organisms use the same carbon and energy sources. There will be a finite number of ways to use the carbon and energy sources, and science is not close to knowing this limit.
A search for organisms that derive some of their catalytic activity from minerals rather than protein enzymes, including organisms that combine mineral and protein catalysis.
A search for organisms from environments that are limited in key nutrients, including phosphorus and iron, and determination of whether they can substitute other elements, such as arsenic, for phosphorus. This effort would involve a search for adenosine tri-arsenate instead of adenosine triphosphate and for DNA with arsenic instead of phosphorus.
A search for life that can extract essential nutrients—such as phosphorus, iron, and other metals—from rocks, such as pyrites and apatite.
A search for anomalous gene sequences in conserved genes, particularly DNA- and RNA-modifying genes. The anomalous sequence in the Nanoarchium 16S rRNA gene may indicate that there are others in other extremophiles, and these may have some significance in the origin of genes and the RNA world.
Study of the resistance of microorganisms that form biofilms on minerals to the harsh conditions of interplanetary transport.
A search for life that stores its heredity in chemicals other than nucleic acids.
The laboratory and field studies that are recommended above are not expensive by any metric; progress can be made with consistent NASA support at the $20 million annual level. That is a small fraction of the cost of a single launch of a space shuttle or of the contribution over the years to the space station, and it is a nearly negligible fraction of the cost of a human mission to Mars. Laboratory and field studies are a necessary component of such a mission. The results obtained from such studies not only will provide an answer to the question, Why go to Mars? but also will be needed to prevent a human landing on Mars from vitiating a key discovery that might have the greatest of effects for science and society.
The committee’s specific recommendations to NASA for space studies are as follows:
Programs that combine the exploration of potential metabolic cycles with the synthetic biology of unnatural nucleic acid analogues and their building blocks and that use the results to guide the design of instruments. This may be one of the principal ways in which ground-based research in astrobiology can inform NASA missions of exploration.
Astrobiology measurements that can potentially distinguish between life on Mars (and possibly other bodies) that arrived via material ejected from Earth (or vice versa) and life that emerged on another body independently of life on Earth. The scientific and societal effects of discovering a second genesis of life, as opposed to discovering another branch on the same tree of life, cannot be overstated.
Inclusion in missions planned for Mars of instruments that detect lighter atoms, simple organic functional groups, and organic carbon to help distinguish between “replicator-first” and “metabolism-first” theories of the origin of life by identifying organic mixtures that differ sharply in composition from the nearby random collections identified in meteorites. Similar considerations should guide inclusion of small-organic-molecule detectors that could function on the surfaces of Europa, Enceladus, and Titan.
Consideration, in view of the discovery of evidence of liquid water-ammonia eutectics on Titan and active water geysers on Saturn’s moon Enceladus, of whether the planned missions to the solar system should be reordered to permit returning to Titan or Enceladus earlier than is now scheduled. 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 unfortunately brief; but this moon of Saturn is the locale that is arguably likely to support exotic life.
Finally, the committee calls attention to the importance of using remote sensing to detect and characterize extrasolar planets that could support alternative carbon-based life. In addition to looking for evidence of water in