Space missions are expensive and we have a lot of valuable information from prior missions. How might we leverage this to ask questions about the possibility of life and what to look for elsewhere in the cosmos.
In my papers I argue that as long as we have no good idea of how life originated, we do not really know what to look for in outer space: early life may not be recognized. It may be present everywhere, even in meteorites.
My approach is that life started as heat engines, and that use of chemicals and light was acquired later. Irrespective of my ideas being correct, we should take a closer look at alternative biological energy sources, both extant in environments in the world around us at present, and extinct, as progenitors of present life.
Again: if we do not know what to look for, we will not be able to see it: the eyes can only see what is already in the mind.
(I wrote an answer this morning, but it seems to got lost in cyberspace; in hindsight I think that I did not answer the question correctly, so I try again).
My answer:
In the absence of a model for the origin of life on earth we cannot prepare tests for whether life is possible on a specific extraterrestrial site (I prefer not to use the word planet, as extraterrestrial dust, comets, meteorites, moons, Kuiper Belt Objects, planetary nebulae and even the atmospheres of cold stars should be considered as potential habitats as well).
We can only check whether life as we know it on Earth could live at a certain site; this already yields an immense variety, as life seems to be present almost everywhere on Earth: from deep down in sediments to high up in the atmosphere. Terrestrial ecology therefore yields many possible shapes of life. But is this set comprehensive?
Schulze-Makuch's book described several shapes that extraterrestial life could take. But again, this set is possibly not comprehensive.
We do not how life on Earth emerged, i.e., the first stages that life on Earth assumed are unknown. Those first stages may be difficult to recognize, as we do not know how they looked like. The eyes can only see what is already in the mind.
Obviously finding out how life on Earth emerged would help. I myself have come up with a model for the origin of life in which the first organisms were essentially heat engines. Such organisms could live on thermal cycling (as in a convection current) or on a thermal gradient; they would NOT need sunlight. My papers state that such conditions are present in many extraterrestrial places (for instance: organisms living in the dark on convection underneath surface ice).
To get back to the stated question. If one could demonstrate that life on Earth could live on thermal cycling or in a thermal gradient, one would have done a convincing test that extraterretrial life is possible almost everywhere. Candidates abound for such life: they range from microorganisms growing in tap water to the jelly fish clogging up the cooling water inlets of Swedish nuclear reactors that are in the news today.
Nice discussion guys. Let me tell you that we have devoted tens of missions to study Solar System objects remotely, but we should not forget that real evidence of life requires "in situ" analyses, and tests. The best tests should be based in a deep knowledge of the peculiar environment chemistry as we know that some non-equilibrium gas species might be key signatures of life. But, the only two bodies that we have "just" started to roughly explore are the Moon and Mars. I agree with my previous colleagues that we miss a real definition of life so this is a real handicap to develop convincing tests. It is not helping either the existence a huge number of life forms on Earth with very diverse metabolisms, and widely distributed in habitats/environments that some decades ago had been ruled out. Interdisciplinary and multidisciplinary science (and cooperative scientists) will give answers in the future. Our recently linked Springer book might be of interest to you in that regard.
On the other hand, we have achieved a reasonable idea of the similar chemical composition of other stars in the Milky Way and other galaxies, and we firmly think that C, H, N, O, P, and S are crucial for living organisms. No doubt that elemental proportions might be locally different in planetary systems of our own galaxy, but why we should think that the powerful organic chemistry doesn't work elsewhere? Anthonie is right that in Earth life organisms are distributed almost anywhere, and this certainly opens the possibility of having life forms in other bodies of our own planetary system. In my books I introduce the idea that (simple) life is also widely distributed, but in those habitable places where had the chance to appear and is able to be preserved . For example, I think that we should explore well all plausible habitats in planetary bodies before ruling out that life also emerged in Mars, Europa, Titan, etc...
http://www.springer.com/astronomy/extraterrestrial+physics,+space+sciences/book/978-1-4614-5190-7
Based upon Earth life I wonder if there are a limited set of signatures for life that could be tested for in soil and atmosphere or are these too diverse to apply?
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