Small Body Grooves, Theories for the formation of grooves on Lutetia and Phobos Options

[Bill Harris]

I agree with tasp. My supposition is that ejecta and debris tends self-organize into thin rings and that the particles in this ring tend to eventually orbitally-decay, creating the lineations. I note that the lineatiosn tend to follow great-circle paths, which says to me "orbital" instead of "tectonic".

--Bill

[bk_2]

I will try to get hold of The New Solar System. Thanks for the leads on the mechanisms for ring decay.

I'm skeptical about the idea of decaying rings being the origin of the grooves. How could a decaying ring leave a family of grooves, parallel but separated by gaps many times the width of a groove? These are most apparent on Phobos, but Lutetia has them as well. If the ring orbits the center of gravity and decays onto the surface of a non-rotating body, it would leave a single groove all the way around. If the main body was rotating, and the ring was at high inclination to the equator, the groove would be smeared out. I can't see a mechanism for the creation of families of grooves, which in the case of Phobos, peter out on one hemisphere.

[algorimancer]

...How could a decaying ring leave a family of grooves, parallel but separated by gaps many times the width of a groove? These are most apparent on Phobos, but Lutetia has them as well. If the ring orbits the center of gravity and decays onto the surface of a non-rotating body, it would leave a single groove all the way around. If the main body was rotating, and the ring was at high inclination to the equator, the groove would be smeared out. ...

I would guess that the ring/surface intersection events are episodic (probably chaotically so). Once the ring begins intersecting the surface, the interaction would throw-up debris which would cause a rapid decay/depletion of the portion of the ring immediately closest to the surface, creating a distinct groove. Over time, this would repeat as the ring continues to decay. Yes, if the ring were equatorial this process would lead to a single ridge about the equator, however the grazing impacts hypothesized to create these rings are unlikely to be oriented on the equator, so they would be expected to have some random orientation to the pole. Rings generated by grazing impacts would be categorically distinct from planetary rings which derive from the breakup of (typically) equatorially orbiting satellites.

I'd love to try doing a simulation to validate the theory, as opposed to the mental simulation I'm doing here, but lack the resources. Perhaps the folks who worked on the Iapetus ring-intersection model would like to give it a go

[Stu]

How about encounters with comets?

These asteroids - and moons like Phobos - are ancient, dating back to the era of planetary bombardment. So, there they are, in orbit... a comet goes past... spinning... throwing off multiple, twisty-turny braids of material... they scour across the asteroid, leaving grooves... the comet either sinks back into the darkness or strikes the planet below...

Later, another comet encounter, same routine... the asteroid is orientated differently, so the fresher grooves are at a different angle to the older ones...

Just thinking aloud, don't mind me

[bk_2]

I would guess that the ring/surface intersection events are episodic (probably chaotically so). ...

Our best example of a grooved body is Phobos. 28Km long, escape velocity ~ 11m/sec. Any ring around Phobos would have been in slow motion, and tenuous since most of the debris from impacts would have escaped. Hardly the stuff to carve those trenches.

[brellis]

In the asteroid belt, and around Phobos, wouldn't it be possible for a substantial collision of two other moderate-sized bodies to produce a retrograde stream of debris that would, eons later, produce a series or multiple series of impacts at higher velocity? You might get parallel rows.

edit: bk2, your post #102 sounds a lot like what I initially meant to say. What if, long ago, Lutetia flew through the aftermath of a recent collision?

[hendric]

Could the grooves have been caused by slow-cooling basalt fracturing into a hexagonal pattern? A cooling protoplanet without plate tectonics seems ideal for this. That would lead to weak points that could be split apart by a large impact, causing grooves. Or maybe even from the heating and cooling caused by the sun? Hexagons have weak points at 120 degree separations, and that could create grooves that cross one another when they reach the top or bottom of the body. Depending on how the cracks propagated, there could be up to three groove families, crossing to form x, (should be X with strikeout line down the middle, normal X's ,Y's, y's, etc. This crop from the MRO picture of Stickney seems to show three sets of groove families intersecting:



Imagine taking a giant, tens of km wide piece of this

http://cumbriansky.wordpress.com/2009/05/1...artian-columns/ (haha Stu! )

Smack it around a bit with some giant rocks, scatter some dust, and let it sit in space heating and cooling for a few billion years, and maybe it would end up looking like Phobos or Lutetia.

Areas that broke up too much, say into hexagons or diamonds, would crumble to the point of losing their grooves.

Depending on the time of cooling and/or fracturing, craters could be made pre-grooves that later get sliced as the grooves widen. Or different groove families could be triggered by different crater impacts, giving them differing ages. It's hard to see what the grooves on Phobos or Lutetia would look like from directly above their crossing point though, so maybe the angles don't work out. I suppose I need to fold up one of Chuck Clark's high resolution constant-scale maps of Phobos!

Phobos is small enough it could have came from a larger parent body, but is Lutetia too big for that? I see that there is some controversy as to its type, an M type would make "sense", sourcing it from deeper in a parent body, near the mantle/core border. But Phobos doesn't appear to be the correct type, since it is likely carbonaceous.

Anyways, just a crazy hypothesis.

[AndyG]

A quick and dirty model of Lutetia suggests (if my maths is right) that the pressure within the asteroid rises to around 100 atm at the core. The sort of pressure people regularly dealt with in coalmines two hundred years ago with limited technology: really not much at all. It's not a value that rock is plastically deformed at.

Following billions of years of impact history, surely every asteroid of this sort of size is therefore just an agglomeration of solid chunks, boulders and fines.

With such low surface gravities, it would seem reasonable to think that the grooves and crater-chains we see are just the badly-filled-in gaps between major cracks, without invoking esoteric ring-impacts in every case.

Andy

[algorimancer]

... Any ring around Phobos would have been in slow motion, and tenuous since most of the debris from impacts would have escaped...

Phobos may be something of a special case, since it's in orbit around a substantial planet. As has been suggested elsewhere, grooves on Phobos may be due to intersection with a ring around Mars.

[algorimancer]

...With such low surface gravities, it would seem reasonable to think that the grooves and crater-chains we see are just the badly-filled-in gaps between major cracks, without invoking esoteric ring-impacts in every case.

I think that in the rubble-pile variety of asteroid, the rubble would be of random sizes randomly distributed about the surface, so any "badly-filled-in gaps between major cracks" would be irregular and centered locally upon the local chunk of debris, not a phenomenon involving neat near-parallel rows with global distribution following great-circle arcs. I gather that Lutetia is large enough to not really follow the rubble-pile analogy, though I'm sure it has a substantial depth of regolith.

[Bill Harris]

Agreed. I think that the real answer will be a combination of the abover hypotheses plus ones that have not yet been thought of. Until we can get there and do some geophysical work with chronologies, we're blind men trying to guess the true nature of that elephant...

--Bill

[bk_2]

Phobos may be something of a special case, since it's in orbit around a substantial planet. As has been suggested elsewhere, grooves on Phobos may be due to intersection with a ring around Mars.

Yes, by me, in the Phobos thread at Unmanned Spaceflight.com > Mars & Missions > Orbiters > Mars Express & Beagle 2. The response was encouraging if muted.

Phobos is our prime specimen of a grooved body (so far), it is the type specimen (in biological terms) and we recognize grooves on other bodies by similarities with the original.

Andy, you say "With such low surface gravities, it would seem reasonable to think that the grooves and crater-chains we see are just the
badly-filled-in gaps between major cracks, without invoking esoteric ring-impacts in every case."

Esoteric they may be, but in the case of the type specimen, ring-impacts look obvious. Why propose an entirely different mechanism for relics of similar grooves families on other bodies?

Hendric, "I suppose I need to fold up one of Chuck Clark's high resolution constant-scale maps of Phobos!" Yes, me too. Better would be a digital 3-D model with Phil's map overlaid, it could clinch, or disprove this idea. If I could turn the model to sight along the grooves I expect find them in planes, with members of families in parallel planes.

[Hungry4info]

Esoteric they may be, but in the case of the type specimen, ring-impacts look obvious. Why propose an entirely different mechanism for relics of similar grooves families on other bodies?

With multiple sets of prallel grooves covering nearly the entire surface of Phobos, some of which intersect more than once, I would say that ring impacts are in no way an obvious solution. Let's look at some bodies for which ring impacts are more likely to have occurred?

I've attached an image of Pan, Atlas, and Prometheus. While the effects of ring interaction on the first two are clear, it isn't as much so for Prometheus.
Either way, there's no real evidence for the kinds of features we see at Phobos and Lutetia.

A moon orbiting a planet in a ring will be at nearly the same orbital velocity as the ring particles. The "impact" will be rather soft. And as we can see from these moons at Saturn, they are overwhelmingly biased toward the equatorial regions of the moon.


So I think it is clear we need to propose an entirely different mechanism.

Attached thumbnail(s)

 

[hendric]

A moon orbiting a planet in a ring will be at nearly the same orbital velocity as the ring particles. The "impact" will be rather soft. And as we can see from these moons at Saturn, they are overwhelmingly biased toward the equatorial regions of the moon.

Well, a moon in a stable, circular orbit maybe. But a moon in an eccentric orbit would definitely have a different velocity than the ring particles. And it's possible the ring could dissipate by the time the moon circularizes its orbit. I like my "basalt hexagons" explanation (I'll make a prediction: We'll see basalt cliffs on Vesta!), but I could see how a captured moon around a planet with a ring system could get tidally locked to face the planet with one side, develop grooves in one direction, then get hit hard enough to break the tidal lock, relock in a different direction, and develop grooves again.

Be interesting to see an experiment using one of those high-powered gas guns, of shooting a plug of fine sand at a round ball of rock.

[bk_2]

A moon orbiting a planet in a ring will be at nearly the same orbital velocity as the ring particles. The "impact" will be rather soft. And as we can see from these moons at Saturn, they are overwhelmingly biased toward the equatorial regions of the moon.

Thanks for the images.

Pan, Atlas and Prometheus are ring-shepherd moons, which probably formed along with Saturn's rings. So their eccentricity was unlikely to have ever been significant, and the difference in velocity between ring particles and moon was low. Low velocity impacts are likely to result in accretion, hence the equatorial ridges.

High velocity impacts however, would throw out debris most of which would escape the feeble gravity, producing grooves rather than ridges.