Comet rock... how?

[Kropster]

Seeing all the pictures of the comet taken by Rosetta and Philae, is was wondering just how the rocks manage to become so solid under the low gravity conditions.... hardly enough to crush the dust and gas particles together enough to form rock.

Or are comets the remains of early proto-planets that were subsequently smashed to smithereens?

In which case, they are not 'virgin' solar system material, but will have been processed at least once or twice.

Maybe?

[jamespels]

Pure guess, but...

If comets are made up largely of water ice (dunno how much but let's go with that idea), and some can melt when the comet is near perihelion, could it re-freeze to form water ice boulders? At minus many degrees they would be hard like this. The colour would come from all the other material.

Not sure how this could happen as liquid water should boil instantly in a vacuum bit could possibly happen deeper inside?

Like I said, pure guess.

[mercboyan]

I think that's a good guess.

Also possible that there is nothing out there to compete with the little gravity there is, but not sure if that would cause hard rock formation. 67p has cliffs and everything, so there is probably a melting/refreezing process happening. I can ask around at work today if you want.

[Knight of Clear Skies]

I believe we're mostly looking at ices that are coated with dust and carbon compounds. As the ices sublimate it leaves behind a concentrated sludge of less volatile compounds on the surface. The comet surface is very dark indeed as this image showing relative brightness shows.

[mercboyan]

Is that Enceledus?

I'm literally editing a video right now about how dark comets really are relative to other objects in the night sky. It's cool, but won't be out until next year.

The darkness is interesting, but still how can comets form such hard, solid rock?

[RichM63]

If you have a little free time and a desire for learning, why not sign up for this free Moons course from the OU. A bunch of us did last year, and it is well worth the effort.

It does go into formation etc.

[mercboyan]

I asked Dr. Bruce Betts. He's smart. Here is his response.

https://twitter.com/RandomSpaceFact/status/532988294930255874

"Why assuming hard rock? Hard ice, mixed in rocks, fluff snow? Ice is rock hard at at low temps."  -BB

[Macavity]

Reminds me of mixing Plaster of Paris etc. (Not my spoon / thumb though!) 

Fine powders can often look like bulk materials? - Form "boulders" and "cliffs", smooth surfaces can suddenly "crack" etc. I seem to remember from somewhere that a vacuum causes powders to clump? Just add the odd bit of water, here and there, and... Instant comet landscape?

[Kropster]

I asked Dr. Bruce Betts. He's smart. Here is his response.

https://twitter.com/RandomSpaceFact/status/532988294930255874

"Why assuming hard rock? Hard ice, mixed in rocks, fluff snow? Ice is rock hard at at low temps."  -BB

Thanks for taking the trouble mercboyan.

I think I was just fooled by the rock-like appearance of the comet.

Looks like it is just a visual similarity after all.

It will be interesting to see the results of the composition analysis of the comet when they are analysed.

[Kropster]

Though...thinking about it... how do these 'dust grains' form?

Unless they are crystals, I don't see why molecules of silicon dioxide and various minerals would clump together in solid form.

They certainly don't possess enough gravity and I would have thought were electrically neutral.

[mercboyan]

Ice as we think of it in our freezer is not that hard, but glacier ice is and under different pressures and temperatures, it can be as hard as rock. Comets are snowballs

[Macavity]

Though...thinking about it... how do these 'dust grains' form?

Unless they are crystals, I don't see why molecules of silicon dioxide and various minerals would clump together in solid form.

They certainly don't possess enough gravity and I would have thought were electrically neutral.

Electrically neutral molecules can still have an attractive (repulsive) potential between them.

http://en.wikipedia.org/wiki/Van_der_Waals_force

Parametrised as the famed Lennard-Jones potential of (long suffering) Physics undergrads.

http://en.wikipedia.org/wiki/Lennard-Jones_potential

[ronin]

Even with just a little attraction, gravitational, chemical or electro-static, it has had 4.5 billion years.

[Astro Imp]

Really interesting thread, thanks to the OP. 

[Kropster]

Electrically neutral molecules can still have an attractive (repulsive) potential between them.

http://en.wikipedia.org/wiki/Van_der_Waals_force

Parametrised as the famed Lennard-Jones potential of (long suffering) Physics undergrads.

http://en.wikipedia.org/wiki/Lennard-Jones_potential

Just as I thought!  

But is that enough to produce a solid 'grain' of dust... or is interstellar and solar dust not infact made of  grains as sand is, but just loosely bound clumps of molecules? A bit like a dust bunny?