Phobos and Deimos Once a Single Moon?

blamanj writes "Phobos (fear) and Diemos (panic), the twin moons of Mars have caused astronomers grief for years, as conventional hypotheses about the moons either violate physical laws or have difficulty accounting for their observed orbits. Now a new hypothesis conjectures that they were once a single moon, that broke apart in an ancient catastrophe."

Comments

FUD

[limekiller4]

Great. Now we just need to find moons Metus (fear), Ambiguitas (uncertainty) and Dubium (doubt) and convince Gates to purchase them...

Doubtful

[CheshireCatCO]

Right, there's a lot here that makes me dubious of the claim. First off, I should point out that I've worked on the capture problem for Mars's moons. (The results haven't been published, although the did land a grant.)

First off, why is synchronous orbit a hint as to their breakup? There's no reason that synchronous orbit is preferred, either as a capture point or as a point for breakup. In fact, synchronous orbit is an unstable equilibrium: a slight perturbation drives everything away from it. (Which is why Phobos is heading inward and Deimos outward.)

Also, he needs to explain why a larger moon orbited there happily (without perturbation!) for billions of years before breaking apart. In the very least, we're witnessing Mars's moons at a very unusal time, and such coincidence make me (and most astronomers) nervous.

Also, Phobos has drifted inward since any such breakup. Why isn't it breaking up more? Unless there's some internal strength (in which case, why did it break up then?), it should.

To be honest, I sort of question his background for this. Besides the fact that he's not an astronomer, he wants to put a base on Deimos? The surface gravity on those moons is virtually non-existant. (For Deimos, being smaller, it's under 1 cm/sec^2, I believe.) No one could even walk around properly. (Although, if he hollowed it out and made a colony ship out of it, we could launch it to Tau Ceti... But it might encounter some hostile, three-eyed aliens.*)

I'd be happy to hear him explain his idea to a group of dynamicists. Hell, I'll volunteer. But I'm very skeptical for now.

(* Kudos to anyone who catches *that* reference.)

Re:Doubtful

[CheshireCatCO]

Glad you asked!

Let's start with moons outside of synchronous orbit. These moons raise a tidal bulge on their planet. (The one on Earth is most apparent in the oceans. Or, rather, at their edges. But there's a bulge in the rock, too.) Now, the planet is spinning and it isn't a perfect fluid. So it will tend to carry the bulge forward with it, before the bulge can move back to under the moon where it wants to be. A balance is struck between these two competing forces where the bulge rides somewhere ahead of the moon.

The moon, then, feels a tug forward in its orbit. This tends to give it angular momentum, so that it drifts outward. (Angular momentum increases as you go out from the central object.) The planet, meanwhile, is being pulled backward so that its spin slows down. (As it must, to conserve angular momentum in the system.) This is why Earth's day in lengthening and why the Moon has drifted about 60 Earth radii from where it formed over the past 4.5 billion years.

What happens of the moon is *inside* synchronous orbit? The opposite happens: the moon moves ahead of the bulge and gets pulled back. So it drifts in.

I'll leave it as an exercise to the reader to work out what retrograde (backward orbiting) moons do. Triton is an example, by the way.