My 12 year old daughter asked me this question:

Dad, why doesn't the moon have an atmosphere, and we do?

I couldn't answer her. I said that the gravity of Earth kept the gaseous atmosphere around but then I began to wonder why it doesn't just gradually go away as individual gas molecules drift up, up and away as the air gets "thinner" up at increasingly greater altitudes?

Why do some moons and planets have atmospheres? What keeps them stable? How do they lose them?

I must say, I've never really thought about this but feel kind up ridiculous for not having done so.

Thanks

Well, you need to explain escape velocity and the kinetic theory of gases first.

An object moving away from Earth or the moon will slow down due to gravity, and if it is not moving at escape velocity or higher, will slow to a stop and fall back to the surface (unless it's also moving sideways fast enough to miss the ground and fly back out again, in which case it's in an orbit). The moon is much smaller than Earth, and has a lower escape velocity.

Also, gases like the atmosphere are composed of many molecules flying around and bouncing off each other. The hotter they are, the faster they fly around. On the moon, the molecules at the top of the atmosphere would often be moving fast enough to just fly off into space. Earth's gravity is enough that this barely ever happens.

There's more to take into account...the relation between different molecular weights and their velocities at a given temperature, and the differences in how much the atmosphere is compressed by gravity, but those are the basics.

Atmospheres are an interesting study. James summarized it nicely. I would mention solar wind, planetary magnetic field, and proximity to the local star also.

I think I've mentioned before in the forum my aha-moment about the mass of air. I wish I had encountered it in school. A friend had a large drycleaner plastic bag. Drycleaners buy a single continuous cylinder on a spool, and cut off pieces to make clothes-size bags. So this was one big polyethylene bag, maybe 5 meters, more, long. (I will deny that we were plotting hot-air balloons to hoax the gullible about the existence of alien spaceships; no, that would be cruel -- but instructive.)

When filled with air and pushed to and fro from one end, one could really get a feel for the mass of that much air, a relatively small amount, just from the inertial effects. It felt like a couple kilograms of stuff. (Edit: about 3.3 cubic meters of air at 1.2 kg per cubic meter, about 4 kg.) Sure, I had been taught that air had mass, and knew it intellectually, but now I had felt it. Aha!

See if your local drycleaner can hook you up with a big plastic bag. It's even worth paying a small amount. You can always use it later for a UFO hoax.

__________________
0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 1 0 0 1 0 1 1 0 0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 0 1 1 0 1 0 0 1 0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 ...

You might also talk about degassing, which is where the atmosphere comes from in the first place. A large body like the Earth will experience more degassing than a small body like the Moon - and being larger, the Earth will be better able to hold onto its degassed materials.

__________________
- Learn a lot teaching others.

And stripping, by occasional large impacts.

Pterodactyls and pteranodons were so flimsy winged, and so large, that I think there is some research on whether or not a higher atmospheric pressure (=greater air density) was required for them to fly.

Which turns out not to be required ...

Grant Hutchison

I may not have been required, but it sure made it easier. The sea-level density of our air today is comparable to what would have been around 7,000 to 10,000 ft altitude during the time of the 'dactyls.

I wonder how much thinner it'll be a hundred million years from now?

On another note,

"Katsufumi Sato, a Japanese scientist, did calculations using current birds and decided that it is impossible for a pterosaur to stay aloft.[29] In the book Posture, Locomotion, and Paleoecology of Pterosaurs it is theorized that they were able to fly due to the oxygen-rich, dense atmosphere of the Late Cretaceous period.[30]" - Wikipedia's entry for Pterosaur

__________________
If I set the budget, we'd have Ares and more. Unfortunately, I don't set the budget, and Ares is just too expensive and too far out for us to accomplish our goals within the budget we were given.

If we halt the ISS, all versions of Ares, and transport Orion and Altair aboard DIRECTv3's Jupiter family of Shuttle-Derived Launch Vehicles, we just might make it back to the Moon by 2020.

Both of these statements turn out to be unreliable.
Sato looked at flapping flight, and in the course of a recent presentation about albatross telemetry appears to have mentioned something that's already well known: the very large pterosaurs are unlikely to have been able to sustain flapping flight. So they'd have trouble getting aloft or staying aloft in still air. The media seems to have run with it from there.
Posture, Locomotion and Paleoecology of Pterosaurs isn't a book: it's a 64-page Special Paper for the Geological Society of America, by Chatterjee and Templin. It demonstrates, in great detail, how even large pterosaurs could stay aloft under normal atmospheric conditions today. It mentions, very briefly, that higher oxygen concentrations would (of course!) have enhanced performance. However, their reference for higher oxygen concentrations in the late Cretaceous is to old, now discarded, work by Berner and Canfield. Berner's current GEOCARBSULF model (700kB pdf), which was published after Chatterjee and Templin's paper, shows oxygen levels staying lower than the current partial pressure right through the Cretaceous.

Grant Hutchison

Ones-and-oohs, did you also consider that some of the "weight" you felt when pushing the inflated bag was actually air resistance rather than the weight of the air itself?

Anyway, it's my understanding that we do lose atmosphereic gass out into space, but that it's not at a rate that would mean anything. (which is supported by the fact that much smarter people than myself have said the same thing up above ).

A laymen's way to look at it (which is how I view everything, being a layman and all) is that there would have to be a reason for the gas to wander out into space. Some force pushing it that way. And whatever that force is, it would have to be greater than the force of gravity keeping the atmosphere around the earth.

__________________

I'm like one of those idiot savants...well, except for the savant part.
"In order to increase awareness of the homeless, security have been given binoculars."

There are interesting questions here about why various bodies have such different atmospheres. I would like to know why Ganymede has virtually no atmosphere and Titan has the atmosphere arguably most resembling Earth, given that the two bodies are close in mass and diameter.

Well, for there to be an atmoshpere, the gasses would have to have been present in the body to begin with (right? or at least supplied after-the-fact from space debris, etc). Also, I'd think in close enough proximity to a large gravitational field, a bigger body might be able to "pull" the lighter elements of an atmoshpere into itself, while the heavier elements remain behind as a moon.

But I'm just guessing here.

__________________

I'm like one of those idiot savants...well, except for the savant part.
"In order to increase awareness of the homeless, security have been given binoculars."

Pushing yes, some, but then the object in motion tended to stay in motion and that was obviously not caused by air resistance, but happened in spite of it.

Try it. Feel the inertia.

__________________
0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 1 0 0 1 0 1 1 0 0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 0 1 1 0 1 0 0 1 0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 ...

Just make'n sure (though I was certian you were sharp enough to consider it).

__________________

I'm like one of those idiot savants...well, except for the savant part.
"In order to increase awareness of the homeless, security have been given binoculars."

Just reminiscing, I think I would describe the inertia of the inflated bag to be much like picking up a large plastic garbage can, holding it horizontally, and pushing it away and pulling it back.

The garbage can's mass would be roughly the same. Its cross section would present about the same cross section that the plastic bag did -- a cylinder with a half-meter radius -- and have about the same wind resistance.

A difference in feeling would be that you'd notice not just the mass but the weight of the several-kilogram garbage can, and its distant nose would require a little strength to keep horizontal. With the inflated plastic bag, it was almost neutrally buoyant in the air, being mostly air, and just circa 100 grams of plastic, and was easy to hold horizontal even being 5 times longer.

canandbag.gif

__________________
0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 1 0 0 1 0 1 1 0 0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 0 1 1 0 1 0 0 1 0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 ...

I suppose the simplest explanation is that Ganymede formed closer to the Sun, so its atmospheric gases would have been warmer than Titan's and therefore able to escape Ganymede's gravity.

Ganymede also formed closer to Jupiter than Titan did to Saturn, so that too would have made things hotter on primordial Ganymede.

__________________
- Learn a lot teaching others.

I have read this assertion before, that proximity to the forming
planet would have some effect on the composition of a forming
moon, but I see no reason for that to happen.

With the Sun, on the other hand, the powerful solar wind of the
young star preferentially blows lighter elements away.

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

A lot of heat would have been generated by the accretion of Jupiter, heating the protosatellite disk around the planet, and giving rise to a condensation sequence.

That's why Io is a rocky-metallic planet, while Callisto is icy. As we move from Io out to Callisto the metallic component decreases and the icy component increases, just as we see with the planets from Mercury out to the giant planets.

__________________
- Learn a lot teaching others.

That's a good point. Presumably the T Tauri solar winds would have been felt more strongly at Ganymede than at Titan.

__________________
- Learn a lot teaching others.

What do you mean by "condensation sequence"? While proto-Jupiter
would certainly heat its protosatellite disk, I can't see it as having a
very large effect. How hot could Jupiter's "surface" have been?

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

Interesting article in this weeks New Scientist (22/11/08, "Back to the drawing board"), about modelling Earth-like planets. Turns out that the Goldilocks Zone isn't the whole story. The circum solar region in which the earth could have an atmosphere and liquid water extends from just 5% nearer the Sun to 1.7 times further away - that's further out than Mars. Mars has water, we now know thanks to Phoenix and other probes, that probably was liquid awhile ago, so why doesn't it have a useful atmosphere now?
The suggested answer is two fold, both due to its mass. Too small to hold onto gases, as above and too small to retain enough internal heat for the volcanic activity that helps maintain the Terrestrial carbon cycle. That's not the original heat of fusing the planet together, I think that idea went a long time ago, but for the heat of internal radaioactivity to be kept in by a large mass/surface area ratio.

The same article describes work on other possible planetary settings - rotation speed and axis, and orbits that produce warming due to tidal stresses. The latter refers to putative planets in eccentric orbits around red dwarf stars. I was going to ask why should, for instance, Io should have this effect, when it is in a nearly circular orbit - but it isn't! See: http://bb.nightskylive.net/asterisk/...hp?f=8&t=14972

John

This gives rise to an ATM theory of life originating on Mars, some of which may have been blasted to Earth via meteoric ejecta billions of years ago, and some, perhaps of humanoid and other ancestry, migrating here millions of years ago.

__________________
If I set the budget, we'd have Ares and more. Unfortunately, I don't set the budget, and Ares is just too expensive and too far out for us to accomplish our goals within the budget we were given.

If we halt the ISS, all versions of Ares, and transport Orion and Altair aboard DIRECTv3's Jupiter family of Shuttle-Derived Launch Vehicles, we just might make it back to the Moon by 2020.

Moving from Jupiter out towards CAllisto, the temperature in the proto-satellite disk decreased. Close to Jupiter it was too hot for ices to condense but cool enough for refractory materials to condense, so Io is a rocky-metallic body, depleted in volatiles. Further out, it was cool enough for ices to condense, so Callisto is rich in volatiles. As we move from Io through Europa and Ganymede out to Callisto we see a gradual decrease in the refractory components and an increase in the volatile components.

This is not just idle specualation on my part. It's quite mainstream. otherwise, how do you explain the condensation sequence that is found in the Galilean satellites?

An Introduction to the Solar System

Check out pages 308-310.

__________________
- Learn a lot teaching others.