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?

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 .

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

Doug

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

I'll stand by my post from December 29, 2005.

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-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.

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)

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.

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.



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.

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

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.

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.

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

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.

When it comes to events 4 Gyr ago, I'm not too "sure." 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.

...."Perhaps there *are* remnants in orbit around Mars!"....

Anybody looking for Black Monoliths?

The resolution of the searches with ground-based telescopes has gotten almost good enough to detect Black Monoliths. If there are any additional moons of Mars, they are very small.

One other extremely half-baked idea -- I have no idea whether it would even work in physical terms. There is speculation now that Mars underwent major polar wander when its Tharsis Bulge formed -- that is, that the Bulge may have formed at a higher latitude, and that centrifugal "force" then caused the entire planet to tilt until the Bulge was at its equator. If Phobos and Deimos were originally in inclined orbits around Mars, could the gravitational field of the migrating Bulge have gradually dragged them along into their current equatorial orbits? Or am I spouting scientifically illiterate hooey?

By spectra, both martian moons look quite different to Mars (and seemingly differnt to each other) and are definenitly not composed my martian mantle material.

They seem to be captured bodies, maybe carbon rich condrite material. Main-Belt-Asteroids, I suppose.

Interestingly, there are a lot of elliptical impact craters on Mars which could be explained as impacts with low velocity and very small inclination. Maybe, there have been more "moons" at all. There could be a mechanism of gravitational influence (Jupiter, Saturn) which leads to asteroid orbits near to Mars and caption after a while.

A possibility is that Phobos and Deimos are the remainders of a captured asteroid which had hit on the Mar's North Pole (Vastitas Borelis).
Does anyone believe that the returned samples by Phobos & Grunt spacecraft will be able to reveal this mystery?

Just brain storming here;

Perhaps Phobos and Deimos are primordial to Mars (or nearly so) but their 'original' orbits or an early evolved form of those orbits might have been something we don't see else where in the solar system today.

(Phobos or Deimos may or may not be remainders of a past 'population' of Martian satellites, I am not qualified to rate the effect that would have on this scenario's probability)

What I am thinking is fairly soon after Mars ( and the rest of the solar system 'settled down" after the late heavy bombardment) Phobos and Deimos might have had their orbits evolve into a resonant condition. But with a 'twist' we haven't seen anywhere else. The resonance was achieved with Deimos outside of the aerosynchronous orbit and Phobos inside.

Has anyone ever modeled a resonance that straddles the 'object/rotation/synchronous altitude ??

I guess I would not be surprised if there is 'sumthin' weird' about a resonant relationship like that, perhaps in allowing (presumably) short lived objects like Phobos and Deimos to persist into our epoch.

Why don't we perceive a resonant relationship now?

Maybe the Stickney impactor deceled Phobos sufficiently to break the resonance in the past <1gy, and we now observe it in a secularly accelerating orbit.

I don't have the mental faculties right now to see if Phobos is just inside a 3:1 (or whatever to 1) with Deimos, and the plausible decel from a Stickney impact would make up the difference . . . .

(but if someone else wants to, no problemo)

I think this is an interesting posibility, which could gain consistency if the elliptical impacts are more concentrated in the equator and with their major axis orientation also parallel to the equator, since this hipothesis assumes that all the ancient Mars satellites which formed those impacts were, as Phobos and Deimos are now, near equatorial orbit.

I haven't any info about the concentration zone (if any) and orientation of elliptical impact in Mars. Have someone any info about this?

Regards

One thing has bothered me for some time about the impact origin theory of moon formation. If a bunch of pieces get thrown into space by the impact, and then later coalesce to form a moon, the orbital momentum of the new moon should be equal to the net angular momentum, with respect to the parent body, of its component pieces of ejecta. It follows that moons can only result from glancing blows, because a symmetric 360 degree spray of debris resulting from a head-on vertical hit can never come together to form an orbiting body, even if some fraction crashes back into the parent body and some other fraction escapes entirely. Tiny close-in moons like the ones around Mars - that I can believe, because maybe they represent only a small fraction of the debris created by a large glancing impact. But big moons like our Luna?

Given a handful of parameters: size of the parent, size of the impactor, their densities, rotational speeds, relative velocity and angle of impact, it should be possible to project the portion of debris that will be too slow to escape and too fast to crash back, and then calculate their net angular momentum. I certainly couldn't do the calculation myself, but it must have been done by someone during the general discussion of moon formation. Whether by calculation or simulation, I would like to know what constraints the basic physics of collisions places on the size of the residual net momentum, and under what circumstances it is really possible to form moons that way. If so, it should then be possible to do the calculation backwards for any known moon, and establish the boundaries of the possible parameter values that could have brought it about. Some people here must know the literature well enough to point out where this has been done.

In the case of the Martian moons, of course, even if you can prove that such small close-in moons could come together from collision debris, they would likely be rubble piles, so the "Stickney objection" would still cause problems.

I can, however, imagine an impactor large enough to succumb to tidal break-up as it approaches Mars.

Bring a body the size of one of the larger asteroids across Mars just within the Roche limit. As the body breaks up, the release of the outer pieces would fling them into escape velocities, and breakup dynamics would allow some pieces to fall into stable orbits. The rest of the body would, of course, impact Mars.

In such a break-up scenario, I can well imagine pieces the size of Deimos and Phobos coming out of the process as relatively intact chunks of rock. Collisions with other pieces of the original parent body during the break-up would knock off the roughest edges, and accretion of smaller chunks over the months and years after the break-up would provide craters and regolith.

Perhaps Phobos looks like it has stretch marks because it was literally stretched in the process of the break-up on the original body? Subsequent mantling and alteration has not been enough to wipe the striations off the face of the little moon, since the cracking also controls impact-shock faulting and reinforces the surface expression of the cracks.

On a gestalt level, it hangs together... at least, for me.

-the other Doug

I don't see a hard dividing line between accretion and impact. Mars was created by many impacts of bodies together. There wouldn't have been any special moment when God clicked a stopwatch and said "DONE!" and thereafter all impacts were impacts, whereas the day before they would have been accretion.

If material were being lofted skywards from Mars, we would see different possibilities as Mars evolved. But the new material being introduced by the impactor would be equally new to the Mars system whether it was this year or 4.5 GYA. And how much of the putative satellite was martian vs. foreign would also be on a continuum.

The elliptical craters on Mars are not really centered on a special strip or plane. I assume that Mars was forced to a polar axis shift over the billenia and to an more or less erratic shift of the whole crust relative to the core. The latter can be explained as an effect of large mantle plumes and it may not have any astronomical reason.

It would be very interesting to get core sampels of Phobos and Deimos and some similar asteroids...

These funny moonx circling very near to the planet and phobos cruising faster that Mars rotates are extremly interesting. It is often said that Phobos will fall down on Mars in some millenia, but there is no complete mathematical theory of the tides in combination with the sun. Maybe Phobos will survive.

Harkeppler

Peter Schultz <Brown univ, Deep Impact mission, etc> proposed some 20 years ago that the oblique impact craters form distinct populations at different inclinations and probably represent the traces of paleo-equators and provide evidence for polar wander on Mars. He proposes an original some 100 km <I think> moon that got shattered or tidally disrupted, Phobos and perhaps deimos are the last survivals.

I thought this was an interesting blurb: perhaps there were once three moons of Mars!

http://news.softpedia.com/news/Mars-039-Sc...oon-96469.shtml

The concept of an ancient moon that broke up is very interesting.

It does seem very strange that Phobos is going to hit Mars in a relatively short (compared to the age of the solar system) time frame. It does seem to imply it may not always have been there. Are there any other bodies like that in the Solar System?

Triton? I think there is a consensus that it was once the largest known TNO.

(136108) Haumea and its two moons look like they were formed fairly recently.

Phoebe and all of the retrograde moons of the outer planets.

Couldn't another mechanism be that Mars captured a binary asteroid system? From what I recall, there are some three-body solutions (in other interacting systems) which can result in dual capture if the parameters are just right. Have there been any attempts to model the Phobos / Diemos / Mars system with this scenario?

(negatives on this proposal would be that Phobos and Diemos aare non-identical in composition, whereas they may have been expected to be more similar if they were a binary system, and likely that this scenario involves some very exacting starting assumptions.)

Phobos and Deimos ( Fear & Terror ): http://www.astroshorts.com/view_video.php?...bf4fc791ba842aa

Martian moon Phobos may have formed by catastrophic blast:

http://www.sciencedaily.com/releases/2010/...00920094804.htm

So finally, it seems not to be a captured asteroid, but formed from pieces of Mars itself !

Marc.

While very interesting, this is not quite as conclusive as the article suggests. The overall reflectivity of Phobos is much lower than of Mars rocks. Carbonaceous chondrites are the best match. Also, it is expected that there is a good collection of rocks blasted off of Mars that have landed on Phobos (hence a sample return mission from Phobos is expected to bring back Mars rocks as well as whatever Phobos is made of). The Mars-like minerals could be from these rocks, not what the bulk of Phobos is made of.

"The overall reflectivity of Phobos is much lower than of Mars rocks."

Well, the albedo of Phobos is lower than the albedo of Mars, but most of Mars is covered with bright dust. I'm not so sure that Phobos is darker than actual Mars rocks, most of which would be of basalt-like composition. I thought the story was a bit misleading in another way... the rejection of a carbonaceous asteroid composition goes back quite a long way, based on things like the Mars Pathfinder multispectral imaging of Phobos, and Phobos-2 spectral data. Several papers have already said Phobos is not asteroid-like in composition.

Phil

One of the quotes in the Science daily article is interesting:

"We detected for the first time a type of mineral called phyllosilicates on the surface of Phobos, particularly in the areas northeast of Stickney, its largest impact crater," says Dr. Giuranna.

The article goes on to imply that this means that the water-bearing minerals must have come from Mars. There are many carbonaceous chondrites that contain -and are even dominated by- phyllosilicates.

So a sample return to and from Phobos might actually be a cheap n nasty way of a very generalised Mars sample return without all that nasty delta V?

P

Aren't phyllosilicates also found on comets? Could they have been delivered by a cometary impactor?

Phyllosilicates are very popular these days. Last time I was at JPL there was a disheveled looking guy leaning on his car a few blocks down the street. As I walked past he said "Pssst, hey buddy, you look like a scientist. You want to see some phyllosilicates?" He popped open the trunk of his car and said, "C'mere man check these out. I got serpentine, micas, clays, talc, whatever you want, real cheap."

We've already had cheap 'n' nasty sample return from Mars, zero delta-V required. But presumably Phobos has picked up some Martian ejecta over eons.

Yes. Comets are possible sources of phyllosilicates too. Stardust oddly has not turned up much in the way of phyllosilicates (i.e. water-bearing `clay' minerals) though.

-pjam

Obviously, don't know much, but yeah, I'm beginning to think that Phobos and Deimos are
chunks of Mars.

I had a thought today about a possible method of estimating the age of Phobos' formation. Since Phobos is slowly spiraling in towards Mars (and is estimated to break up in 30-50 million years), could not a calculation be done in 'reverse' to see how long ago Phobos would have been in/near a areo-synchronous orbit around Mars? In other words, we know that Phobos must have been formed/captured somewhere inside a synchronous orbit, otherwise, it would never have spiraled in-ward in the first place. Therefore running the calculation backward would give a maximum age in which Phobos would have formed or captured.

Right now, Phobos orbits approximately 6,000k above Mars and the areo-synchronous altitude is 17,200km, a distance of 11,200km. I understand that Phobos is descending at a current rate of ~1.8cm per year. That rate of orbital decay is not constant (it increases the closer it descends), but I am sure someone has an equation (differential formula?) which could calculate that.

Any ideas about this?

Running time backwards we see Phobos spiraling back out, but as it does so it crosses various resonances with Deimos, and Deimos might be subject to forces changing it's orbit too. I'd think it would get difficult to know how long a particular resonance might have persisted, and which ones might have been active.

Tough problem, just thinking about the math makes me dizzy.

The problem is that simply reversing the orbit dynamics in time gets to the point where you require another body in the equation to effect the orbital capture. And it's impossible to tell what those dynamics were.

Myself, I prefer the theory that a fairly large and rapidly spinning body broke up when it passed within Mars' Roche limit. Part of of it impacted Mars, part of it achieved escape velocity, and two pretty big chunks ended up in stable orbits. Other chunks ended up in unstable orbits and eventually hit Mars.

At what time this happened is probably most easily constrained by looking at the age of craters/basins that could have been caused by such a catastrophic impact. You can place the event at almost any point in the time-reversed orbital history of the moons by simply adjusting the size, speed and trajectory of the body that came apart, so such a reverse-time orbital analysis would be less useful for constraining the timeframe, I would imagine.

-the other Doug

Good point which I did not consider. But would resonances be significant for such low mass moons? Unlike the Galilean moons, which have considerable mass and are in resonance, these moons are only 22km (Phobos) and 6km across (Deimos). Even if we put Phobos near the areo-synchronous altitude of 17,000km, it would still be ~6,000km from Deimos. I would think gravitational disturbances with such low masses would be very minor at that distance...

Even if there were some sort of resonance, would it prevent the kind of tidal orbital decay of a moon under the areo-synchronous altitude?

Extrapolate a tiny tiny force, over millions and millions of years.

What do you get? A big difference.

You even need to include things like solar pressure etc etc. Very long period orbital extrapolation is fraught with nature's subtle influence, much of which is very hard to simulate.

The real problem here is that these kinds of orbital extrapolations only work for limited periods. The system becomes chaotic especially near any resonances, and as Doug pointed out there could have been other short-lived chunks which would make it even more chaotic. Even now we can't predict precisely where the two moons will be from one decade to the next - the MSL Mastcam images on about sol 42 showed the moons a bit off where they were expected (there's an LPSC abstract on it somewhere). So we can't possibly extrapolate backwards very far.

Phil

In the debate over the origin of Phobos and Deimos, it looks like several recent papers have been published supporting the 'giant impactor' theory. Here are the abstracts to a couple very interesting papers on this subject:

"Are Phobos and Deimos the result of a giant impact?" - Robert A. Craddock Icarus Volume 211, Issue 2, February 2011, Pages 1150–1161

"On the formation of the martian moons from a circum-martian accretion disk" Pascal Rosenblatt Icarus Volume 221, Issue 2, November–December 2012, Pages 806–815

I am not too sure if this is to be called a follow-on to the demise of Nozomi.

Today's Asahi newspaper here in Japan talks about JAXA sending a sample
return mission to one of the Martian satellites. What follows is my translation.

JAXA reported to the Space Activities Commision on 9 June 2015 (yesterday)
that they woud like to launch a sample return mission to one of the Martian
satellites during the early part of 2020's. SAC accepted it.

The proposal is based on JAXA's ISAS's judgement that given Hayabusa experience and
the expected experience from the sample return mission proposed for 2019 from
the Moon the sample return mission form a Martian satellite is within the capability
of ISAS.

This is part of the next 10 year's space programme agreed by the government
in January this year that three medium sized projects will be conducted
during the next 10 years.

P

This will be among the topics covered in this dedicated NASA seminar series on Phobos and Deimos, beginning September 14th. These can be seen live or via archive.

http://sservi.nasa.gov/event/planetary-evo...bos-and-deimos/