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Upon reading on this site (and many others), one can think that Mars might have supported life in a distant past (discovery of liquid water, valleys, mountains...).

On the other hand, Venus is the perfect example of the consequences of the greenhouse effect. A planet where life is simply impossible because no heat can escape the planet's ultra-thick, CO2 filled atmosphere.

So, with a lot of exageration, one can say that both Mars and Venus could be the Earth's distant future.

Since the Earth is actually facing global warming and greenhouse effect, I was wondering if the Earth would one day be like Venus ? Or if it is going to be like Mars, with no more magnetosphere and almost no atmosphere left ? Or if it is not going anywhere near those two planets' fate ?

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up vote 6 down vote accepted

If you are asking about short-term effects related to human's effect on the climate, the answer is (obviously) unclear. But in the very long term, Earth is likely to evolve to a more Venus-like state.

Over the next billion years or so, the Sun's luminosity will slowly increase, which will heat Earth's surface. As a result, more water vapor will evaporate into the atmosphere. Since water vapor is a greenhouse gas, this will compound the heating. It's not entirely clear what the new equilibrium temperature will be after this runaway greenhouse effect runs its course. But it will clearly be more Venus-like than Mars-like.

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This is essentially what I'd have said. A Mars outcome is unlikely because the Earth's gravity and magnetic field makes losing it's atmosphere and becoming a Mars very unlikley. Becoming a Venus, over a billion or two years, is entirely possible, but likely with more water than Venus, but closer to Venus than Mars certainly. The hotter sun and water being a greenhouse gas suggests a hot future in 1-2 billion years is highly possible. A repeat of "snowball earth" is also possible if CO2 drops low enough, but . . . I think that's unlikely. Technology could prevent either outcome too. – userLTK Feb 3 at 3:36

Either or neither. It's impossible to tell from the present.

If runaway climate change occurs, then yes, the conditions on Venus could be a potential analogue for the kind of environment on Earth due to the greenhouse effect.

Mars's atmosphere is assumed to have been much thicker in the past, otherwise it could not have sustained liquid water on the surface; it would have evaporated away. This water is required to explain the gorges and riverbeds we see on the Martian surface today. Researchers are still unsure where the atmosphere went, but the prime candidate is through top-loss to outer space, rather than sequestration through minerals on the surface (see here).

The likelihood of such an atmospheric escape process becoming dominant on Earth is small due to the Earth's greater mass and strong relative magnetic field, which prevents ion escape. In fact, the dominant loss process on the earth is sequestration. Some estimates put the reservoirs of sequestered carbon from original CO$_2$ at 250000 times the size of the existing atmosphere.

The alternative is that the Earth remains in its current steady state, or close to it, far in to the future. There are many mechanisms to enable such an equilibirum, such as oceanic sequestration of C0$_2$ highlighted above. There are also more contested theories such as the Gaia hypothesis. This proposes that the global biological ecosystem on the Earth is self regulating, helping to maintain life enabling conditions on the Earth. It's pretty much a dead-cert that life doesn't exist on either the Martian or Venusian surfaces, so we can accept that if the Gaia hypoethesis is true, the Earth will never reach these conditions.

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Thanks for the answer. I'll ask a little bit more :p One day the Earth will either be tidally locked to the moon, either lose the moon. Since the moon plays a great role in the Earth's geological activity, in both cases, the Earth will have no more geological activity. If I'm right, that means the Earth's magnetic field will decrease. Couldn't that lead the earth to the fate of Mars ? – Nico Feb 2 at 13:38
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I'm not sure what you mean by lose the moon, but I'm going to assume you mean it leaves the Earth's orbit. This is unlikely, as a result of the tidal locking you mention (see here). I'm not sure what effect the Moon has on geological activity; plate tectonics are primarily cause by the earth's contraction and convection current's in the mantle, heated by radioactive processes in the Earth's core. I think the ocean's sublimation of CO_$_2$ has a greater effect on atmospheric conditions, I'll update my answer. – polyphant Feb 2 at 13:46
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@Nico Don't forget that Venus' atmosphere is toxic to life for more reasons than simply CO2, it's full of other nasty chemicals like ammonia that don't exist in such high concentrations on Earth. We might get a runaway greenhouse effect eventually, but it will likely never be as "extreme" as Venus in terms of general lethality to life as we know it. – thanby Feb 2 at 23:04
    
@Nico, I think you're incorrect on the Earth's rotation having to do with it's geological activity. It's the heat in the earth's core and the convection currents that are driven by that heat that causes the Earth's magnetic field. en.wikipedia.org/wiki/Dynamo_theory The Earth's core will eventually cool and the magnetic field will weaken (in a billion or few billion years) but not because of tidal locking. – userLTK 6 hours ago

This is a question that cannot be accurately answered.

However, the closest to a honest and accurate answer would be that neither, since the Earth is going to "evolve" (curious choice of words) towards the Earth's fate.

The presence of humans means any purely physical projections need to be taken with a grain of salt. Our ability to influence the planet will only increase. With it, so will the range of possible future states.

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Excuse me for the poor choice of words, I'm French and I don't know that many words. Thanks for your answer ! – Nico Feb 2 at 15:26
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I know what you meant and couldn't restrain my nitpickery, excuse me too. – Zbyněk Dráb Feb 2 at 15:29
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Why is it a curious choice of words? What other word would mean the gradual development over time? – corsiKa Feb 2 at 19:07
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@Nico That was a perfect choice of words. – DCShannon Feb 2 at 20:30
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@DCShannon, corsiKa, Nico: Evolve (M-W definition) "to change or develop slowly often into a better, more complex, or more advanced state [emphasis added]: to develop by a process of evolution". That's probably why it is curious, since the states of Venus and Mars are probably none of these three compared to current Earth. But agree that it was nitpickery. – Erwin Bolwidt Feb 3 at 4:16

Since the Sun's temperature will continue to increase and it is estimated that in ~2 billion years it will be so hot on Earth that life will be impossible, I'd say it's going to evolve more toward Venus.

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