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?

[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