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A Traveler from a very advanced civilization has visited the Solar system. He found the humans likable, and decided to leave them a boon. For some reason, he couldn't do anything directly to the Earth or humans, but what he conducted was a compete atmospheric terraforming of Venus and Mars.

On day X, Venus and Mars' atmospheric composition and surface pressure has turned to be an exact match of the Earth's. Moreover, they got enough water vapor to form seas and even oceans. However, for the same unstated reasons, or completely other ones, the Traveler didn't do anything to the planets' interiors or even the minerals at the surface.

The question: for how long would Venus and Mars stay habitable after terraforming?

P.S. My definition of habitability is that some of the Earth animals and plants would be able to proliferate and humans can walk around without spacesuits (but some protective clothing is Ok).

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  • $\begingroup$ Yep, must change my spectacles confirmation. $\endgroup$ – Tantalus' touch. Feb 19 '19 at 22:39
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    $\begingroup$ What a lazy Traveler! $\endgroup$ – NofP Feb 19 '19 at 22:39
  • $\begingroup$ Have you been playing TerraGenesis? Seems like a familiar scenario, if not I’d certainly check it out if I were you! Btw, unless you give them an ozone to protect them from the sun, those rocks are still uninhabitable $\endgroup$ – Bett Struble Feb 20 '19 at 4:53
  • $\begingroup$ @Bett Struble yes, ozone will be available at the appropriate altitude, similar to the Earth. $\endgroup$ – Alexander Feb 20 '19 at 8:35
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Neither are habitable, but there are more reasons than just radiation that was the main focus of Tim's answer.

Another key factor would be temperature. Even assuming (unstated in the question) that the traveler ALSO adjusted the atmospheric temperatures to match Earth temperatures at the same time he dumped ocean sized buckets of sea water on the planets and did the atmosphere swaps, it would be a very short time (probably just hours or days, almost certainly shorter than a week) before the distances between the planets and the Sun caused HUGE temperature issues.

Temperatures here on Earth drop significantly just in the (relatively) tiny shade of a lunar eclipse.

Venus would be especially similar to Earth, initially, due to similar size. But with Venus' slow rotation, the night side would freeze solid quite quickly while the day side oceans boiled.

Mars, being so far away, would just freeze over entirely, day or night side.

In both cases, the temperature of the atmosphere, even if breathable by composition, is no longer breathable by temperature. Cooked or frozen lungs are the result, without an environmental suite, which seems to be beyond the spirit of "humans can walk around without spacesuits" requirement of "habitability"

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It would take millions of years for solar stripping to cause problems, and the atmosphere would be entirely sufficient as radiation shielding. This is especially so on Mars, which receives about half the solar radiation on Earth to begin with, and which will require over 2.6 times as much atmospheric mass per unit of ground area to achieve a full atmosphere of pressure due to its weaker gravitational field. Such an atmosphere would probably be more effective radiation shielding than Earth's atmosphere and magnetosphere combined.

The "atmospheric composition and surface pressure has turned to be an exact match of the Earth's" part is actually where you run into trouble. Mars would get very cold...not as much as you might expect due to the very deep atmosphere, but probably inconveniently so. You'd want an atmosphere with an enhanced greenhouse effect.

Venus has it worse: its lower gravity means it'd still have a deeper atmosphere than Earth's, better at trapping solar heat, while it gets twice as much heat in the first place. But that's not even the big problem: unless this mysterious visitor also adjusted the rotation rate of Venus, the solar day will be 117 times as long as Earth's. The daytime temperatures will quickly rise to something unreasonable even if not quite as extreme as they are today, and it might get colder than an Antarctic winter during the night. The poles would end up being the least extreme environments on the planet, but "habitable" might be a stretch. You'd want atmospheric adjustments to deal with the increased solar input, but that wouldn't be enough without also dealing with the rotation.

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Neither are habitable from day one.

Neither Mars or Venus have a magnetosphere. If your traveler did nothing to the internals of the planet, they didn't create one either. That means two things;

1) Your terraformed planets will slowly but regularly lose atmosphere thanks to the solar wind - you've brought Venus' atmospheric pressure down to a little over 1% of it's current levels, and you've increased Mars' by 100x, but both are currently losing atmosphere at a steady rate to solar winds and this won't change; all you've really done is changed when the atmospheres are depleted.

2) You've left the big X-Ray machine called the Sun full access to anything on the surface. That means, your humans on Earth simply can't live there unprotected by some form of shielding or dome, no matter how temperate or oxygenated the environment may be.

Edit; *It should be noted that if there is an ionosphere on these new planets (and there would be) then at least some of the radiation would be deflected and I don't have exact numbers on me as to what percentage of the solar wind would be deflected in the process of stripping away the atmosphere and what would still reach the surface of the planets. The Aurorae would be spectacular and life as we know it would be impossible because of EM interference, but it's possible that the surfaces would be habitable to at least some degree, although I suspect still dangerous over the long term.*

It should also be noted that life doesn't just magically appear in complex form because the environment is right. It took over a billion years for life on earth to reach any reasonable level of complexity, and your planets haven't started yet. Also, there's no oceans (no net change to minerality) so with the lack of a magnetosphere especially you don't have an ideal environment on which it could form on either planet. Add to that the fact that you can't just plant trees on Mars and they'll grow; trees are just one part of a complex biome including soil bacteria and other life forms breaking down humus that if those things are not present, then your tree dies in short order.

The web of life on Earth is a amazingly complex and fragile and we often don't appreciate what we have in that regard. Just changing atmospheric pressures and temperatures does not a biome make and the worlds your traveler has made are no exception to that.

It might be a lot easier to colonise either planet, but that doesn't make either habitable.

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  • $\begingroup$ Thank you for your answer. I wanted to get somewhat accurate idea about the concern #1, and I'm not sure if Earth's magnetosphere does anything to stop Sun's x-rays (#2). $\endgroup$ – Alexander Feb 19 '19 at 23:11
  • $\begingroup$ @Alexander well, probably the easiest way to describe it is that the magnetosphere blocks the high speed charged particles in the solar wind (which would do us damage anyway and is the reason Apollo Astronauts kept seeing flashes of light - it was those particles damaging their retinas - but it also protects the atmosphere, especially the ozone, which protects us from UV. So, losing it would be a double whammy. $\endgroup$ – Tim B II Feb 19 '19 at 23:16
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    $\begingroup$ I presume the atmosphere itself should provide the screen for charged particles (while taking damage in the process). $\endgroup$ – Alexander Feb 19 '19 at 23:33
  • $\begingroup$ @Alexander I've edited to reflect what I know about the Van Allen Belt now. $\endgroup$ – Tim B II Feb 19 '19 at 23:56
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    $\begingroup$ Nitpick: since you call the Sun a big X ray machine, a magnetosphere cannot stop X rays, but only charged particles. $\endgroup$ – L.Dutch - Reinstate Monica Feb 20 '19 at 6:48
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Terraforming Mars is a wash, there's not a lot of stuff on the planet. It's mass, gravity and lack of a magnetic field renders it useless even if it were habited; not without artificial schemes to thwart radiation.

Venus is potentially terrform-able. But challenges include - Lowering it's average temperature from 864 degrees fahrenheit to 60. - Eliminating or sequestering it's co2 and sulfer dioxide rich atmosphere. - Finding a major source of Nitrogen to support plantlife and biological activity. - Introduction of water

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    $\begingroup$ well, about Venus atmosphere, there are 3 to 4 times more nitrogen than Earth $\endgroup$ – Rodolfo Penteado Jun 10 at 5:34
  • $\begingroup$ Yeah, Venus has more nitrogen than you need already. And sequestering the CO2 and introducing water are the same thing, you need to import hydrogen. And the lack of a magnetic field would be a bigger issue for Venus than for Mars due to its thinner terraformed atmosphere (due to its gravity and need to stay cool) and ~4x received solar radiation...the atmosphere will probably still provide adequate shielding to the surface, but satellites and space stations that close to the sun might appreciate a magnetosphere. The rotation is the big problem though, and makes fixing the atmosphere look easy. $\endgroup$ – Christopher James Huff Jun 10 at 12:49

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