why do comets look like barren deserts in close-up pics when they’re described as dirty snowballs?

Question

The images don’t seem to show a lot of the activity we’d expect to see from gases and bits breaking off to grow the comet’s tail.

In: Physics

Answer 1

Nearly all of the water, ice and other volatiles on a comet are BENEATH the surface.

So ya, that’s basically it!

That’s the main straight forward answer to your question, as to why you don’t see water-ice in most upclose photos of comets.

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However, just to go into a bit more deeply

So… as the comet comes soaring into the inner solar system (where Earth is), is does the following:

– At a certain point it crosses over the “frost-line” of our solar system.

– The frost-line is the point that is close enough to the sun to begin heating and warming the surface, along with a few layers directly beneath the surface, to the point where ice melts, and water begins to evaporate/vaporize.

– So at that point, that’s when the water and other volatiles start to vaporize, and begin escaping from beneath the surface.

– Sometimes they can’t escape right away from below the surface and get trapped or blocked, and form pockets of gas.

– The pockets of gas then build up more and more pressure, then burst upwards, causing a small eruption on the surface, and a jet of gas.

– These jets of gas/volatiles can even act like mini rocket engines and change the speed, course of the comet!

– Anyways as the comet gets closer and closer to the sun, all these vaporizing volatiles begin to leave a hazy cloudy trail behind. That’s why you get these beautiful hazy tails with comets.

– NEXT, the comet swings around the sun, and then begins moving further and further away from the sun.

– It then goes back across the frost-line, freezing in a deep cold state, so that no more water and volatiles are escaping very much anymore.

– And so, if you visit the comet in the cold part of the frost-line border, then all you will see is a rocky surface, because on it’s last approach to the sun, the sun vaporized all the surface ice, and the stuff directly below.

– IN THE END, that means gradually with each crossing of the frost line and approach to the sun, the comet will loose more and more of its water/ice/volatiles.

– So it’s hazy comet tail will become less and less spectacular and impressive with each future orbital pass.