Flyby a Comet!

[thomastech]

Here is NASA's Picture of the Day!

http://antwrp.gsfc.nasa.gov/apod/ap040119.html

Amazing detail from deep space!

[VanderL]

Here is an update on what the ideas are about the comet nucleus.
http://www.spaceref.com/news/viewpr.html?pid=13448
Surpsise, surprise a comet doesn't look like an icy snowball. Just like comet Borrelly, Wild 2 has strange features. In the text it is said that Borrelly didn't have interesting terrain, but there were also circular features and boulders. I see a surface that has been carved by electrical discharges and this process is still active, the active regions should all be on relatively high terrain.
Cheers.

[damienpaul]

It looks as if this comet is a KBO knocked into orbit or a moon thrown out of orbit by Jupiter, I would love to see a trajectory of its orbit. I wonder if that were the case what exactly would have catapulted it into a comet orbit?

[VanderL]

KBO, Kuiper belt object right? Just showing off again?
In the press release it is said that Jupiter nudged it in a new orbit, so the trajectory was at the same distance as Jupiter's orbit in one point, that's not where the Kuiper's are, correct?
Cheers.

[Tinaa]

No surprise, looks exactly like a dirty snowball. Parts of the comet has thawed faster than other parts and therefore we see the sink holes. In fact, back in the days when I lived where I got snow, it looks just like what happens to snowmen when they partially thaw and refreeze. I did not see any lightning!

[VanderL]

Come on, snowballs don't have 100 meter cliffs or circular holes with flat floors (how can the floors be flat?) and terraced terrain, and loose boulders. I don't see vents or crevices, but I don't know exactly what the resolution is. Maybe the place where the jets originate will tell us more.
Cheers.

[damienpaul]

Differential melting methinks, just as tinaa said, different rates of thawing can cause all sorts of terrain - and it is conceivable that the comet is a mish mash of different components - similar, well analogous to the moon Miranda in my books anyway.

[Tinaa]

Most snowballs aren't 5 km wide either. Those little tiny features on a 5 inch snowball become huge when blown up to 5 km size!

[VanderL]

So, where do the flat floors come from?

[VanderL]

By the way, In my opinion there is no dirty snowball, a comet is, just like an asteroid, a piece of solid rock, and I hope the return samples from Stardust will show this. All kinds of particles and elements (shortlived isotopes among them) and little if any sign of water or any other ice.
Cheers.

[Tinaa]

It has indeed been suggested that once the comet's icy material is boiled off it does become an asteroid. Also, comets tend to reflect light much better than asteroids, hence frozen gases and/or water.

[VanderL]

If only the mechanism of erosion was changed, it is the electrical view precisely. Apart of course from the ice part, if there is ice in a comet, it should be found throughout, not just the surface. And what about the craters 1 km wide on a 5 km object? They can't be impact craters, can we agree on that?

[Tinaa]

That is what they are! impact craters. Why wouldn't an object, flying through space be hit by another object? I most certainly know it is not from a a bolt of lightning that has travel through the galactic arm.

[VanderL]

Impacts on such small targets must be very rare, and what will be left of a 5 km body after an impact when a 1 km crater is produced? Unless there is direct evidence that it was an impact (the story until now is always, what else forms craters), the possibility it was a discharge shouldn't be discounted.
Cheers.

[damienpaul]

Large impact craters of that ratio are quite common, Phobos and Mimas for instance. If the impact is on ice, then one would expect that the meltwater would settle in a flattened floor. I am sorry, i see no reason for electrical doo-hickeys.

[VanderL]

What I meant by "should be very rare" is not that we don't see them very often, we see them everywhere, but my thought is that they should actually have destroyed the object.
Sorry, but those examples you mention, including the other rocks and moonlets with comparable cratering are all solid bodies. There seems to be no ice, at least I haven't seen any comments showing us that there is ice. Hmm, I really should find a way to show what I mean in pictures. That's what started me thinking. I'll try to find the pictures and explanations.
Cheers.

[damienpaul]

Actually, for example Mimas only has a density of 1.17 (almost the same as water) as seen on this website:

http://seds.lpl.arizona.edu/nineplanets/ni...nets/mimas.html (http://seds.lpl.arizona.edu/nineplanets/nineplanets/mimas.html)

Many others that I was alluding to also have very low densities

[VanderL]

You mean that low density is an indicator of ice? We know of ice on a lot of moons (especially the bigger ones). But I'm not sure we have direct evidence of ice on the cratered moonlets and comets. I'll look it up.
Cheers.

[damienpaul]

This is because if we take ice as haveing a density of around 1, then Mimas' 1.17 is very similar or it'll be a homneycomb, look at taht site, there are links to an incredible database of the entire soalr system.

[VanderL]

Thanks, great link, still leaves the question open how big a crater can get before the moon gets destroyed. The reason that electric cratering doesn't destroy a moon is obvious, but what about impact craters, are there any experiments with that famous canon?
Cheers.

[damienpaul]

well there is information from that site that alludes to the maximum size, Miranda had been hit pretty hard as had Mercury, with the Caloris basin...interesting question though.

[VanderL]

I found a link to the lab doing those experiments:
http://ares.jsc.nasa.gov/Education/website.../gunlabtour.htm (http://ares.jsc.nasa.gov/Education/websites/craters/gunlabtour.htm)
But I haven't found any reference to the maximum impact possible for a moon yet. I guess if there is any information on how difficult the visible craters are to reproduce in a laboratory we would know more the chances they are really formed by impactors.
Cheers.