Phobos Options

[JohnVV]

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 .

[Phil Stooke]

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

[Phil Stooke]

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

[JohnVV]

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

[bk_2]

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.

[Phil Stooke]

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

[Shaka]

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

[bk_2]

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?

[Hungry4info]

JohnVV has provided us with this spectacular model for Celestia.

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

[imipak]

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: ..?

[JohnVV]

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

[bk_2]

@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.

[Phil Stooke]

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

Phil

[elakdawalla]

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*)

[volcanopele]

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...