Printable Version of Topic

Posted by: bk_2 Jul 11 2010, 06:25 AM

The similarities with Phobos are striking, the photos clearly show two families of roughly parallel grooves, in two different planes. But the grooves seem to have been obliterated over most of the surface by later big impacts.

Once again I have to say they look like the tracks of intersection with rings, edge on. What else could carve a long smooth trench on the surface of a large object in space? Where Lutetia might have encountered rings is not going to be easy to answer, the chaos of the early Solar System is way beyond our scrutiny. The grooves do seem to be very old features, pockmarked with small craters, as well as restricted to areas clear of debris from the big ones.

Posted by: Mr Valiant Jul 11 2010, 12:06 PM

Darn, I know the 'grooving' is similar to features on Phobos, but it
also reminds me of the layering pictured by Apollo 15, on Mt Hadley, for example...

http://en.wikipedia.org/wiki/File:Silver_Spur_Apollo_15_telephoto.jpg

Posted by: Hungry4info Jul 11 2010, 01:51 PM

Interesting, Mr Valiant

Grooves on Lutetia, grooves on the moon, grooves on Phobos. Why are Phobos' so much more apparent? That may be a clue.

Posted by: alan Jul 12 2010, 12:35 AM

Thought experiment:

Take an asteroid, shatter it via an impact, reassemble into a ruble pile. Allow small impacts to generate a regolith to bury the pieces until it appears to be a solid object. Now you have an object made up of many piece with some hidden empty spaces.

Hit the end of the object (define this point as the pole) with a large body. Will the pieces making up the interior of the object shift in such a way that the outer dimensions change, for example, shrinking lengthwise while expanding the diameter of the equator? Could this produce parallel groves running away from the point of impact?

Posted by: kenny Jul 12 2010, 10:20 AM

Only if the constiuent pieces of the re-assembled atseroid were aligned in a precise way. If they are random (as seems likely) then parallel grooves also seem improbable.

Posted by: Phil Stooke Jul 12 2010, 11:01 AM

Organized grooves seem to me to require a fractured monolith. Impact jostling of the pieces could open those fractures to produce the observed bulk porosity, but the grooves could be linear.

Phil

Posted by: bk_2 Jul 13 2010, 11:22 AM

Phil,

Some of the grooves on Lutetia, and many on Phobos, are obviously crater chains that must have been formed by contemporary impacts of similar sized chunks, spaced quite close and somewhat regularly. Others appear less lumpy but still bear the marks of many smaller impacts, with some ribbing along the length. Surely these are of the same origin but with smaller chunks. Then there are the smooth grooves with no sign of individual impacts. Isn't it the same thing but with dust? Why propose a separate origin for the smooth grooves from that of the crater chains. They occupy the ends of a continuum.

And dust in a line dense enough to carve a groove implies a ring, encountered edge on, by a body coplanar with the ring but in an elliptical orbit.

I feel like the Ancient Mariner, buttonholing anyone who'll listen. But Phil, I just can't see impact jostling producing the surface of Phobos.

Posted by: Phil Stooke Jul 13 2010, 11:53 AM

Let's see if any of the objects that actually are close to rings today have similar features - objects like Atlas, Prometheus and Pandora. That might be a good test. Pandora does have a few, but oriented normal to the rings, I think. Still, careful study of these objects would be useful.

Phil

Posted by: tasp Jul 14 2010, 03:53 AM

{It's late here and I am tired, STTFWIW}

However, lets say good old Lutetia is made of fairly sturdy stuff. And from time to time, something that impacts Lutetia is going to be a very unsturdy, volatile rich object. So, if by chance, the volatile rich impactor manages to hit Lutetia on an existing crack, or fault, or fissure, the sudden release of pressurized gases from the impact might be channeled away from the crater site by the crack, and it blows the detritus in the crack out for a considerable distance. Depending on the crack geometry, the stuff might blow out kind of randomly over whatever distance it can.

Sort of like blasting the sandy, dirty joints in your sidewalk with an air hose. Only scaled up a bit.

Posted by: bk_2 Jul 14 2010, 08:37 AM

The ring-shepherd moons you mention have circular orbits, like the ring particles. At this stage of their lives they don't plough through anything, though they might have done in the past. Interestingly enough Emily posted a great picture of Janus today, and it also shows some evidence of grooves, though much eroded. Perhaps the phenomenon is fairly common.

If the orbits of these moons of Saturn were even slightly eccentric during the formative stages of the rings, they might well have encountered the proto-rings edge on.

I find it hard to stretch the notion of stratified rock jostled by impacts to all the objects with grooves. They are too diverse.

Posted by: algorimancer Jul 14 2010, 01:07 PM

I like the ring-intersection theory for creating the grooves. I would interpret them as being the result of the decay of one or more non-equatorial ring systems, which I could easily envision having formed as the result of grazing impacts on Lutetia. This is similar to the ring system decay speculated to have formed the equatorial bulge around Iapetus, except for a non-equatorial ring. We might anticipate that these would be common on asteroids large enough to support a ring system following grazing impact (enough gravity to keep the fragments in orbit). With this in mind, watch for grooves around Vesta and Ceres. We'd probably see them on the Moon if not for the instability of lunar orbit.

Posted by: fredk Jul 14 2010, 03:55 PM

What's the orbital speed like for Lutetia? Would decaying ring particles be travelling fast enough to produce much in the way of impacts?

Posted by: AndyG Jul 14 2010, 04:08 PM

~50 m/s for a low near circular orbit. Less than ~80 m/s for anything bound to an orbit of Lutetia.


Andy

Posted by: bk_2 Jul 15 2010, 07:58 AM

Your idea of rings around Lutetia (and others) in decaying orbits is new to me. What mechanism would cause the decay?

My theory is that the grooved body was in orbit around a much larger one, and that the rings were associated with the large mass. The grooved body had an elliptical orbit co-planar with the circular rings, and ploughed through them edge on as it swooped in for each close approach. For a tidally locked satellite this would also account for the lack of grooves on the trailing face, as we see so clearly on Phobos.

Posted by: tasp Jul 15 2010, 02:29 PM

Among other effects (Poynting Robertson effect, solar wind/plasma drag, magnetic field interactions, etc. Also, posit that Lutetia is strictly airless) ring particles in adjacent orbits (assuming their diameters exceed the difference in orbital radius) will gently 'bump' each other as their periods are different. Cumulatively, these 'bumps' transfer angular momentum across the ring. The lower edge of the ring will tend to lower further (till it contacts the surface at the highest point along the ground track) and the high edge will tend to expand, however, the other drag effects will probably constrain the expansion of the ring system at the high side. We might expect most of the ring to be eventually deposited on the low side.

Joseph Burns has a wonderful, detailed article on rings and ring processes in the excellent book, The New Solar System

Posted by: Bill Harris Jul 15 2010, 05:59 PM

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

Posted by: bk_2 Jul 16 2010, 08:13 AM

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.

Posted by: algorimancer Jul 16 2010, 01:09 PM

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

Posted by: Stu Jul 16 2010, 01:45 PM

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

Posted by: bk_2 Jul 17 2010, 04:56 AM

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.

Posted by: brellis Jul 17 2010, 06:59 AM

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 http://www.unmannedspaceflight.com/index.php?s=&showtopic=6371&view=findpost&p=162076 sounds a lot like what I initially meant to say. What if, long ago, Lutetia flew through the aftermath of a recent collision?

Posted by: hendric Jul 18 2010, 07:34 AM

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/2009/05/11/great-crumbling-martian-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.

Posted by: AndyG Jul 18 2010, 03:17 PM

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

Posted by: algorimancer Jul 19 2010, 01:08 PM

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.

Posted by: algorimancer Jul 19 2010, 01:16 PM

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.

Posted by: Bill Harris Jul 19 2010, 03:14 PM

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

Posted by: bk_2 Jul 20 2010, 10:56 AM

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.

Posted by: Hungry4info Jul 20 2010, 11:30 AM

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.

Posted by: hendric Jul 20 2010, 06:01 PM

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.

Posted by: bk_2 Jul 21 2010, 10:09 AM

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.

Posted by: Hungry4info Jul 21 2010, 11:18 AM

While I can see how that would apply to Phobos (though reorientation events would be required to explain the observed sets of grooves), I'm not sure it could be stretched to cover Lutetia. Lutetia, as far as we know, hasn't orbited a planet, and getting a situation to work where the asteroid would spend enough time in the vicinity of one to experience reorientation events would be difficult.

Furthermore, for ring impacts to create lines, the moon's orbit must be coplanar with the ring plane, otherwise an entire hemisphere gets blanketed when puncturing through the ring(s). In the case of Phobos, coplanarity with a ring would be best explained if Phobos formed from the ring. IIRC, Mars and Phobos are not believed to have the same composition, so having Phobos form from the ring is implausible. Another idea is that a hypothetical third moon could come in and get disrupted at the altitude where Phobos passed though on its current tidal inspiral toward Mars. This requires the hypothetical third moon to be coplanar with Phobos' orbit. If this is satisfied, we need a mechanism for reorientating Phobos. Interaction with a fourth moon that was since ejected?

Posted by: algorimancer Jul 21 2010, 03:06 PM

Here's the mechanism proposed for Lutetia. Envision a low angle (non-equatorial) impact which hurls a debris cloud into orbit, then the debris condenses into a ring. The mechanism for this is well established. For a variety of reasons, the ring orbit decays over time until the ring intersects the surface, forming a groove and depleting the ring material closest to the surface. Repeat until multiple grooves have formed, bearing in mind that Lutetia is rotating beneath the ring. Here's an illustration:

For Phobos, the ring is postulated to have formed about Mars, not Phobos, an entirely different scenario.

Posted by: Hungry4info Jul 21 2010, 04:19 PM

The bit about the body rotating beneath the ring puts a twist on the idea, I can see how it would create parallel grooves.

Such grooves would be distorted by the irregular shape of the body though. Do we see evidence for this on Lutetia?

Posted by: tasp Jul 21 2010, 04:41 PM

Ring materials will preferentially strike the highest elevated spot(s) along their ground track, assuming circular orbits. As we consider primary objects that are successively smaller in size, the irregularity of the object, on average, will increase. For altitudes that are sufficient high, even a quite irregular object will manifest a gravitational field as emanating from a point source, as the altitude decreases, however, the irregularity of the object and the resulting gravitational field will cause increased dissipative losses in the ring plane materials as the orbiting particles 'feel' the irregularities and experience velocity changes, and even out of plane effects. The ring 'particles' will grind amongst each other more, and the angular momentum transfer process will increase in efficiency in dissipating the ring structure.

For scenarios that might require considerable time for successive realignments of the spin axis, the rapid dissipation of the ring materials (either from the high or low side) would seem to present a difficulty in having sufficient duration while the materials are available.

Another complication, for orbiting materials to collapse to the LaPlacian plane, my understanding is the oblateness of the primary is the key factor in facilitating the process. For grossly non-spherical objects, I am not sure how oblateness might be characterized. Perhaps there might be some guidelines on how close the measurements of a triaxial ellipsoid have to be to each other for the LaPlacian process to commence.

Posted by: Phil Stooke Jul 21 2010, 05:03 PM

The first point of difficulty will be explaining how the debris forms a ring. Normally one would say that every piece of ejected material will be on a path that either escapes the object or falls back to the surface after less than one orbit. How is that changed to leave an orbiting ring?

Phil

Posted by: algorimancer Jul 21 2010, 05:22 PM

I would postulate that these ring systems, formed by grazing impacts, may be relatively easy to form on bodies of the right size range ... say 100 to 1000 km (pure speculation) diameter airless bodies. Thus there is no need to realign the spin axis ... one grazing impact will form a ring yielding a distinct set of grooves, then assume that there have been multiple such impact scenarios, each yielding a ring in a slightly differently inclined orbit.

[from Phil]...The first point of difficulty will be explaining how the debris forms a ring. Normally one would say that every piece of ejected material will be on a path that either escapes the object or falls back to the surface after less than one orbit...[...]
It's been quite a few years since I read the papers on the lunar origin theory which bases the origin of the moon on a grazing impact (I recall a major paper in Icarus, plus I have a book on the topic somewhere), but as I recall: during a grazing impact there develops a plasma interaction between grazing impactor and the ground, the net result of which is a momentum exchange such that proportion of the impactor ends up in orbit, rather than being lost to escape velocity or impact. Presumably this generalizes to larger asteroids. Considering that many smaller asteroids are rubble piles, it may be even easier for a grazing impact by rubble pile asteroid to leave a substantial amount of material in orbit to form a ring.

Assuming that all of this isn't complete navel-gazing, there is presumably some optimal size/mass of asteroid for which this sort of thing is most common. Some group with the appropriate computational resources (say those who worked on the Lunar origin scenario or the Iapetus ridge-via-ring scenario), would be in a good position to determine this through a series of simulations.

Posted by: charborob Jul 21 2010, 05:38 PM

I've been more or less following this discussion about ring systems around asteroids. Considering the number of asteroids out there, what would be the probability that a few actually exist at the moment? I suppose we would have to take into account the frequency of impacts and the life expectancy of a ring system.
Personally (and unscientifically), my feelings go to the "partially filled cracks" theory.

Posted by: fredk Jul 21 2010, 09:33 PM

How is the groove supposed to form? Impact of ring particles? At the very low Lutetia orbital velocity, wouldn't you expect the ring material to pile up in ridges, Saturn-moon style?

Posted by: tasp Jul 22 2010, 04:57 AM

I did a little math on the the possible ring derived Iapetan equatorial ridge system, (volume vs. deposition rate), and assuming a few simplifications, a deposition rate of 1 cubic meter per second can 'install' the ridge system in less than a thousand years.

I have insufficient math skills to come up with some estimates for how fast a ring system can redistribute angular momentum to shed material onto the primaries' surface from the low side, but I suspect the 1 cubic meter per second to be extremely fast. But even a limit of .01 cubic meter per second only gets you out to 10,000 years duration when there would be something 'fun' to see. In a 4 billion+ year old solar system, you would need a pretty serious asteroid collision rate to assure something 'fun' to watch in our era. I also note, in the case of Iapetus, it appears an extreme upper limit for the number of times it plausibly had a ring system would be <or = to 1.

As for splatting collisional materials into orbit, grazing collisions would seem to offer a substantial 'braking' effect while the contact between the bodies is occurring. As the impactor grazes, and breaks up, it seems plausible a significant portion of the impactor might end up in the correct speed regime to achieve some kind of orbit. Granted, materials lost on hyperbolic trajectories following the graze are lost for good. I think examining the surface of the primary 180 degrees around from the grazing collision sight would be interesting. As the lofted materials 'process' per the LaPlacian collapse to the equatorial plane (see the Planetary Rings chapter in The New Solar System for a fuller description of this) undoubtedly, considerable materials are going to clobber the primary, but unfortunately for us wanting to study the process, these tertiary craters are going to be spread over a huge area on the primary and are going to be difficult to sort out from all the other craters. As we see from the depositional rate, an essentially planar ring system does not require much mass to be interesting, so even a somewhat inefficient process for orbiting the raw materials, and another somewhat inefficient process for cranking those materials down to the equatorial plane does not seem to be an insuperable barrier.

Grazing collisions are quite interesting, the degree of overlap of the primary and the impactors radius, the approach angle, the differential velocity, the similarity of the compositions of the two bodies, are all going to make modeling these events rather complicated.

Posted by: bk_2 Jul 22 2010, 08:24 AM

That is the sine qua non of this hypothesis. I see two possible origins for rings coplanar with a moon. A grazing impact, as described by Tasp above, would produce a cone of debris with some distribution of particle size and velocity but essentially symmetrical. If some of the debris that didn't escape or fall back, coalesced into a moon, while the rest formed a ring, they might well be coplanar.

The other possibility is a disruptive capture of an asteroid, with some of the original mass torn off by tidal force in passes beneath the Roche limit, providing the raw material for the ring. This too is likely to result in coplanar ring and moon.

Posted by: Hungry4info Jul 22 2010, 10:37 AM

The Roche limits of the kind of bodies we're talking about are probably close enough to it that the irregular shape of the body is going to pose an immediate obstacle to forming a ring system.

Posted by: bk_2 Jul 23 2010, 09:55 AM

If the grooves on Lutetia were caused by the mechanism I propose for those on Phobos, the question is how Luetia could ever have been in orbit around a large body with rings.

We have evidence for a candidate large body in the asteroids, M and C types. The differences imply the existence of a large, fully differentiated body with a metallic core, that subsequently disintegrated. Perhaps there were several of these large doomed globes. Their disintegration must have involved catastrophic impacts, but it seems likely that before that there were many smaller, but significant grazing impacts giving rise to moons and rings in coplanar orbits. Lutetia could have been part of this.

It will be interesting to see how common the phenomenon is as we explore more of the asteroid belt.

Posted by: Hungry4info Jul 23 2010, 01:54 PM

There's still the problem of forming the ring in the first place. The irregular shape of the body (Lutetia in this case) would being any such rings down quickly unless the impactor hit the highest elevation point, which in itself is an improbable event. Impact velocity is also an issue. These bodies have very low (and irregular) surface gravities, getting the particles to make a nice neat line is going to be a problem.

Speaking of nice neat lines, how do you propose for the ejecta to form such a narrow, confined line? Debris spreads out after a collision, yet the grooves on Phobos, Lutetia are linear instead of fan shape.

And why would your mechanism be confined to small bodies? Why not larger bodies like the various moons in the solar system, who have stronger surface gravities to help encourage such secondary impacts and as well as pull in impactors?

Posted by: bk_2 Jul 23 2010, 11:28 PM

Perhaps I didn't make myself clear. Impacts on Lutetia played no part in the formation of its grooves. It was an impact on the body it was orbiting that formed rings and Lutetia itself. The ejecta coalesced into moon and rings, with the moon in an elliptical orbit coplanar with the rings.

I'm worried now that there may not be enough matter in the asteroid belt to account for a planet of sufficient size to support this hypothesis. It was big enough to differentiate. What is the minimum mass for a body to differentiate?

Posted by: DFinfrock Jul 24 2010, 12:54 AM

I was showing these images of Lutetia to my son and he remarked that the grooves were reminiscent of the concentric "shock waves" that appear when you toss a rock into viscous mud.

Could an impact into a compacted rubble pile produce shock waves that would show up as "grooves"? If that is possible it would make sense to see if any of the grooves align concentrically at a distance around large impacts. Of course if the grooves are really straight, rather than showing slight curvature, then that hypothesis dies quickly.

Posted by: AndyG Jul 24 2010, 10:44 AM

They'd be "straight" - well, great circles/great circle arcs - at the "impact equator" measured halfway between the impact point and the anti-impact point.

Hmmm...If your idea were right, there could be all sorts of interference patterns developing from a shook-up rubble pile following an impact. (I'm reminded of the wonderful phrase that the Moon "rang like a bell for hours" after Apollo hardware impacts).

Definitely a job for computer modelling, this.

Andy

Posted by: bk_2 Aug 17 2010, 08:23 AM

The Cassini image of the crater Penelope on Saturn's moon Tethys, from the recent flyby of three moons, Enceladus, Tethys and Dione, shows grooves crossing the lighted rim and within the walls. The crater is ~90Km across, the grooves look wider than those on Phobos, but they have the same characteristic family grouping, in at least one place. And there are a few curved grooves, the best long groove on the crater floor is one of them. The ring-whacker hypothesis predicts that the locus of each of these is in a plane.

Posted by: cassioli Aug 31 2010, 08:52 PM

This cool picture from our moon could possibly suggest an alternate explanation for Lutetia and Phobos trails:


Compare it with trails in this picture:


Rolling stones both and our moon, Mars moons and Lutetia?

Could a low-mass object like a moon or an asteroid cause such a "light" impact that material coming from crater, rather than being thrown away, just starts rolling around the body?

But, again, I don't know where rocks would be right now...

Posted by: algorimancer Sep 1 2010, 05:55 PM

I like this idea, with caveats. I wouldn't rule it out in all cases, but the neat parallel rows makes me think it unlikely.

Phobos is just really weird. Those grooves appear to slice through the moon, even through deep craters, and they intersect at arbitrary angles. There's no way I can envision those grooves as fractures, and the only way I could realistically see them forming is for Phobos to have had a few transits of a very thin ring about Mars. A ring around Phobos itself couldn't have created these grooves. And I find it hard to believe that a ring around Mars would be that thin.

It's almost like there are clouds of oriented strings of particles which occasionally intersect asteroids. Here's a way-out notion... perhaps nickel-iron asteroids with residual magnetic fields are able to maintain clouds of particles aligned along the magnetic field lines, perhaps with the assistance of electrostatic levitation (which has been observed on the Moon). I could envision a near-pass or impact from such a system onto an asteroid yielding something like what we see on Phobos. Modeling such a system is beyond me, but if they're in the asteroid belt they should be detectable by remote sensing or transits. There would presumably be an optimal combination of mass and magnetic field strength to create such a system. Alternately, perhaps such a system can be temporarily generated by the combination of induced magnetism and debris due to an impact, such that the resulting grooves are the result of the decay of the magnetic field dropping the suspended debris onto the surface.

[edit](These last couple of posts should probably be in the Small Body Grooves section)

Posted by: cassioli Sep 1 2010, 06:31 PM

what happened to last posts?!?

Posted by: centsworth_II Sep 1 2010, 06:34 PM

Are http://www.unmannedspaceflight.com/index.php?s=&showtopic=6683&view=findpost&p=163652 the ones? Moved to the "Small Body Grooves" thread.

Posted by: cassioli Sep 1 2010, 06:41 PM

without any warning?
this forum is magic!

Posted by: bk_2 Sep 6 2010, 10:02 AM

The grooves left by the rolling boulders on the moon are superficially similar to the grooves on Phobos, but they are recognizably different. For starters, there are no boulders at the ends of the grooves on Phobos, nor are there families of parallel grooves among the boulder tracks on the moon.

Algorimancer, you see the grooves as I do, the result of "a few transits of a very thin ring about Mars". But then you say you think such a thin ring improbable. Why? Saturn' rings are 10m thick across most of the extent. There are irregularities, collections of bigger boulders and wiggles on the edges caused by resonance with shepherd moons, shown so clearly at the recent equinox. If the putative (and now absent) rings of Mars were like that, you would expect the variety in the grooves we see on Phobos, from crater chains to smooth trenches.

Posted by: Hungry4info Sep 6 2010, 10:45 AM

You cannot argue that a thin ring will produce a thin line of craters without requiring the moon's orbit to be coplanar with the ring. Otherwise, when the moon impacts the ring, it does not just transit it, it's blanketed by the ring. An entire hemisphere gets hit.

Another problem is that unless the body is in a high eccentric orbit, the ring particles...


wait....

I've already mentioned all this.

I'm done with this thread.

Posted by: Fran Ontanaya Sep 6 2010, 01:25 PM

Hi.

How close should Phobos have orbited in order for Mars gravity to make one loose rock do suborbital jumps? Maybe it eventually broke apart, or reached escape velocity.

Posted by: ElkGroveDan Sep 6 2010, 07:46 PM

The Admin team has concluded that we've whipped this horse long enough and it doesn't appear to be going anywhere. This discussion is now closed.

If and when a published paper or other reputable study provides something new on these features, then a new topic can be started.