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Captain Kidd
2004-Jan-23, 03:16 PM
Ok, this has bugged me off and on over the years, not enough to lose sleep over but enough to annoy me each time I think of it.

It is supposed that at one time Mars had an atmosphere and it bled off into space due to the weaker gravity.

That I understand. What I don’t get is; how’d it get one in the first place if its gravity was too weak to maintain one?

Was it, A)the gasses escaped out of the surface at a rate equal or close to that which it bled off into space or B) the formation of Mars created the atmosphere and the bleed-off rate was slow enough to support the supposed bodies of water long enough to carve the canyons and whatnot? (C) I'm waaaay off in my understanding of how it lost its atmosphere, if it had one in the first place.) Is there even a timeframe yet as to how long Mars had an atmosphere?

cyswxman
2004-Jan-23, 03:35 PM
Not trying to be a nit, but Mars does have an atmosphere, albeit a very thin one. Surface pressure is about 6 millibars (Standard surface pressure on
Earth at sea level is about 1013mb).

As to if it had a thicker atmosphere in the past, and what happened to it, I am not sure. :-k

wedgebert
2004-Jan-23, 05:07 PM
Lots of things can cause the gravity to disappear. For example, the lack of a magnetic field means that the solar wind can blow away the atmosphere over time.

aurora
2004-Jan-23, 06:25 PM
Try The Life and Death of Planet Earth by Ward and Brownlee.

It covers the topic of atmospheric loss by a planet.

Antice
2004-Jan-23, 06:42 PM
Both Mars and Earth had a thicker atmosphere in the past.
Due to the low Gravity of Mars and the smaler magnetic fields the loss of atmospheric gases to space would be much greater that what has happened on earth.
A lot og the lost "air" would of cource be replenished with gases evaporating out of the ground and any open bodies of wather.
However. it all ads up over time, and over millions of years the atmosphere has gotten thinner and thinner. On earth we are apear to be lucky enough to have the right conditions to preserve a fairly dense atmosphere without to large a loss per year. (note: i could not find any research delving into how large the actual losses are. anyone?)
Research do show however that the present density of earth atmosphere is incapable of suporting creatures as large as the dinosaurs where, and it is reasonable to asume that the density of earth's atmosphere might have been quite a lot denser than it is at present.


ref:NASA news article (http://science.msfc.nasa.gov/newhome/headlines/ast08dec98_1b.htm)

informant
2004-Jan-23, 07:01 PM
Research do show however that the present density of earth atmosphere is incapable of suporting creatures as large as the dinosaurs where [...]
Can you give a reference for that?

Captain Kidd
2004-Jan-23, 08:03 PM
Hmm, ok. I was wondering, esp. when I read books like the Mars series (whatever the official name is) Red Mars, Blue Mars, Green Mars. My thought was, well, that's fine and dandy to reintroduce an atmosphere (btw, I knew it has one already, but sitting next to your boss tends to make you type fast :) ) but it'll just bleed off again.
However, I guess if it takes millions of years then they'd have enough time to keep it maintained.

aurora
2004-Jan-23, 08:26 PM
A lot og the lost "air" would of cource be replenished with gases evaporating out of the ground and any open bodies of wather.


Volcanoes, too.

Here's a Q&A session on the topic of gravity and the atmosphere:

http://www.newton.dep.anl.gov/askasci/phy00/phy00589.htm

Antice
2004-Jan-24, 12:31 AM
Antice wrote:
Research do show however that the present density of earth atmosphere is incapable of suporting creatures as large as the dinosaurs where [...]

Can you give a reference for that?

After doing quite a bit of searching, i could not come up with anything reliable in evidence for this. :oops:
and there is plenty of oposing theories on how the dinosaurs could manage to operate with such huge bodies, so I'm going to leave it at that for now. #-o
I am going to search more about this, but not before I've gotten my hands on The Life and Death of Planet Earth

btw. googling this topic generates an enormus amount of dubious websites. ](*,)


edited for spelling and bbcode

George
2004-Jan-24, 01:21 AM
"In the Beginning" is the chapter in the BA's book that helps explain atmospheres (pg. 194).

Essentially, the lighter elements get kicked by larger elements/molecules to escape velocity easier than the more massive ones. The amount of gravity determines if you'll have any. More gravity, higher escape velocities.

Also, the temperature is a factor. The colder the gas, the less "kicking" action in the gas that would bring them to the escape velocity.

Mar's gravity is much weaker but the lower temperature helps some.