So Space?

Exploration of hypotheses regarding the existence of life on other planets using knowledge from early Earth and modern extremophiles.
Associated readings:
- Rothschild and Mancinelli, 2001. Life in extreme environments. Nature 409: 1092-1101.
- Krisasen-Totton et al., 2018. Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life. Science Advances 4, eaao5747.
Recall prior lectures on detecting early life on Earth.

Life alters atmospheric compositions via waste products released, indicating the potential to detect similar gases on exoplanets.
Key points from Krisasen-Totton et al. (2018):
- Understanding life’s influence on atmospheric gas ratios helps infer life on other planets.
- Discussion questions on candidate gases for detection and problematic aspects of this approach.

Potential candidate gases include:
- Oxygen (O2)
- Methane (CH4)
Challenges to this approach include:
- Many gases are produced by specific metabolic processes, necessitating convergent evolution for life elsewhere.
- How can we analyze atmospheric gas data to infer life presence?

Methane (CH4) as part of the modern O2-CH4 redox couple:
  - Displays unique ratios in absence vs. presence of life.
  - Methane’s atmospheric kinetic lifetime is approximately 10 years.
  - In absence of O2, methane can persist for about 30,000 years without a sustained source.
  - Life is a known source of methane, making it an easier evolutionary trait compared to oxygen generation.

If methane has a mantle-derived source, carbon monoxide (CO) would also be produced:
- Microbes would consume CO if life is present.
- Abiotic sources of methane are likely to be minimal and unconvincing.
Researchers discuss the probability distribution of abiotic methane production from serpentinization on Earth-like planets.

Historical presence of chemical disequilibria since the Archean, increasing in magnitude with biomass and oxygen levels.
Conclusion:
- Detecting simultaneous high levels of CH4 and CO2, along with the absence of CO on habitable exoplanets, suggests biological activity.

Phosphine detected using atmospheric spectral fingerprints:
- Product of anaerobic bacteria on Earth.
- Detected in a region of Venus consistent with minimal hostility for life.
Considerations for using phosphine as a proxy for life:
- Exclusivity to living organisms required for it to be a valid biosignature.
- Alternative, non-biological processes may create phosphine, leading to misinterpretation.

Challenges in detecting exoplanet atmospheres:
- Data gathering is complex and methods often calibrated using ground-based telescopes.
- Multiple observation flights (Cordiner et al., 2022) failed to replicate phosphine results, suggesting if present, it might be in minuscule quantities.
Similar spectral signatures for different gases (e.g., phosphine vs. sulfur dioxide) complicate detection:
- Reanalysis showed phosphine signals may derive from less stable atmospheric layers of Venus.

Recent findings (2023) indicated dimethyl sulfide (DMS) alongside methane and carbon dioxide, hinting at potential oceanic presence under a hydrogen-rich atmosphere (K2-18 b).
Phytoplankton major contributors to DMS production in photic zones; heterotrophic bacteria significant in aphotic zones (Zheng et al., 2020).

Difficulties in identifying fossilized microbial life on Mars compared to Earth, as highlighted in Smithsonian Magazine.

Controversial claims about Mars being a living planet with prokaryotes, lichens, and fungi:
- Analysis of images using a rating system to assess biological probability (Joseph et al., 2019).
- Evidence considered circumstantial, needing extraordinary proof for extraordinary claims.

Analysis of images claiming life on Mars:
- Many formations identified as non-biological, and misinterpretations are common (3 cm photo; hematite-rich concretions).

Claims of diatoms found in Mars meteorite, allegations of species being all freshwater varieties:
- Lack of peer-review and questions regarding the meteorite’s authenticity raise concerns.

Supporting evidence for claimed microbial fossils:
- Isotopic differences, morphology, evolutionary context, environmental feasibility, and contamination considerations.

Crystal structures thought to signify life found to be similar to non-biogenic carbonaceous materials (Golden et al., 2004).

Wolfe-Simon et al. (2011) revealed a bacterium utilizing arsenic in place of phosphorus, showcasing unique biosynthetic capabilities.
Claims of arsenic’s integration into macromolecules raising profound evolutionary discussions.

Following the research on arsenic, substantial skepticism arose:
- Claims deemed unreplicated, confirming the requirement for phosphorus much more than arsenic, contrary to initial findings.

Insights into tectonic styles influencing life:
- Contrast between single lid tectonics and plate tectonics delineating bioactive supplies, oxygen levels, climate control, and habitat formation.
- Implications for the emergence of complex life forms and potential for communication among civilizations in determining extraterrestrial existence (Stern and Gerya, 2024).

Comparative analysis of the influences of tectonic activities on biological evolution and the effects on abiotic and biotic interactions across geological timescales.

Acknowledgment of the complexities in identifying life on other planets and continuing necessity of both rigorous scientific evaluation and skepticism in examining claims of extraterrestrial life.