not a storm.
The Great Dark Spot on Neptune may be a hole in the clouds, not a storm.

But never fear, the Great Dark Spot has appeared again in the northern hemisphere.  It seems that these spots disappear regularly and another appears elsewhere on the planet.  They are certainly not permanent.  And neither is the Great Red Spot on Jupiter.

My Favourite Planet

The Solar System has eight planets, if you don’t include Pluto.  I’ve sometimes thought about what my favourite planet is.  Sometimes, I thought Jupiter, because of its moons.  Sometimes, I chose Saturn, because it’s the current giant planet being explored with its remarkable moon Titan.  Other times, I thought about Mars, as it’s the most explored planet in the Solar System other than the Earth, and is also our next manned destination.

Beautiful blue Neptune and its Great Dark Spot. Photo is courtesy of NASA.

But you know what?  I like Neptune.  It’s the most distant planet, and it’s an ice giant with a beautiful vibrant blue colour.  The winds are extremely fast, and as it is so far, it is probably the least understood planet.  Triton is fascinating, making me wonder if it’s anything like Pluto.  We’ll find out next year.

So, what’s your favourite planet?

A Wishlist for Solar System Exploration

Space exploration has been quite exciting in recent years.  Mercury, Venus, the Moon, Mars, and Saturn currently have probes in orbit or on the surface.  There are probes en route to Pluto, Ceres, Mars, and Jupiter.  There are planned probes to Venus, Mars, the Moon, asteroids,  and Jupiter’s moons.  There’s a lot going on.  However, I feel there could be more.

I have six missions in mind that I would like to see happen.  These are currently not planned, but some have been discussed in recent years.

240px-Venus_globeVenus Lander and Flyer

Venus currently has an orbiter, and has had several orbiters and landers in the past.  It has been completely mapped by radar, and the atmosphere is currently being studied.  Although there have been landings in the past by the Soviet space program, those landers succumbed to Venus’ incredibly high temperatures and volatile atmosphere.  I’d like to see a lander built to survive on the surface and provide us with several weeks of data.  I’d also like to see a flyer that can observe both the atmosphere and the landscape.  Maybe it can also see lightning.

Europa-moonEuropa Lander

Galileo studied Europa, and there’s currently a probe on its way to Jupiter to study the planet, as well as the Jupiter Icy Moons Explorer planned to launch in 2022, but nothing planned to land on Europa.  This is one of those places in our solar system that may have life.  It has a liquid ocean under its icy crust, and that ocean is covering a silicate mantle.  It’s quite possible that it could be geologically active with hot smokers like in Earth’s oceans, where microbial life may exist.  But we have to be careful not to contaminate the world if we drill through the ice into the ocean.

Titan_natural_colorTitan Lander

Saturn’s largest moon has already been visited by a lander, the Huygens probe, but it only lasted a short time.  Cassini has done a great job seeing through the clouds to study this remarkably Earth-like moon’s hydrocarbon lakes in the polar regions.  Titan needs a dedicated lander that can study the conditions for a much longer time.  Perhaps a flyer would be useful, as well.  I’d love to see some of Titan’s landscapes.

240px-Uranus2Uranus Orbiter

Uranus has only been visited once, by Voyager 2.  This is a fascinating planet that’s sitting on its side.  It has a ring system and plenty of very interesting moons, like Miranda, Titania, Ariel, Umbriel, and Oberon.  This is a system that needs to be studied up close by a dedicated orbiter.  We also need to understand more about the interior of the planet, as it’s most likely very ice.  It’s called an ice giant, not a gas giant.

244px-NeptuneNeptune Orbiter

The other ice giant, Neptune, has also been visited by Voyager 2.  This planet has a more dynamic-looking atmosphere, but most of the moons are quite small.  However, it has a unique moon, Triton, that is likely to be a captured Kuiper Belt object.  It’s also geologically active.  The most distant planet in the solar system needs an orbiter.

Eris_and_dysnomia2Eris Probe

The largest dwarf planet (larger than Pluto) should be explored in the future, as well.  It’s a lot farther away than Pluto, but it could be reachable by a probe within our lifetime, I hope.  Pluto will be explored next year, but I’d love to see what Eris is like, too.  What is a world so far away like?  It would be amazing to see it.

These are the missions I would love to see in my lifetime.  I’m anxious to know about these worlds.  What would you like to see?