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t@nn
2006-Aug-17, 12:25 PM
After the moon was formed, did it have a thick atmosphere, however briefly (due to outgassing after the object cooled down)? Or did the way it was formed mean there were no volatiles to outgas?

cjbirch
2006-Aug-17, 04:05 PM
I do not believe the Moon having formed from the Giant Impactor Scenario would have any atmosphere let alone been able to form a thick one. I believe this to be the case due to the fact that the Moon does not retain the mass to keep an atmosphere from escaping into space, being geologically inactive the lost gases would not have been replenished!

CJ

cjbirch
2006-Aug-17, 04:18 PM
An informative link on how Saturn's moon Titan managed to retain an atmosphere in comparison to Earth's satellite:

http://www.pbs.org/wgbh/nova/titan/atmosphere.html

t@nn
2006-Aug-17, 05:26 PM
I do not believe the Moon having formed from the Giant Impactor Scenario would have any atmosphere let alone been able to form a thick one. I believe this to be the case due to the fact that the Moon does not retain the mass to keep an atmosphere from escaping into space, being geologically inactive the lost gases would not have been replenished!

CJ

So, the moon would never in its entire history have any more atmosphere than it has now, even before the gasses had escaped? I'm talking about the initial period after it had cooled, maybe the first few decades/centuries or so. If you got into a time machine and went back to that period of the moon's history what would you see?

cjbirch
2006-Aug-17, 06:10 PM
So, the moon would never in its entire history have any more atmosphere than it has now, even before the gasses had escaped? I'm talking about the initial period after it had cooled, maybe the first few decades/centuries or so. If you got into a time machine and went back to that period of the moon's history what would you see?

Okay, if we were to somehow travel back in time to visit the early Moon, I believe that in the period in which the Moon had cooled there would not have been any more of an atmosphere than there is now.
However, if we were to assume that for the first 10 or so million years of the Lunar timeline that the surface of the Moon had a magma ocean, due to the thermal heat released when the impactor struck the Earth, it is likely that the core of the Moon would have been molten, this core would be subject to currents that may have produced a magnetic field, hypothetically this could protect an early atmosphere, such as believed on early Mars. Yet once the core had cooled the moon would not have been able to generate these currents and thus maintain any form of atmosphere.
Yet as I stated previously the mass of the moon precludes it from retaining any form of atmosphere, I cannot see this being any different for any time period on the moon, except for the aformentioned scenario. Not only is the moon not capable of retaining an atmosphere, due to its weak gravity every impact that strikes the Moon, results in the fact that it will lose more mass than it should subsequently receive.
Hope this helps!

CJ

Peter Wilson
2006-Aug-17, 10:54 PM
...Not only is the moon not capable of retaining an atmosphere, due to its weak gravity every impact that strikes the Moon, results in the fact that it will lose more mass than it should susequently receive.
Hope this helps!

CJWhoa! :hand:

Who decides how much more mass it should susequently receive?

Nereid
2006-Aug-18, 12:54 AM
There are several different factors at work, most of which have already been mentioned, one way or another, in this thread.

First, under the currently favoured model, the Moon condensed from the debris left over from the collision of a Mars-sized object with the proto-Earth - that debris was composed of almost entirely refractory minerals (ones which have a high melting and boiling point) - this matches well with the observation that the Moon is extremely 'dry' - there are no volatiles, even in the form of chemically bound water ("hydrated"), and is considered a big plus for the 'Big Splat' hypothesis of the Moon's formation.

So, there was, and could not have been, any significant outgassing, because the Moon never had any significant amount of C, H, or N (and the only O was tightly bound to the Si, etc).

Or did it? After the Moon formed, and its surface cooled to solidity, it was battered, repeatedly, by a very large number of impacts ... the period of Late Heavy Bombardment. During this time, many of the impactors would have been rich in volatiles, like the comets which occassionally deign to pay our part of the solar system a visit. Some of those volatiles would have been buried deep into the crust; some would have been captured by the Moon, if only via condensation on the night side; and so on.

Could this have lead to a transitory, if thin, atmosphere? I don't know. But if it did, it would have been been gone quickly - months? years? decades??

Slightly OT, the Moon does have an "atmosphere" today (http://sirius.bu.edu/planetary/moon.html) ... albeit not one that we would normally think of when we hear the word "atmosphere".

t@nn
2006-Aug-18, 04:52 AM
There are several different factors at work, most of which have already been mentioned, one way or another, in this thread.

First, under the currently favoured model, the Moon condensed from the debris left over from the collision of a Mars-sized object with the proto-Earth - that debris was composed of almost entirely refractory minerals (ones which have a high melting and boiling point) - this matches well with the observation that the Moon is extremely 'dry' - there are no volatiles, even in the form of chemically bound water ("hydrated"), and is considered a big plus for the 'Big Splat' hypothesis of the Moon's formation.

So, there was, and could not have been, any significant outgassing, because the Moon never had any significant amount of C, H, or N (and the only O was tightly bound to the Si, etc).

Or did it? After the Moon formed, and its surface cooled to solidity, it was battered, repeatedly, by a very large number of impacts ... the period of Late Heavy Bombardment. During this time, many of the impactors would have been rich in volatiles, like the comets which occassionally deign to pay our part of the solar system a visit. Some of those volatiles would have been buried deep into the crust; some would have been captured by the Moon, if only via condensation on the night side; and so on.

Could this have lead to a transitory, if thin, atmosphere? I don't know. But if it did, it would have been been gone quickly - months? years? decades??

Slightly OT, the Moon does have an "atmosphere" today (http://sirius.bu.edu/planetary/moon.html) ... albeit not one that we would normally think of when we hear the word "atmosphere".

So the moon probably never had more atmosphere than it did now, and even if it did it would be transitory and thin. Pretty boring. Do you think this is common to most moons that form via impacts in this way?

Van Rijn
2006-Aug-18, 05:24 AM
So the moon probably never had more atmosphere than it did now, and even if it did it would be transitory and thin. Pretty boring. Do you think this is common to most moons that form via impacts in this way?

It definitely has had more atmosphere than it has now. The moon landings increased it significantly, for a few weeks anyway.

More seriously, lunar atmospheric loss has been looked at in a little more depth. In Terraforming by Fogg, there is a reference to an article on the possibilities of a lunar atmosphere that was written by Richard Vondrak and published in Nature in 1974. According to Vondrak, the current mass of the lunar atmosphere is on the order of 104 kg. Currently, there are so few molecules that they rarely meet each other. They are on ballistic paths and have a staying time on the order of weeks or months. But for a relatively thin atmosphere, on the order of 108 kg, the rules would change. This would protect the surface and lower atmosphere from solar wind and loss would primarily be thermal. This would dramatically lower the loss rate so that there would still be significant atmosphere for thousands of years. This much atmosphere could be created by introduction of about 100 kg/s for around a year. Lunar air pollution could eventually be a real issue if we build large lunar settlements.

Anyway, 108 kg is about the minimum for transition to thermal loss, but it could be much thicker (keep in mind that 108 kg of gas would still be extremely thin by earth standards). Geologically, thousands or tens of thousands of years might as well be a blink of an eye, but it is likely that the moon had a noticeable atmosphere at some point, for at least a little while.

t@nn
2006-Aug-18, 07:08 AM
It definitely has had more atmosphere than it has now. The moon landings increased it significantly, for a few weeks anyway.

More seriously, lunar atmospheric loss has been looked at in a little more depth. In Terraforming by Fogg, there is a reference to an article on the possibilities of a lunar atmosphere that was written by Richard Vondrak and published in Nature in 1974. According to Vondrak, the current mass of the lunar atmosphere is on the order of 104 kg. Currently, there are so few molecules that they rarely meet each other. They are on ballistic paths and have a staying time on the order of weeks or months. But for a relatively thin atmosphere, on the order of 108 kg, the rules would change. This would protect the surface and lower atmosphere from solar wind and loss would primarily be thermal. This would dramatically lower the loss rate so that there would still be significant atmosphere for thousands of years. This much atmosphere could be created by introduction of about 100 kg/s for around a year. Lunar air pollution could eventually be a real issue if we build large lunar settlements.

Anyway, 108 kg is about the minimum for transition to thermal loss, but it could be much thicker (keep in mind that 108 kg of gas would still be extremely thin by earth standards). Geologically, thousands or tens of thousands of years might as well be a blink of an eye, but it is likely that the moon had a noticeable atmosphere at some point, for at least a little while.

A little less boring then. :) So it seems that it is possible impacts might have led to more significant atmosphere for at least a period of time (a thousand years?), though any time-travelling humans would still be wearing space suits.

cjbirch
2006-Aug-18, 10:40 AM
Whoa! :hand:

Who decides how much more mass it should susequently receive?

For one, SMART-1 principal scientist Bernard Foing, has written that every impact upon the moon causes a loss in the overall mass of the moon due to its weak gravity. This would determine that the moon is slightly smaller than it was in its early past. On the other hand, the same number of impactors which struck the Moon could have struck the Earth, albeit at a faster velocity. The Earth however is big enough to retain more mass from these impacts.

Kaptain K
2006-Aug-18, 05:37 PM
Do you think this is common to most moons that form via impacts in this way?
Most moons do not form this way. In fact, it is such a "Goldilocks" event (not too fast-not too slow, not too big-not too small, not too shallow-not too steep, etc.), that it is unique in our solar system.

aurora
2006-Aug-18, 07:18 PM
Most moons do not form this way. In fact, it is such a "Goldilocks" event (not too fast-not too slow, not too big-not too small, not too shallow-not too steep, etc.), that it is unique in our solar system.

Or almost unique. The Pluto system seems to have been formed via collision as well.

Kaptain K
2006-Aug-18, 09:00 PM
I was under the impression that the Pluto-Charon system was, most likely, formed by a multi-body exchange/capture process.

Bad jcsd
2006-Aug-18, 09:34 PM
I've been to the moon, it's alright foor a vistit, but if I'm honest there's not much atmosphere there.

neilzero
2006-Aug-18, 10:32 PM
I think it is likely that our Moon, briefly, had a million times more atmosphere than now; but that is still a thin atmosphere. That fits most of the usual seanarios for how we got our moon. Neil

t@nn
2006-Aug-19, 05:27 AM
Most moons do not form this way. In fact, it is such a "Goldilocks" event (not too fast-not too slow, not too big-not too small, not too shallow-not too steep, etc.), that it is unique in our solar system.

I know most moons do not form this way. I was asking whether moons forming via impacts of this sort, have a similar lack of volatiles.

aurora
2006-Aug-19, 01:40 PM
There was a paper this year in Nature on the formation of Pluto's moons.

PDF Paper on formation of Nix and Hydra (http://www.boulder.swri.edu/plutonews/stern_et_al_nature.pdf)

Abstract in Web format (http://xxx.lanl.gov/abs/astro-ph/0512599)


we show that P1 and P2's proximity to Pluto and Charon, along with their apparent locations in high-order mean-motion resonances, likely result from their being constructed from Plutonian collisional ejecta

And the Charon article in Wikipedia is currently stating some uncertainty on the formation of Charon.

Formation of Pluto and Charon (http://en.wikipedia.org/wiki/Charon_(moon)#Formation)


Simulation work published in 2005 by Robin Canup suggested that Charon could have formed by a giant impact around 4.5 billion years ago, much like the Earth and Moon. In this model a large Kuiper belt object struck Pluto at high velocity, destroying itself and blasting off much of Pluto's outer mantle, and Charon coalesced from the debris. However, such an impact should result in an icier Charon and rockier Pluto than we find. It is now thought that Pluto and Charon may have been two bodies that collided before going into orbit about each other. The collision would have been violent enough to boil off volatile ices like nitrogen but not violent enough to be disrupted