False negatives for remote life detection on ocean-bearing planets: Lessons from the early Earth (1702.01137v1)

Abstract: Ocean-atmosphere chemistry on Earth has undergone dramatic evolutionary changes through its long history, with potentially significant ramifications for the emergence and long-term stability of atmospheric biosignatures. Though a great deal of work has centered on refining our understanding of false positives for remote life detection, much less attention has been paid to the possibility of false negatives, that is, cryptical biospheres that are widespread and active on a planet's surface but are ultimately undetectable or difficult to detect in the composition of a planet's atmosphere. Here, we summarize recent developments from geochemical proxy records and Earth system models that provide insight into the long-term evolution of the most readily detectable potential biosignature gases on Earth - oxygen (O2), ozone (O3), and methane (CH4). We suggest that the canonical O2-CH4 disequilibrium biosignature would perhaps have been challenging to detect remotely during Earth's ~4.5 billion year history and that in general atmospheric O2/O3 levels have been a poor proxy for the presence of Earth's biosphere for all but the last ~500 million years. We further suggest that detecting atmospheric CH4 would have been problematic for most of the last ~2.5 billion years of Earth's history. More broadly, we stress that internal oceanic recycling of biosignature gases will often render surface biospheres on ocean-bearing silicate worlds cryptic, with the implication that the planets most conducive to the development and maintenance of a pervasive biosphere will often be challenging to characterize via conventional atmospheric biosignatures.

• The paper highlights the significant risk of false negatives in remote life detection on ocean-bearing planets, informed by lessons from early Earth's atmospheric history.
• Ocean-atmosphere interactions on potentially habitable planets can recycle biosignature gases internally, preventing them from accumulating detectably in the atmosphere and leading to cryptic biospheres.
• The findings suggest a need to re-evaluate conventional biosignatures, explore non-traditional detection methods, and incorporate Earth's complex biogeochemistry into astrobiological models for a more accurate search for life.

False Negatives in Remote Life Detection on Ocean-Bearing Planets: Insights from Early Earth

The paper conducted by Reinhard et al. presents a detailed analysis of atmospheric biosignatures with a focus on the possibility of false negatives in detecting life on exoplanets with ocean-bearing surfaces. The research highlights the evolution of Earth's atmosphere and offers a cautionary perspective on conventional biosignatures used for remote life detection. While much progress has been made in identifying false positives, this paper emphasizes the importance of acknowledging false negatives, where a planet's active biosphere may remain undetected.

Overview of Atmospheric Biosignatures and Historical Context

The canonical O₂-CH₄ disequilibrium, often used as a potential biosignature, might have been challenging to detect for a substantial portion of Earth's history. The paper revisits geochemical proxy records and Earth system models, leading to the conclusion that atmospheric oxygen levels have only served as a robust proxy for the presence of Earth's biosphere in the last ~500 million years. The authors suggest that methane detection would have been equally problematic for the preceding ~2.5 billion years due to internal biosignature gas cycling within ocean-bearing worlds, potentially rendering surface biospheres cryptic.

Implications of Ocean-Atmosphere Coupling and Internal Biosignature Cycling

The paper presents the concept of oceanic recycling of biosignature gases, resulting in cryptic biospheres that are difficult to detect remotely. This recycling process implies that planets conducive to the development and sustenance of a biosphere may also face challenges in their characterization via atmospheric signatures. This is particularly concerning for ocean-bearing silicate planets, where internal recycled biosignature gases might prevent detectable signatures from reaching the atmosphere.

Practical and Theoretical Implications

The findings from this paper have both practical and theoretical implications. Practically, they necessitate a reevaluation of our reliance on conventional atmospheric biosignature gases, such as O₂ and CH₄, for life detection. The paper underscores the importance of developing non-traditional methods to detect biospheres that might not leave detectable atmospheric traces. These include exploring alternative biosignatures that take into account the presence of exoplanetary oceans and their biogeochemical impacts on potential life signatures.

Theoretically, the paper calls for a comprehensive inclusion of Earth’s dynamic biogeochemistry history into astrobiological models. A deep understanding of Earth's evolutionary phases is crucial to inform the search for biospheres on extrasolar planets. Additionally, the interplay between ocean chemistry and atmospheric biosignatures as documented in Earth’s history can serve as a valuable analog for predicting biosignature stability and detectability on other habitable planets.

Speculations on Future AI Developments

Advanced AI models could facilitate more sophisticated simulations that incorporate complex biogeochemical interactions akin to those highlighted in this paper. Future developments in AI may enhance our ability to simulate hypothetical scenarios involving the evolution of atmospheric biosignatures across various planetary conditions. This could significantly refine our approach to identifying life on exoplanets by improving the interpretation of spectral data from future telescopes such as the James Webb Space Telescope (JWST) and other exoplanetary observation platforms.

Conclusion

Reinhard et al.'s paper provides an informed perspective on the complexities of atmospheric biosignature detection, emphasizing the necessity of acknowledging potential false negatives. This introspection enriches the ongoing discourse on astrobiology and the search for life beyond Earth, inviting further research into novel atmospheric markers and the dynamic interactions between a planet's atmosphere and its potential biosphere.