The Nature of Life
The life as we know it is the one that exists over the surface of our planet and, possibly, may also exist in the Jupiterís satellite Europa or even in Mars Ė life based on the carbon element (C) and using the water (H2O) as the solvent.
Titan: Ethane and Methane
However, nothing tells us that this is the only possible life form. On the Saturnís satellite Titan, where the methane (CH4) assumes the role that water does on Earth (existing under gaseous, liquid and solid states and falling on the ground under the form of rain), a primitive chemistry is being developed. But, given the slowness of the reactions (because the low temperatures Ė about Ė180 ļC or 90 ļK Ė determine so), it hasnít yet overtaken the pre-biotic state, even when 4500 million years have already elapsed since the beginning of its existence.
Nevertheless, there can be found there uncountable organic molecules, like the hydrocarbons (ethane, propane, acetylene, etc) and nitrogen composites (hydrogen cyanide, cyanoacetylene, etc). Itís probable that Titan is covered by ethane and methane oceans, where many of those molecules, namely the hydrocarbons, may be found dissolved.
Profile of the atmosphere of Titan (University of Michigan)
Ammonia and Other Solvents
Other solvents could also serve as a base for the emergence of life forms, as the alcohol or the ammonia. The ammonia melts at a higher temperature than the methane and, therefore, it could harbour life forms in planets with higher temperature planets, more favourable to quicker reactions.
Itís thought, nevertheless, that the existence of organisms in such environments is less probable than in the water, because this is the most abundant solvent and it has a great ability to concentrate solid elements at its edges.
Silicon and Germanium
Life can also be based in molecules other than carbon. Silicon and germanium are candidates to that role, because they allow the formation of polymers, which are long molecular chains that are the base of the life.
However, carbon is much more abundant in the Universe than silicon and amazingly more abundant than germanium. Besides that, silicon forms excessively strong links with the oxygen, which prevents it from having the required versatility for the metabolic reactions, which are based on the constant formation and breaking of links inside the environment of a living cell.
One hypothetical form of life based on silicon would resemble more a crystal than any plant or animal that is known on Earth. Something similar to a robotic insect would feed from rocks and would live under a very dense and thick atmosphere (far above the boiling point of water). The atmospheric pressure would be very powerful Ė the air would weigh there more than a terrestrial ocean at a deepness of several thousands of metres. A crystal-like vegetation would grow constantly over the surface of such planet, feeded by the lava flows.
The tweezers of a lithovore (silicon creature that is feeded by stone) would have the power to break and grind the crystal rock in order to ingest it more easily. Hard minerals could be melted or dissolved through jets of strong acids. The lithovoreís life is based on the electric energy generated by an internal reactor and, therefore, itís important that they are endowed with tools (like the Geiger counters) that allow them to find radioactive minerals. Itís also possible that they are a sub-surface species, spending most of their lifes excavating, breeding and hunting in complex nets of tunnels.
The lithovores are not born like us. They are manufactured by a "queen" (as in the case of the termites or the bees), which looks much more like a factory than like a living creature.
Lithovore (Adolf Schaller)
Life may not even be based on molecular matter, as we know it. Itís possible that on the crushing surfaces of the neutron stars there are creatures based on a macronuclear chemistry, in which complex and gigantic structures of neutrons have the same role as the molecules (structures of atoms) have on Earth. Such creatures would be microscopical and, because their vital functions would be processed at the light speed, they wouldnít probably have life expectancies higher than a fraction of a second. Even if we visited them, they wouldnít see us as life forms, since we would look like being frozen in time.
The Black Cloud
It may also be conceived a creature living in the inter-stellar space, where it would nearly face no limitations concerning to its size, because the gravity force is very dimly felt there. So, such a creature should be huge (measuring several kilometres), in order to be able to filter a sufficient quantity of the scarce dust (the interstellar space is almost empty) and light photons (the interstellar space is cold and dark as a new moonís night) that it would meet in itís path. Even the low energy levels that are necessary to keep the metabolism require giant collecting panels, analogue to the solar panels.
In the interstellar space such creatures could be extremely thin, because they would seldomly collide with whatever. The fact that they are so thin would reduce their mass to minimum values and, that way, their needs for matter and energy would be limited. However, they would have to preserve a minimum consistence so they wouldnít become disrupted in space, the only environment where their existence would be possible. Itís noticeable that one of these creatures would immediately collapse if it was faced with the terrestrial atmospheric pressure and gravity.
The movement of these creatures in space could be entirely passive, like the plankton adrift in an ocean. On the other hand, it can be imagined a creature that would need to be transported in the direction of the energy and matter sources (used during its growth). However, that behaviour would require higher matter and energy consumption, which could be revealed to be strong barriers against the feasibility of such a way of life, precisely due to the big distance between such sources. It is, therefore, unlikely that such living creatures actually exist.
Finally, we can mention some ideas about artificial living creatures. In the future, the humanity may create entirely artificial brains that will replace the biological tissues through the use of nanotechnology (technological manipulation at an atomic or molecular scale). Thatís what already happens with the computers with which we are familiarized today, although at a still quite modest level.
If our brain could achieve the thinking speed (information processing speed) of these machines, we would become far more intelligent and we would have the perception of the world at a much slower motion that we have today (a half minute would look so long as a year).
If some day we achieve to create artificial brains, we finally may be able to get rid of the biological restrictions, to increase our lifesí expectancies (perhaps close to immortality) and to endow ourselves with prodigious capacities.
Nanotechnology: microscopical engine (Nanotechnology magazine)