What other bodies in the solar system are most likely to contain forms of life?

Question

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?

Answer 1

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.

Answer 2

Technically all planets except Mercury might host life and almost all spherical moons and "dwarf planets" too. As for my opinion, I'd go with that order:

1.) Europa

Jovian moon Europa is covered by a water ice crust beneath which there is a subsurface liquid ocean which might harbor life near volcanic plumes on the ocean's ground. Europa's ocean reportedly contains more liquid water than the Earth's oceans altogether. Thus Europa has the most clearly liquid water of all celestial bodies in this system obviously. It is hot enough near the plumes for life to survive there.

2.) Titan

On the largest moon of Saturn, methane-based life is likely on the surface or in the methanic seas while carbon-based life might be possible in Titan's subsurface water ocean. Contrary to Europa however Titan is not assumed to contain subsurface volcanoes. I rather believe that methane-based life is very likely on Titan.

3.) Enceladus

Another moon of Saturn, same situation as Europa. However, Enceladus is much smaller, less massive, less dense and has a very low gravity (surface acceleration is 0.114 m/s² or 0.012 g). Europa has a more Earth-like interior.

4.) Ganymede

The biggest moon of Jupiter has layers of subsurface liquid water where life is possible. Its orbit still is close enough that its subsurface oceans get tidally heated by Jupiter and maybe there's an internal source of heat within Ganymede.

5.) Mars

The red planet has water ice polar caps. Some vales on Mars reach an atmospheric pressure beyond the triple point of water so that liquid water is possible outside at the correct temperatures. These temperatures are reached at certain times of the Martian year, Mars is barely in the habitable zone.

6.) Callisto

Same situation as with Ganymede but I doubt its subsurface oceans get enough tidal heating from Jupiter. Callisto is much farther from Jupiter than Ganymede. But perhaps there is life and maybe heat from another source within Callisto.

7.) Venus

Extremophile organisms might exist in the upper atmosphere of Venus where the air pressure and temperature are Earth-like.

As for other spherical moons, most of them likely have subsurface water oceans so life might exist there too. The only moon on which life obviously cannot be possible is Jupiter's Io and perhaps the Earth's Moon though certain astronauts and probes found organisms on the Moon that may or may not hail from the Earth.

The four gas giants might host extremophile organisms in their upper layers just like Venus.