An Approach to Searching for Life on Mars, Europa, and Enceladus

摘要

Near-term missions may be able to access samples of organic material from Mars, Europa, and Enceladus. The challenge for astrobiology will be to determine if this material is the remains of dead microorganisms or merely abiotic organic material. The remains of life that shares a common origin with life on Earth will be straightforward to detect using sophisticated methods such as DNA amplification. These methods are extremely sensitive but specific to Earth-like life. Detecting the remains of alien life—that does not have a genetic or biochemical commonality with Earth life—will be much more difficult. There is a general property of life that can be used to determine if organic material is of biological origin. This general property is the repeated use of a few specific organic molecules for the construction of biopolymers. For example, Earth-like life uses 20 amino acids to construct proteins, 5 nucleotide bases to construct DNA and RNA, and a few sugars to construct polysaccharides. This selectivity will result in a statistically anomalous distribution of organic molecules distinct from organic material of non-biological origin. Such a distinctive pattern, different from the pattern of Earth-like life, will be persuasive evidence for a second genesis of life.