Astrobiology Fundamentals: Dating, Habitability, and Life’s Origins

Posted on Jun 3, 2025 in Biology

Radiometric Dating: Concepts and Equations

• Not all nuclides are stable; some split apart through radioactive decay. A nuclide is a species of an atom with a specific combination of protons and neutrons.
• Radiometric Dating of Meteorites: The Solar System is estimated to be 4.53-4.58 billion years old. Carbonaceous chondrites provide a sample of early Solar System ‘dust’.
• Decay Constant and Parent Atoms: The rate of decay is given by the equation: (dN/dt) = -λ * N.
  • λ (lambda) = decay constant (fraction of atoms decaying per unit time)
  • N = number of parent atoms
  • t = time
  Each radioactive nuclide has a typical timescale over which it will decay. Given a large population (N), a fixed proportion of atoms will decay during each time interval (t).
• Radioactive Decay Law: From (dN/dt) = -λ * N, we derive N = N₀ * e^(-λ * t).
  • The number of parent radioactive atoms decreases exponentially with time.
  • The number of daughter (stable) atoms increases.
  • The half-life (t₁/₂) is the time it takes for half of the current number of parent atoms to decay.
• Fundamental Equations of Geochronology:
  • N = N₀ * e^(-λ * t)
  • D = D₀ + N(e^( λ * t) – 1)
  We can measure N (parent atoms), D (daughter atoms), and λ. If we know either N₀ (initial parent atoms) or D₀ (initial daughter atoms), then we can solve for t (the age). For some systems, we can assume D₀ = 0, which simplifies to: t = ln((D/N) + 1) * λ⁻¹ where (D/N) = daughter atoms / parent atoms, and λ = decay constant.

Zircons: Ideal for Radiometric Dating

• Zircons are resistant to erosion, burial, and high-temperature metamorphism. They incorporate large amounts of uranium (U) but no lead (Pb) during crystallization.
• Jack Hills zircons are the oldest materials found on Earth so far.
• Uranium-Lead Dating of Zircons: Unstable atoms, such as uranium, eventually change into stable atoms, such as lead. The original version is called a parent atom (or isotope), and the new version is called a daughter atom. Parent atoms transform into daughter atoms.

Earth’s Timeline: Major Geological & Biological Events

• Earth’s Formation: ~4.5 billion years ago
• Core Formation: ~4.5-4.4 billion years ago
• Moon-Forming Impact: ~4.4 billion years ago
• Hadean Earth: 4.5-4.0 billion years ago (Moon was ~15x closer to Earth than today!)
• Earliest Evidence of Life: 3.8-3.5 billion years ago
• Oldest Fossils – Stromatolites: ~3.4 billion years ago (layered structures formed by trapping/binding of sediment by microorganisms in shallow ocean habitats)
• Archean Earth: 4.0-2.5 billion years ago
• Rise of Oxygen: ~2.4 billion years ago
• Cells with Nucleus (Eukaryotes): ~1.8 billion years ago
• Animals: ~540 million years ago
• Proterozoic Eon: 2.5-0.538 billion years ago (Earth underwent several glaciations)
• Dinosaurs: Appeared ~245 million years ago; went extinct after asteroid impact ~66 million years ago
• Oldest Evidence of Humans: ~300,000 years ago

Understanding the Circumstellar Habitable Zone (CHZ)

• Habitability is a measure of a planetary body’s potential to develop and sustain life.
• The Circumstellar Habitable Zone (CHZ) is the region around a star in which liquid H₂O could exist on the surface of an Earth-like planet.
• The CHZ distance varies depending on star mass and temperature, with more massive and hotter stars having a wider and more distant habitable zone.
• Why Liquid Water is Essential:
  • Water is an excellent solvent for organic chemistry.
  • Water allows for strong climate regulation through processes like the carbonate-silicate cycle (e.g., CaSiO₃ + CO₂ + 2H₂O → CaCO₃ + H₄SiO₄).
  • Liquid water is stable over a wide range of temperatures at 1 atmosphere of pressure.

Limitations of the Habitable Zone Concept

• Habitable for what type of life?
• Are there solvents other than water?
• Is liquid water stable underground?
• Are there other sources to heat up a planetary surface?
• Does the chemistry of the initial stellar nebula affect planetary properties?
• Stars and planets evolve with time!
• Extremophiles can survive under a wide range of temperatures, pressures, pH, salinity, and energy availability, expanding the definition of habitability.

Planetary Habitability Evolution Over Time

• The Sun’s luminosity increases through time, causing the habitable zone to shift outwards.
• Internal processes during planetary evolution also control habitability.
• Planets can evolve to become uninhabitable over time; for example, Venus would have been in the habitable zone when the Sun was less luminous and may have had oceans.
• Some exoplanets move in and out of the habitable zone due to eccentric orbits.
• Tidally Locked Planets: These planets have no rotation relative to their host star. If Earth were tidally locked with the Sun, precipitation patterns would create localized habitable environments.

Five Defining Characteristics of Life

• 1. Homeostasis: Regulation of internal conditions.
• 2. Growth and Reproduction: Increase in size and creation of new organisms.
• 3. Dynamic Information Transfer: Genetic information encoded in DNA/RNA.
• 4. Chemical/Structural Organization: Complex, ordered internal structures.
• 5. Metabolism: Harnessing energy from the environment to sustain processes.

Four Essential Ingredients for Life

• 1. Carbohydrates (Sugars): Primary source of energy.
• 2. Lipids (Fats): Form cellular membranes.
• 3. Amino Acids: Building blocks of proteins.
• 4. Nucleotides: Building blocks of nucleic acids (DNA/RNA).

The Urey-Miller Experiment: Significance

• The Urey-Miller experiment demonstrated that organic molecules could form spontaneously under conditions thought to resemble early Earth.
• The experiment used a simple gas mixture and spark discharge to yield a range of important organic molecules.
• The evolution of Earth’s atmosphere is critical here; this process would not work under modern atmospheric conditions. Its significance lies in understanding the origins of ancient life.

Carbonaceous Meteorites: Organic Delivery

• Carbonaceous meteorites deliver significant amounts of organic molecules to planets, including lipids, amino acids, and sugars, along with smaller quantities of nucleotides.

Water: An Excellent Solvent for Life

• 1. Essential for Organic Chemistry: Water provides the medium for biochemical reactions.
• 2. Polarity and Interactions: Water is a polar substance, capable of dissolving many compounds due to its ability to form appropriate non-covalent interactions.
• 3. Liquid Over Wide Temperature Range: Water remains liquid over a wide range of moderate temperatures, crucial for biological processes.
• 4. High Heat Capacity: Water possesses a high heat capacity, helping to regulate temperature.

Biomolecule Organization and Function

• Polymerization: The process of linking monomers to form larger biomolecules. This includes:
  • Nucleic Acid Synthesis: Linking nucleotides to form DNA/RNA.
  • Protein Synthesis: Linking amino acids to form proteins.
  Mineral surfaces can catalyze polymerization, and wet-dry cycles can also stimulate the condensation of amino acids.
• Membrane Formation: Lipids interact with water to form structures like bilayers and micelles, which are fundamental to cell membranes. Lipids are amphiphilic, possessing a hydrophobic (water-fearing) carbon tail and a hydrophilic (water-loving) head group.
• Information Transfer: Genetic information flows from DNA to RNA to protein, enabling the synthesis of functional molecules within cells.

Cellular Foundations: LUCA, Ribozymes, RNA World

• Ribozymes: Relatively small RNA molecules capable of catalyzing chemical reactions, including self-replication.
• The RNA World Hypothesis: This hypothesis proposes that early life used RNA for both genetic information storage and enzymatic catalysis. The proposed stages include:
  • 1. RNA forms inorganically.
  • 2. Some RNA molecules (ribozymes) can self-replicate and, in some cases, synthesize other proteins.
  • 3. RNA can encode information for both DNA and proteins.
  • 4. Proteins eventually take over most cellular catalysis.
  • 5. DNA eventually takes over information transfer, becoming the primary genetic material.
• LUCA (Last Universal Common Ancestor): The most recent common ancestor of all current life on Earth.

Metabolism: Catabolism and Anabolism

• Metabolism encompasses all chemical reactions that sustain life, largely driven by electron transfer between compounds.
• Catabolism: Breaks down molecules, releasing energy (similar to discharging a battery).
• Anabolism: Builds new molecules, requiring energy.

Electron Transfer (Redox) Reactions

• Redox reactions always involve two compounds: an electron donor (reductant) and an electron acceptor (oxidant).
• These reactions produce an oxidized product (which has lost electrons) and a reduced product (which has gained electrons).
• Oxidation: The removal of electrons, requiring an oxidant.
• Reduction: The addition of electrons, requiring a reductant.