How would that explain the closeness of the craters (Surely, the impactors would have spread out)? How would that explain the presence of the linear features all over Phobos? What about the lines that cross and curve?

Hungry4info

a late night thought

but i do think they are the result of tidal stress

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i have been working on something else and came across a "crater chain "
luna north pole but ...

Absolutly stunning images from orbit 6906 (21.05.2009)

2592 x 2248 pixels
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5184 x 4488 pixels
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1008 x 1018 pixels
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"I thought there was a theory that the sets of parallel lines were caused by ejecta from impacts on Mars? "

There was - still is in some minds, but I've never felt it was very satisfactory. My preferred idea is that the grooves are the surface expression of jointing (families of intersecting fracture planes) in the interior, possibly caused by large impacts, though not necessarily Stickney (the south polar depression is probably a very old crater larger than Stickney). They might also be caused by pressure release after excavation from the interior of a parent body, and opened by later impacts. The porosity we now expect can be explained as caused by open jointing.

Phil

Another stunning images from orbit 6916 (23.05.2009)

5184 x 5040 pixels
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1008 x 1018 pixels
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..... and from orbit 6926 (26.05.2009)

5184 x 2728 pixels
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1008 x 1018 pixels
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Wow, ESA just won a little more of my heart.

The fracture hypothesis Could be more likly because Phobos IS a rubble pile and surely tidal force from Mars might along with the larger impacts have caused some of the spaces between the rocks to be filled in by small particles and like a sinkhole here in Florida the above rock then collapsed to fill in the new space vacated by the particles that filled in the cracks and formed the holes on its surface.
The recent flyby could have been to determine if this is true.

Don't get too excited - these are all old images - May '09 mostly.

Even so, they are new to me.

Could the tidal fracture interpretation of the linear features on Phobos explain why some of the close lines are slightly curved and intersect each other? Or why the linear features are seen across the entire moon? I was browsing through the Phobos images I've so far downloaded over the years and came across this one. It hurt my confidence in the tidal fracture hypothesis.

Photos taken at the end of May 2009 and published in February 2010. It's an absolute record for ESA. It is cause for excitement and enthusiasm.

Looks like some large boulders rolled or bounced on the surface of Phobos, possibly disappearing in space afterwards

Hi, I've been lurking here a while but at last I've been prompted to post.

The old images of Phobos set me thinking again about the grooves. I worried about them for months a few years back. Now I’ve had a new idea. The trouble with the ploughing through debris theory [John B Murray 2006] is that at orbital altitude there won’t be strings of ejecta narrow enough to make those regular grooves, it has to involve some fine ring material. But if Phobos was ever orbiting in a ring it would have the same velocity as the ring material in which it orbits, it wouldn't have ploughed through anything. So here’s the new idea. If, during the period when Mars had these putative rings, Phobos had a more elliptical orbit, but in the same plane as the rings, it would have come screaming in and ploughed right through the rings at periapsis on each orbit. To create the grooves, Phobos would have had to be tidally locked as it is now, to present the same face to the flak, though it must have wobbled a bit, going by the angles of the grooves. (cf. Murray)

Perhaps the rings even formed in the same event that created Phobos, whether impact or capture, presuming there was enough material dislodged by the tidal disruption if it was the latter.

I understand that the orbits of smaller particles and dust circularise and flatten out relatively quickly by collisions, while a large chunk like Phobos, would slowly circularise by tidal forces, so if they formed together there would be a time when the orbits were different. Another point is that a common source for Phobos and the rings would be most likely to put them in the same plane.

The involvement of rings of fine material is crucial. How else could a groove form that extends the length of the flank in serene uniformity, than by collison with a narrow linear obstacle like a ring edge on? There are some less well defined grooves that are clearly crater chains, like the five little craters almost identical in size, equally spaced in a line parallel to one of the huge smooth trenches. These could have been caused by irregularities in the rings, bits where larger and fewer ring particles accumulate.

Obviously the rings didn't persist for long, they aren't there now, and Phobos must have disrupted their structure by it's regular visits. But they lasted long enough to leave the grooves.

Does it have any merit?

Would there even be any evidence on the Martian surface of rings? They'd have to be large enough to make it to the surface, and if the atmosphere was thicker in those days than there'd be no way of knowing.

We'll get new photos in a couple days anyway though.

I'd have to say it tickles my fancy, but I don't know enough about orbital dynamics to pick out showstopper flaws.
Certainly it strikes me as more in keeping with the overt appearance of the grooves, than mechanisms invoking tidal stresses or big impacts on Mars.
What you do need to explain is the quite uniform width of particular grooves, together with the significant variations in width between grooves. The widest grooves are clearly chains of individual impact craters that overlap to a limited extent. Your hypothesis requires impacts with a family of objects of relatively large and uniform size. Small craters are not evident. The narrower grooves aren't so easily resolved into individual craters, but are presumably the result of numerous impacts with families of smaller objects; again quite uniform within grooves, but with negligible 'contamination' by the large objects forming the wide grooves. Somehow you have to postulate a size distribution of objects within your rings that either spatially or temporally sorts out size classes.

I'll be hornswoggled if I can readily visualize that, but an orbital mechanics guru might be able to. It would be groovy if we could work out the sequence of deposition of the grooves. My first-glance WAG would be that the narrowest grooves deposited earlier than the widest, but I wouldn't bet my surfboard on it.

Best of luck with your "ring-whacker" hypothesis!

i just tossed out the idea as a late night idea . I was thinking of my bad brakes and in the past they have ground the disks and drums .

seeing as Phobos is just a ball of rubble ( like a chocolate chip cookie - with almonds)
the stresses from Apogee/Perigee ( .3 Km) small but add in meteor hits ( Stickney) and it is not a surprise that there are there .

I'm pleased to see new discussions of the formation of these unusual grooves. The ring hypothesis is unique, as far as I know. My concern would be that it would have to account for grooves in multiple orientations, including north-south oriented groove sets crossing the north pole from 90 to 270 longitude.

Also, don't forget there are grooves on Gaspra, Ida and Eros... they are not as numerous or organized as those on Phobos, but they still exist. Do we need separate explanations for grooves on different bodies, or can one be found that works for all of them?

Phil

I am just starting a new photomosaic map of Phobos, using the best modern images. Positional control will be from the Viking mosaic made by Damon Simonelli, Peter Thomas et al. at Cornell in the 1990s, but this will be 3x the resolution and using much better images. I'll post sections of it as they are completed, in a new thread. Maybe it will help the groove origin discussion, though I'm doing it for other reasons.

Phil Stooke

hi phil i found that the new isis shape model of Phobos i did was of not much use on the "low" res images
but if you want it let me know

I would like to elaborate a little on my "ring-whacker" hypothesis (thanks Shaka for the monicker).

Although the grooves generally extend from the leading face along the flanks of Phobos, they are not all parallel. They vary widely near the apex, they even cross eachother. But along the flanks they are nearly parallel as far as I can see, varying by ten degrees or so at most.

The explanation could be that as Phobos slammed into the rings on each pass, it altered the distribution of mass in the main body, changed its shape a bit. Being tidally locked, its orientation would have to change to accommodate the new shape, and on the subsequent pass it would be travelling in the new orientation, presenting a slightly different face to the flak. The shape seems to have been constant enough to keep the same hemisphere in the direction of travel, all the grooves peter out as they approach the trailing hemisphere, but that's about all you can say. Near the leading apex they are confused, with some grooves cutting others. (This presents an ex-archaeologist like me with an interesting example of stratigraphy, the groove that does the cutting must be later than the one cut.)

But the compelling evidence for this hypothesis is the morphology of the grooves. The best of them are smooth trenches a hundred meters wide and twenty kilometers long, straight as a Roman road, impervious to the terrain traversed, crossing crater walls even into the hollow of Stickney. How could such a feature arise? A ring would do it. A flat thin ring, met edge on. It would be like slicing the top off your boiled egg with a knife, only this knife didn't penetrate. The one good example of rings we have in the Solar System is Saturn's glory. I have read that they are of the order of ten meters thick. That sounds about right for a hundred meter wide trench. If Mars' putative rings were similar to Saturn's, they would have been ideal for trench-gouging.

Phobos' orbital velocity is currently about 2Km per second. If we consider that during the time of its elliptical orbit, its orbital velocity at periapsis was double that, while the ring particles were orbiting at it's current velocity, the delta v was about the same, 2Km per second. The impact of millions of tiny particles, in a ten meter thick layer, at 2Km/s would create just such a trench. The escape velocity of current day Phobos is less than 12m/s. Much of the ejecta from the impacts must have departed the scene, but there may also have been accretion. Either way, the distribution of mass would have been altered.

Some of the grooves are obviously crater chains, where the impactors were much larger than particles of dust. These are in much the same orientation as the long smooth grooves. I read that the particle size in Saturn's rings varies from ring to ring, and that some like the Anthe Ring Arc are limited in extent. During the formation of Mars' putative rings there may have been similar variation and irregularity, particularly with the disruption by Phobos. The larger pieces of debris from the formation event had to go somewhere. Maybe there is a mechanism in the formation of rings from a debris cloud that sorts out the particles by size. If so, the nice regular chains of similar sized craters at similar spacing could be the result of collision with such a size-sorted ring or arc. (I'm afraid I'll have to go and read some more about the theory of ring formation.)

That's all I can think of now.

I don't think the reorientation idea would stand up to a dynamical analysis, but it's ingenious and worth studying. A couple of other things to think about... would the process dig grooves or build ridges? (i.e. is the impact velocity large enough to do what you want?) And what about topographic shadowing? At the edge of the disk the particles would hit the forward-facing surfaces on ridges or crater walls, but not the leeward sides of them. Can we find examples like that?

Phil

No telling what we might find in a new high-resolution photomosaic map of Phobos!

Phil, I don't think I see any examples of topographic shadowing. It's something I hadn't thought about.

On refelction I have to concede that the shape-shifting idea is a bit lame. It doesn't look as though much matter was lost or accreted by the groove formation, at least not a significant proportion of the mass of the main body. If you filled the ridges alongside the grooves into the hollows it looks like it would restore the ungrooved surface. Not enough to change the centre of gravity significantly.

But collision with a ring could affect its orbit. Remember that this hypothesis rests on the idea that the orbit of the main body and the rings were co-planar. That in itself is a big ask, and maybe it is answered by a common original impact. Even so, co-planar is not that well-defined in a rubble cloud trying to settle down into rings. Phobos' orbit could have been perturbed sufficiently on each pass at periapsis to nudge the next pass a few kilometers higher or lower, the sort of separation we see between the grooves. Like aerobraking, if you get the angle just a bit wrong, it send the craft off-course.

If Phobos ploughed through the rings with its equator above or below the plane, making grooves in the middle latitudes, the path of the grooves should trace the intersection of the ring-plane and the surface, they should follow a contour. I have been trying to verify this using the folded paper model of Phobos published on this site some time back, but the angular shape is too crude, and anyway I couldn't figure out how to glue those last seams. A high-res digital 3D model would be really helpful. But if it proves true that the rings trace out the contour of a plane intersecting the surface, it would clinch the ring-whacker theory in my opinion. What else could Phobos hit that's flat and a few meters thick?

The other problem is the criss-cross pattern of grooves on the leading apex. It seems that the orientation about the direction of travel varied between passes, as if Phobos was oscillating about the gravitational minimum, mainly in latitude. We see something like that in the libration of the Moon, it wobbles in its orbit. The angles of intersection of the grooves means Phobos must have wobbled quite a bit, around 90 degrees. Perhaps the wobble was amplified by off-centre passes.

Does anybody know where to find a high-res digital 3D model?

JohnVV has provided us with this spectacular model for Celestia.

http://celestiamotherlode.net/catalog/show...p?addon_id=1359

Extremely interesting idea, bk_2! Even I can grasp it and visualise it without understanding orbital dynamics. Therefore I think it's elegant, minimal, and retrospectively obvious - all attractive properties. (Then again, I have a notorious tin ear... ) A few questions that occurred to me reading the very interesting discussion:

1. What would a ring/moon impact look like? I suspect it shouldn't be pictured as a series of point-events - multiple distinct impacts, releasing a burst of energy and creating a crater. Instead there'd be a continuous rain of impacting particles at any given point. (A curtain of dust raining down in a ruler-straight line... what an image!) How would the end-result of lots of small impacts in one spot compare to one big one, assuming the same (cough) aggregate mass for the particles as for the monolithic impactor? Does the outgoing ejecta interact with the incoming ring material?

2. How do the masses of the moons where groove-like structures have been observed compare to those of moons without grooves? Larger objects might have more energetic internal processes renewing the surface, or perhaps there's an upper bound to the gravity of moon. (Wouldn't stronger gravity fields mean the ring particle orbits would be rapidly disrupted?) I'm thinking of the wake images of the Saturnian shepherd moons and trying to visualise the dynamics with a larger shepherd, but I'm handicapped by ignorance of the calculus; and ISTR that these phenomena can be startlingly unintuitive!)

3. [EDIT: remove some of the foolishness.]

4. With respect to the crossing / intersecting angles of the grooves (and come to think of it, parallel grooves are presumably a special case of the general phenomena: something about the relative motion of ring and moon must have changed for the track to have moved across the moon's surface):
- perhaps the groove-creation isn't continuous. Perhaps it was only one orbit of the moon in 100,000 intersects the ring. Or 1000 consecutive orbits, but with each episode happening only every 10^2, 10^3, 10^4 years.

6. Perhaps the groove-creation happened before Phobos was tidally locked?

7. Why are the grooves discrete structures, rather than very broad bands? If the moon/ring aspect angle was changing, presumably it would happen at a steady state. If every orbit the moon makes intersects with the ring, whilst it's rotational axis is slowly precessing (or it continuous slowly rotating prior to being tidally locked), that would make the impact zones broad strips, or the shape of two very thin triangles touching at the apex. Instead, we see distinct grooves of similar widths, separated by apparently virgin surface typical of any common or garden rubble-and-dust-pile. To me this suggests that either:
i. whatever changes the location of the ring's intersection with the mooon was a sporadic, short-duration event; or,
ii. that the actual ring/moon intersection that caused the rings

8. As I understand it, Saturn's rings are largely fine dust and ice grains - 10^-3cm -- 10^2 cm or thereabouts. The masses of particles in rings resulting from impacts on Mars, or from tidal disruption of passing rubble piles, might be expected to have a different mass distribution curve.

9-99: ..?

Hungry4info but that cmod of phobos ( i have other formats) is a sculpted model .

done by hand and not made with 3d stereo ( or lidar) data

this is the same argument i have with t00fri
I am artistic and he is 100% scientific

@imipak

I told that to my five-year-old grandaughter, "elegant, minimal, and retrospectively obvious". She laughed like a drain.

But to the matter.

> 1. What would a ring/moon impact look like?

Dusty, I expect, ejecta from the buzz-saw of the ring would have obscured the view. Would it have interfered with the incoming ring? It depends on the trajectory of the ejecta. If you drop a pole flat in the pool, the splash it makes is a sheet on either side, angled away from the direction of impact. I expect the impact of the thin, colimated ring would do something similar, most of the ejecta would get clean away, from the incoming ring and the feeble gravity, and return to the rubble cloud.

> 4. ... Perhaps it was only one orbit of the moon in 100,000 intersects the ring

If the rock of this idea is sound, the grooves formed in the early period when the orbits of the main body and the rings were different, but co-planar. I think the series of grooves were lain down in short order, relatively speaking, before the rings were totally dispersed by the repeated passes. So I think the intersects would have been on each pass.

> 6. Perhaps the groove-creation happened before Phobos was tidally locked?

What we are discussing is the grooves that we can see on the current surface. There have been no significant changes in the shape of the moon since they were laid down, no scarps or groove discontinuities. So the c.o.g. hasn't changed much since the grooves were laid down. It had to be tidally locked in its current shape to present the same leading hemisphere. My point is that the tidal lock equilibrium could have been disturbed by an intersection. If the ring hit in the northern middle latitudes I can imagine it would rock it back on its heels, and set it rocking about the balance point.

> 7. Why are the grooves discrete structures, rather than very broad bands?

Yes, that's tricky. It depends on the angular velocity of the moon around the axis of its direction of travel at the time of intersection. How quickly did it wobble, or nod up and down around the balance point? That would depend on how close the c.o.g. was to the geometric centre, how unbalanced it was, and the inertia of the gross mass. I have to propose, without any visible means of support, that the nod was slow enough that the impact of each individual ring was not smeared out by the shifting terrain. But maybe the ring system consisted of many narrow rings, and intersection with each was brief enough to leave a narrow groove even though the moon was nodding.


> 8. As I understand it, Saturn's rings are largely fine dust and ice grains - 10^-3cm -- 10^2 cm or thereabouts.

Sure, but Saturn's rings are mainly ice and have a different origin. I don't know what it was, but it definitely wasn't an impact on the planet below. An impact on Mars would have thrown up a lot of rock, in chuncks of various size. They might have all been pulverized to dust but it seems unlikely. I suspect the proto-groove, the crater chains, are attributable to the remnant rocks.


There, some of them answered.

I hear hints that the Phobos images will be released on the 15th.

Phil

All right, then we're going to have to divide forces. Which of us UMSFers is going to go after the Phobos pics, and which are going to dive into the deep end of the LRO PDS release?

(*raising my hand for Phobos*)

I'll be at a meeting but it might give me a chance to do some light processing on the netbook with Gimp

My vote is for Phobos, since I doubt I could do much with the LRO images from that machine...

seeing as i already have a working folder for phobos and mro isis files
I would also need to install the lro isis files

And these things are BIG

http://www.esa.int/SPECIALS/Mars_Express/SEMK17CKP6G_0.html

Phil

With the latest spice kernels for Mars Express I just simulated the Phobos flybys with Celestia, am I right that the Phobos-Mars Express geometry is almost the same for every flyby?
The flyby distances were pretty accurate.

The ESA page says "Its origin is debated. It appears to share many surface characteristics with the class of ‘carbonaceous C-type’ asteroids, which suggests it might have been captured from this population. However, it is difficult to explain either the capture mechanism or the subsequent evolution of the orbit into the equatorial plane of Mars."

If it was a capture of a carbonaceous asteroid, possibly involving another body which was ejected in the process (which makes the capture hypothesis more plausible) could it not have shed a lot of matter by tidal disruption in passes below the Roche limit? The debris could have formed rings co-planar with the initially elliptical orbit of the main body.

If it was an impact, the impactor could have been a carbonaceous asteroid.

Although the ring-whacker idea has nothing to say about the evolution of Phobos' equatorial orbit, it must have been a factor in it's circularisation. Aerobreaking has been mentioned as a possible contributor. Ringbreaking would have been more effective than aerobreaking at transferring momentum, and would have occurred at a greater distance from the planet.

I wish there were a few more images on ESA page.

The lumpy appearance of the lineations on Phobos remind me of the boulder trails inside Victoria crater. Could this be a possible explanation? Are they lined up in the right direction? I note that Phobos is inside the Roche limit for a strength-less object so the boulder would be lost due to tidal forces once they reached the inner or outer parts of Phobos, relative to Mars.

Phobos grooves origin was an old thinking issue for me. I never believed about the explanation of being the result of crater ejecta sent to space from Mars impacts, because it would probably formed craters in Phobos rather than grooves.

I think Phobos is a captured asteroid. I know that a capturing mechanism is hard to explain unless an orbital energy loss of the captured asteroid should be introduced. Something like ours probes, which their retrorockets have to be operated in order to loss orbital energy to be captured by Mars instead of making a fly-by.

Phobos, as a prior free asteroid, had its own rotation. Now Phobos is tidally locked, so its rotation rate is equal to is orbital angular speed. Any moon tidally locked to its planet has some very know properties (Cassini laws):

- Its principal axis of inertia with minimal inertia is pointing directly to the center of mass of its mother planet.

- Other principal inertial axis is perpendicular to its orbital plane.

- The last principal inertial axis is orthogonal to the other two axes (in the direction of its velocity along the orbit).

There must be a time in which Phobos should have experienced tidal torques because of the non-alignment of its principal axis of inertia in the above mentioned way and because of its previous freely rotation rate before been captured were different from the tidally locked one. All this resulted in tidal torques applied to Phobos trying to force it to orientate its principal axis of inertia. This process generated internal energy inside Phobos and, also, differential tidal stresses in its ground and its interior.

The energy released in this process is the one that captured Phobos, circularized its orbit and tidally locked Phobos to Mars (and perhaps emanated volatiles along the grooves, forming what appear chains of craters).

At some point, as its rotation rate was decreasing, it should have experience a period of alternative torques (like a pendulum). All these effects generated dynamical differential tidal torques all along the moon (these stresses can be calculated, and I will do it when I´ll have enough spare time) which maybe can be the origin of the grooves. If the calculation of this mechanism predicts shear stresses in Phobos which alignments are consistent which the parallel pattern of the grooves, it will be a good theory for the origin of the grooves.

If this is a plausible mechanism, I guess it can explain some grooves evidences we see:

1. Why grooves are parallel and with origin in the closest point to Mars?, because are the lines of higher shear stress when Phobos was still in the process of been tidally locked to Mars. They converge in the point of the principal axis of inertia, which is the closes point to Mars, close to a point of the summit wall of Stickney crater.

2. Why are different families of grooves with small angle between them?, because Phobos could have had a small variation in internal density (is very porous) or because an impact, that change a little the orientation of its principal inertial axis.

3. Why Phobos has grooves crossing in almost right angle?, because in the past the principal inertial axis could be one orthogonal to the real now.

4. Why grooves cross almost all of the craters?, because craters were formed in Phobos when it was a free asteroid and grooves were formed later, in the process of beeing captured by Mars.

This is only a theory of mine (maybe I can be totally wrong), that I worked out a little, but still not enough to have minimal results.

Thanks. Hope explanation is easily understood.

The Mars Express blog has a post about the grooves of Phobos and their possible origin: http://webservices.esa.int/blog/blog/7. At the bottom of the post, there is a link to an article on the subject.

Here's a cross-eyed stereo version of the red-blue anaglyph released by ESA:

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http://webservices.esa.int/blog/post/7/1073

Looking at the images cross-eyed produces inverted relief. You need to swap the images for correct cross-eyed viewing.

Thanks for the heads-up charbobob! I've adjusted the image and replaced the incorrect version in my previous post.

ESA's Planetary Science Archive (PSA) - Mars Express new batch (2010-06-01). Orbits 7105-7697.
Nothing particularly exciting except Phobos and Deimos mutual event.
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A quick amination of that sequence.
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EDIT: A second version.
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Phobos thirty-eight years later. I'm not sure why the image taken by Mariner was published with a strange characteristic texture. I love this picture, it was with me all my life.
H6916_0000_S12
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Below:
all the elements of surfaces recognizable on both images have been marked by arrows.
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Q: Phobos is tidally locked; as Mars' North pole and rotation shift over the course of millenia, so will Phobos' orbit?

A: No.

Tidal lock has driven the rotation axis to be parallel to the orbital axis (and Phobos' long axis to point to Mars). This is irrespective of Mars' rotation axis orientation. Tidal locks will force obliquity to either 0 (north pole "up") or π (north pole "down").

Edit: Another thing of note: If Mars' obliquity caused meaningful changes to the inclination of the orbits of Phobos, Deimos, then it would be no mystery as far as their origins are concerned as to why their orbits are coplanar.

The rotation axis of Mars precesses and varies in obliquity. So is it a remarkable coincidence that Phobos and Deimos orbit in very close to equatorial orbits, or are their orbit planes tied to the planet's equatorial plane? That seems to be what brellis is asking, rather than a question about the rotation axis of the satellite. Alas, I know nothing about orbital dynamics.

Phil

FURTHER ORBITAL MECHANICS DISCUSSION MOVED HERE - ADMIN

Orbits 7701-8312 released. (ftp://psa.esac.esa.int/pub/mirror/MARS-EXPRESS/HRSC)
Phobos over Mars, the view more beautiful than before. The image is sharp as a razor.
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Orbit 7982

The most beautiful image sequence of passage of Phobos over Mars
Orbit 7982
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High resolution camera.

Bra-VO, ESA!!! Absolutely gorgeous!

Very spectacular!

Phil