Lecture 33: Searching for life in our solar system and elsewhere – where and how

Searching for Life in Our Solar System and Beyond

Plate Tectonics and Habitability

  • Influence of Plate Tectonics on Earth:

    • Major environmental impact through recycling of materials and stabilization of climate.

  • Comparative Lack on Venus and Mars:

    • Venus:

    • Excessive volcanism with CO2 accumulation.

    • Absence of chemical weathering to remove CO2.

    • No plate tectonics means no subduction of carbonates, leading to a one-way carbon cycle.

    • If it was ever habitable, it couldn't maintain conditions for long due to lack of carbon cycling.

    • Mars:

    • Similar absence of plate tectonics.

    • Early volcanism ceased, leading to loss of volatiles (water and CO2).

    • These factors contribute to Mars' challenging conditions for long-term habitability.

Implications of Current Research

  • Mars vs. Europa:

    • Preference for Mars as a potential habitat due to past liquid water presence ("warm little ponds" model).

  • Commonality of Life Forms:

    • Simple, single-celled organisms likely more prevalent than complex multicellular forms.

  • Biosignatures:

    • Atmospheric oxygen (O2) and ozone (O3) are strong indicators but can yield false negatives.

    • Organic molecules do not indicate life but their molecular weight distribution and chirality could serve as useful biosignatures.

Searching for Evidence of Life

  • Liquid Water as a Criterion:

    • Focus on celestial bodies with or having had liquid water, primarily Mars and Europa.

  • Challenges in Understanding Life Origin:

    • Uncertainty impacts where to search; origins at hydrothermal vents suggest certain bodies might harbor life (e.g., Europa, Enceladus) if polymerization processes are involved.

  • Current Findings on Mars:

    • Curiosity Rover Findings:

    • Detection of methane bursts suggesting local sources.

    • Complex organic compounds in ancient sedimentary rocks indicate potential for past life.

  • Basic Life Requirements:

    • Water, organic matter, and accessible energy sources identified on Mars do not confirm life, but support the hypothesis.

Organic Compounds and Life Indicators

  • Amino Acids Discovery:

    • Over 500 exist, with 96 found in Murchison meteorite but only 22 utilized by Earth organisms.

    • Palette of specific organic compounds is critical for clues on biological origins in Martian sediments.

  • Molecular Weight and Complexity:

    • Distribution of organic compounds suggests biological vs. non-biological production. Non-biological reactions favor simple compounds; life favors complex ones.

  • Chirality as a Distinctive Feature:

    • Left- and right-handed organic molecules differ based on synthesis; biotic processes predominantly yield left-handed ones, marking a potential biosignature.

Future Missions and Technologies

  • ESA ExoMars and Its Objectives:

    • Planned for 2028, to analyze chirality of organic molecules; implications for determining biological origins.

  • Potential Investigations on Europa and Enceladus:

    • Ice layers where liquid water might exist could provide organic matter biomarkers.

Detecting Life on Exoplanets

  • Examining Spectral Properties:

    • Use of light passing through atmospheres of exoplanets to detect life-signature elements.

    • Challenges in confirming life existence solely based on Earth-style characteristics.

  • Example of Earth Observed from Space:

    • Indicators include chlorophyll absorption, presence of O2/O3, co-occurrence of methane and organic compounds

    • Chemical disequilibrium indicative of life: simultaneous O2 and methane presence is a signature of biological processes.

Notable Findings and Skepticism

  • Galileo Spacecraft Observations:

    • Detection of atmospheric gases suggestive of life on Earth (oxygen, methane, chlorophyll).

  • James Webb Space Telescope (JWST):

    • Launched with the goal of analyzing exoplanet atmospheres to look for signs of life.

  • Latest on Exoplanet K2-18b:

    • Detection of dimethyl sulfide (DMS) raises possibilities, but caution advised on interpreting such findings without considering abiotic processes.

Final Considerations

  • Future of Life Search:

    • The prevalence of building blocks of life suggests commonality of life, but verification is essential.

    • Advanced techniques important for assessing hundreds of potentially life-sustaining exoplanets.

  • Scientific Vigilance:

    • Importance of skepticism towards findings, ensuring rigorous examination for false positives and negatives in the search for extraterrestrial life.