I have simple questions about neptunian system

Question

I have two simple questions about Neptunian system.

I understood the text just iterally, but I can't grasp the mechanism of them. Could you explain them?

Answer 1

The sections of your book are brief and I understand your confusion.

The most important aspect is the difference between the 3 kinds of moons. Impact moons, Moons that formed with the planet at the time of formation and captured moons.

Impact moons (like our moon, or Pluto's Charon) can be quite large compared to the planet. Charon at 1/11th Pluto's mass and our moon at 1/81st the Earth's mass are the 2 largest ratios of satellite to central object in our solar-system (ignoring small binary meteor/asteroid/comet systems).

Gas giant planets are unlikely to have impact moons because of their gaseous atmosphere. It's much less likely to blow debris off a gaseous atmosphere in such a way as it forms into a new moon.

Moons that form with the planet (Formation moons) - I'm not sure that's a term or not, but I like the term. Many of the Moons in our solar system likely formed with the planet. Jupiter's Galilean Moons, Many of Saturn's and Uranus' Moons.

The nature of a spiraling disk of matter is that it all doesn't end up in the center, but a percentage of the material, due to angular momentum ends up in far smaller, separate units orbiting around the center-mass. Explaining why this happens is very mathematical and above my ability, but all coalescing bodies, whether stars or planets, have an angular momentum. (see fun video here). Not all the material gets caught in the center of mass, but some form into orbiting satellites. The ratio of mass between planet and moon of formation tends to be very planet dominant, hundreds or thousands to one. "Formation" moons need to orbit in the same direction that the planet rotates, though, theoretically, a giant impact could reverse a planet's rotation, so there may be rare cases where the planet rotates in the opposite direction, but in that case, the Moons would still orbit in the "right" way. Wrong way moons are rare and are thought to only be possible via capture (or giant impact), not formation.

Captured Moons

It's difficult for a planet to capture a moon. It might seem easy that a massive body would capture moons left and right but the mathematics of gravity doesn't work that way. As an object falls towards a massive body like a planet it gains velocity and the gained velocity helps it accelerate away from the planet. In a simple 2 body Newtonian system, capturing a body into orbit isn't possible. The object is either already in orbit, or it can't be captured because there's no force slowing it down. A capture is only possible if there's 3 bodies and 2 of the orbiting bodies interact, making the capture possible, or if there's some kind of atmospheric drag or orbital debris that slows down the new object as it passes by the planet. There's also, I suppose, relativistic effects that make capture possible, but lets not go there. None of that is really important. What is important is that capture of an asteroid is very difficult and it happens rarely and when capture happens, it's usually with a very eccentric orbit.

Triton, because it's a wrong-way moon, capture is the most likely scenario because a formation moon can't be a wrong-way moon and an impact moon, like a formation moon, also needs to orbit in the direction that the planet rotates.

If Triton was captured (most likely scenario), it's relatively high mass to most moons in the solar system would make it dominant and as it's highly eccentric orbit was circularized over time due to tidal forces with Neptune. It now has a nearly circular orbit, but it's wrong way orbit and it's eccentric to circular orbital change over time, wrecked havoc on the other moons of Neptune and cast out a lot of the. It's the reason Neptune has comparatively few moons.

In any system with multiple orbitals around a central body, there's greater stability if all the larger orbitals move in the same direction and in roughly the same plane and if they are mostly circular. A large body (Triton is very large for a moon), orbiting in the wrong direction with presumably a very elliptical orbit would cause chaos among the Moons around the planet.

Another property of a highly elliptical orbit is strong tidal variation that stretch and squeeze the moon, heating it's insides. Triton no longer has those tidal forces because it's orbit is nearly circular, but when it was captured and it had to have a very elliptic orbit, it would have experienced tidal heating and volcanic eruptions as a result. Over time as it's orbit circularized this tidal heating decreased to the point now where it's very low.

It's worth pointing out that it might be difficult to tell if a moon is captured or was there at formation if it orbits in the expected direction due to orbits circularizing over time. Triton's wrong way motion is strong evidence that it was captured. Mars' 2 tiny moons are thought to be captured and several of the smaller gas giant moons were probably captured.

That's the basic explanation anyway. Hope that helps & corrections are welcome if I made any errors. I'll try to add some links later to make this answer a bit better and clear up some of the wordiness.