adenine—preferable to hydrogen cyanide—especially in the presence of trace amounts of hydrogen cyanide. Nucleoside phosphates, thermodynamically understandable with respect to hydrolysis in water, can be synthesized in formamide. Furthermore, formamide is a liquid over wide ranges of temperature and pressure; it can form a liquid phase on Mars and can be dehydrated at low pressure, high temperature, or combinations of the two. RNA duplexes need not be unstable in formamide-water mixtures. Small-molecule (metabolism-first) theories of origins avoid such paradoxes but can be expected to be viable with respect to alternative solvents as well as water. That offers an opportunity for a broad research program to explore formamide as a potential prebiotic solvent—a simple task for Earth-based laboratory research.

The discovery of evidence of liquid water-ammonia eutectics on Titan provides a context for the potential for polar fluids outside what is normally regarded as the “habitable zone.” The stay of the Cassini-Huygens mission on the surface of Titan was brief, but this moon of Saturn is the locale that is most likely to support exotic life. The committee believes that it is important to consider whether the planned missions to the solar system should be reordered to permit returning to Titan earlier than now scheduled.

Essentially nothing is known about the solubility of biomolecules in cryogenic solvents. Therefore, research into the possibility of life in cryogenic solvents needs to begin by making fundamental measurements and establishing a database describing the solubility of organic species in such solvents over a range of pressures and temperatures that are relevant to locales in the solar system.

Concepts of life in the gas phase are speculative, but questions related to organic chemistry in ices are experimentally approachable and are components of NASA missions. With sample return from comets imminent, NASA missions have the potential to generate fundamentally new insights into the kinds of organic compounds that might have been present on early Earth. Indeed, the committee concluded that the least expensive mission to the solar system that would have a high potential for fundamentally altering current understanding of how life originated would be the return to Earth of samples of cometary ice. Laboratory studies on Earth can provide sensible simulations of interstellar and solar system ice bodies that would be key to guiding these missions.

6.5
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