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-   -   Shouldn’t ice on comets burn up quickly? (http://www.physicsforums.com/showthread.php?t=185929)

ZeuZ Sep20-07 04:58 PM

Shouldn’t ice on comets burn up quickly?
 
I see that the current explanation for a comets tail is that as a comet approaches the sun, it starts to heat up. The ice transforms directly from a solid to a vapour, releasing the dust particles embedded inside. Sunlight and the stream of charged particles flowing from the sun – the solar wind – sweeps the evaporated material and dust back in a long tail.

I just noticed that the tails on comets are huge, sometimes tens of millions of kilometres in length, it appears that loads of ice and vapour are coming off them. As the ice is continually melting all the time, shouldn’t many comets reach the point where the ice melts completely and only rock is left? They do get very close to the sun, and ice does melt quite easily.

If all the ice did burn up, and it stopped having a tail, would it then not be classified as a comet, or does it still count as a comet due to its elliptical orbit?

----------------

Maybe it’s due to the fact that bodies in a circular orbit (meteors) are travelling around the sun at a constant electric potential, whereas comets (having an elliptical orbit) are travelling directly through the electric field causing an electrical discharge 'tail' as the comet adjusts to the localised change in electric potential?

The only reason I say that is that Nasa's Galaxy Evolution Explorer recently took a picture of the rare sight of a star moving at 80 miles per second through a galaxy and it has a huge tale of electricity behind it. You certainly can’t claim that its ice coming off a star! - http://news.bbc.co.uk/1/hi/sci/tech/6947607.stm

Janus Sep20-07 05:48 PM

As large as the tail of a comet is, it is very, very thin. It is not much more a vacuum, so a typical comet doesn't lose a large percentage of its mass as it passes perhelion.

For example, Comet Halley at its closest approach to the Sun loses about 30,000,000 grams of water a sec. This seems like a lot, but it only adds up to a loss of a 1 meter depth of its surface per orbit. Since Comet Halley is over 5 km in radius, a loss at this rate would have Halley totally evaporated in about 1700 orbits or in something over 13,000 yrs.

Comets do eventually melt away and this happens faster for the short term comets. The dust trails they leave behind in their orbit are called comet remnants. From time to time, the Earth passes through these remnants and we get meteor showers. Two of the more familiar ones are the Perseids and Leonids, which occur annually.

ZeuZ Sep20-07 06:36 PM

Quote:

Quote by Janus (Post 1437181)
Comets do eventually melt away and this happens faster for the short term comets.

Thanx for the info. I didn't know comets were that big and had that much water/ice on them. out of interest, why don't any meteorites have tails of ice? they're made of the same stuff are they not?



also i found this: http://www.guardian.co.uk/science/20...aceexploration

it says (in brief):
The first examination of the dust particles collected from the tail of a comet, which were collected by the Nasa probe on a 2.8bn-mile (4.6bn-kilometre) round trip to comet Wild 2, has revealed minerals that could only have formed at blistering temperatures close to the sun. The finding has surprised mission scientists as comets are known to form in the frigid outer reaches of the solar system, at least 40 times further away from the sun than the Earth is.
"The interesting thing is we are finding these high-temperature minerals in materials from the coldest place in the solar system," said Donald Brownlee, the project's Washington University-based lead scientist. "It's certain these materials never formed inside this icy, cold body."

any ideas of how blistering temperatures close to that of the sun can be seen on a dead ball of ice?


and i also found an interesting paper on 'Spectroscopic Proof of the Repulsion by the Sun of Gaseous Molecules in the Tail of Halley's Comet' which looks into detail at the strong repulsive effect the sun has on the particles left after comets, and specifically looks at the tail of Halley's comet that you mentioned.
http://links.jstor.org/sici?sici=000...3E2.0.CO%3B2-Z this effect was predicted by plasma cosmology, and i think it may be a good indication that they were correct about the electrical nature of comets tails, which would also explain the high temparatures observed by the Nasa probe.

are there any pictures that NASA has taken of close up images of the water and ice vaporizing from the surface of a comet that would disprove that theory? i would like to see them. They claim comets are ionizing particles in the 'ion wind' and are charged bodies moving through a varying electric field produced by the sun, and thats the sole cause of the tail. Im not sure which is more plausible but both seem good explanations, the tails do look electrical in nature sometimes and their brightness is known to vary greatly.

ideas?

mgb_phys Sep20-07 07:23 PM

Quote:

Quote by ZeuZ (Post 1437244)
why don't any meteorites have tails of ice? they're made of the same stuff are they not?

Meteorites are lumps of rock/metalic minerals they aren't the same as comets.

Quote:

any ideas of how blistering temperatures close to that of the sun can be seen on a dead ball of ice?
The quote said - these minerals were FORMED at high temperatures, they aren't at high temperatures now. They were obviously formed close to the sun and either picked up by comets as they passed close in or the minerals were blown to the outer part of the solar system were comets form.


Quote:

They claim comets are ionizing particles in the 'ion wind' and are charged bodies moving through a varying electric field produced by the sun, and thats the sole cause of the tail. Im not sure which is more plausible but both seem good explanations, the tails do look electrical in nature sometimes and their brightness is known to vary greatly.
I think you are misunderstanding the article. There are two tails on a comet. One trails behind the comet and is bits of ice, dust and gas left behind the second is material stripped from the comet by the solar wind which forms a tail pointing directly away from the sun. On the way back out of the inner system these tails can point in opposite directions.

ZeuZ Sep21-07 04:53 AM

Oh right thanks for clearing that up, i suppose most things originate from stars anyway so finding that sort of temparature on a comet would be expected.


out of interest then, why are most comets icy, and no meteors are? I would expect the meteors to have the most ice as they are furthest away from the sun so are less likely to melt (ones in the asteroid belt at least)

mgb_phys Sep21-07 09:53 AM

Quote:

Quote by ZeuZ (Post 1437811)
out of interest then, why are most comets icy, and no meteors are? I would expect the meteors to have the most ice as they are furthest away from the sun so are less likely to melt (ones in the asteroid belt at least)

Meteors are much closer to the sun - the asteroid belt is between Mars and Jupiter only 3-4 times as far from the sun as we are - right next door in solar system terms.
The comets are in the Oort cloud about 50,000 times as far from the sun as us. At this distance the temperature is low enough that water and pretty much all 'gases' are solids.
Also there isn't much out there except comets so while all the space ice/dust/rock etc in the inner solar system has been swept up by hitting passing planets, out there it is undisturbed unless picked up by a comet.

We only see comets when one of them is disturbed somehow and begins to fall toward the sun. Most pass by the sun and head back out to the cloud on a very long period orbit. Only a very few are caught in an orbit that brings them back quickly like halley.

ZeuZ Sep21-07 02:47 PM

Do some meteors have tails then? I guess what i'm asking is if a comets ice was all melted by a star and it stopped having a tail is that comet then classified as a meteorite or still a comet?

i just want to try to disprove the proponents of the 'electric universe' model who claim comets tails are created solely due to their highly elliptical orbit taking them through the earths electric field creating an electrical/plasma discharge. http://www.rense.com/general63/elele.htm . They say this theory fits nicely as meteors that travel in a circular path never have tails due to the fact they are staying at the same electrical potential in the suns electric field.


this could be disproved if there are bodies in the solar system that take a similar path to other comets, but have no tail as they have no ice on them. Are there any comets like that?

mgb_phys Sep21-07 02:57 PM

Meteors don't have tails because they are in a pretty stable orbits the same distance from the sun. Any volatile material on them that was going to boil off and make a tail has long since done so.
Comets have tails because they move from an area where ice is stable to nearer the sun where the ice is vapourised and is lost from the comet.

Comets do eventually evaporate, I think Halley loses something like 50tons/second when near the sun. But since comets are mostly small chunks of rock held together by ice, when they lose all the ice they end up as a loose cloud of a few random bits of rock. They may follow the same path for a while but will eventually get swept up by something - typically Jupiter.

Comets tails are due to their orbital paths - comets don't have tails while out in the Oort cloud, but this is nothing to do with Earth or electric fields.

Janus Sep21-07 05:26 PM

Quote:

Quote by ZeuZ (Post 1438273)
i just want to try to disprove the proponents of the 'electric universe' model who claim comets tails are created solely due to their highly elliptical orbit taking them through the earths electric field creating an electrical/plasma discharge. http://www.rense.com/general63/elele.htm . They say this theory fits nicely as meteors that travel in a circular path never have tails due to the fact they are staying at the same electrical potential in the suns electric field.


this could be disproved if there are bodies in the solar system that take a similar path to other comets, but have no tail as they have no ice on them. Are there any comets like that?

The asteroid Icarus has a highly ellipitical orbit which has a perihelion closer than Mercury and a aphelion about twice the rafius of the Earth's orbit and it shows no tail.

Not only that, but the behavior of comet tales themselves disputes this theory. As a comet nears perhelion, the path of the orbit becomes more and more perpendicular to the radial line of the orbit. IOW, the rate at which the distance between the comet and the Sun changes decreases until, at perihelion, it stops altogether. If a comet's tail was due the Comet moving across the electric potential of the Sun, then the tail would shrink as the Comet approached perihelion and disappear entirely at perihelion. We do not see this happening.

mammalian Feb13-11 07:56 AM

Re: Shouldn’t ice on comets burn up quickly?
 
The comet Tempel was found by a NASA probe to have only 6% water ice on the surface. To me, a comet core just looks like an asteroid. The NASA Deep impact pages really seemed to be trying to imply that they were correct all along. See for yourself - it just sounds very suspect. They still seem to be saying that comets are made of comglomerations of dust and ice. They couldnt/didnt measure the proportions of substances identified. It does show that the comet is markedly layered (like a rock) and contained the silicates olivine and spinel also clay minerals and carbonates.
The Deep impact site explanation for the layering is "comets might be formed in layers. Imagine two small proto-comets smashing into one another, sticking together and flattening like pieces of playdough," Hmm
What would occams razor suggest if - it looks like rock. It has layers like rock. It is at least partly composed of rock (ice chambers inside are only infered and only tiny crusts of ice on surface).
Also.. It is cratered like an asteroid.. do craters look the same in ice as they do in rock?

I think a good possible explanation is that comets are mostly rock. The layers could be formed on a planet or moon etc and was blasted into space by collisions or whatever.

Tyrannical Feb18-11 12:35 AM

Re: Shouldn’t ice on comets burn up quickly?
 
Comets have enough gravity to trap some of the surrounding water vapor. Once it cools (comet moves away from the Sun) it could re-accumulate on the comet.

mammalian Feb21-11 09:22 PM

Re: Shouldn’t ice on comets burn up quickly?
 
Quote:

Quote by Tyrannical (Post 3143921)
Comets have enough gravity to trap some of the surrounding water vapor. Once it cools (comet moves away from the Sun) it could re-accumulate on the comet.

Comets are very small (like a few kilometers diameter i.e low gravity) and moving at speed throughout thier orbit. I would imagine these combined factors would not be conducive to accumulation of ice/dust. A stationary (relative to other bodies) asteroid maybe... I can see how matter (that is very close by) could be (very slowly) accumulated then ..but not at speed with such weak gravity. I think NASA are trying to make reality fit into theory rather than being unbiased scientists.

This lack of ice (or alot less than was first assumed) must be a problem in the current theory of how Earth got its water.
Its a shame the comets arent balls of dusty ice as we thought (well, all the ones closely observed anyway)... all that water would have come in handy in the future...


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