Early Atmosphere

Initial Conditions and Atmosphere Overview

• Overview of early Earth's atmosphere to provide a context for later discussions.

• Focus on broad ideas rather than minute details.

• Acknowledgment that our understanding of the Archean atmosphere is limited.

• Reference to a comprehensive review paper as the primary source.

• Timeline:

  • Heavy bombardment (possibly, but not a major factor).

  • Life emerges.

  • Earth resembles its current form.

  • The Great Oxidation Event (GOE).

• Early Earth features liquid alterations, continents, and life.

Constraints on the Archean Atmosphere

• Difficulty in constraining the Archean atmosphere.

• Three primary pieces of evidence:

  • Fluid inclusions and 36Ar:

    • Used to constrain nitrogen levels.

    • Indicate a partial pressure of nitrogen less than one bar (similar to or slightly less than present values).

  • Physical volumes in basalt flow:

    • Suggest a total atmospheric pressure of 0.23 bar.

    • High error margins.

    • Implies a very low-pressure atmosphere.

  • Fossil record imprint:

    • Disputed evidence.

    • Suggests partial pressure of nitrogen between 0.52 and 1.2 bar.

• Overall conclusion:

  • Sparse evidence with significant errors and limited constraints.

  • Nitrogen levels were likely the same or slightly less than today (log-wise, around 0.1).

Carbon Dioxide (CO2)

• Importance as a greenhouse gas and regulator of ocean pH.

• Estimation methods:

  • Weathering analysis:

    • Estimates CO2 levels needed for observed weathering.

    • Suggests 7. 1 to 69 present atmospheric levels.

  • Weathering by aqueous solutions:

    • Considers temperature dependence.

    • Assumes the greenhouse effect is primarily due to CO2.

    • Indicates potentially much higher CO2 levels.

• Consideration of the faint young Sun paradox:

  • Less solar heating in the past.

  • Requires a stronger greenhouse effect.

• Refined CO2 estimates using climate models:

  • Incorporating methane assumptions.

  • Results in a pCO2 range of 0.006 to 0.6 bar.

  • Based on a Covenant circuit cycle model.

Methane (CH4)

• Modern methane flux is limited by the oxidizing environment.

• Archean methane:

  • Potentially much higher levels.

  • Linked to sulfur production.

Oxygen (O2)

• Sulfur production requires low O2 levels (2 to 20 ppmv, parts per million by volume).

• Iron washing out of paleosols also indicates a reducing environment.

• Limited oxygen oases may have existed.

• Estimating oxygen levels:

  • Volcanic SO2 breakdown by UV radiation.

  • UV radiation in the troposphere implies a small ozone layer.

  • Low ozone necessitates low oxygen levels.

  • Presence of SA (polymerized sulfur) indicates reducing conditions.

  • Resulting O2 estimation: 10^{-6} present atmospheric levels.

• Debate on the cause of the reducing atmosphere:

  • Lack of oxygen versus other reducing agents.

Hydrogen (H2)

• Hydrogen levels are challenging to estimate due to methane reduction.

• Microbial evidence suggests significant hydrogen concentrations.

• Hydrogen as an alternative explanation to oxygen for the reducing atmosphere.

Atmospheric Trends

• Nitrogen: Flat or slightly increasing around the GOE (reasoning unclear).

• CO2: Decreasing significantly.

• Methane: Decreasing drastically, with significant error margins.

Hydrogen Loss Mechanism

• Magnetic field influences atmospheric escape.

• Ions accelerated by the magnetic field escape if they lose their charge.

• Alternatively, particles at the poles can escape along magnetic field lines.

Hydrogen-Rich Atmosphere Model

• Exoplanet observations indicate frequent hydrogen-rich atmospheres in early stages.

• Thermodynamic model to simulate interaction with magma ocean.

• Model parameters:

  • Based on echondrites and orbrights for early composition.

  • Initial hydrogen atmosphere composition: 0.2%.

• Hydrogen retention depends on planetary mass (explains loss on Mercury and Mars).

• Mixing rates are critical for model equilibration.

Model Results

• Explains:

  • Earth's water origin.

  • Core density deficiencies.

  • Mantle oxidation.

• Hydrogen interaction with iron leads to transfer to the metal phase.

• Correlation between hydrogen incorporation and H2O delivery to the atmosphere.

• Hydrogen atmosphere of 1.2% can produce Earth's ocean volume.

• Reduces the need for extraterrestrial water sources.

• Core density:

  • The iron-nickel core has an expected density.

  • Observed density is 10% less.

  • Hydrogen inclusion in metal phases reduces core density.

  • 0.2% hydrogen atmosphere leads to 0.5% hydrogen in the core, resulting in a 8.7% density difference.

• Mantle oxidation:

  • Earth's oxidation state changes over time.

  • Measured by FEO content in bulk silicates.

  • Silica in the core can oxidize the mantle, but requires significant amounts.

  • Hydrogen inclusion required to reconcile model with observations.

• Model assumptions and limitations.

• Hydrogen in the early atmosphere plays a key role in Earth's development.

• Hydrogen loss influences oxidation states.

Evidence of Atmospheric Loss

• Xenon depletion compared to lighter noble gases (e.g., Krypton).

• Non-thermal escape mechanisms are necessary to explain Xenon loss.

• Ten times the modern solar output post-GOE is required for necessary mixing.

• Mixing rates necessitate at least 1% radiation.

Hydrogen Escape and the Great Oxidation Event (GOE)

• Hydrogen loss as a major contributor to Earth's oxidation.

• Cyanobacteria evolved to produce oxygen, but the GOE occurred later.

• Reducing agents on Earth's surface initially consumed oxygen produced by cyanobacteria.

• Oxidation requires an oxidized surface for oxygen stability.

• Hydrogen loss oxidizes the atmosphere by removing reducing agents.

• Mars as an example of oxidation via hydrogen loss but does not have oxygen due to other reasons.

Evidence for Hydrogen Escape

• Mars has an oxidized surface due to hydrogen escape.

• Disappearance of BIFs (banded iron formations) around the GOE.

• Requires a reducing environment to form.

• Red beds indicate an oxidized environment but don't necessitate oxygen.

• Multicellular life appeared because oxygen is required to build up and become eukaryotes since aerobic respiration is a much more efficient pathway, and it allows you to build up more complex life.

• Multicellular life requires oxygen for aerobic respiration.

Timeline of the GOE

• Occurred around 2.4 to 2.2 billion years ago.

• Steady loss of xenon and hydrogen reached a critical point.

• Oxygen buildup by cyanobacteria.

Evidence for GOE

• Mass fractionation of sulfur isotopes shows oxygen and oxygen.

• Cyanobacteria change as evidence of why we oxidized driven GOE, since they can survive in oxygen free environments.

• Oxygen destroys methane, potentially driving global glaciations.

• Hydrogen loss is an important factor in the GOE.

• Evolution of oxygen-producing bacteria occurred before the GOE.

Implications and Future Research

• Understanding atmospheric evolution aids in the search for life on other planets.

• Studying exoplanets of different ages provides atmospheric data.

• Need to study agnostic sphere, an area in the atmosphere where we believe all things of concern are happening to find out what is happening there.

Open Discussion

• If hydrogen is included and interacts with water, why dont we see water in life on other planets.

• Is Earth special, or have we not found other life yet?