I don't think I am stating anything too controversial with the following when you consider the speculation that Europa may harbour life. I am puzzled by the many references to habitable zones in articles dealing with the search and discovery of exoplanets, for example, http://www.space.com/12918-habitable...fographic.html. There seems to be undue importance placed on an idea, which frankly, seems fairly meaningless.
Generally, the idea of a habitable zone appears to be based upon three factors:
- Our solar system, Earth, its distance from the Sun and the difference in temperatures between Earth and Venus, and Earth and Mars.
- Our observations of these three planets, Venus, Earth and Mars, and where liquid water might exist on a planet (based upon our observations of these three planets). Earth being the one with liquid water, so we conveniently conclude Earth is in the middle of this idea of a habitable zone relative to our sun.
- Liquid water being necessary for life because that's the way it occurs here on Earth.
Assuming three to be correct, I'd contend the fact that Venus is too hot, and Mars too cold is more a matter of coincidence rather than reflecting any idea of a habitable zone or their distance from the Sun. The coincidence of the order of hot to cold appears to be blinding us a little to what is far more important - a planet's atmosphere and geological processes with its ability to heat and cool.
If Venus was located where Mars is, it may not be as hot as it is now, but it would still be too hot for life given its atmosphere. How controversial is that idea? If it isn't controversial, doesn't it really mean we have to dismiss this idea of a habitable zone as meaningless? If Mars was located where Earth is, would it be significantly warmer given its atmosphere and lack of geological processes? If Earth was located where Mars is, would it necessarily be a different temperature or very much cooler? Is it possible that it may be warmer?
The idea of a habitable zone, as far as I can tell, doesn't fit the data except in a most general way. When thought about in detail the data appears to contradict the idea because Venus is far too hot and Mars is far too cold. In other words, they aren't just a little too cold and a little too hot as a result of being just a little outside the habitable zone in our solar system like they should be if the idea of a habitable zone held water - excuse the pun.
The important factor is atmosphere and geological processes which must either heat or cool the planet - there is no neutrality in the equation, as appears to be assumed. If this is the case, a planet (or moon) can be almost anywhere in a solar system to be habitable depending upon its atmosphere and geological processes. For example, Europa.
The search for liquid water only makes sense when considered in the context of a planet or moon ie. not just sitting out in space by itself. Liquid water doesn't exist in space. This is why the idea of a habitable zone seems to become meaningless, because of the immense variables when considering a planet such that liquid water can exist almost anywhere, regardless of distance from a star. It appears what is important is rather the state of the planet, or the state of the planet combined with the distance from a star and its heat - the way the two interact. Without detailed knowledge of the state of an exoplanet, something which we are a long way from discovering given the difficulties we still have with our own planet, the idea of habitable zone is comforting at best, distracting at worst..
I'm interested to be educated otherwise by others. Thanks.