Atmosphere

Puke · #1 January 31st, 2001, 11:28 PM

since we have completely destroyed this unsuspecting little thread, i will ask:

does anyone remeber what the gas giant hydrogen breathers were like in the David Brin books (uplift/startide/sundiver)? i dont, but i remember he had 'em.

Cybes · #2 January 31st, 2001, 11:48 PM

i only mentioned it is an aside. a mild attempt at humour, really. never expected it to spark such controversy.

as i said, it's not my theory, and i'm not the one to take it up with. it's also an older theory ("cosmos" was early 80s, i think), and may have since been discarded while i wasn't watching.

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"Just think of it as Evolution in action" - 'Oath of Fealty', by Larry Niven & Jerry Pournelle.

apache · #3 February 1st, 2001, 01:44 AM

Well, I looked up planet surface gravity, and all kinds of other stuff just to find out where you got your information. Yes, I did find plainly given stats that claimed the surface gravity compared to earth's surface gravity. Now, I calculated through the numbers using the universal law of gravitation, and the maximum radius of Jupiter. This gives the aforementioned statistics that Jupiter's surface gravity is approximately 2.5 times that of earth's. However, the fact is that the 'surface gravity' here is the gravity at Jupiter's outer atmosphere. We don't know where the surface of Jupiter actually lies, and as such, using such a statistic to calculate gravity at the 'surface' of jupiter is pretty ridiculous. If Jupiter had a solid core the size of earth, for example, the gravitational acceleration there would be about 3520 times that of Earth's.
Oh, and I still have no idea where you found that gravity is some function of density, because its absolutely untrue. It is a function of mass and distance only. This is related by the equation, a=GM/(r^2) where M is the mass of the planet in kilograms, r is the distance from the center of that planet in meters, and G is the univerersal gravitational constant, 6.672*10^-11. This gives you the acceleration due to gravity.

Puke · #4 February 1st, 2001, 01:53 AM

quote:
Originally posted by apache:
Oh, and I still have no idea where you found that gravity is some function of density, because its absolutely untrue. It is a function of mass and distance only. This is related by the equation, a=GM/(r^2) where M is the mass of the planet in kilograms, r is the distance from the center of that planet in meters, and G is the univerersal gravitational constant, 6.672*10^-11. This gives you the acceleration due to gravity.

im with you on all your statements, and it really supports what i was thinking. i have to point out though, that you could derive density from mass and distance (although you could obviously not do it the other way around).

Krakenup · #5 February 1st, 2001, 07:32 AM

quote:
Originally posted by apache:
Well, I looked up planet surface gravity, and all kinds of other stuff just to find out where you got your information. Yes, I did find plainly given stats that claimed the surface gravity compared to earth's surface gravity. Now, I calculated through the numbers using the universal law of gravitation, and the maximum radius of Jupiter. This gives the aforementioned statistics that Jupiter's surface gravity is approximately 2.5 times that of earth's. However, the fact is that the 'surface gravity' here is the gravity at Jupiter's outer atmosphere. We don't know where the surface of Jupiter actually lies, and as such, using such a statistic to calculate gravity at the 'surface' of jupiter is pretty ridiculous.

Well, if you want to be picky

, but remember that when you get all the way down to the center of Jupiter, gravity is zero because you have an equal amount of mass pulling in every direction. The pressure is pretty high though. The point, however, is that raynor and Barnacle Bill posted about how high gravity would be on a gas giant, but it isn't. It's similar to Earth's in many cases because of the low density of gas giants.

quote:
If Jupiter had a solid core the size of earth, for example, the gravitational acceleration there would be about 3520 times that of Earth's.

I'll have to question your math here. Jupiter has 318 times the mass of Earth so if it were the same size as Earth, it would have 318 times the density and, consequently, 318 times the gravity.

quote:
Oh, and I still have no idea where you found that gravity is some function of density, because its absolutely untrue. It is a function of mass and distance only. This is related by the equation, a=GM/(r^2) where M is the mass of the planet in kilograms, r is the distance from the center of that planet in meters, and G is the univerersal gravitational constant, 6.672*10^-11. This gives you the acceleration due to gravity.

Perhaps you weren't aware that mass is a function of density, namely density times volume, and that the volume of a sphere is a function of r^3 (4/3 pi r^3 IIRC). Therefore, the simplest form of the gravitational equation is a constant (4/3 pi G) times density times radius. So if you used this form of the equation, gravity would depend only on density and radius. In reality, the three variables (mass, density and radius) are interrelated, and the equation can be written using any two of the three. You can, however, have two planets with identical masses but different densities that have widely different surface gravity. In that case, gravity would vary with density to the 2/3 power.

dmm · #6 February 1st, 2001, 09:32 PM

Krakenup and Apache,
You're both confused. Fortunately for you, I'm a physicist, and I'm here to set you straight.

Both of your equations are correct, but you are both making bad assumptions.

If you use the equation that has only mass and radius, then you have to keep in mind that you only use the mass INSIDE the radius. Jupiter's solid surface, if it has one, is obviously way below the gaseous surface, so you would not use the total mass of Jupiter for your M. (It would be way too high.)

On the other hand, if you use the equation that has only density and radius, then you have to keep in mind that you have to use the AVERAGE density INSIDE the radius. Since the density increases as you get closer to Jupiter's core, you clearly should not simply use Jupiter's overall average density. (It would be way too low.)

But, of course, all this talk is silly anyway, because the heat and pressure at Jupiter's solid surface (if it has one) would be enormous. Nothing could live there. Any life would have to be in the atmosphere. At what radius? Who can say?

apache · #7 February 1st, 2001, 09:38 PM

First off, yes, the 3520 was a typo, one I noticed and thought I fixed, but apparently did not.
Now, no no no, density has nothing to do with gravity. It is only distance and mass, and density is a function of mass and volume. At a distance of R from some mass M, the gravitational acceleration at that distance is constant, no matter how dense the mass is.
If you have a black hole with the mass of the sun, it will definitely be a few million times denser than the sun. However, the gravitational force on the earth from the black hole will be exactly the same as the force the sun has on the earth, if the earth orbited a black hole at the same distance it orbits the sun.
Now, if you were at the center of Jupiter, theoretically you would either experience infinite gravity or gravitational force based on the mass of the planet. Logic says its no gravity, but then again, there cannot be zero gravity, in fact there would at least have to be a force completely pulling you apart because you are completely surrounded by the mass of Jupiter.
But then again, we really do not know since we have never been to the center of any planet or moon or anything to see what the gravitational forces would be.