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There have been many candidates stated as potential habitats for life outside of Earth, but what are considered to be the most likely to harbor life?

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    $\begingroup$ Fun fact: Apollo 12 retrieved Surveyor 3 camera that was later established contained bacteria that had been dormant for years on the Moon, and still alive. The camera was presumably contaminated prior to launch with Streptococcus Mitis. This was later challenged that the sample was contaminated in laboratory, but this never really accounted for the fact that only a single bacterial species was cultured out of the sample, and that it showed signs of stunted mitosis like irradiated sample from the Moon would. So in a sense, there is highest likelihood that the Moon contains some local life. :) $\endgroup$ – TildalWave Nov 26 '13 at 1:00
  • $\begingroup$ Woah, I have never heard that. Very interesting. Simple bacteria are amazing in their resistance and ability to live in extreme environments. So are Tardigrades. $\endgroup$ – Stu Nov 26 '13 at 13:11
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For my money, I'd go with Jupiter's moon Europa. First, it has a thin oxygen atmosphere, and oxygen, as we've found, is not something normally found hanging around by itself. Given any chance, it bonds with natural "oxygen sinks" like iron and silicon. Europa has quite a bit of silica as well, and yet there's free oxygen in the atmosphere.

We have deduced from our own planet's geology that the relatively abundant oxygen in our air is in fact a byproduct of early anaerobic microorganisms, which respirated in a similar way as modern plants; taking in the abundant carbon dioxide, and producing excess oxygen more or less as waste. With anaerobes being the dominant life form on this early Earth, the only consumers of oxygen were chemical; iron deposits, the water oceans, etc. Once those "oxygen sinks" were filled, atmospheric oxygen built up to toxic levels tfor the dominant anaerobes, causing the Great Oxygen Catastrophe, the largest mass extinction event in Earth's history.

So, we're pretty sure that a planet with a significant percentage of diatomic oxygen in its atmosphere means that there was some active process to produce it, and the one we like best is life.

Also importantly, its surface, composed mostly of water ice, is very smooth and regular. This means several things of significance. First, it means the ice most likely formed when a layer of liquid later froze, meaning that Europa was, however briefly, in the "Goldilocks" temperature band (most likely a result of its own residual heat). Second, the fact that the ice is still smooth and hasn't been noticeably chewed up by meteorite impacts and exposure to low pressures over hundreds of millions of years, along with features like the Conamera Chaos which are indicative of movement of plates of ice over the surface, like our own mountain ranges are a result of tectonic shifts, both point to a liquid ocean under the ice. Where there's liquid water, there is the potential for life.

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    $\begingroup$ According to the Wikipedia article, "Unlike the oxygen in Earth's atmosphere, Europa's is not of biological origin. The surface-bounded atmosphere forms through radiolysis, the dissociation of molecules through radiation." The surface pressure is only about a trillionth of Earth's. Follow the link for more details. $\endgroup$ – Keith Thompson Nov 27 '13 at 21:18

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