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scienceteacher42
I am merely a science teacher and have a question that I can't get answered so I thught I would try here. The question is: If Saturn's rings are made of ice as are comets, why does the ice not sublimate as it is in a vacuum?
z
Hi,

One possible simple explanation is this. Ice sublimates in the earths atmosphere, because the density of the sublimating molecules is less than the density of the atmosphere.

This is not the case in space. The density of the ice molecules is greater than the surrounding space.

The acltual situation is probably more complex involving the kinetic energy of the molecules and the bond strengths of the ice matrix.

z
scienceteacher42
Well that sounds good but then why does freeze drying work so well in a partial vacuum? Isn't this accelerated sublimation?
ARtone
Hi scienceteacher42

As you mentioned comets I must put my opinion here.

It is MY belief that the movement of large bodies (anything of atom size and above) has a maximum speed which is very much less than the speed of light.

Research into this shows the maximum speed mentioned anywhere is 600km per sec 1440000 MPH for a comet passing within the acceleration distance of a Star.

Such comets are reportedly giving off 20 tons per second of particles which I believe is caused by the breakdown of the strong force of the atom nuclious at this speed which could well be the maximum large body speed possible.

This would mean that saturns rings are just not moving fast enough.

model t provides an answer to comet tails

AR
z
Hi Science Teacher,

You are right.

After considering my explanation, it couldn't be correct. It would have just the opposite effect. Sorry about that. I answered too quickley. I sometimes have a bad habit of doing that.

It may be that the temperature in space being so low, the molecules of water are bound much more tightly in the ice crystal matix. In this case very few if any of these molecules would have enough kinetic energy to escape the said matix.

z



ARtone
Hi z

Sorry that wont do either, you are forgetting comets. 20 tons per second.

AR
scienceteacher42
If this is the case then sublimation must stop at some temperature. I have never heard of a reference to this temperature. Space doesn't have temperature only the matter that is in it. Pluto it is said is about four kelvin so even there the particles have some kinetic energy. Saturn's rings are closer to the sun which has been shining on them for about 4.5 billion years and yet the ice (astronomers tell us) is still there. I just don't get it. I guess I was hoping for a simple answer to a question that has bugged the hell out of me for many years.

st42
ARtone
teacher42

there are 2 options

it isnt ice all the time it freezes and then it melts where would it go?

or its nothing to do with the temperature

AR
ARtone
hI 42

Lets examine this a little further

you have a Ice cube in space and presumable it is sometimes in the sun and sometimes shaded by a planet therefore it freezes and melts freezes and melts etc etc and it is being dragged relatively slowly through space by the attraction of the planet.

The ice melts possible even vapourizes where do you think the water or vapour goes? it doesnt go anywhere it simply freezes again. To remove particles from the mass it has to be travelling at a speed whereby the nuclionic attraction is broken down i.e. comet speed

AR
scienceteacher42
OK here it is: It is ice (let us assume) then through sublimation it becomes a gas. A gas then through brownian movement tries to fill the vessel. The vessel is infinite space. Ergo it cannot condense back into ice since it is on its way to infinity.
Why is it still ice. I still do not see an anser to my question. Why does space ice not sublimate (solid to gas). This is my quest and I am not going to give up until I get some ratioanl aanswer. I believe there is one but it has to convince me the eternal skeptic. Maybe the astronomers have it wrong and there is no ice. I mean, who has visited the rings or the comets and come back to verify the fact?
dirak
Hi all.

Why sublimation should occur there? It probably depends on an object temperature, which in space is dependent on how efficiently the object adsorbs and emmits radiation.

So, let me assume that temperature there is around tens of K (the mean temperature on Saturn (at the cloud tops) is 88 K), so there should be no sublimation at all (or probably negligible).

PS. I am not a specialist in space science, just my opinion unsure.gif .
z
Hi all,

I would say that temperature is the only variable in all these situations. The average kinetic energy of the molecule in the ice crystal is a function of temperature. There is also a variance in the kinetic energy that has a similar form at all temperatures.

The is a temperalture below which the highest energy molecules still do not have a enough energy to escape from the forces that bind it into the ice crystal.

Comets outgas due to the absorption of solar radiation which raises their internal average temperature They are "dirty" balls of ice so they absorb more solar radiation than pure ice crystals. . They constantly absorb the radiation for a great deal of their path time to and from the sun.

Ice crystals around Saturn would not absorb that much radiation. They are blocked sometimes by the planet Saturn for one. Also, they have a high degree of reflectivity, so they reflect much of the incoming radiation. Thirdly, due to smaller size, much of the absorbed radiation is reradiated into space in a short time, thus preventing internal temperalture rise.

Finally, the temperature of empty space as measured by the cosmic backgroung radiation is only around two degrees Kelvin.


z
dirak
Hi z,

So, you then agree that there is no sublimation , just because the temperature is too low there?


z
Hi drak,

Yes in general iIagree. Sublimation is unlikely.

There MIGHT be instances of SMALL amounts of sublimation at certain times, but due to the fact that the molecules have nowhere to go (they are in orbit the same as the rest of the ice crystal, they probably would just reenter the same crystal matix or possibly another one.

z
scienceteacher42
Thank you gentlepersons. You have given me much to think about. A lot of what you say makes sense. I will continue to try to wrap my mind around this problem
and now I know when I have another I will know where to go to ask for intelligent opinions.

Science rules

st42
ARtone
Hi z

Why if the comet asumptions are correct are the particles not being carried along with the comet. They remain in situ where they were ejected. Outgassing would only produce a minor force which I believe inadequate to overcome the foward momentum and this is in vast distances from the sun.

Outgassing to produce documented volumes of 20 tons of particles per second would need vast amounts of gas. And having been to Iceland and seen old natural ice it really dosnt contain gas or vast amounts of particles in fact its mostly translucent.

And in reality what is outgassing? 12 months ago I did a small project at the university which measured the products of melting ice. The only outgassing produced was from ice made of carbonated products.

AR
z
Hi AR,

Comets are not only made of ice. They contain many other elements and compounds.

z
Capracus
BigFairy at work.
http://www.worldofwar.net/gallery/data/500/southpark.jpg
yor_on
"One of the more important recent applications of vacuum technology is in large-scale industrial refrigeration. The rate of evaporation of water is accelerated in vacuum conditions and the process is used for freeze-drying foods (see Food Processing and Preservation). The water in the food is removed by sublimation into ice that, at the same time, freezes the food. "
When we freeze dry goods we keep the temperature near 273K (0.Celcius) and apply a strong vacuum. At that temperature the water gives of about 4mm Hg of pressure. What they do is to use a vacuum to turn ice directly into a vapor without having to go through the liquid stage. "If the temperature drops to 190K (-83.15C), the vapor pressure will go down to 0.0002mm Hg -or a drop of 4 orders of magnitude"

It seems that the temperature of deep space is around around 5K(-268.15C) which stops the sublimation process.

"The liquid phase, by the way, can exist only under a narrow range of conditions. For example, one can easily conceive of a situation in which the pressure is so low that liquid water cannot exist: at very low pressure, only at temperatures low enough to freeze will water have a vapor pressure low enough to remain condensed. This is, in fact, the situation in outer space; liquid water vented into space will boil and freeze simultaneously!"

You can take a look at this phase diagram to see what that water pressure will be at different temperatures. " The mean surface conditions on Earth, Mars and Venus are indicated."

And if you find it as confusing as I did you can check this first and then this ( which hopefully gave me a rudimentary understanding of it :)

Ps: In Sweden the funeral directors are planning to to use this instead of burning as it will be environmentally friendly :)
(It's Done by dry freezing the body and then shake it into a powder)
Kind of creepy if you ask me, humans as 'replaceable parts' sort of.
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