Okay -the standard answer is first Mars - could be surface - either everywhere at extremely low concentrations - or perhaps more likely, in a few "special regions", such as salt deposits and the warm seasonal flows.
It could also be a few meters below the surface in caves or geologically heated "hot spots" of trapped water - or deep underground in its "hydrosphere" a perhaps 100 meter or more layer of water trapped kms below the surface - or all three.
Then, Europa (subsurface ocean with water vapour plumes) and Encladus (ditto - with plumes sent into space).
Also the subsurface oceans of various icy moons. So there may also be life deep underground on Ganymede, even Callisto. And Titan also may have a sub surface ocean as well as its methane / ethane surface oceans.
Europa most interesting of all because it's ocean is thought to be oxygen rich through dissociation of the ice by the intense radiation from Jupiter.
The larger asteroids like Ceres also may have life though this is less likely - but it is likely enough so that Ceres is a borderline category in the Planetary protection workshops - marked with a note that when we get to study it close up - then the situation needs to be re-evaluated.
And - the larger Kepler objects and Pluto could potentially have life underground if they are warm enough beneath the surface.
Another possible location for life is in the upper Venus atmosphere. This is a major challenge - in many cases the most habitable location in our solar system - almost Earth like in temperature, pressure, and atmospheric composition (without the oxygen of course) - but with droplets of concentrated sulfuric acid.
However we do have acidophiles on Earth that survive in conditions not far off the acidity of Venus clouds - in sulfuric acid outflows from mines on Earth.
So - it's possible that there is sulfuric acid tolerant life in the Venus clouds. The planetary protection workshop on Venus came to the conclusion that this was unlikely - but their conclusions were questioned later by some exobiologists. So it's possible a new workshop held today might come to different conclusions.
The other main problem with the high Venus clouds is that there are no solid surfaces of course. So could the life find some way to stay aloft? The residence time of particles is months rather than days - so - that makes it easier - but it's still quite a challenge.
Very remote possibility - but might be life in the Mercury ice caps - perhaps beneath a protective layer of organics that would hold in the water and prevent it from evaporating.
For larger comets - this is like the earlier suggestion of life in interiors of the larger KBOs. Possible. And some smaller comets maybe traces as result of fragmentation of larger bodies?
Then - moving to more exotic forms of life not exactly like DNA. There might be life on Io, sulfur based, in underground pools of liquid SO2, with the life chemistry probably also using H2S.
There might be life on triton using polysylanes instead of polycarbonates.
Life on Titan is a challenge for us to model because of the cold, and the novel chemistry - no oxygen to speak of, no water, no CO2. But - nitrogen might take the place of oxygen - or it might use reactions involving the energy in double and triple bonded carbon.
With all these exotic chemistries - the thing is - that nearly all our experiments on Earth are conducted in a very limited temperature and pressure range. So we know a huge amount about organic chemistry at around room temperature - and then extrapolating to do selected experiments at higher and lower temperatures. But - we don't know much at all about what happens at ultra low temperatures and using long chain chemicals that are rare on Earth.
After all many of organics we study were originally formed only through life processes, though of course many are synthesized nowadays. Would we even know about many of the more complex organic molecules if we were not surrounded by plants and animals and microbes that use them?
It's hard to figure out how much weight to put on this observation. Does it mean we know almost nothing about complex chemistry outside of the range of temperatures humans are comfortable in, and outside of the range of chemical processes carried out by life itself? Or do we know a lot, through the extrapolations we can do from the experiments that are easy for us, and occasional experiments in more extreme conditions?
One of the most challenging forms of life would be silicon based life living in liquid lava.
If this was possible, we might find it on Venus - which has entire rivers of liquid lava flowing in lava tubes - for huge distances across its surface.
These could use Silicones - organosilicon polymers with a silicon-oxygen backbone.
These are stable at temperatures so high they would destroy any organics. But - would it remain stable at the temperatures of even cooling magma pools on Venus?
If life is possible in magma pools - then why hasn't it evolved on the Earth also? Why have we never found silicon based life fossils in lava flows on the Earth?
So - that last one seems unlikely - but still worth mentioning.
For more about the more exotic ideas here, see Cosmic Biology
Some extracts available online here:
Louis Neal Irwin, Dirk Schulze-Makuch
It could also be a few meters below the surface in caves or geologically heated "hot spots" of trapped water - or deep underground in its "hydrosphere" a perhaps 100 meter or more layer of water trapped kms below the surface - or all three.
Then, Europa (subsurface ocean with water vapour plumes) and Encladus (ditto - with plumes sent into space).
Also the subsurface oceans of various icy moons. So there may also be life deep underground on Ganymede, even Callisto. And Titan also may have a sub surface ocean as well as its methane / ethane surface oceans.
Europa most interesting of all because it's ocean is thought to be oxygen rich through dissociation of the ice by the intense radiation from Jupiter.
The larger asteroids like Ceres also may have life though this is less likely - but it is likely enough so that Ceres is a borderline category in the Planetary protection workshops - marked with a note that when we get to study it close up - then the situation needs to be re-evaluated.
And - the larger Kepler objects and Pluto could potentially have life underground if they are warm enough beneath the surface.
Another possible location for life is in the upper Venus atmosphere. This is a major challenge - in many cases the most habitable location in our solar system - almost Earth like in temperature, pressure, and atmospheric composition (without the oxygen of course) - but with droplets of concentrated sulfuric acid.
However we do have acidophiles on Earth that survive in conditions not far off the acidity of Venus clouds - in sulfuric acid outflows from mines on Earth.
So - it's possible that there is sulfuric acid tolerant life in the Venus clouds. The planetary protection workshop on Venus came to the conclusion that this was unlikely - but their conclusions were questioned later by some exobiologists. So it's possible a new workshop held today might come to different conclusions.
The other main problem with the high Venus clouds is that there are no solid surfaces of course. So could the life find some way to stay aloft? The residence time of particles is months rather than days - so - that makes it easier - but it's still quite a challenge.
Very remote possibility - but might be life in the Mercury ice caps - perhaps beneath a protective layer of organics that would hold in the water and prevent it from evaporating.
For larger comets - this is like the earlier suggestion of life in interiors of the larger KBOs. Possible. And some smaller comets maybe traces as result of fragmentation of larger bodies?
Then - moving to more exotic forms of life not exactly like DNA. There might be life on Io, sulfur based, in underground pools of liquid SO2, with the life chemistry probably also using H2S.
There might be life on triton using polysylanes instead of polycarbonates.
Life on Titan is a challenge for us to model because of the cold, and the novel chemistry - no oxygen to speak of, no water, no CO2. But - nitrogen might take the place of oxygen - or it might use reactions involving the energy in double and triple bonded carbon.
With all these exotic chemistries - the thing is - that nearly all our experiments on Earth are conducted in a very limited temperature and pressure range. So we know a huge amount about organic chemistry at around room temperature - and then extrapolating to do selected experiments at higher and lower temperatures. But - we don't know much at all about what happens at ultra low temperatures and using long chain chemicals that are rare on Earth.
After all many of organics we study were originally formed only through life processes, though of course many are synthesized nowadays. Would we even know about many of the more complex organic molecules if we were not surrounded by plants and animals and microbes that use them?
It's hard to figure out how much weight to put on this observation. Does it mean we know almost nothing about complex chemistry outside of the range of temperatures humans are comfortable in, and outside of the range of chemical processes carried out by life itself? Or do we know a lot, through the extrapolations we can do from the experiments that are easy for us, and occasional experiments in more extreme conditions?
One of the most challenging forms of life would be silicon based life living in liquid lava.
If this was possible, we might find it on Venus - which has entire rivers of liquid lava flowing in lava tubes - for huge distances across its surface.
These could use Silicones - organosilicon polymers with a silicon-oxygen backbone.
These are stable at temperatures so high they would destroy any organics. But - would it remain stable at the temperatures of even cooling magma pools on Venus?
If life is possible in magma pools - then why hasn't it evolved on the Earth also? Why have we never found silicon based life fossils in lava flows on the Earth?
So - that last one seems unlikely - but still worth mentioning.
For more about the more exotic ideas here, see Cosmic Biology
Some extracts available online here:
How Life Could Evolve on Other Worlds (Google eBook)
Louis Neal Irwin, Dirk Schulze-Makuch
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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