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Small Body Grooves, Theories for the formation of grooves on Lutetia and Phobos
bk_2
post Jul 11 2010, 06:25 AM
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QUOTE (Phil Stooke @ Jul 10 2010, 09:15 PM) *
The best set of grooves on any object since Phobos. This has to put an end to the 'grooves caused by Mars ejecta' argument. fantastic object and a wonderful data set. And this is just the highest priority data, all the rest still to come.

Phil


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.

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Mr Valiant
post Jul 11 2010, 12:06 PM
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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_S...5_telephoto.jpg

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Hungry4info
post Jul 11 2010, 01:51 PM
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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.


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alan
post Jul 12 2010, 12:35 AM
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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?
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kenny
post Jul 12 2010, 10:20 AM
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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.
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Phil Stooke
post Jul 12 2010, 11:01 AM
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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


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bk_2
post Jul 13 2010, 11:22 AM
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QUOTE (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,

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.


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Phil Stooke
post Jul 13 2010, 11:53 AM
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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


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tasp
post Jul 14 2010, 03:53 AM
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QUOTE (Hungry4info @ Jul 13 2010, 08:13 AM) *
. . . . . Anyone have any idea as to what caused this?




{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.


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bk_2
post Jul 14 2010, 08:37 AM
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QUOTE (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


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.

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algorimancer
post Jul 14 2010, 01:07 PM
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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.
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fredk
post Jul 14 2010, 03:55 PM
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QUOTE (algorimancer @ Jul 14 2010, 02:07 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.

What's the orbital speed like for Lutetia? Would decaying ring particles be travelling fast enough to produce much in the way of impacts?
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AndyG
post Jul 14 2010, 04:08 PM
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~50 m/s for a low near circular orbit. Less than ~80 m/s for anything bound to an orbit of Lutetia.


Andy
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bk_2
post Jul 15 2010, 07:58 AM
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QUOTE (algorimancer @ Jul 14 2010, 02:07 PM) *
I like the ring-intersection theory for creating the grooves.


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.
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tasp
post Jul 15 2010, 02:29 PM
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QUOTE (bk_2 @ Jul 15 2010, 01:58 AM) *
Your idea of rings around Lutetia (and others) in decaying orbits is new to me. What mechanism would cause the decay?



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
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