Why do comets have a dust layer on their surface?

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fred20
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I was just watching the time-lapse footage of the Philae landing and was wondering:
How does a comet acquire and keep a dust layer on it's surface?
Can anyone say?

It just seems a bit odd to me.
 

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Dust is just small grains of particulate matter, and all matter is subject to gravity
Every object in the solar system originally formed from a gas and dust cloud condensing due to gravity.
In the case of large planetary bodies like Earth, gravity became sufficient that these small grains accumulated into a solid mass, while the gases became an atmosphere. There are still parts of the Earth's surface though (deserts) where dust is a predominating feature, though it's mostly there because of erosion of rocky material, it's not the primordial dust. A fair bit gets into the atmosphere too during events like sandstorms, but it can't escape Earth, the gravity is too strong.
On Mars dust is a predominant feature nearly everywhere on the surface.

Some dust from the primordial cloud still is out there in interplanetary space though, and some of it continues to be accumulated by Earth and other bodies. The larger grains are sometimes visible when falling to Earth as micrometoerites.

Smaller bodies like comets have very little gravity and are not very solid to begin with, they easily lose material when near the Sun (the comets tail) or may completely disintegrate when perturbed by another very large gravity field like Jupiter. Much of the dust gets returned to the mostly empty space from which it came.

The simplest answer to your question is that the dust is there because it's a principal component of what formed the comet to begin with, the other major one being water ice.
Comets have been appropriately described a 'dirty snowballs'.
 
Last edited:
rootone said:
Dust is just small grains of particulate matter, and all matter is subject to gravity
Every object in the solar system originally formed from a gas and dust cloud condensing due to gravity.

Smaller bodies like comets have very little gravity and are not very solid to begin with, they easily lose material when near the

Thanks for the reply but this really just states my confusion ...

This is how I see it - perhaps you can correct what I am getting wrong:
A comet is a small object with low gravity so is unlikely to have formed just by gravity spontaneously - and is more likely to have been formed
by ejection from a larger body somehow (collision/explosion etc) and therefore unlikely to be able to hold a surface dust layer.

As it is a small body with low gravity it is unlikely to have attracted sufficient dust particles to form a surface layer. Collisions with other
small objects would just scatter particles away wouldn't they?

I can see how a surface layer can happen with something as large as the moon - or Mars say ... but comets are very small by comparison.
Perhaps I am underestinating the force of gravity or I'm missunderstanding something more fundamental?
 
Comets are thought to representative of the very earliest stages of planet formation.
Although their gravity is miniscule it's just enough to hold them together.

When a comet is close to the Sun a lot of what little solidity it has is reduced further as ices begin to vaporize.
The comet develops a temporary gaseous atmosphere with a few bits of the heavier material mixed in.
Much of this is blown away by the solar wind, (which is a very weak force), forming the comets tail.
As the comet leaves the vacinity of the Sun what remains of this 'atmosphere', falls back to the comets main body, and that probably accounts for most of the powdery surface.
 
In addition to what rootone has said, it isn't just gravity that helps hold a comet together, there is a bit of cohesion also. The dust layer on the comet comes mostly from dust that was in the comet when it formed. ( one model has ice forming around dust particles and then colliding and sticking together to form the comet.) As the comet makes its close approach to the Sun these frozen gases and water begin to boil away. If the approach is not too close, this process can be weak enough that it does not blow away the heavier dust particles and they are left behind on the surface. (somewhat like what you see with a dirty snow bank as it melts). So the surface dust is dust that once was inside the comet and is left on the surface as the ices boil away. Of course, the closer the comet approaches the Sun during perihelion, the more violent the boiling action and the less dust is left behind. Thus the thickness of the dust layer on a comet can be used as an indicator of how close it has passed from the Sun in the past.
 
Thanks for the replies.
Somehow I'm still not convinced. Something just doesn't feel right about this when I try
to think through the life cycle of a comet. (A rolling stone... etc.)
I can't see a comet being created through gravity at all. And I still can't see how an
explosive creation would allow large amounts of dust to remain attached.
I can understand "vacuum welding" but presumably that would entail the comet passing through
vast amounts of dust to create the layer we see. That much dust doesn't seem to be there.
Collisions with large objects would create dust but a small comet wouldn't have the gravity to
return it to the surface - especially at speed.

I'm having a lot of trouble with the explanations of comets.
Is there a pc model maybe that demonstartes the current theories I wonder.