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nat
2004-Nov-10, 11:30 PM
why is the moon so baren?as it has been around nearly as long as earth
so it should realy be like a minny earth.
is it becaus it is to small?
or is it because it does not have a molten core?
:D

Doe, John
2004-Nov-10, 11:32 PM
Because it doesn't have any water or atmosphere, both of which are necessary to support life as we know it.

pghnative
2004-Nov-10, 11:37 PM
Welcome to the board!!

And to add a few details, one reason the moon has no water or atmosphere is because it is so small. So your guess was on the right track.

It is a little more complicated that that. As you post and read this bulletin board, you'll learn that smaller bodies farther from the Sun do have atmospheres and/or water. Basically, the closer you are to the Sun, the larger a planet/moon needs to be to hang on to light elements like water.

edited to note that our postings have crossed.

nat
2004-Nov-10, 11:38 PM
i know that but why doesnt it ?it is after all a part of earth.
why didnt it develop one like the earth did?

nat
2004-Nov-10, 11:45 PM
thanks native :)

Evan
2004-Nov-11, 12:54 AM
i know that but why doesnt it ?it is after all a part of earth.
why didnt it develop one like the earth did?

Not enough gravity.

zebo-the-fat
2004-Nov-11, 07:30 PM
You need the gravity to hold the atmosphere in place against the solar wind. It may only loose a small fraction of atmosphere every year... but it has had a VERY long time to looose it.

pghnative
2004-Nov-11, 07:32 PM
Is that the major effect? I thought it had more to do with low molecular weight compounds easily achieving escape velocity from low gravity planets/moons.

zebo-the-fat
2004-Nov-11, 07:45 PM
Is that the major effect? I thought it had more to do with low molecular weight compounds easily achieving escape velocity from low gravity planets/moons.

A bit of both i suspect!

Evan
2004-Nov-11, 11:44 PM
A bit of both i suspect!

Same thing.

JustAGuy
2004-Nov-12, 04:12 AM
Well, Titan is not too terribly larger than the moon, yet has a sustained 1.5 bar atmosphere. I'd venture to say that the moon's situation is a combination of lower gravity, distance from the sun (increased solar wind vs Titan = more atmosphere loss) and Earth, which being so near by will probably "steal" any atmosphere the moon might have.

Van Rijn
2004-Nov-12, 06:51 AM
It is likely solar wind would be a substantial issue for Earth over geological periods if it didn't have a magnetic field, especially in regards to hydrogen loss (Earth isn't massive enough to hold onto gaseous hydrogen). It is separate from thermal and impact loss. Titan is much colder, so that reduces the average molecular velocity which reduces loss. Also, it likely has large reserves of volatiles.

When impacts were more common, this was a significant loss method for Mars, again because of the planet's lower mass.

The moon probably didn't start with many volatiles, but it doesn't really matter: It would be too warm to hold onto a significant atmosphere for geological periods. There have been suggestions of "terraforming" the moon - and it could have a reasonable pressure atmosphere for thousands of years, but not millions. Not without further help, at least.

Evan
2004-Nov-12, 07:39 AM
Actually, our moon does have a trace atmosphhere.

http://www.iac.es/galeria/mrk/atmo_lun.html

jfribrg
2004-Nov-12, 01:39 PM
The main factor is whether the light elements can achieve escape velocity. Closer to the sun, the molecules are warmer and therefore faster. They are moving fast enough that they exceed the escape velocity of the moon (which isn't all that fast). Farther away from the sun, it is colder and the molecules are moving slower, so the same molecules would not escape from the gravitational influence of a moon-sized object.