Yesterday's announcement by NASA 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.
"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. The stay of the Cassini-Huygens mission on the surface of Titan was unfortunately brief; but Titan is the locale that is likely to support exotic life, which could be discovered using robotic remote sensing devices.