Glacial erratic comparison is interesting but check out the right angles.

Of course, it’s possible Phobos’s orientation is chaotic; that its axis of rotation shifts from time to time… or that one of those impacts flipped it to a new orientation, leading to the two sets of grooves.

They’re landing strips for alien spaceships!

(Yes, it had to be said)

The sedimentary layering hypothesis belies the size of the damn thing: those would be some pretty big duneforms (note cross-cutting relationships), or if we are suggesting unconfromities, then we would expect some indication of changing rock type, wouldn’t we?

On the grooves, I question how they could all be caused by wandering boulders when they mostly fall into nearly parallel groups.

From a comment I posted previously:-

Phobos (and as it happens Deimos too) is tidally locked with Mars, that is to say it always shows the same ’side’ to Mars just as our Moon always shows the same ’side’ to the Earth. So tidally locked moons rotate once on their axis for each orbit around their planet. That means Phobos has a leading face and a trailing face as it orbits Mars. So if the current theory about the grooves being caused by debris thrown up from massive impacts on Mars is correct, that explains why the many grooves are in parallel. The rocks that drew the lines would not have had to arrive all at the same time. Even if Phobos encountered a rock in its path today, it would still roll the rock along its flank and the groove that it made would be in parallel with hundreds of existing grooves. The fact that there are other ‘families’ of grooves on Phobos suggests that it used to be oriented differently, when its leading ‘face’ was not that part of the surface that leads today.
Bob(Big)

None of these stands up to examination.

I propose a new idea. Isn’t it obvious? They are the tracks of rings. Phobos ploughed through a set of rings edge on early in its existence. No other mechanism could give rise to those long sublime trenches, straight as Roman roads right along the flanks.

Rings, what rings? There aren’t any rings around Mars now. But then rings are thought to be transitory phenomena, Saturn’s rings may not be all that old. And anyway, Phobos must have knocked them to bits if this is what happened.

I propose that Phobos and the rings were formed in the same event. Either a capture of an asteroid, or an impact on the surface which threw up a lot of material. If it was a capture I have to plead that the material which went to form the rings was dislodged from the original asteroid (or its partner if it was a binary system) by tidal forces on a close pass, below the Roche limit. If it was an impact there would have been plenty of stuff left over after the main body coalesced, to from rings.

Either way, the debris must have settled down into circular and planar orbits relatively quickly, through collisions. But the main body would have had an elliptical orbit which circularized more slowly.

The important consequence of both possible origins is that the orbits of the rings and the main body would be coplanar. During the period when the orbit of the main body was elliptical and the rings more circular, proto-Phobos would have intersected the rings at high speed on each pass at periapsis. With its tidally locked orientation presenting the same face to the flak each pass, you would expect to see the grooves starting on the leading face and continuing down the flanks, and none on the trailing face. A series of passes a few kilometers higher or lower would create parallel families of grooves.

Of course there is the difficulty that the grooves, and families of grooves, are not all equatorial and cross eachother. In fact just south-west of the crater Drunio (?) there is a field where they cross at right angles. The stratigraphy is tantalizing. One of the best grooves in the picture goes north-south, skirting the N pole. If it was the result of of intersection with a narrow coplanar ring, Phobos must have been turned through 90 degrees around its direction of travel since the the groove was created.

For these difficulties, I have to propose that the mass of the main body was altered so that the tidally locked minimum shifted. Perhaps Stickney was the cause. However there is one groove that extends into Stickney’s interior. It must have been laid down since Stickney. And thank goodness, it seems to be roughly equatorial.

The crux of this theory is the shape of the grooves. Bouncing boulders leave me cold, but a ring edge on, maybe ten meters thick, of dust and sand, coming in at kilometers a second, would do it. A cosmic buzzsaw sandblaster. It doesn’t look like there was much exchange of mass, the ejecta would have escape the feeble gravity, but it would have exchanged momentum, helping to circularize the orbit of the main body.

I put this idea up on UMSF some weeks ago and it was well-received, but it doesn’t seem to have got out into the wild. Let’s hope this generates some discussion.

BTW it’s called the “ring-whacker” theory.

Really cool imagery though. thanks for posting it Phil.

Those martian folk should paint it or something. Maybe we could send some paint up with the Russian probe.

Seriously – I wish every space picture would come with a little color guide, so I could tell what the colors mean and as in this case tell me if this is true color or just a b&w image.

In a science themed blog, shouldn’t it be about time imperial measures get kicked out the door for good?

On the grooves, I question how they could all be caused by wandering boulders when they mostly fall into nearly parallel groups. There’s the largest group that go around the body from Stickney to the other end, then there’s another bunch at the top of this pic that also seem to be parallel to each other. Also, the wandering track is actual craters rather than grooves. I can’t see that being caused by the same mechanism as caused the grooves.

I hope the Russian mission to retrieve a sample works well. No more lens caps dropping in the wrong places!