Question of the Week

How long does it take for comets to "melt"?

Thu, 4th Apr 2013

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Question

Jane Maddin asked:

How long does it take for a comet to completely 'melt' away from repeated exposures to the sun? (Composition and size would certainly have something to do with it, I'm sure) Will there come a time when all the comets are gone from the Solar System because they have sublimated?

Answer

We put this question to Katrin Ros, a researcher in Astrophysics at Lund University, Sweden...

Katrin -   Comets are actually large clumps of ice and rock that were left over when the planets were formed.  We can think of them as huge dirty snowballs orbiting the Sun.  Being snowballs, this means that they can melt and they also actually do melt.

Comet Hale-Bopp as it flies over the sky of Pazin in Istria, CroatiaEvery time a comet passes close to the Sun, part of its sublimates, meaning that the snow and ice turn directly into vapour and this is the vapour that we can see as one of the tails of the comet.  So, since part of the comet sublimates every time it passes the sun, it cannot live forever.

For example, we have Haley’s comet which is quite well-known since it passes the Sun once every 75 years or so, and this comet will be completely sublimated and disappear after only 10,000 years or about 100 rotations around the Sun. 

This is a typical lifetime for a comet.  But still, this does actually not mean that there will come a day when all the comets are gone and that’s because there are huge hidden supplies of comets waiting beyond the orbit of Neptune.  That’s the Kuiper belt and the Oort cloud.  And these regions continuously replenish the inner Solar System with new comets.

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At one limit, just one approach, if the comet has an extremely elliptical orbit that takes it directly into the Sun.
At the other extreme, a comet on a circular orbit well beyond Pluto will hardly sublimate at all. evan_au, Fri, 22nd Mar 2013

There are some estimates that periodic comets such as Halley's Comet will eventually melt away.  Estimates vary from 10,000 years to 10 million years, and that it may have already lost as much as 90% of its mass during the last few million years.

Measurements of the age of Comet Wild 2 indicate that it likely was formed at the same time the solar system was formed, or or about 4.5 billion years ago.  So, either the comets orbits vary significantly over their lifetime, or they may have some extraordinary longevity.

One possibility with a comet such as Halley's comet is that it might spend a few months loosing material as it passes the sun, then it may spend the next 75 years vacuuming up micrometeorites, and small molecules from its path.  And, thus the overall loss may not be as much as might be expected.  However, with a return every 75 years, it could take centuries of precise observations to get precise data about its loss.
CliffordK, Fri, 22nd Mar 2013

Comets have been called "dirty snowballs", and it is evaporation of the volatiles as they approach the Sun which give comets their characteristic tails.

If a comet on a very elliptical orbit meets with another orbiting object (eg micrometeorites), they are likely to have a closing velocity of 1-30 km/sec. This is more likely to turn all volatiles in the micrometeorite into a plasma, as well as a similar volume of material from the comet. This plasma will most likely be lost to the comet since it has almost no gravity. So I suspect that any vacuuming will be mutually destructive.

On the other hand, dirty snowballs in the Oort cloud which have nearly circular orbits could collide at much lower speeds, and succeed in producing one large snowball (plus some splatter which goes into independent orbits). evan_au, Sat, 23rd Mar 2013

I think ill eat pizza on friday Bob Zimbobway, Tue, 10th Dec 2013

makes sense Andy, Sat, 24th Oct 2015

Any rocks in the comet will be left over as asteroids. Steve. Meyers, Thu, 22nd Sep 2016

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