The Search for Alien Life in Our Solar System: Strategies and Priorities

The search for extraterrestrial life, particularly life with a separate origin from Earth life, is a scientifically significant endeavor. Discovering such life would revolutionize our understanding of biology, as it would reveal whether the biochemical components of life are unique solutions or merely contingent upon Earth's specific geochemical conditions. The paper argues that finding life that migrated from Earth (panspermia) would be interesting but less impactful than discovering truly alien life. The discovery of a separate origin of life would also have profound implications for cosmology and philosophy, supporting or refuting the hypothesis of cosmic evolution—the idea that life's emergence is a common occurrence in the universe given the right conditions. The authors acknowledge the financial constraints on future space exploration and advocate for a strategic, prioritized approach to maximize the chances of success.

The authors review existing literature on the search for extraterrestrial life, discussing various perspectives on the origin of life, including the 'metabolism-first' hypothesis which posits that life emerged gradually from a flow of free energy through an appropriate chemical mixture. They highlight past controversies surrounding evidence of extraterrestrial life, such as the interpretation of the Viking mission results on Mars and the debate over Martian microfossils. The authors also touch upon the possibility of a second, independent origin of life even on Earth itself, suggesting that our current understanding of biology might be limited to a single branch of life. They reference several publications discussing these ideas, including work by Koonin, Davies, and others.

The paper proposes a four-pronged strategy for searching for extraterrestrial life, acknowledging budgetary limitations. First, the authors champion the 'Cosmic Evolution' hypothesis, which assumes that life's origin is a relatively common event. Second, they emphasize the importance of finding conclusive evidence of extant alien life, prioritizing the discovery of living organisms over ambiguous signs of past life. Third, they recommend a 'middle-of-the-road' approach, focusing on carbon-based life with biochemical systems different from Earth's, avoiding both too familiar (likely panspermia) and too exotic life forms. Fourth, they suggest a "follow-the-carbon" strategy, focusing on locations with anomalous concentrations of carbon compounds sufficient for analysis. This involves identifying unique patterns or 'carbon signatures' of organic monomers not expected from abiotic processes. The chirality of monomers, if present, would be a strong indicator. They contrast the sparseness of organic molecules in Earth life with the diversity found in meteorites. The authors highlight that analyzing monomers is more practical and informative than analyzing complex macromolecules in extraterrestrial samples. They discuss several classes of organic compounds relevant to Earth biochemistry (e.g., fatty acids, amino acids) and their characteristic patterns, which differ from those in meteorites. The authors advocate for robotic missions to conduct initial analyses of organic materials, reserving sample return missions for cases showing promising results.

The paper proposes a prioritized list of celestial bodies for searching for extraterrestrial life based on their potential for harboring life with a separate origin, balancing the probability of success with the cost of exploration. The top three priorities are: 1. Titan: Its rich organic chemistry, liquid methane lakes, and potential for diverse metabolic pathways make it a high-priority target. It also offers the possibility of finding exotic hydrocarbon-based life or alternative water-based life, due to the possibility of water-ammonia oceans beneath the surface. 2. Mars: Past and present evidence suggesting microbial life (Viking results, methane detection, potential liquid water) makes it a logical choice. However, careful analysis would be needed to distinguish indigenous life from panspermia. 3. Europa: The strong evidence for a subsurface ocean, potentially with hydrothermal vents as energy sources, makes it a promising candidate, although its organic carbon inventory remains unknown. The paper also mentions Enceladus and Venus as promising, albeit lower-priority locations due to the presence of water plumes in Enceladus and the possibility of life in Venus's acidic atmosphere. The authors emphasize that the most significant discovery would be evidence of a separate origin of life, which would be supported by findings of carbon-based life forms with biochemical systems substantially different from those on Earth.

The prioritized approach suggested in the paper addresses the limitations imposed by economic constraints on future space exploration while maximizing the likelihood of finding extraterrestrial life. The strategy of focusing on a smaller number of promising locations with intensive investigation is more efficient than a broad, less focused survey. The emphasis on finding conclusive evidence of extant life prioritizes the scientific value of the discovery. The focus on carbon-based life with different biochemistry increases the chances of finding life that is truly independent from Earth life, rather than evidence of panspermia. The 'follow-the-carbon' strategy provides a practical and effective approach to analyzing extraterrestrial samples.

The authors conclude that a focused, prioritized search for extraterrestrial life is crucial given economic limitations. They propose a strategic framework centered on identifying carbon-based life forms with distinct biochemical pathways to minimize the likelihood of detecting panspermia and maximize the chances of detecting a separate origin of life. Further research should concentrate on developing advanced in-situ analytical instruments and refining the identification of specific biosignatures. Future missions should prioritize Titan, Mars, and Europa based on the presented rationale.

The paper's main limitation is its inherent speculative nature. While based on existing scientific understanding, the conclusions regarding the probability of finding life at various locations are largely based on inferences and extrapolations from known data. The success of the proposed strategy relies on technological advancements in robotics and in-situ analytical capabilities. The authors acknowledge the difficulty in definitively ruling out panspermia in certain cases, requiring rigorous investigation to ensure that discovered life forms are truly of independent origin.