So – You’re Still Interested in Astronomy?
Last time, we looked at the incredible moons of Uranus. As promised, this time around we’ll take a look at the moons of Neptune and see how strange things can get the further out from the Sun you go.
1. So – which particular places are we looking at today?
Today we’ll take a look at all 14 of the moons belonging to the last planet out from the Sun, Neptune. As you will see, they are all named after minor Greek water deities, and they come in two distinct groups, cunningly labeled – Regular and Irregular moons.
2. The Regular moons
In order of distance from Neptune, the regular moons are:
Naiad, Thalassa, Despina, Galatea, Larissa, S/2004 N 1, and Proteus.
3. Are there any interesting facts about these moons we need to know?
Not really. It would appear that most of the regular inner moons lie within what is called the synchronous orbit radius. Basically, their altitudes will gradually decay due to tidal deceleration (whereupon they will impact the surface) – or they will break up and add their essence to the already existing rings.
At 420km (260 miles) in diameter, Proteus is the second largest moon of Neptune. It is about as large as a body of its density can be without being pulled into a spherical shape by gravity.
5. Is there anything else about Proteus we might like to know?
There certainly is. Proteus is heavily cratered and shows signs of geological modification. The largest crater – Pharos – is 260km (161 miles) in diameter and is up to 15km (9 miles) Deep. Despite its small size, Pharos is littered in large impacts sites, averaging 50 – 100 km (31 – 62 miles) in diameter.
Proteus also features a number of other landform features, most noticeably – scarps, valleys, and grooves. Lots of detail for such a little moon.
6. The Irregular Moons
In order of distance from Neptune, the irregular moons are:
Triton, Nereid, Halimede, Sao, Laomedeia, Neso, and Psamathe.
This group of moons includes both prograde and retrograde bodies, and as was the case with the regular satellites, there isn’t much to say about these moons apart from Nereid and Triton
Nereid is the 3rd largest moon of Neptune and orbits in the same direction as the planet’s rotation – prograde.
It is thought that Nereid was once a regular moon that might have been knocked out of orbit when Triton arrived. Either that or it is a captured asteroid or Kuiper Belt object.
NOTE: The Kuiper belt is a region of the solar system beyond the planets, extending from the orbit of Neptune (At 30AU) to at least a distance of 50AU, outward from the Sun.
At 2,700 km (1,700 miles) in diameter, Triton is the largest Neptunian moon and the 7th largest in the Solar System. She was discovered on October 10th, 1846 by English astronomer William Lassell.
Triton possesses a tenuous Nitrogen atmosphere that contains trace elements of carbon monoxide and methane. The surface is comprised mostly of frozen nitrogen (55%), with the rest a complex blend of water ice and frozen carbon dioxide.
With a surface temperature of 35.6K (-273.6˚C) she has one of the coldest surfaces in the solar system. Despite that fact she is geologically active, and the entire surface is cut by complex valleys and ridges as a result of tectonics and ice volcanism.
9. What else do we know about Triton?
Because of the nature of its orbit, it is thought Triton might have been captured from the Kuiper Belt. (As moons in retrograde orbit cannot form in the same region of the solar nebula as the planet they orbit).
Fact: It is the only large moon within the solar system that has a retrograde orbit. (It travels in the opposite direction to Neptune’s rotation)
10. Any other interesting facts?
Most certainly. The distribution of mass amongst the Neptunian moons is the most lopsided of all the family groups in the Solar System. Triton possesses 99.67% of the total mass. 99.67%! The remaining fraction is made up of all the other moons combined.
A crazy ratio, until you remember this might be due entirely to Triton’s capture.
She is a big moon, and so, her arrival probably caused the already existing satellites to collide and shatter, and/or get reduced into dust to become disc material.
Fact: Triton is larger than the dwarf planets Pluto and Eris
But that’s what makes our small Solar System so fascinating.
So there we go. That’s our brief look at the chilly moons of Neptune.
Next time, we’ll move on to take a closer peek at the moons of the Dwarf Planets…starting with a former ‘planet’. Pluto.
I can’t wait to see you then…