Captured Moons, How the heck do planets capture moons> Options

[Chmee]

A question that has bugged me for a long time is how planets capture asteroids, etc into an orbit. My understanding of orbital dynamics is that a body approaching a planet would need to be "braked" in order to be captured into orbit. In the same manner that our space probes use their rockets to slow them down enough or they would shoot past.

So for moons like Triton, Deimos, and Phobos (as well as the small, distant moons of Jupiter/Saturn) how were they captured? What provided the braking?

[dvandorn]

Gravity. In specific, gravity from a third (and even fourth or fifth) bodies. Around Mars, I'd guess that Jupiter generated a gravitational resonance that braked Diemos and Phobos enough to be captured. Tidal influences can then work to circularize the orbits.

I find it fascinating that, according to one current theory, Uranus and Neptune may have formed *between* Jupiter and Saturn. I can just image the gravitational resonances that must have disturbed the entire Solar System when those two giants were flung out into the outer reaches...

-the other Doug

[helvick]

My understanding of orbital dynamics is that a body approaching a planet would need to be "braked" in order to be captured into orbit.

Things don't always need to be braked to be captured - as I understand it if the velocity relative to the target is less than the escape velocity at that point then it will be captured. The reason spacecraft need to be braked is because of the fast transfer orbits that need to be chosen to get them to their targets in a reasonable length of time\fuel efficient manner.

[tedstryk]

A question that has bugged me for a long time is how planets capture asteroids, etc into an orbit.  My understanding of orbital dynamics is that a body approaching a planet would need to be "braked" in order to be captured into orbit.  In the same manner that our space probes use their rockets to slow them down enough or they would shoot past.

So for moons like Triton, Deimos, and Phobos (as well as the small, distant moons of Jupiter/Saturn) how were they captured?  What provided the braking?

A lot would depend on relative velocity. If an object were in a similar orbit moving at a similar speed, not a lot of breaking would be needed. Atmospheric drag also can play in, but such objects probably would decay quickly. It is also possible, in the case of Triton, that it was a combination of a low relative velocity encounter with a collision with some primordial Neptunian moon.

[ljk4-1]

Gravity.  In specific, gravity from a third (and even fourth or fifth) bodies.  Around Mars, I'd guess that Jupiter generated a gravitational resonance that braked Diemos and Phobos enough to be captured.  Tidal influences can then work to circularize the orbits.

I find it fascinating that, according to one current theory, Uranus and Neptune may have formed *between* Jupiter and Saturn.  I can just image the gravitational resonances that must have disturbed the entire Solar System when those two giants were flung out into the outer reaches...

-the other Doug

Those moons are the leftovers from a Kardashev Type 2 Civilization astroconstruction project that had to be abandoned when the Galactic Empire switched ruling political parties.

Apparently they didn't count all the votes on Floridon 12 and Ohyox 3 until it was too late to reverse the decision.

[Bob Shaw]

Those moons are the leftovers from a Kardashev Type 2 Civilization astroconstruction project that had to be abandoned when the Galactic Empire switched ruling political parties.

Apparently they didn't count all the votes on Floridon 12 and Ohyox 3 until it was too late to reverse the decision.

LJK:

According to *my* issue of the Guide (42nd Edition, Megadodo Publishing House, Ursa Major Beta) it was actually because of a mix-up between Galactic Imperial Quarts and the exchange rate of the Trigellian Pu. The Management Consultants brought in afterwards made sure that poor old Slartibartfast got the blame for the whole affair, and then the mice naturally promoted him to deal with some project called Dirt, or Mud, or something like that. Compost, was it?

Obviously, no such mistake could happen here!

Bob Shaw

[dvandorn]

I wonder if it's friendly?

-the other Doug

[abalone]

A question that has bugged me for a long time is how planets capture asteroids, etc into an orbit.  My understanding of orbital dynamics is that a body approaching a planet would need to be "braked" in order to be captured into orbit.  In the same manner that our space probes use their rockets to slow them down enough or they would shoot past.

We use gravity assist for probes like Voyager, Cassini etc to transfer kinetic energy from the planets orbital motion to that of the craft. When we want to put something like Cassini into orbit we can do the reverse as well i.e. transfer some of the kinetic energy from the craft to the planet to slow it down by approaching from a different angle and then add some rocket full to complete the task. The maximum energy removal happen when a object approaches from a direction that puts it into a retrograde orbit ( as I understand it) and that is why many captured outer moon are like this.

The whole process is difficult to explain easily. Velocity is a vector and therefore defined by magnitude and direction. The planets gravity does not change the speed of the probe but by changing its direction it can change its velocity relative to the sun and therefore can add or remove orbital energy.

[Guest_BruceMoomaw_*]

If the relative velocity of the planet and the potential moon is low enough, third-body gravitational effects can jockey the moon into orbit around the planet -- that is, the fact that both objects are orbiting the Sun at about the same distance out can by itself allow the moon to approach the planet slowly enough to be captured into orbit by it when it reaches the planet's vicinity. But for this to happen, their orbits and thus their velocities must be very close to begin with -- which was always one problem with the "capture" hypothesis for the origin of Earth's own Moon.

Instead, most captured moons are captured by one of two mechanisms: either they brushed through the cloud of gas that was still around a forming planet and were slowed down by friction, or they collided with a hunk of debris from the large cloud of solid debris that was still orbiting (or being pulled into) the planet while it was forming and got braked into orbit that way. There is a debate as to which of these mechanisms was more important in the early Solar System, and whether we'll ever be able to estimate whether a particular captured moon was captured by one or the other. (This is also one reason for the continuing dispute over the origin of Phobos and Deimos: it's hard to conceive of either mechanism being strong enough around the forming Mars for their capture to be likely.)

[Bob Shaw]

If the relative velocity of the planet and the potential moon is low enough, third-body gravitational effects can jockey the moon into orbit around the planet -- that is, the fact that both objects are orbiting the Sun at about the same distance out can by itself allow the moon to approach the planet slowly enough to be captured into orbit by it when it reaches the planet's vicinity.  But for this to happen, their orbits and thus their velocities must be very close to begin with -- which was always one problem with the "capture" hypothesis for the origin of Earth's own Moon.

Bruce:

The recent classic instance of a more-or-less (Solar) co-orbital object being captured by a planet must be the Apollo 12 S-IVB, but without some actual change in velocity such captures will be quite 'loose' - like the outermost semi-moons of the giant planets, which may not know quite whether they are asteroids, Trojans or moons. Phobos and Deimos, however, seem to me to defy all logic, though the very fact there's two of them surely suggests some common factors...

Do you have any good links regarding capture mechanisms?

Bob Shaw

[ljk4-1]

Bruce:

The recent classic instance of a more-or-less (Solar) co-orbital object being captured by a planet must be the Apollo 12 S-IVB, but without some actual change in velocity such captures will be quite 'loose' - like the outermost semi-moons of the giant planets, which may not know quite whether they are asteroids, Trojans or moons. Phobos and Deimos, however, seem to me to defy all logic, though the very fact there's two of them surely suggests some common factors... 

Do you have any good links regarding capture mechanisms?

Bob Shaw

Though Phobos and Deimos look like planetoids and have therefore been considered captured ones since at least Mariner 9, what about the possibility that they are the remains of a larger moon that was formed with Mars long ago? Perhaps another moon smashed into it and Phobos and Deimos are what's left of the cosmic collision.

[Rob Pinnegar]

A question that has bugged me for a long time is how planets capture asteroids, etc into an orbit.

The following is _very_ vaguely remembered information from a term project I wrote up about seven years ago -- my apologies if any of it is obsolete or just plain wrong:

In addition to three-body effects, atmospheric friction and collisions, there's also internal tidal dissipation to consider. If a fairly large "proto-satellite" makes a close enough pass by a larger planet, the side of the proto-satellite that is closest to the planet will be significantly closer to the planet's centre of mass than the side farthest away. The "near side" will then want to zip past at a different velocity than the "far side". This is what leads to deformation of the proto-satellite, and it generates *a lot* of frictional heat (assuming the proto-satellite doesn't rip apart entirely). That heat energy has to come from somewhere and it ends up coming from the kinetic energy of the proto-satellite, relative to the planet. If enough is lost, the beast can be slowed down to the point where capture can occur.

Part of this vague recollection of mine is that this model was at one time proposed for Triton's capture around Neptune. Right after capture, the initial planet-centred orbit inevitably ends up being _very_ elliptical, so this model works best when you're far, far away from the Sun's greedy grasping fingers. Once the moon is securely captured, further tidal dissipation can round-out the orbit over time. This last point is of course crucial in Triton's case since its orbit is nearly circular. I believe that interaction with Neptune's atmosphere was also considered in this model, but, as I said, it's been a while.

As regards Luna: There's a book called "Origin of the Moon" from about 1986 in which the capture hypothesis of the origin of the Earth's Moon is described in some detail. However this book has been superseded by one or two more recent volumes. All of them are conference proceedings.

[Guest_PhilCo126_*]

Gravity is certainly the answer here...
The large planet Jupiter (gas giant in some way a failed star) acts as a vacuum cleaner and sucks in matter that comes to close (remember 1994... Shoemaeker-Levy comet).
Pioneer 10 had a dust particle counter and when it closed in towards Jupiter, the particle count went X100

[Bob Shaw]

Gravity is certainly the answer here...
The large planet Jupiter (gas giant in some way a failed star) acts as a vacuum cleaner and sucks in matter that comes to close (remember 1994... Shoemaeker-Levy comet).
Pioneer 10 had a dust particle counter and when it closed in towards Jupiter, the particle count went X100 

Phobos and Deimos still make no sense, though!

Bob Shaw

[Guest_AlexBlackwell_*]

Phobos and Deimos still make no sense, though!

I've always thought the main question regarding Phobos and Deimos is: What is their origin? The two main models are (1) the two moons are captured asteroids or (2) they co-accreted with Mars. Not surprisingly, there is evidence to support both. While both models have attractive components, however, they also have some rather glaring holes.

For a more rigorous treatment of the subject, I would refer the reader to Joe Burns's chapter in the classic reference work Mars [H.H. Kieffer et al., Eds. (Univ. of Arizona Press, Tucson, AZ, 1992)], which, while a little of out date being published in 1992, is still de rigueur reading on anything related to Mars.

At first glance, the "captured asteroids" model seems to be the more attractive of the two. The two moons, for all intents and purposes, do "look" like asteroids. And the close proximity of the asteroidal main belt offers a convenient source. That said, though, even first order observations supporting this view are somewhat puzzling. For example, the spectra of the leading hemisphere of Phobos (i.e., the Stickney-dominated region) best fit the curves for T-class asteroids, while Phobos' trailing hemisphere (and, incidentally, Deimos' leading hemisphere) match spectra from D-class asteroids.

Even assuming these spectral observations are truly indicative of captured asteroids, as Burns points out there are problems in the capture mechanism. With aerocapture, presumably by the primordial Martian nebula or proto-Mars atmosphere, the problem is not so much with its mechanics, which, though problematical, can be made to work, but rather with its timing. Moreover, capture scenarios should, ideally, show a good fit to the observables.

For example, tidal evolution theory vis-à-vis Phobos's secular acceleration needs to account for the timing of the Sun's putative T-Tauri stage and associated stage solar wind, which narrows the window for aerocapture and prevention of rapid orbital decay. In short, if the T-Tauri stage came first, then the captures most probably would not have happened (i.e., no extended atmosphere). If the T-Tauri stage came afterwards, then the moons should have decayed a long, long time ago. This is a true puzzle.

Looking for a way out, Burns modelled the particular case of a planetesimal that was captured by the proto-Mars nebula and subsequently evolved down to areosynchronous orbit. At this position, orbital decay would virtually cease due to the low relative velocities between the planetesimal and the Martian nebula. Subsequently, the planetesimal was shattered by another, resulting in two or more fragments that resulted in Phobos ending up below areosynchronous orbit and Deimos above. The former would undergo secular acceleration (i.e., orbital decay), which has been documented and is well known. The latter, Deimos, would undergo relatively little orbital evolution, which is consistent with the observables. Indeed, given the nature of orbital dynamics, it is possible to integrate Phobos' orbital history backwards in time to infer that the moon, even under an accretionary origin model, originated at ~5.7 Martian radii (Rm). This, of course, assumes that its orbit has always been roughly circular and conveniently ignores chaotic evolution, resonances, etc.

Of course, one will note that the above model relies on a series of rather unique events to account for what we see today. Mainly, such a model contains rather precise timing, and I'm not sure it does not avoid the dreaded "Tooth Fairy" hurdles (i.e., one is allowed to invoke "miraculous" events only once per model). That said, it still does not mean it did not happen.

It's obvious that highly detailed in situ and/or sample return studies are needed to progress further, else the "modellers" will continue to dominate the literature. To approach a resolution, especially on the co-accretionary model, one needs a dedicated mission(s). Hopefully, a sample return concept such as Gulliver: Deimos Sample Return Mission or something similar to the Aladdin mission concept (for details click here and here), which was proposed a couple of times for the Discovery Program, gets approved. The Russians have also made noises with their PHOBOS-GRUNT mission concept but, as I mentioned elsewhere, I'll believe in this mission when I see it.

This post has been edited by AlexBlackwell: Dec 29 2005, 02:17 AM