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Origin of Phobos and Deimos, Where did these guys come from?
Chmee
post Mar 25 2006, 02:49 PM
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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 didnít do perigee burns to brake them into orbit) wink.gif

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?
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djellison
post Mar 25 2006, 04:11 PM
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I've always thought it was 2 that was the most widely accepted theory? I'm sure Phil can fill us in though being a small-body-guru smile.gif .

My un-pro take on thigs ; Deflection by jovian gravity could put a small asteroid into an orbit whereby a close slow flyby of Mars would result in a capture. Over millions of years - just about any orbit ends up being near circular and equatorial.

They are in decaying orbits - they wont be around for many hundreds of millions of years - so I don't think they could have formed when the planet did or that process would have already occured.

If they accreted from ejecta, a coming together of debris would produce a loose rubble pile - then Phobos surely wouldnt be strong enough to have withstood the impact that caused Stickney?

Interesting debate all the same smile.gif

Doug
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Phil Stooke
post Mar 25 2006, 06:16 PM
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Well... this is an interesting subject, but not one we can yet resolve.

First, we don't know the compositions at all well. The old statement that they are carbonaceous asteroids is not well supported with evidence. They are too faint and too close to Mars for good spectral studies from Earth (I might not be up to date on that, though, my attention has been elsewhere). Very little compositional work has been done by spacecraft. Phobos 2 IR spectroscopy is probably best. But contamination by Mars ejecta and also by light scattered off Mars complicates the issue. In truth we can't say yet whether these moons are the same composition or different, or if they are asteroidal, bulk Mars or more like Mars regolith in composition. Actually there could be a mixture of all these components on the surfaces of the moons.

Second - only Phobos is in a decaying orbit. But that fact does suggest it might not have been there for 4.5 Ga. I don't see the equatorial circular orbit as too serious a problem, since close orbits of oblate planets ought to evolve in that direction if they have time. But capture is VERY difficult to make work. Ejecta seems more likely to me than capture.

Maybe we can try to put this together: a very early large impact (Hellas or northern plains) puts a lot of debris into orbit. Actually most would not stay in orbit, but complex dynamics may allow a bit to end up in an equatorial ring which can then accrete into a moon. This would be very early, and maybe would have to be out near the synchronous orbit area. It sits there for a while. Then it is fragmented by a large impact. Phobos and Deimos are the only two remaining fragments of that disruption. One was thrown just inside the synchronous orbit and has slowly drifted inwards - Phobos. One was pushed out a bit and has slowly evolved outwards - Deimos. This disruption had to happen quite a long time ago, so Phobos has had time to acquire its dense population of craters. Oh - and Stickney is probably not the cause of the grooves. That set of internal fractures is more likely to date to the disruption, and I'm assuming that the parent body was quite well consolidated in order for that to be true. Stickney might have shaken them open a bit.

We really need a Phobos and/or Deimos sample return mission. I believe it is now the highest priority sample return mission for which we actually have the technology today.

Phil


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Guest_AlexBlackwell_*
post Mar 26 2006, 05:40 PM
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I'll stand by my post from December 29, 2005.
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Guest_AlexBlackwell_*
post Mar 26 2006, 09:25 PM
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To keep this thread active, I've copied my above-mentioned post.

==========================

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-ŗ-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.
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Guest_AlexBlackwell_*
post Mar 26 2006, 09:40 PM
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QUOTE (tasp @ Mar 26 2006, 07:05 PM) *
Wow. The D-class asteroids get around. Or rather the material with the distinctive signature does. From the outer asteroid belt, to the Jupiter Trojans and outer satellite horde of Jupiter, and the Cassini Regio on Iapetus (and I am willing to bet the dark crater floor material of Hyperion) D- type spectrums are very well represented.

I had wondered how close to the sun the 'material' would be stable. Mars distance is quite a jump from the outer belt. Would the D-type spectra be more of a tracer for a 'ubiquitious' material that can form across a wide range of solar distances rather than for a common source (like an ancient odd asteroid that was disrupted) of it?


I guess I should have included the reference for this:

Near-Infrared Spectrophotometry of Phobos and Deimos
A. S. Rivkin, R. H. Brown, D. E. Trilling, J. F. Bell, III and J. H. Plassmann
Icarus 156, 64-75 (2002).
Abstract
Reprint (307 Kb PDF)
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nprev
post Mar 26 2006, 10:14 PM
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QUOTE (AlexBlackwell @ Mar 26 2006, 01:25 PM) *
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.


I understand your reluctance in this regard, but evidence seems to be mounting that the early Solar System was an extraordinarily dynamic environment, and collisional processes are the best explanation for its current configuration.

To put Phobos & Deimos in context, we also must consider the Earth's Moon and the entire Saturnian system as well as those of Uranus and Neptune as "fossils" from a far more active era in terms of orbital dynamics. Even Pluto and other KBOs are now beginning to reveal additional satellites that presumably arose from collision events. The absence of satellites, anomalous or otherwise, for Mercury & Venus has been well explained by their proximity to the Sun, but the rotation period of Venus is yet another piece of the puzzle that might be best understood as an artifact from an early collision or other interaction with another massive body. Jupiter seems to be the odd man out in many ways as far as peculiar dynamical behavior or origins for its satellite system, and I suspect that this is a direct consequence of both its position in the Solar System and its mass. In fact, from a causal perspective, Jupiter probably originated rather than 'suffered' from collisions throughout its history by disturbing the orbits of passing bodies so that they interacted with the other planets or merely absorbing them whole al a Shoemaker-Levy 9.

The whole point here may be that the number of unaccreted large planetisimals in the early system may well have been much larger than currently thought, and/or the T-Tauri phase of the Sun may not have been energetic enough to purge the system of 'debris' as efficiently as is currently believed. In either case, enough wandering bodies were apparently present to produce a profound influence on the modern layout of the Solar System. Didactic views of key events may be misleading; some of these assumptions should be reassessed against emerging evidence.


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Guest_BruceMoomaw_*
post Mar 27 2006, 02:23 AM
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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.

QUOTE (Chmee @ Mar 25 2006, 02:49 PM) *
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.
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Bob Shaw
post Mar 27 2006, 12:17 PM
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QUOTE (BruceMoomaw @ Mar 27 2006, 03:23 AM) *
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.


Bruce:

And, to answer the question of why the Earth doesn't have rings (the planetary norm, rather than the exception!) we only have to look at (1) the Moon's gravitational effects and, in the case of very fine debris, perhaps (2) the Earth's magnetosphere. Plus (3) the effects of the Sun's tides etc, although on a lesser scale than with regard to the other inner planets.

Bob Shaw


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antoniseb
post Mar 27 2006, 03:58 PM
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I think that option 4 is a serious possibility, and it doesn't have to hve been 4 billion years ago either. Of course we'll know more after a sample return, or very advanced in situ probe. I think it is no coincidence that the Hellas Basin is on the opposite side of the planet from the giant crack and giant new volcanos. I think Deimos and Phobos (and a long-gone ring between them) could have formed as recently as 200 million years ago.

Getting some samples from the Hellas basin could tell us some time-line details that would prove or disprove this idea.
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Guest_BruceMoomaw_*
post Mar 27 2006, 08:24 PM
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QUOTE (djellison @ Mar 25 2006, 04:11 PM) *
If they accreted from ejecta, a coming together of debris would produce a loose rubble pile - then Phobos surely wouldn't be strong enough to have withstood the impact that caused Stickney?


Doug


Uh-uh. Consider those utterly gigantic craters seen by NEAR on the rubble-pile asteroid Mathilde. Rubble piles, it turns out, can tolerate much bigger impacts, capable of producing much bigger craters, than solid asteroids can without splitting up, because their loose structure serves as a shock absorber -- like firing a bullet into a pile of sand. This also means that such craters toss out far less ejecta, because the shock of the impact tends to compress the local material instead -- which, if John Murray's theory as to the real origin of the "Stickney grooves" is correct, would explain why Stickney itself is a very big crater that has actually thrown out very little ejecta.
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Guest_AlexBlackwell_*
post Mar 27 2006, 08:31 PM
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QUOTE (BruceMoomaw @ Mar 27 2006, 02:23 AM) *
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.
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Bob Shaw
post Mar 27 2006, 09:51 PM
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QUOTE (AlexBlackwell @ Mar 27 2006, 09:31 PM) *
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.


Perhaps there *are* remnants in orbit around Mars!

Hmmmm....

Bob Shaw


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Guest_BruceMoomaw_*
post Mar 27 2006, 10:02 PM
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QUOTE (AlexBlackwell @ Mar 27 2006, 08:31 PM) *
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.
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Guest_AlexBlackwell_*
post Mar 28 2006, 12:25 AM
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QUOTE (BruceMoomaw @ Mar 27 2006, 10:02 PM) *
How sure are we of that?

When it comes to events 4 Gyr ago, I'm not too "sure." cool.gif However, given Phobos' and Deimos' size, and assuming they are end-member representatives of accretitionary processes, some models indicate the existence of extant orbital debris, either discrete bodies or thin disks/belts.

This post has been edited by AlexBlackwell: Mar 28 2006, 12:36 AM
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