Moons

[KvickFlygarn87]

How do moons get captured? I've been eluded by this for a long time, and know I'm asking you.

Because you're travelling at escape velocity when you enter a gravity well, the moon should merely have escaped.

How did it reach orbit? One vague theory of mine (which is highly unplausible) is a

that the moon got bombarded by asteroids, and they slowed it down sufficiently for it to orbit.

But then again, no. This is not likely to happen at all.

So what happened?

[Nikolai]

How do moons get captured?

If the only gravitational bodies under consideration are the planet and the potential moon, it can't. However, planets can already have existing moons -- or, in the case of asteroidal moons, it's not unreasonable that a potential moon might have a companion. Or, if it's close enough, even the Sun might play a role. The interaction of more than two bodies can allow a moon to be captured.

Note, too, that capture is not the only way for planets to have moons.

[PakledHostage]

There was a related discussion about this a couple of weeks ago. Capture of any object requires interaction with (at a minimum) a third celestial object. The captured object's orbit may not be stable, however.

A gravitational assist from an existing moon is one mechanism for capture. Another is passing through a Lagrange point at low energy. This is how J002E3, suspected to be the S-IVB stage from Apollo 12, was recaptured from solar orbit. Yet another mechanism is two objects orbiting each other passing close to a third much more massive object. If the trajectories are just right, one of the pair of objects may be captured while the second of the pair is flung away at higher speed. This is the mechanism that is believed to give hypervelocity planets and stars their incredible speeds.

[Faark]

Tidal forces can be another way to transfer energy from one body to another. Its surely not enough to capture a fast body, but might help to stabilize it over time.

[Anton P. Nym]

Then there's always the currently-accepted theory of how Earth captured the Moon; lithobraking.

-- Steve

[jwenting]

hmm, thought the currently accepted theory is the moon being ejecta from an impact on the earth of a Mars sized planet...

[Karriz]

Triton for example is believed to be a captured Kuiper belt object. I'm not sure if other moons of Neptune have enough gravity to help in the capture, but that's probably the only possible explanation.

The way how this works confused me as well, but it indeed works if there are more than two objects involved.

[Anton P. Nym]

hmm, thought the currently accepted theory is the moon being ejecta from an impact on the earth of a Mars sized planet...

Well, the ejecta came from both bodies... so I consider that a variety of Extreme Lithobraking.

-- Steve

[mdatspace]

Usually, the larger moons around a gas giant form from accretion of the material left behind after the gas giant forms. That is one way they can form too.

[peadar1987]

On that subject, I wonder are there any moons in the solar system that could have been aerocaptured, some of the smaller Jovian or Saturnian ones perhaps

[Nibb31]

Interesting article here:

http://www.nature.com/news/planetary-science-lunar-conspiracies-1.14270

[KvickFlygarn87]

Thank you for all the answers. The core of the question was however, how was our own moon captured.

Also, aerobrake is no alternative as you would have to raise periapsis after capture.

[Ralathon]

Thank you for all the answers. The core of the question was however, how was our own moon captured.

Also, aerobrake is no alternative as you would have to raise periapsis after capture.

Our own moon wasn't so much captured as it was ripped from our mantle.

The best theory we have for the formation of our own moon is that we got hit by a mars sized object at some point. It hit us at a shallow angle so it scooped up a lot of our mantle and put it into orbit. There it formed a saturn style ring that merged into our moon after a few million years. That's why the rocks that Apollo brought back had pretty much the same chemical makeup as the rocks we find on earth.

After the moon got made it was really close to earth. it must've been pretty impressive, with the moon looking huge and volcanoes erupting as it rises (due to tidal effects). But the earth rotated faster than the moon orbited, back in those days a day only lasted about 10 hours. So over time tidal effects slowed down the rotation of the earth while boosting the orbit of the moon. So over time the moon has slowly drifted away from us. It's still moving away at a rate of about 4 cm a year and slowing the earth's day accordingly. IIRC the moon will escape the earth in about 2 billion years.

So the moon essentially got captured by lithobraking. A truly kerbal way to make a moon.

[brdavis]

So the moon essentially got captured by lithobraking. A truly kerbal way to make a moon.

Agreed

There is, sort of, an aerobraking way of capturing a moon too (this is a mechanism for the outer moons of Jupiter and Saturn, for instance). A passing object is captured into orbit via gas drag from the pro to-Jupiter nebula, and then the remainder of the nebula gets blown away as the Sun "turns on". The result is that extremely distance moons are often as much prograde as retrograde.

The inner gas giant moons (Io, Europa, Titan, Dione, etc.) likely formed in situ via accretion in mini-nebulas that a gas giant planet could form.

You can have moons captured via likely traumatic three-body interactions (up to and including lithobraking with a pre-existing moon). Triton around Neptune is a likely candidate (which would also explain why Neptune doesn't have much of a "regular" moon system; it was likely disrupted by the capture of an object as large as Triton into a retrograde orbit)

You can have moons generated via the "giant impact" mechanism (Earth and Pluto would both fall into that category, as likely would a whole lot of asteroids with moon lets).

You can have planets that tidal effects would have removed the moons - a moon of Venus or Mercury above a certain (rather small) size wouldn't exist until the current epoch, because it would tidally lock to the planet below, and then solar tides would actually start moving it down towards the planet, resulting in the once-moon either deorbiting or reaching escape from the smaller SOI of these inner worlds by the current time.

And then you can have things like Phobos & Deimos around Mars, which… well, umm… they look like captured asteroids. So we can try that. Even if the dynamics really doesn't work out. So, yeah... jury is largely still out on those.

[mdatspace]

snip

I think titan is a special case. Why? Jupiter has a large moon system that easily fits co-accretion. They have similar sizes and masses and a clear trend in density. But titan is a massive moon with no other saturnian moon to match it.

http://www.space.com/11604-saturn-moon-titan-impacts-atmosphere.html

http://www.natureworldnews.com/articles/3714/20130829/new-analysis-titan-challenges-current-theories-regarding-saturns-largest-moon.htm

I shall stop nitpicking. Triton did completely wreck Neptune's pre-capture moon system, and you are right. Even without such disruption, moon systems can be a mess. Look at Uranus(don't joke)! Most of its moons will collide with each other and most of the irregular inner moons are spiraling inwards.

Also, there is another way small moons can form: Collisional disruption of a parent body. There is a reason there are moon families. This does not apply frequently to large moons, but most retrograde moons orbiting the gas giants may be the result of collisional disruption. Moon gets captured, moon crosses one moon2's orbits, moon fails as soft landing and gets blown to bits on similar orbits.

Edited December 12, 2013 by mdatspace