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It is estimated that in about five billion years, our sun will enter its red giant stage. During that phase it will expand and completely engulf Mercury, Venus and possible Earth. Earth will either be destroyed or become uninhabitable.

Will there be a phase during this development where the sun's energy at the distance of Jupiter becomes strong enough to turn the surface of Europa into a liquid ocean but not yet strong enough to completely evaporate it? When that happens would Europa be habitable for humans and if yes for how long?

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This question asks for hard science. All answers to this question should be backed up by equations, empirical evidence, scientific papers, other citations, etc. Answers that do not satisfy this requirement might be removed. See the tag description for more information.

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    $\begingroup$ What's your threshold for habitability? For instance, Europa is less than 1% the mass of the Earth. That means it can't support any real atmosphere, and that humans would be subjected to significantly less gravity than their bodies are designed for. Those are potentially solvable problems, but not without hypothetical advances in technology. Or in other words, Europa itself will be inhabitable, but certain technologies could still allow humans to live there. Is it habitable if we can build a habitat, or only if a human could survive directly on the surface with no artificial life-support? $\endgroup$ – aroth Mar 4 '16 at 12:33
  • $\begingroup$ Just for clarity's sake, you probably should define "habitable", since both aroth's comment above and guildsbounty's answer are partly asking what you really mean by that. That is even more important given that you have tagged this hard-science. Voting to put on hold as unclear in the interim. $\endgroup$ – a CVn Mar 4 '16 at 15:50
  • $\begingroup$ I feel like I read a question like this on this site, and it said human-like intelligent life would not be able to develop in the time Europa would be in the habitable zone. Also, @aroth I would assume he is talking about Wuropa being in the earth's habitable zone. $\endgroup$ – Xandar The Zenon Mar 4 '16 at 16:04
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    $\begingroup$ Bah, just realized that "habitable" and "inhabitable" are actually synonyms. I supposed I was going for...inhospitable? Or actually, "uninhabitable" is what I meant in the second-to-last sentence there. $\endgroup$ – aroth Mar 4 '16 at 16:21
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I am assuming that by 'habitable' you mean 'in the Circumstellar Habitable Zone of the star,' (aka Green Zone) since that is what is going to change when the sun turns into a Red Giant.

Sol's current CHZ is estimated to range between 0.5AU and 3.0 AU. This wide band is dependent on a lot of things. Earth-like planets can function in the nearer reaches of this, while greenhouse-planets can potentially sustain life further out. (ref)

According to the write up "Can Life develop in the expanded habitable zones around Red Giant Stars?" (ref) They provided a new estimate that says that when Sol enters the Red Giant phase, its new CHZ will range from between 9AU to 22AU. Jupiter is situated an average of 5.2 AU from the sun. This positions it too close to the sun to be within the green band, all the water on Europa would boil off. So no, Europa would not be habitable...too close to the sun.

On the other hand, Saturn orbits the sun at an average distance of 9.6 AU, which plants it firmly in the 'close-range' green band of the new CHZ, which is where Earth-like planets may exist. Titan, one of Saturn's moons, is presently wreathed in an atmosphere of hydrocarbons and has a methane cycle analogous to Earth's water cycle...however, the planet itself is composed of a significant amount of water ice. It would take a lot of work to clean the place up, but once the sun turned into a Red Giant, Titan would be a potential terraforming candidate...if you could get its methane levels under control and stabilize the atmosphere into something humans could breathe.

Do note that this does not mean Titan would be habitable by humans on its own...just that it would be the right temperature for humans to potentially live on. As it is, Titan's atmosphere is not friendly to human lungs, even if it wasn't ludicrously cold. We would have to terraform it first.

The sun would stay in its Red Giant phase for about one Gigayear (a billion years)...so that's how long you'd have to live on Titan.

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  • $\begingroup$ But the expansion into red giant state isn't immediate, isn't it? Wouldn't there be a period during the expansion where Jupiter is in the CHZ? How long would that period be approximately? $\endgroup$ – Philipp Mar 4 '16 at 14:45
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    $\begingroup$ It is not immediate, no. There is a several thousand year period where the star is growing into a Red Giant after Hydrogen fusion in the core has started to fail. At first, the growth is barely noticeable, but it is an exponential curve. The longer the process, the faster it goes. Look at the graph on page 4 of this document: arxiv.org/pdf/0801.4031v1.pdf There would, of course, be a window of time where Europa would be in the green zone for maybe a thousand years or so at the most. But the steady temperature changes would make global warming look like a joke $\endgroup$ – guildsbounty Mar 4 '16 at 14:59
  • $\begingroup$ If I understand the reference correctly, the 9 to 22 AU is the range within the latter stages of the star's Red Giant phase, and there will be a period before that during which the habitable zone will be from 2 to 9 AU, which actually suits Europa perfectly. $\endgroup$ – aroth Mar 4 '16 at 16:15
  • $\begingroup$ That is correct...the period of time would not be long (astronomically speaking; a few thousand years at most), and it would not be stable. Temperatures would steadily rise as the sun got steadily bigger. The stabilization at 9-22AU is what happens when it is done growing, and it will stay that way for about a billion years before it crumples down into a white dwarf. But, that could be a long enough time to tell a story...assuming you can get Europa terraformed...which will actually be notably harder than terraforming Titan. $\endgroup$ – guildsbounty Mar 4 '16 at 16:23
  • $\begingroup$ This is because Europa is significantly smaller, does not already have an atmosphere, and has to deal with the hellish magnetic fields and tidal forces hurled at it by Jupiter. $\endgroup$ – guildsbounty Mar 4 '16 at 16:24

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