Origin of Phobos and Deimos, Where did these guys come from? Options

[Chmee]

So what is everyone's thoughts on the origin of Mars' moons Phobos and Deimos? They are a bit of a mystery.

Here are the different theories:

1. They formed along with Mars when it accreted out of the plantary nebula.

Pros: explains how both are in the same circular, equatorial orbit around Mars.
Cons: Seems a strange coincidence that we are around to witness Phobos in such a low orbit that it is about (in a couple million years) to crash out of orbit. Also this would be the only case in the solar system where such small "asteroid-like" moons formed around such a large body.


2. They were captured into orbit around Mars.

Pros: This would explain their similarity to asteroids out in the Belt.
Cons: The probability that they would be both be captured into circular and equatorial orbits is virtually zero. Also, there is no know mechanism for asteroids to be captured by such a small body like Mars (after all the moons didn’t do perigee burns to brake them into orbit)

3. They were once part of a larger moon that that broke up into several pieces. Phobos and Deimos are the last remnants of it.

Pros: This would explain how both moons have circular and equaltorial orbits (since they started from the same body). Theoretically, there would have been many more moons at one time, but they have crashed into Mars one by one, as Phobos is on course to do.

Cons: Phobos and Deimos do not appear to be very similar compositionally, which is strange if they came from the same moon. Of course it was large enough, the large proto-moon may have been differentiated.

4. The moons were formed from a large impact early in Mars history, perhaps from the impact that created the Hellas basin or the northern lowlands. This impact formed a small debris field around Mars which accreted into the moons.

Pros: Explains the circular orbits of the moons and Moons created from early gigantic impacts seems to be a re-occurring theme we see in the rest of the solar system (i.e. Earth's Moon and likely Pluto's moons)

Cons: While it explains the circular orbits, it does not explain how they are equatorial.


I believe the favored theory this decade is number 3, where a large body was present, but was broken up.

What is everyone's thoughts?

[Guest_BruceMoomaw_*]

In this connection, there's an extremely interesting new LPSC abstract ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2195.pdf ) claiming that Mars Express' new full-surface photography of Phobos has solved the problem of the surface grooves -- which are definitely NOT cracks or ejecta trails from Stickney, but ejecta trails from several giant impacts on Mars itself that tossed debris upward to hit Phobos in various places! If so, then the idea that Phobos and Deimos themselves are composed of accreted debris tossed into Mars orbit by really giant impacts becomes more plausible.

Also, there's one tantalizing new EGU abstract ( http://www.cosis.net/abstracts/EGU06/05330/EGU06-J-05330.pdf ) announcing that the results of the first MARSIS examination of Phobos (from only 239 km distance) will be revealed at the EGU meeting. They got very high-quality data, but there's not a hint given as to what it will show.

4. The moons were formed from a large impact early in Mars history, perhaps from the impact that created the Hellas basin or the northern lowlands. This impact formed a small debris field around Mars which accreted into the moons.

Pros: Explains the circular orbits of the moons and Moons created from early gigantic impacts seems to be a re-occurring theme we see in the rest of the solar system (i.e. Earth's Moon and likely Pluto's moons)

Cons: While it explains the circular orbits, it does not explain how they are equatorial.

Actually, I think it would mesh very well with equatorial orbits for them. If the debris from the impacts was originally tossed into inclined orbits (as it certainly would be), the orbits of the different pieces of debris would precess around the planet relative to each other -- so the paths of the various debris pieces would then cross at the equator, which is where collision would be most likely. There would then be exactly the same kind of process that gradually flattened out Saturn's rings into a near-perfect plane around its equator -- the difference being that Mars' equatorial ring of debris, being beyond the planet's Roche limit, would then continue accreting into a couple of lumps. In fact, this theory is the only one that explains really well why their orbits are so close to the equator.

[Guest_AlexBlackwell_*]

Actually, I think it would mesh very well with equatorial orbits for them. If the debris from the impacts was originally tossed into inclined orbits (as it certainly would be), the orbits of the different pieces of debris would precess around the planet relative to each other -- so the paths of the various debris pieces would then cross at the equator, which is where collision would be most likely. There would then be exactly the same kind of process that gradually flattened out Saturn's rings into a near-perfect plane around its equator -- the difference being that Mars' equatorial ring of debris, being beyond the planet's Roche limit, would then continue accreting into a couple of lumps. In fact, this theory is the only one that explains really well why their orbits are so close to the equator.

There are holes in that model. For example, where is the remaining debris? No one seriously believes that Phobos and Deimos represent all of it. And even decay processes, when integrated over Mars' lifetime, should still result in remnants in martian orbit.

[Guest_BruceMoomaw_*]

There are holes in that model. For example, where is the remaining debris? No one seriously believes that Phobos and Deimos represent all of it. And even decay processes, when integrated over Mars' lifetime, should still result in remnants in martian orbit.

How sure are we of that? It certainly didn't happen in the case of our own Moon -- although, of course, our Moon is far more massive and so would have allowed runaway accretion to occur to a greater degree.

As for Bob Shaw: there HAS been a pretty thorough search for additional debris in Martian orbit -- including Earth telescope surveys (and Viking Orbiter photo surveys) that should have revealed any object down to a few dozen meters in size, and an actual search using the MER cameras for any sign of skyglow from a ring of debris. All of them have come up empty-handed.