http://www.nasa.gov/feature/goddard/phobos-is-falling-apart

Has the mystery of the grooves finally been solved once and for all?

Not really, we just have another suggested explanation. The suggestion that Triton is similar is absurd.

Phil

New paper out in Scientific American that lends weight to the collision theory for the origin of Phobos and Deimos.
Such a large collision would also potential explain the Northern flatlands as the remnants of a vast crater from the collision:

http://gizmodo.com/a-badass-new-theory-on-...oons-1783059254

There are sufficient craters on Mars large enough to have ejected dust and rubble beyond the Roche limit where it would have accreted to form the two poorly cohesive moons. A single massive impact forming a large moon with an atypical degrading orbit is not required although is of course a possibility. Why Phobos, though tidal locked is orbiting faster than Mars' rotation and thus having a degrading orbit is a question although.

Tidal locking itself is not too surprising, esp. if Phobos' internal mass distribution isn't very symmetric. However, in light of this new hypothesis, I wonder if Phobos' orbital history can be traced backwards to approximate the age of the purported impact event that created it & perhaps identify a candidate crater. Considering that it will hit Mars in 50 million years or less maybe Phobos hasn't existed for very long at all, geologically speaking...hundreds of millions instead of billions of years.

If that admittedly wild speculation is correct, then Deimos may have arisen from a different impact entirely.

" I wonder if Phobos' orbital history can be traced backwards to approximate the age of the purported impact event that created it & perhaps identify a candidate crater."

No. The system is far too chaotic and unpredictable for that. Among other factors, mass redistribution via volcanism and polar volatiles cycling would alter the gravity field of Mars enough to introduce small orbital changes we can't model. Also the suggested existence of other moons in the past makes for interactions which can't be modelled.

Also,"There are sufficient craters on Mars large enough" - no, there are not lots of craters big enough, only a few huge basins, and they are very old. Craters like Lyot or Lowell produce lots of ejecta, but only a small fraction of it could end up in a debris ring (if it's even possible for impacts that size). For those craters we might look at sources of SNC meteorites, not moons.

Another point to consider. Phobos and Deimos are in orbits very close to the equator. But the obliquity of the axis of Mars swings wildly between about 0 and 60 degrees - where ours varies very little. So either the present low inclination orbits are a coincidence or the orbit planes also vary significantly over relatively short periods.

Phil

That's coming close to my idea that both Phobos and Deimos are remnants of a single geosynchronous moon. The big ancient impact could have resulted in a single geosynchronous moon (like Charon) which was shattered by a later and smaller collision, leaving some of the debris spiralling in and some spiralling out - as we see today.

You all seem to have missed a very recent paper: Nature Geoscience (2016) doi:10.1038/ngeo2742 Behind paywall.

Abstract: Phobos and Deimos, the two small satellites of Mars, are thought either to be asteroids captured by the planet or to have formed in a disc of debris surrounding Mars following a giant impact1, 2, 3, 4. Both scenarios, however, have been unable to account for the current Mars system1, 2, 3, 5, 6, 7. Here we use numerical simulations to suggest that Phobos and Deimos accreted from the outer portion of a debris disc formed after a giant impact on Mars. In our simulations, larger moons form from material in the denser inner disc and migrate outwards due to gravitational interactions with the disc. The resulting orbital resonances spread outwards and gather dispersed outer disc debris, facilitating accretion into two satellites of sizes similar to Phobos and Deimos. The larger inner moons fall back to Mars after about 5 million years due to the tidal pull of the planet, after which the two outer satellites evolve into Phobos- and Deimos-like orbits. The proposed scenario can explain why Mars has two small satellites instead of one large moon. Our model predicts that Phobos and Deimos are composed of a mixture of material from Mars and the impactor.

These are what I was alluding to. Isidis, Hellas, Argyre, possibly the older Utopia impact; sufficient to provide accretion material without having to invoke an earlier, massive collision.

If the Nature Geoscience article is accurate, then what we ought to look for, perhaps, would be the impacts of the two larger moons that their theory predicts spiraled in and impacted Mars.

Two of the best candidate sites for such impacts, I would think, are Hellas and Argyre. Interestingly, if these are impacts from the two moons predicted by the above-mentioned theory, and if those moons, like Phobos and Deimos, were in low-inclination orbits, then at the time of the impacts Mars itself had different rotational poles, with a lot of what is now the southern hemisphere sitting along the equator. Considering it has been theorized that Mars did have a significant rotational pole shift after the huge piles of Tharsis lavas all got built up on one side of the planet, the whole thing re-orienting to place the greatest mass of the Tharsis bulge along the equator, this could account for seeming high-inclination impacts which may actually have been from low-inclination moons -- it was the surface of Mars that changed in inclination, so to speak, rather than the moons' orbits.

-the other Doug

I remember as a youngster in the 1950s, being taught that the Pacific basin was probably the remnant of an impact that provided the material from which Luna was formed. Continental drift was discussed but only as a hypothetical in the context of the east coast of South America and the west Coast of Africa with the jigsaw fit pretty much dismissed as a coincidence. Mars today is in the same space. Lots of conflicting models and hypotheses that cannot be adequately tested and limited empirical data until such time as humans rather than robots get to ground proof Mars, if ever.

Or we get a nice sample return from Phobos and Deimos. Would they settle the matter once and for all in an Earth-based lab, anymore than samples from the Moon constrained theories of its origin?