[X] My Assistant

[Phil Stooke]

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

[bk_2]

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.

[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

[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.

[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.

[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.