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I'm doing an open university course in planetary science, and I've been thinking again about Lunar terraforming as a result. It seems likely that Luna was formed from the same hydrated minerals that make up the Terran mantle, and since the Terran atmosphere is probably derived from outgassing, and since Luna was volcanically active early in it's history... could Luna have originally possessed an atmosphere that was later dispersed by the solar wind, or reacted with the surface?
I looked at the previous thread on Lunar terraforming using a mirror, and found a link to this article about Luna gas pockets:
What might these volatile substances be? Before the recent lunar missions flew, it was common to declare that water was not a possibility. However, we recently discovered from study of the lunar samples that water was present in the deep interior of the Moon during the epoch of mare volcanism three billion years ago; water could still be present in the subsurface. There are many other volatile substances that could be responsible as well, including carbon monoxide, hydrogen sulfide, gaseous sulfur, as well as other more exotic gases. Because the compositions on Mercury are poorly known, the possibilities for exotic materials there are even more extensive.
Could Luna have been mischaracterised all these years, and have an extensive volatile inventory hidden beneath the barren service? Could there have been *actual* Lunar seas - and if so, could carbonates have precipitated out?
Use what is abundant and build to last
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Seems to me, you could build a Bishop Ring around the Moon and use the Moon's gravity and structure to hold it together. Using the Moon's natural gravity to hold in atmosphere means you have to constantly replace the gases you lose into space. From the looks of the Moon's surface, it doesn't appear the Moon ever had a substantial atmosphere, and if it did, it didn't last long.
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After scientists studied Apollo Moon rocks, they concluded the Moon was formed by impact. A planet about the size of Mars impacted proto-Earth at an angle. Crust material from both planets was thrown up, creating a ring around Earth. A lot rained down. The majority of that other planet was absorbed into Earth, making our planet bigger. That was the last major impact to form our planet. The resulting debris quickly coalesced into a single body. If you can call 10 million years "quick". One belief is that accretion of the debris was not even, it formed two bodies. For a while, Earth had two moons. The second, smaller moon collided with the larger moon. That collision was not vigorous, the smaller moon "smooshed" into the back side, spreading out. That's why the far side has thicker crust than the near side.
One reason for this conclusion is that minerals are the same as Earth igneous minerals. And isotopes are the same too. This means the Moon did not form from Earth mantle material, it formed from Earth crust material.
The dark "seas" were lava lakes. But there's the question: how did the lava remain liquid long enough to form those "mares"? It may have required an atmosphere to prevent lava from cooling immediately. Or did the vacuum of space simply allow magma to out-gas, then remaining lava remained hot in the vacuum of space until it radiated? Don't know. Haven't heard/read any definitive answer to that.
But this process of breaking crustal material into chunks, then the chunks accreting to form the Moon? That's violent. A lot of volatiles would have escaped into space in the process. It is possible for some to have been part of proto-Moon. But the smaller Moons before the final accretion collision would have had even lower gravity. Such low gravity would have permitted rapid loss of volatiles to space. Hard to say if the Moon ever had an atmosphere.
Current theory is that ice at the poles is caused by impacts of small comets, snowball size ice meteoroids, or C-type carbonaceous chondrite asteroids or meteoroids. This delivered volatiles to the surface of the Moon long after formation. Most of the volatiles boiled off to space. However, some would boil to form gas near the surface during the 2 weeks of sunlight, then freeze as frost during 2 weeks of night. It would boil again during the next 2 weeks of sunlight. This would continue until the gasses either escape to space, or migrate to a place where sunlight never touches it.
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The moon has comparatively (compared to other volatiles) high levels of sulphur and little if any native water. A nascent atmosphere may have been dominated by sulphur dioxide, as there would be no water to assist the formation of sulphates. It may have been protected by a magnetic field in the early days.
So far as terraforming is concerned, a shell world approach would allow the most habitable surface for a minimum of invested gas. You could do it incrementally too, building masonry domes from baked regolith or even carved lunar stone. The pressure at the base of a dome is equal to density x g x radius. So if density is 4000kg/m3, g = 1.6m/s2 and allowable pressure is 10MPa, then the radius of the dome could be 1.5km. Of course, when pressurised much of that compressive stress would disappear, but you would need enough residual stress to keep friction between the blocks. More likely, the dome will be relatively thin and counter-weighted with lunar surface regolith and rock. You could add more and more domes interlinked by tunnels until you gradually build up a planet wide biosphere.
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