Consider the present day Earth. Now, through some event (magic, a giant maid with a vacuum cleaner, a disgruntled alien did it, ...), our planet is stripped of its atmosphere. Assume that this happens in such a way that other features of our planet are not significantly affected, and that the Earth is actually stripped of its atmosphere (it isn't merely replaced by some other gas(es)). The Sun is still there, and the solar system as a whole is unaffected by this little antic.

After a reasonable length of time (maybe a few hundred million years into the future), an alien civilization comes zooming past the solar system in their superspace-drive spacecraft, and decide to make a rest stop at what used to be the Earth.

  • What would the Earth look like when these visiting aliens arrive?
  • What would be the primary driving factors in the transition from what we have today to what they would be encountering?

Particularly consider what the surface would look like, including the effects on plate tectonics and the effects of meteor bombardment from space.

I'm not tagging this as hard-science, but the harder the science in the answers, the better.

  • Are you wondering if there is liquid water left? – user6760 Aug 19 '15 at 12:23
  • @user6760 That isn't my primary consideration, but in a way yes because presence or absence of liquid water would definitely affect in many ways what the surface looks like. – α CVn Aug 19 '15 at 12:29
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    Thinking about it... i have a feeling that an answer might depend on the time frame for the removal of the atmosphere. Are we talking instantly, days, months, generations until the atmosphere is completely gone? – Burki Aug 19 '15 at 12:51
  • Day becomes night, earthquakes still occurs due to convection in mantle, to replace mass of atmosphere the uppermost layer of ocean will boil away so that the sea level drops by probably less than a meter(mass of water is a couple of hundred times larger) radiation level increases exponentially killing most land based life so I think you have got your alien a new fishing ground!😁 – user6760 Aug 19 '15 at 12:53
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    @corsiKa Seriously, a giant maid with a vacuum cleaner. And this one can move at ludicrous speed. – α CVn Nov 26 '15 at 22:31

Second 1: The oceans on both the sun side and the dark side of the Earth instantly start to boil, reacting to the vacuum (absence of) pressure.

water state chart

The mass of the atmosphere of Earth is $5.15×10^{18}$ kg, which I guess is heavy by our standards, but the Oceans are $1.4×10^{21}$ kg, so a lot more mass there.

Second 2: 7.3 billion people suddenly realize they cannot breathe

Second 30: Most of the 7.3 billion people are now unconscious, soon to die.

...

The oceans will continue boiling for hours or days, my guess would be until the new water vapor atmosphere reaches the triple point around 1 kPa (1% of current pressure) or until the ocean surface all freezes due to heat of evaporation cooling, whichever comes first. This would probably take less than 0.1% of the ocean mass.

All the nitrogen is gone, and the contribution from volcanoes and other sources is minimal. The water vapor will slowly dissociate in the upper atmosphere, slowly leaking hydrogen off into space.

I suspect that absent the greenhouse effect of the atmosphere, the ocean surface would eventually freeze. The rest of the land surface will be covered by a layer of ice too.

Plate tectonics is powered by radioactive decay, so that would continue unabated. Meteors would plummet into the Earth hindered only by the water-vapor atmosphere. Volcanic eruptions would slowly add other trace gasses as well.

Edit: There's a fair bit of Nitrogen at the bottom of the ocean, and with geological processes, it may over a few hundred million years end up back in the atmosphere.

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    I read somewhere that water vapor is a good insulator so the oceans may not freeze. Maybe. I don't have any of the math required to figure that out. – Green Aug 19 '15 at 13:37
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    But even in a pure vacuum at 255K (-18.5C), water is still going to sublimate into a gas. I guess it's race to see if the oceans boil enough water to form enough insulation to prevent cooling to 255K. – Green Aug 19 '15 at 13:51
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    This looks really familiar... – Samuel Aug 19 '15 at 16:51
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    @Samuel, great minds link alike. ;) – Serban Tanasa Aug 19 '15 at 16:57
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    Basically, if this ever happens and you somehow have advanced warning, don't hold your breath. – zfrisch Aug 19 '15 at 20:44

What would the Earth look like when these visiting aliens arrive?

I have no clue, because humanity could quite possibly survive.

Based on the effects of a vacuum on humans and animals, almost everyone will be dead within two minutes. If buildings don't have the air within them removed, people in large building in coastal cities may survive long enough for the boiling oceans to replenish their air before they die due to vacuum exposure.

What about someone swimming in a large lake or the ocean? They're going to be rather surprised when the water they are swimming in starts boiling, but it's not going to burn them. An article I found with a quick search mentions this:

At equilibrium, the percentage of each gas in the water would be equivalent to the percentage of that gas in the atmosphere – i.e. its partial pressure13

With no atmosphere above it, the dissolved oxygen (and CO2, nitrogen, etc) would boil out faster than the water itself. This means that the water vapor coming off of large bodies of water should be breathable. This will only delay death for those near insufficiently large bodies of water (such as a mountain lake) as the vapor will likely spread out enough to not be useful.

People playing in the ocean when the atmosphere disappears are going to be in good shape. They may be unable to breath for a moment or two, but the seawater will immediately provide them with at least something to breath. After not too long, there will be enough air in their immediate vicinity to survive long-term.

Do you know who will have it even better than people playing in the ocean? Scuba divers! Unless they're very stupid divers, they won't let their tanks get all the way empty. Even then, they would still have some air in their tank - if a diver "runs out" of air, swimming up a little bit can lower the pressure enough that they can get more air out of the tank. In a vacuum, you'd be able to get air out of a completely "empty" tank. Also, the amount of air per breath depends on the surrounding pressure, so in a vacuum a full scuba tank would last extra long. So thanks to the combination of having a supply of breathable air and being in or near a reasonably large body of water, scuba divers would be able to deal with the initial few minutes.

Do you know who would have it even better than scuba divers? Submarine crews. Their first indication of something weird happening would be the radio silence. By the time they came up to check things out, the boiling oceans will have already made near-sea-level areas livable again.

So humanity is likely to have enough survivors of the initial atmospheric disappearance to continue, but how will they move forward? Fortunately for them, plants are able to handle vacuums better than animals are. From the article about the effects on humans and animals:

An experiment indicates that plants are able to survive in a low pressure environment (1.5 kPa) for about 30 minutes.

This allows for more time for the boiling oceans to help coastal forests and prevent total devastation. Trees and other land-based plants will take a major hit, but the survival rate right along the coast will be reasonable - they won't go extinct altogether.

Life will go on. Inland areas will become desolate for thousands of years, but gradually the atmosphere will be replaced and life will return.

Conclusion

The visiting aliens arrive to find a booming tourism industry. As they visit one history museum, they see a chart with six mass extinction events - the Cretaceous–Paleogene extinction event, the Triassic–Jurassic extinction event, the Permian–Triassic extinction event, the Late Devonian extinction, the Ordovician–Silurian extinction events, and the atmospheric disappearance event.

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    I think you're misreading the linked article. The gas mix dissolved in the oceans is roughly the same as the gas mix in the atmosphere, but there's about 100,000 times as much water as gas, and that's going to boil out. The resulting atmosphere will be almost all water vapor, with nowhere near enough oxygen to maintain life. It's possible that the water vapor will recondense faster than the gases as pressure reasserts itself, but I don't think it's going to do so fast enough to save anyone not in a submarine, and the new atmosphere is going to be much thinner with a different composition. – Russell Borogove Aug 19 '15 at 19:05
  • @RussellBorogove Oh, I forgot to point out that dissolved gases will boil out even faster than the water itself. – Rob Watts Aug 19 '15 at 19:20
  • Thank goodness that in 2010 the US Navy decided that female officers could finally serve on submarines! We'll need more women. – Mohair Aug 19 '15 at 22:44
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    Haha - "the atmospheric disappearance event" - thats the funniest thing I've heard all day. – Richard Venable Aug 19 '15 at 23:45
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    Buildings are usually permeable. I'm pretty sure almost all buildings will lose their air at almost the same moment the atmosphere disappears. Especially if your windows are open. Also, you're overlooking the massive cooling that would result from the evaporation - your swimmers will probably freeze, and the coastal cities will likely have the same problem. It's hard to tell exactly how much this would affect the whole planet (the water would stop boiling as the air pressure increases, but how much heat does that mean?). – Luaan Aug 20 '15 at 12:57

Approach #1: The atmosphere is stripped instantly.

If the rest of the planet remains unaltered by your galactic maid, i think after your reasonable length of time this planet would look a lot* like it did some 10'000 or 20'000 thousand years ago:
Since the magnetic field remains intact, and there are processes like for example volcanic activity that emit gases, there will be a new atmosphere that is being generated over time.

Some of the extremphile microorganisms will not even have noticed the absence of the atmosphere, so there will still be some form of life on the planet right after the atmosphere was removed. From those, new life forms will evolve.

While at first the missing atmosphere will leave the earth defenseless against asteroid bombardment, thus greatly altering the surface, the re-forming atmosphere will soon lead to erosion. So, the general appearance of the geology will look similar in features, although obviously not in detail, to what it is like today. It might look smoother, since the atmosphere will reform quite quickly, thus adding your reasonable amount of time of extra erosion.
Serban Tanasa suggested the missing atmosphere will be replaced within hours by water vapor, so, provided this is correct, the lack of protection against celestial rocks would even be negligible. The same was true if it was not hours but months for an atmosphere to re-form: a few more craters, but nothing that millions of years of erosion would not be able to turn into pelasant valleys, if plate tectonics didn't remove them anyway.

Serban also pointed out that plate tectonics will be unaffected, so no change to be expected from that side. Although i have a vague feeling that the sudden missing of the atmosphere, and thus of the pressure on the plates, might result in a few more earthquakes and maybe even the odd volcano being triggered, but i am pretty sure that has no consequences that would be noticeable after the very long timespan you suggested before the alien visitors dropped by.

*) "a lot like before" needs to be stretched quite a bit, i guess.
This was supposed to mean: A lot of flora and fauna, although not the exact flora and fauna we know, maybe not even remotely similar, since it will have evolved from a different base and species will have been favored or hindered by different events than before.

Approach #2: The space maid is very thorough

If on the other hand, your space maid does not stop after having vacuumed our atmosphere away, but stays at least for a year or so, to clean up all the vapor rising from the oceans, things will turn out quite differently.
First, our determined maid removed a large portion of the liquid water from the planet. The missing atmosphere will expose the frozen parts, especially the glaciers, but also the ice caps at the poles, to sunlight that is suddelny a lot more intense. While the missing of the insulating effect and the greenhouse effect from the late atmosphere result in a lot of cooling on the dark side of the planet, the half that lies in sunlight receives more energy than it did before. The absence of liquid water removes a buffering effect, which in turn again increases the temperature variations of the day-night-cycle.
At first, the radiation from the sun will kill off almost all surface life that had the indecency to survive the absence of atmosphere (which should pelase our maid a lot).
Also, the greater day-night-temperature swings will lead to stronger erosion, since tension through heating will crack a lot of rock, while on the other hand the redesign of the planetary surface due to rocks from the sky will be a lot stronger than in the first scenario.

Assuming the space maid will eventually call it a day and move on, the bombardment from space should trigger a lot of volcanic activity. this should re-build the atmosphere fairly quickly. After that, much the same as in scenario one should apply, since it can be assumed that at least some extremphiles survived even that, just to re-start the forming of life as if nothing had happened.

But in this scenario, the greatest difference to the first, and to now, would be a much lower ocean level, resulting in much larger dry areas.

Also, the beautiful fjords Slartibartfast had so lovingly designed will be gone. Which is a crying shame.

I agree with the posts that say that an atmosphere mostly composed of water vapor would reappear with the "boiling" of the water bodies. It would not only be water as nitrogen, oxygen and carbon dioxide are found dissolved in water. Water itself would only start boiling if at 0 pressure the temperature would be above boiling which I don't think is the case, it would more likely evaporate rapidly. The gases inside might be another story, and would definitely start bubbling much like a soda drink does when one opens it. The quick evaporation of the water would lead to a cooling of liquid water as the water molecules take away energy through evaporation.

The temperature of the atmosphere would therefore also be much colder than the previous temperature.

One thing that has not been said in the previous posts is that removing the atmosphere will remove a load from the surface of the earth. A quick removal of the atmosphere would certainly cause some amount of earthquakes and tectonic readjustments, specially cracks in the plates, as I would imagine the earth would expand from the release of the atmospheric weight. It is important to consider that the atmosphere is actually very thin, 75% of its mass being concentrated on the first 10km.

Immediately after the removal of the atmosphere, one can imagine that a sudden reduction in pressure on the surface of the planet would have disappeared uniformly from all the surface of the planet. As ocean water evaporates, however, that will lead to an asymmetric change in the pressure across the surface of the planet. Because the ocean will have evaporated this would lead to reduced pressure on the surface that was covered by water bodies. That would be another source of tectonic readjustment.

There are a couple of interesting pictures coming to mind from this idea. 1) Large steam clouds would originate from bodies of water, less densely from land due to underground water bodies. In other places water would not evaporate such as deserts and ice covered regions. - this would lead to small streams disappearing all together leaving empty stream beds

To answer your question, if aliens would visit the planet many millions of years later they would probably find a very similar world to the one we have now, minus all complex life. Many microbes would probably survive the ordeal, but more complex organisms such as animals would die from lack of oxygen, the plants would die from the inability to do photosynthesis, due the blocking of sunlight by clouds.

Earth would regain its atmosphere at the expense of the oceans.

If a vacuum were somehow instantaneously introduced by completely evacuating all of Earth's atmosphere, then the oceans would boil off to form an atmosphere of mostly water vapor. UV light breaks down H2O into hydrogen and oxygen so eventually a hydrogen/oxygen atmosphere would come back. Aerobic bacteria would provide some CO2. Anaerobic bacteria may contribute nitrogen or other gases to the atmosphere.

All non-single cellular terrestrial life dies. The oceans don't boil fast enough to provide breathable atmosphere.

I suspect the atmosphere would be replenished quite a bit faster than most people expect. The replenishment of an atmosphere composed of water vapour and dissolved atmospheric gasses will provide an initial atmosphere, but the big refilling will occur when the sudden removal of weight due to the removal of the atmosphere and upper layer of the ocean triggers tectonic events in the Earth's crust.

A large part of the atmospheric and hydrological cycles of the Earth actually take place over millions of years as atmospheric carbon is absorbed by the oceans and converted into calcium carbonate (for example the building of billions of shells by sea life), then eventually trapped in the ooze at the bottom of the oceans and converted into limestone. Under normal circumstances, it is eventually sub ducted into the mantle and dissolved by the great heat, with carbon dioxide being emitted by volcanos millions of years later. so as the crustal plates are disturbed by the asymmetric removal of weight, we should expect more volcanic activity, pumping millions of tons of carbon dioxide and other gasses into the atmosphere (indeed, a large volcanic eruption can put as much carbon dioxide into the Earth's atmosphere at once as a year's industrial output of human activity).

The "new" atmosphere will be quite different, with a high concentration of water vapour, carbon dioxide and various sulphides and sulphates. Since water vapour is the best of all possible "greenhouse gasses", the surface of the Earth will warm up rather dramatically, melting any remaining icecaps and glaciers, causing more tectonic disturbances and probably more vulcanism. Plants which surveyed the initial removal of the atmosphere will have a generally better environment (current plant life seems to grow best at CO2 concentrations of between 800 to 1200 ppm, as opposed to the current CO2 concentration of just under 400 ppm). Countering this is the lack of oxygen means the ozone layer is gone, so the surface will be bombarded with ultraviolet radiation. Plants will "burn" under the sun, and any surviving animal life will also suffer severely.

Plants which do survive will mutate rapidly, and most of the surviving plant life will be plankton under the surface of the oceans. Fish and sea going mammals will die due to the lack of oxygen in the water, so the Earth will be covered with some sort of tough moss, mutated versions of plant life that exists now and filled to the brim with algae and plankton in the oceans and seas. Perhaps a billion years after the event, animal life will have re-evolved from whatever survivors were underground or otherwise shielded from the lack of O2 and UV radiation, so the forms of life on Earth will be vastly different from what we see today: something like the weird life from the Burgess Shales. Convergent evolution may eventually develop analogues of the creatures we see today in similar ecological niches (shark shaped oceanic predators, for example), but a lot of that may depend on what the "base" stock of multicellular creatures was (if the first new creatures were anything like Hallucigenia, then all bets are off).

So aliens arriving on Earth millions or billions of years in the future will see a populated planet teeming with organisms, all of which seem to have radiated from a limited number of base stocks in the relatively recent past.

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