At the 50th anniversary celebration of NASA on October 1, 2008, Stephen Hawking, then Newton's heir as the Lucasian Professor of Mathematics at the University of Cambridge, was asked the question, “Are we alone?” His answer was short and simple: "probably not."
Hawking outlined three possibilities: One, that there is no life out there; and two, somewhat pessimistically, that when intelligent life gets smart enough to send signals into space, it is also busying itself with stockpiling nuclear bombs.
What we normally think of as 'life' is based on chains of carbon atoms, with a few other atoms, such as nitrogen or phosphorous, Hawking observed in his lecture, Life in the Universe. We can imagine that one might have life with some other chemical basis, such as silicon, "but carbon seems the most favorable case, because it has the richest chemistry."
The Earth was formed largely out of the heavier elements, including carbon and oxygen. Somehow, Hawking observes, "some of these atoms came to be arranged in the form of molecules of DNA. One possibility is that the formation of something like DNA, which could reproduce itself, is extremely unlikely. However, in a universe with a very large, or infinite, number of stars, one would expect it to occur in a few stellar systems, but they would be very widely separated."
Other prominent scientists have warned that we humans may be blinded by our familiarity with carbon and Earth-like conditions. In other words, what we’re looking for may not even lie in our version of a “sweet spot”. After all, even here on Earth, one species' “sweet spot” is another species' worst nightmare. In any case, it is not beyond the realm of feasibility that our first encounter with extraterrestrial life will not be a solely carbon-based fete.
Alternative biochemists speculate that there are several atoms and solvents that could potentially spawn life. Because carbon has worked for the conditions on Earth, we speculate that the same must be true throughout the Universe. In reality, there are many elements that could potentially do the trick. Even counter-intuitive elements such as arsenic may be capable of supporting life under the right conditions. Even on Earth some marine algae incorporate arsenic into complex organic molecules such as arsenosugars and arsenobetaines.
Several other small life forms use arsenic to generate energy and facilitate growth. Chlorine and sulfur are also possible elemental replacements for carbon. Sulfur is capable of forming long-chain molecules like carbon. Some terrestrial bacteria have already been discovered to survive on sulfur rather than oxygen, by reducing sulfur to hydrogen sulfide.
Nitrogen and phosphorus could also potentially form biochemical molecules. Phosphorus is similar to carbon in that it can form long chain molecules on its own, which would conceivably allow for formation of complex macromolecules. When combined with nitrogen, it can create quite a wide range of molecules, including rings.
So what about water? Isn’t at least water essential to life?
Not necessarily. Ammonia, for example, has many of the same properties as water. An ammonia or ammonia-water mixture stays liquid at much colder temperatures than plain water. Such biochemistries may exist outside the conventional water-based "habitability zone". One example of such a location would be right here in our own solar system on Saturn's largest moon Titan.
Hydrogen fluoride methanol, hydrogen sulfide, hydrogen chloride, and formamide have all been suggested as suitable solvents that could theoretically support alternative biochemistry. All of these “water replacements” have pros and cons when considered in our terrestrial environment. What needs to be considered is that with a radically different environment, comes radically different reactions. Water and carbon might be the very last things capable of supporting life in some extreme planetary conditions.
NASA's recent controversial announcement of the discovery of the possibility of arsenic-based life in Mono Lake fits hand-in-glove with NASA's strategy to expand the search for life beyond Earth to extreme non-carbon-based life. No discovery that we can make in our exploration of the solar system would have greater impact on our view of our position in the cosmos, or be more inspiring, than the discovery of an alien life form, even a primitive microbial one.
The discovery over the past decade of extreme life forms thriving on Earth at the super-heated walls of Ocean volcanic vents and in the interior regions of the planet's crust, led to a seminal 2009 report, The Limits of Organic Life in Planetary Systems, by the National Research Council (NRC). The NASA sponsored report recommended that the search for beyond Earth’s solar system should be widened throughout the Universe to include the possibility of “weird” life.
"Nothing," the report concludes, "would be more tragic in the American exploration of space than to encounter alien life and fail to recognize it.”
Earth did not accumulate oxygen during the first roughly 3 billion years, or form an ozone layer until about 1.5 billion years ago. There is considerable emphasis on looking for contemporary Earth atmospheres that have oxygen and an ozone layer, but, the report hits home, we should also be using models with different anaerobic microbial non-carbon ecosystems, atmospheres that might parallel the different stages in the evolution of Earth's atmospheres over 4 billion years, and conditions that could indicate the presence of a tectonically active planet.
The report pointed out that the exploration of the planet is concentrated on looking for places where liquid water exists -- which goes along with the idea of where life is found on the Earth. However, they emphasize that liquids such as ammonia, methane, and formamide could also be the building blocks for life.
Saturn's moon, Titan is a perfect candidate: the discovery of evidence of liquid water-ammonia on Titan provides the potential for life-bearing polar fluids outside what is normally regarded as the habitable zone.