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So the mass of air around the earth has doubled. How it doubled isn't part of the question, but for the curious let's say a space station floating around earth processed all the nitrogen, oxygen, carbon, etc from some comets and pumped it down to sea-level. The process took a week to complete and the new gas temperature was at 15C(the earth-average). By the end, there was twice as much air(by mass) around the earth.

What I'm wondering is how this actually affects the earth. How much would the pressure increase(if any) and would the ground-level composition of the atmosphere change?

Any information helps, but I'm most interested in how the change would affect pressure and the balance of the atmosphere on the macro scale.

Thanks for your help!

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  • $\begingroup$ @JBH I changed the question to be more specific. I'm really just focused on how the increased amount of air would affect the earth on a macro-scale. $\endgroup$ – chase leffers May 4 at 1:28
  • $\begingroup$ @JBH I simplified it to two core questions. The two questions are closely related, and each by itself doesn't tell much, so they have to remain together. Any impact on the environment can be inferred by the answer, so that question is necessary. Hope that clears it up! $\endgroup$ – chase leffers May 4 at 2:17
  • $\begingroup$ The title says mass but the body says volume, does that need to be clarified? $\endgroup$ – rek May 4 at 2:57
  • $\begingroup$ Are you looking for how many years until the Earth is fully cooked and hits a temperature equilibrium? Otherwise, what point in time after the attack occurs are you interested in? (Assume that question is referring to Earth's atmosphere as it is today, but would be good to confirm that too.) $\endgroup$ – KerrAvon2055 May 4 at 5:04
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1 + 1 = 700(ish)

Assumptions and approximations:

  1. Earth has a mean radius of 6371 km. The Kármán line for the existing, unmodified Earth atmosphere is 100 km up. Even if doubling the mass of the atmosphere doubled this to 200 km (it would not), the effective atmospheric surface area that can radiate energy back into space would change by a trivial amount and will be disregarded.
  2. The solar radiation intercepted by Earth remains at current levels.

  3. Human attempts to stop the additional atmosphere being added or do anything of relevance subsequently can be disregarded in the face of technology this overwhelming.

  4. Any short term effects of the additional atmosphere being added such as temperature fluctuations, hypersonic shockwaves scouring the surface etc are trivial on the macro level.

Energy budget: Earth has a radiation budget (nicely illustrated in a NASA poster here). In order to maintain a constant temperature at the planetary level, the same amount of energy from solar radiation that Earth intercepts must be radiated or reflected back into space. If Earth changes its characteristics so that it is no longer radiating or reflecting the same amount of energy back into space then its temperature will change until a new equilibrium is reached.

Imagine that you are lying down trying to sleep in a summer weight sleeping bag in freezing conditions - you simply cannot get warm. Now imagine that you are in a sleeping bag twice as thick designed for those conditions - now you are warm. Your body is still generating the same amount of heat and over a long timescale the sleeping bag is still radiating the same amount of heat to the freezing conditions outside but a comfortable temperature can be maintained inside the sleeping bag.

Doubling the mass of the Earth's atmosphere would have the same effect as getting into a thicker sleeping bag, except that the conditions on Earth would go from "normal" to "lethally hot". Earth would suddenly stop radiating as much net energy to space while still intercepting the same amount, which means that the temperature goes up. The increase in temperature just as a result of adding more atmosphere would compound with:

  • melting polar icecaps, which would decrease the amount of surface reflection and increase surface absorption; and
  • increased greenhouse effect as more water vapor is added to the atmosphere.

Given the drastic changes being modelled by climate scientists based on relatively tiny changes to Earth's atmosphere as a result of human activities, it is highly likely that adding an entire extra atmosphere worth of gas would trigger a runaway greenhouse effect, with surface and atmospheric temperatures rising above the boiling point of water. This would then turn Earth's hydrosphere of an estimated 1.4 x 10^21 kg into part of the atmosphere, meaning that the atmosphere that originally had a mass of 5 x 10^18 kg would not mass twice as much as it did originally but around 700 times as much, with water vapor being the large majority initially. (Chemical reactions will start occurring in the high temperature environment to change at least some of the water into other compounds, but that is a whole separate question.) The nearest point of comparison would be the atmosphere of Venus but the massively changed Earth atmosphere would have even higher pressure at ground level.

Timeframes: Wild guesses here, but conservatively assuming that adding a second atmosphere to Earth will result in an average 1% net absorption of sunlight per year for a couple of centuries at least:

  • It will somewhere between a few decades to a century to raise the atmospheric temperature to 100C (depending on how effectively the crust acts as a heat sink). During this period the mass of the atmosphere will only increase gradually (as the water vapor mass increases) but the temperature will be increasing constantly. Looking how this corresponds to the ideal gas law, the "sea level" pressure will only increase very gradually above the starting pressure of 2 atm, but the "bubble" of atmosphere around the Earth will expand.
  • Once the atmosphere reaches the boiling point of water then it would take about another century or so to boil the hydrosphere completely. During this period the atmospheric temperature would remain relatively constant at around 100C but the pressure at the former sea level would increase continually as the mass of the atmosphere increases.
  • After the hydrosphere has finished boiling, the pressure at the former sea level will remain constant while the atmospheric temperature will continue to increase until an equilibrium is reached. Given that Earth is further from the sun than Venus, the equilibrium temperature is likely to be lower than Venus' 462C. During this period the atmosphere will expand outwards again as the temperature increases.

What weather systems will exist at each stage along this progression are outside my area of expertise. However, given that the increased energy absorption will be uneven across the Earth's surface and will change over time (eg as icecaps melt and eventually the oceans boil), I would confidently predict an increase in high energy events. On the bright side, after the first few decades there will be few lifeforms left on Earth to be inconvenienced by them and within a century they will bother no one.

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The pressure would increase by a lot.

From the ideal gas equation (nRT=PV) doubling the quantity of gas (n = number of molecules * a constant) would double the pressure assuming a constant volume. It is unlikely that the volume of the atmosphere would remain constant, so I suspect the increase would be much less than double, but the pressure increase would be between 1-2 atmospheres and probably much closer to 2 than to 1.

The composition of the atmosphere would be roughly the same with twice as much carbon dioxide to trap heat at least initially. But it would trap a lot more heat increasing temperature which would allow more water vapour to enter the atmosphere. As water vapour is a greenhouse gas this would increase temperatures further still and could easily cause a runaway greenhouse effect with average global temperatures soaring possibly by many tens of degrees or more.

As a side note adding all of that atmosphere within one week would require some fairly fancy process to say the least. The volume of air required could easily create hypersonic shock waves and increase temperatures by hundreds of degrees unless very well managed (no doubt you have that in hand…)

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  • $\begingroup$ I'm curious, what might happen to the various levels: troposphere, stratosphere (-60 °C), mesosphere (-85 °C), thermosphere (1500 °C), ionosphere? Kármán line would be pushed out I assume, but definitely not doubled (20% maybe?). Is this space station doubling the complex gases, or just major ones (water vapor/methane/nitrous oxide/ozone vs. nitrogen, oxygen, CO2/monoxide)? Ozone needed to keep stratosphere calm (at a larger radius!) may more proportionally, than a doubling would allow. Much less if it would propagate in a timely fashion. So much going on here. Not including weather. $\endgroup$ – user3082 May 4 at 8:33
  • $\begingroup$ Most of the gas would go into the troposphere as that contains 75% of the atmosphere and the stratosphere contains most of the rest (23%).But the whole atmosphere would be moved far from equilibrium by adding so much gas and it would take a long time to settle down again. For example the water content would have to increase substantially in the new environment $\endgroup$ – Slarty May 4 at 9:08
  • $\begingroup$ I was reading troposphere had 50%. Bottom gets thicker, and top moves outward, but doesn't double distance, because: sphere. If no water vapor in the second atmosphere, instant drying as everything gets sucked up. But also, all of this is different temp than existing atmosphere, the parts that move out absorb/shed heat? But is density of atmosphere solely controlled by gravity? Second atmosphere would add some mass, so be able to make it denser. If composition is same, way more weird stuff close to ground, as that should stratify in upper layers. $\endgroup$ – user3082 May 4 at 12:22
  • $\begingroup$ enotes.com/homework-help/… that's my reference re the atmosphere. The initial temperature will depend massively on the way in which the second atmosphere is added. It could add or remove heat depending on assumptions made. $\endgroup$ – Slarty May 4 at 13:33
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Potentially huge weather effects, depends on how you're adding atmosphere, etc, etc.

One effect, would be that any low-orbiting satellites would like start experiencing friction and would have to be raised in orbit, the remainder would start crashing. You would ground all human space flights until spacecraft are redesigned; they would need (probably) more than double their current thrust capacity (longer time to get thru more atmosphere carrying more fuel - 90%+ of all fuel is used to move the fuel needed to the height and speed at which it needs to get used).

You might also be stranding (or killing them on re-entry) all orbiting astronauts, as re-entry now has twice as much air to get thru (and perhaps frictional heating lasts a lot longer?).

Mass of the Earth 5.972x10^24 kg
Mass of the Atmosphere 5.1480×10^18 kg
Mass of the Earth after 5.97200515 × 10^24 kg (negligible increase)

Would have an insignificant impact on gravity, as barely nudges the overall mass.

Depending on what you're adding (ozone?), and if you add it all at sealevel, could result in a higher percentage of things which are more stratified in the upper atmosphere (currently). These are trace amounts of the bulk of the atmosphere, but would probably be considered pollutants. This would take some detailed science to figure out. As a single example, the current ozone is stretched so thin that we have an ozone hole, if you double the atmosphere, the amount you need to cover the sphere at that density more than doubles. So you would end up vastly increasing the ozone hole - even if you were injecting the ozone at the appropriate layer of the atmosphere. If you inject it all at sealevel, at an average temperature, it will take time, and have temperature changes to get where it needs to go - however the need for it to be where it needs to be will be immediate. And, AFAIK ozone is not available in Oort-cloud material, so is your space-station manufacturing this?

Also, this is a huge space station to process so much material, so quickly. I'm assuming it has antigrav, as it would have to be well out of a LEO yet needs some means to transport its product?

There are a lot of potential answers, which all depend on the choices you make with how you're adding the gases and particulates.

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