Atmospheres are simply an outer coating of gases wrapped around planet's solid cores. Sometimes, with large planets, there might not be any boundary because the pressure is so great the lower part of the atmosphere is past the critical point where there is a difference between gas and liquid.
It's much simpler on a planet like Earth where there is mostly solid, a bit of liquid sloshing around on the surface, and then a very thin layer of gas. When the planets consolidated from the pre-planetary disk, the solids around proto-Earth solidified, maybe some water oozed out to form the liquid contribution, and the light stuff that wasn't cold enough to condense got trapped in the gravity well of the solid core and made an atmosphere. Simple, simple, simple.
If there was more gas, there would be more atmosphere. There is nothing magical about what stays in the atmosphere. The lightest gas, hydrogen, escapes first from lighter planets, then helium, then others in order of molecular weight. Some gases, chemically reactive ones like oxygen and the halides, react and join the solid mass. Nitrogen and carbon dioxide are pretty heavy compared to hydrogen, and stay around longer. If their residence time is longer than the age of the planet, they are still there.
The amount of the atmosphere depends on how much of these atmosphere makers was in the gas cloud which made up the planetary disk. There was clearly some ratio of atmospheric gases to heavy mass, and that ratio, less any chemical reactions with those elements that do that, should be the one that dictates how much atmosphere is on a planet.
Here's the question. Venus is a a neighbor planet and was near Earth in the pre-planetary disk when it formed into planets. So Venus and Earth should have about the same amount of atmosphere. We need to take into account that plants grew on Earth and sucked up the carbon dioxide, and spewed the excess oxygen back into the atmosphere. Fine, maybe that's a factor of two or something. But, Venus has a lot more atmosphere than Earth does. Why is that? Where's the rest of Earth's atmosphere?
Venus' atmosphere is mostly carbon dioxide. The claim is that chemical combination made the carbon dioxide on Earth into carbonates, mainly calcium carbonate, and that solid is common near the Earth's surface. Water is pretty low in atomic weight, and the atmosphere of Venus is much hotter than Earth's, so it should have evaporated faster there. Venus' gravity is also less than Earth's, so water would escape a bit easier. The Earth has a mesosphere, a cold, rarefied layer of gases, where water vapor condenses into ice clouds. Water has high intermolecular attraction, and forms ice easily, which serves as a barrier to escape. No similar barrier has been identified on Venus. It has a cold, rarefied mesosphere, but the clouds there are sulfuric acid.
That leaves nitrogen. There is four times as much of it in Venus' atmosphere as in Earth's. Because Venus gravity is less, its temperature is greater, and it has no internally generated magnetosphere to protect the upper atmosphere from the solar wind, there should be less not more. There are also possibly factors related to the very long day there also indicate more nitrogen would be lost per milennia from Venus than from Earth. So, did Venus get hit by a passel of nitrogen meteorites? No. Earth has lost most of its nitrogen, when it should not have.
Earth has a large moon and Venus has none. However, it is simply not close enough or large enough to draw off most of Earth's nitrogen in recent gigayears. However, that may not be true for the impact processes that formed it.
There has been speculation, perhaps since humanity started looking at the sky, about where the moon came from. One idea is that it is a binary planet, formed like binary stars do. But that requires a strange distribution of angular momentum in the preplanetary disk. So other theories have arisen where it was formed when a large planet, nicknamed Theia by some, plowed into it. Theia may have been jostled out of its orbit by one of the giant planets and crossed Mars' orbit to intersect Earth's.
If the hit was a head-on collision, the energy involved might have broken up both of them if the relative speed was of the scale of orbital velocities, or might have resulted in a merged planet if they were very small. If the hit was just a glancing blow, Theia would have proceeded on past Earth and done some other interactions elsewhere. If the hit was between these, and the velocity less than of the scale of orbital velocities, there would have been a temporary merger as the mutual gravity tore the crusts off both of them and merged the mantles and cores. But a non-direct hit has huge angular momentum in the center-of-mass frame and this must be preserved over the short period it takes for the interaction. An elongated blob of molten matter would form and then split in two. The Earth would be one blob, the big one and the moon, the other one. At that time, the moon would be in close orbit over the surface of the new Earth.
The division of mass between the two is such that the Earth's gravity could hold onto the moon and the angular momentum would be divided between the two. However, almost instantly afterwards, the mass that was between the two would fall back to the Earth as it resumed its spherical shape. That mass would have been rotating at the same speed as the dual blob that formed the two bodies, the Earth and the moon. As it falls back, the Earth would have to speed up to maintain the total angular momentum. That means the Earth would have to be rotating faster than the orbital rotation rate of the moon around the new Earth. With the two bodies so close, tidal interactions would be extreme, and the new Earth would begin slowing down its rotation, while the moon would be pushed out further and further with the new angular momentum it was collecting through tidal interactions.
What happens to the atmosphere of the old Earth through this process? There is a huge shock wave rushing through the old Earth, leading the atmosphere to act like a spallation layer and move outward. Some would keep going and be lost, and the rest would fall back. The temperature of the atmosphere would be greatly elevated, leading to escape losses as well. Lastly, the moon in its orbit, starting only a few Earth diameters over the Earth, would be tearing off gas readily. It would be no surprise if only 10 percent remained.
The Theia theory has some other interesting features. One is that the atmosphere, being mostly carbon dioxide, nitrogen, some hydrogen still left, some sulfur chemicals, plus vaporized rock, would be turning itself into a chemical stew. That great heating episode would lead to chemical reactions occurring at a rapid rate and making more varieties of molecules that anyone could list. Among them would be plenty of organics, which could condense into a liquid when the atmosphere cooled down. Maybe even a layer of immiscible ones might cover the re-condensed oceans. Wouldn't that be nice for life?
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