We generally make the assumption that carbon and water are fundamental ingredients required for the advent of living systems in the universe. Given that all the data we have to work with is based on the behavior of biologic systems operating on the surface of a single planet, this conclusion, from a purely scientific perspective, is based on insufficient understanding. If we define life as a pattern of physical behavior that is independent of its specific physical and chemical ingredients, then our current universal view of life’s required chemistry and supporting environment is truly suspect. One consequence of this change in perception will be seen in the Sagan-Drake “equation” for the estimation of intelligent life in a galaxy.
The estimation of the number of worlds harboring intelligent life would increase due to an increase in the number of planets capable of supporting biological systems (ne; see below) if we remove the carbon and liquid water requirements (which implies specific temperature ranges) for the advent and evolution of life. There is no good reason why Carbon and water should be deemed universally necessary ingredients for the advent of living systems up to and including life forms that are capable of thinking in unusally abstract ways. After all, thinking is an electrochemical process (so we think, anyway). There is, however, one obvious reason to think Carbon and water are required components for life. It's what we have found to be the case here on Earth. So, better to rephrase what we are looking for out there: Life like ours. That focuses the question. It's always easier to look for things when you know what you're looking for. Still, if you want to look for something as broad as the notion of life in the universe you certainly can't base your search criteria on what works for a single planet. On to the Sagan-Drake equation (which is necessarily composed of some rather subjective variables).
The Sagan-Drake equation:
N = R*fpneflfifcL
N is the number of intelligent communicating civilizations in the galaxy at present
R* is the average rate of star formation in our galaxy (stars/year)
fp is the fraction of stars that have planetary companions
ne is the number of planets per planet-bearing star that have suitable ecospheres (that is, environmental conditions necessary to support the chemical evolution of life)
fl is the fraction of planets with suitable ecospheres on which life actually starts
fi is the fraction of planetary life starts that eventually evolve to intelligent life-forms
fc is the fraction of intelligent civilizations that attempt interstellar communication
L is the average lifetime (in years) of technically advanced civilizations
The notion that microbial life is abundant in the universe is certainly a compelling possibility ( requires a high value for ne ) if life is in fact an endemic planetary surface property with an evolutionary pattern that is tightly coupled to that of its planet. Perhaps the degree of bio-environmental coupling is a significant factor in determining if the development of intelligent complex life is possible. Certainly, if a planet harbors substantial life (present globally like here on Earth) then the evolution of the planet’s surface will be strongly coupled to that of its biology and the planet will maintain a surface environment capable of supporting life for periods of geologic time. Time, and lots of it, is a critical ingredient in advanced biological evolution. Or is it? I just ranted about the inherent problems with requiring Carbon and water for the advent and evolution of biological systems. Why this geologic time business? That's a good question. I don't have the answer. For now, let's just say that, regardless of specific chemistry and physics, it takes a long time for life starts to blossom into thinking creatures. I agree that this restriction may be too harsh.
Perhaps Mars is an example of a rocky planet that had sparse microbial life (relative to Earth) and therefore Martian biology had little net effect on the evolution of the Martian surface and atmosphere leading to a relatively short geobiologic lifespan and therefore no chance for the advent of complex life. Clearly, this is wild conjecture, but in the next 5 or so years we will probably know the answers to these Martian questions.
For detecting extraterrestrial life we should not only focus on whether or not carbon/water-based life forms can be supported on a rocky planet (geologically active and rocky surface (not a gas giant), like Earth, Mars, Venus and Saturn's wildly interesting moon, Titan), but whether or not a planet possesses surface properties that demonstrate a predictable pattern of behavior over time ( for example, a substantial and dynamic atmosphere (like the consistent addition and removal of Methane from Earth's or Titan's atmosphere) ) which is independent of the specific chemistry and physics operating on the geobiologic level. Now, if places like Jupiter's Europa support biological systems, then throw this paticular solution out of the nearest window since there is no way using this technique to remotely detect the presence of life that lives beneath the surface of a moon with no atmosphere in an ocean of salty water. It's certain that just looking at geologic patterns on the surface of a place like Europa will not provide enough evidence for the existence of life.