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

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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

The words “dirty snowball” don’t actually mean a snowball in the sense you’re thinking. It’s just a way to describe that the vast majority of the comet is water ice mixed with various “pollutants” I.e. not h2o

Also most pics are taken in the deep – far enough from the sun that the energy required for melting the ice and creating the spectacular show we associate comets with is not there.

As such the tail is not something that’s always there since it’s matter escaping from the comet which requires energy input (sunlight) of a certain magnitude.

. Well, technically you could say there is a kind of tail even in the deep since some of the matter persists in the comets orbit. Most of it is gone “soon” enough though.

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.