Terrestrial Planets: Atmospheres, Water, and the Carbon Cycle

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Conducted at PollEv.com/uoftastro. Topics include colors, personality traits, concepts about blankets, and other fundamental questions related to terrestrial atmospheres.

Introduction to Terrestrial Atmospheres

Course: AST101: The Sun and its NeighboursLecture: Class 16: Terrestrial AtmospheresProfessor: C. Barth Netterfield

Characteristics of Terrestrial Planets

General Features:

• Terrestrial planets are generally classified as small and rocky, having relatively thin atmospheres or, in the case of some, no atmosphere at all.

• Most of these planets have a limited number of moons that orbit them, influencing their gravitational fields.

• They are primarily composed of heavy elements, including various forms of rock and metals, giving them unique geological characteristics.

Planets Discussed:

The planets explored in detail in this lecture include:

• Mercury: The closest planet to the Sun, known for its extreme temperature fluctuations and lack of atmosphere.

• Venus: Often referred to as Earth’s twin due to its similar size but with a thick, toxic atmosphere.

• Earth: The only planet known to support life, featuring a diverse range of environments and a protective atmosphere.

• Mars: Known for its reddish color due to iron oxide on its surface, it has sustained interest in the search for past life.

• Earth's Moon: Unique for its impact history, providing insight into the geological processes of terrestrial bodies.

Impact of Craters

Craters Present on Mercury and the Moon:

• Craters on both Mercury and the Moon vary significantly in size, with many overlapping due to multiple impact events over eons.

• The presence of these craters is strongly indicative of historic impacts, providing a timeline for geological activity in the solar system.

Volcanism's Role in Atmosphere Formation

Comparison of Planetary Volcanism:

• Earth's Moon: Exhibits abundant craters, with few signs of volcanic activity, indicating a lack of recent geological activity.

• Mars: Retains some ancient craters; evidence suggests it had volcanic activity in the past, which may have contributed to its atmosphere.

• Venus: Displays very few visible craters, which suggests a more active geological history with processes like volcanic resurfacing.

• Earth: Features few craters due to active erosion and plate tectonics, demonstrating dynamic surface changes.

Planetary Size and Atmospheric Chemistry

Small vs. Large Terrestrial Planets:

• Smaller terrestrial planets tend to cool quickly, leading to a cessation of tectonic and volcanic activity, resulting in the retention of ancient craters.

• Larger planets, on the other hand, retain heat due to their size, allowing for continuous volcanic activity, which can significantly shape their atmospheres.

Outgassing Role:

• Large terrestrial planets can maintain thicker atmospheres due to gravitational forces that retain atmospheric gases more effectively after volcanic outgassing.

Changing Surface Dynamics

Forces Affecting Planetary Surfaces:

• High-speed impacts: Create craters and can lead to localized damage and surface changes.

• Volcanism: Leads to the formation of new landforms through lava flows and emissions of gases into the atmosphere.

• Erosion: Caused by wind, ice, and water reshapes the planetary ground, leading to substantial changes over geological timescales.

• Internal stresses: Can generate mountain ranges and canyons through tectonic activity.

Atmospheres of Terrestrial Planets

Summary of Atmospheric Composition:

• Mercury: Has an almost nonexistent atmosphere measured at approximately 0.00000000000001 bar, making it incapable of supporting life as we know it.

• Earth's Moon: Similar to Mercury, it exhibits an almost imperceptible atmosphere, unable to support life.

• Mars: Possesses a thin atmosphere measured at around 0.007 bar, with a composition heavily weighted towards carbon dioxide (95%) and minor amounts of nitrogen (2.7%).

Atmospheric Chemistry Overview

Main Components of Atmospheres:

• Oxygen (O): 16 (atomic weight)

• Carbon (C): 12 (atomic weight)

• Nitrogen (N): 14 (atomic weight)

• Hydrogen (H): 1 (atomic weight)

Chemical Structures:

• Fundamental compounds like CO2 (carbon dioxide), H2O (water), and N2 (nitrogen) were introduced along with their significance in contributing to atmospheric dynamics.

Instant Quiz on Methane vs. CO2

• This segment compares the characteristics of methane (CH4) against carbon dioxide (CO2), focusing on factors like molecular composition and average molecular speeds which affect their behavior in the atmosphere.

Atmosphere Example Profiles

Observations:

• There is a discernible trend where larger terrestrial planets tend to have thicker atmospheres.

• Notable Observation: While hydrogen is one of the most abundant elements in the universe, it is conspicuously absent in the atmospheres of larger planets, indicating specific formation and retention processes.

Escape Velocity and Molecular Behavior

Concepts:

• Escape Velocity: This determines whether a gas can leave a planet's atmosphere. Larger planets typically have a higher escape velocity, allowing them to retain gaseous elements more efficiently.

• Temperature Effects: Increased temperatures result in faster gas movement, thus enabling lighter gases to escape more easily.

Mars and Atmospheric Loss

Mechanisms of Loss:

• Mars undergoes significant atmospheric loss mechanisms where ultraviolet light and solar winds act to strip its atmosphere away.

• Lighter gases like hydrogen, due to their low molecular weight, escape more readily, contributing to a critical loss of potential liquid water on the planet.

Earth’s Magnetic Field

Significance:

• Earth’s magnetic field is crucial in protecting its atmosphere from solar winds. This shield helps maintain the atmosphere and supports the presence of liquid water on the surface.

• Mars' core cooling leads to a loss of its magnetic field, contributing to its atmospheric degradation over time.

Venus Atmospheric Comparison

Key Atmospheric Features:

• Venus has a dense atmosphere with pressure around 92 bar, comprised predominantly of carbon dioxide (95%), resulting in extreme surface temperatures averaging around 464 °C, mainly due to a runaway greenhouse effect.

Earth's Unique Atmospheric Composition

Overview:

• Earth's atmosphere is measured at 1 bar pressure and composed of:

  • CO2: 0.04%

  • N2: 78%

  • O2: 21%Critical factors such as oceans play an essential role in regulating atmospheric CO2 levels, providing a balanced system.

Earth's Carbon Cycle Overview

Mechanism:

• The carbon cycle includes processes whereby CO2 dissolves in rainwater, flowing into the oceans and forming carbonate rocks through tectonic actions.

• Volcanism also plays a role in the release of CO2 back into the atmosphere, completing the cycle.

CO2 Levels Over Time

Influences on Levels:

• CO2 levels have been influenced significantly by biological activity and volcanic actions, showing a declining trend over the past 50 million years.

Climate Change Implications

Rising CO2:

• The increase in atmospheric CO2 levels, driven by fossil fuel burning, poses significant impacts on global climate and weather patterns. Historical data illustrates links between CO2 levels and temperature variances, leading into cycles of ice ages and warmer interglacial periods.

Future Predictions

Projections:

• Models predict various transitions for future global temperatures based on carbon emissions, underscoring the urgent nature of addressing climate change.

Evidence of Water on Mars

Findings:

• Geological surveys and photographic evidence reveal dried riverbeds and delta formations, suggesting the presence of water in Mars's historical context, which raises questions about past life.

Photodissociation Effects

Definition:

• Photodissociation refers to the process whereby UV light from the sun breaks apart atmospheric molecules, leading to the escape of lighter atoms into space over time.

Summary on Earth, Mars, and Venus

• Earth successfully retains its atmosphere thanks to its magnetic field and active geological features, whereas Mars loses its atmosphere to solar wind threats, while Venus endures extreme conditions due to its thick CO2-rich atmosphere, making it inhospitable to life as we know it.

Plants and Photosynthesis

Photosynthesis Process:

• The process of photosynthesis, wherein plants convert H2O and CO2 into O2 and carbohydrates, is critical for the production of oxygen in Earth’s atmosphere, supporting life forms that rely on oxygen.