Lecture 15 - Terrestrial Planets (Atmosphere, Water, Carbon Cycle)

Atmosphere

The layer of gases surrounding a planet, retained by gravity.

Which terrestrial planet has almost no atmosphere?

Mercury

What is the main gas in Venus’ atmosphere?

Carbon dioxide (≈96% CO₂)

Mars’ atmosphere is mainly:

Carbon dioxide

Bar (unit)

Measures atmospheric pressure; Earth’s average is 1 bar.

Which four elements make up most planetary atmospheres?

Hydrogen, Oxygen, Carbon, Nitrogen.

Which gas escapes a planet’s atmosphere most easily?

H₂

Escape Velocity

The minimum speed needed for a molecule to escape a planet’s gravity.

Why don’t terrestrial planets have hydrogen atmospheres?

Hydrogen molecules escape too easily

What three factors affect whether a planet keeps an atmosphere?

Planet mass, temperature, and molecular mass of gases.

Greenhouse Effect

Process where gases trap infrared radiation, warming a planet’s surface.

Without greenhouse gases, Earth’s average surface temperature would be about:

-16°C

Name Earth’s three main greenhouse gases.

Water vapor, carbon dioxide, methane.

Venus’s extreme surface temperature (~460°C) is due to:

Runaway greenhouse effect

Why does Earth have little CO₂ compared to Venus?

CO₂ dissolves in oceans and forms carbonate rocks.

Carbon Cycle (short version)

CO₂ dissolves in water → forms carbonate rock → recycled by volcanism.

What happens to CO₂ dissolved in rainwater?

It forms carbonate rocks

Radiative Equilibrium

When energy absorbed from sunlight equals energy emitted as infrared radiation.

What returns CO₂ to Earth’s atmosphere?

Volcanic activity and human emissions.

Photosynthesis

Plants use water and CO₂ to make carbohydrates and release O₂.

What causes Venus to lack oceans today?

The Sun’s heat evaporated them and the water was lost to space.

What protects Earth’s atmosphere from being stripped by solar wind?

Earth’s magnetic field.

Why did Mars lose its atmosphere?

No magnetic field after its core cooled

Photodissociation

UV light breaks apart molecules like H₂O, allowing lighter atoms to escape.

Why can hydrogen escape more easily than oxygen?

Hydrogen atoms are much lighter.

Which gas has risen rapidly in the last century due to fossil fuel burning?

CO₂

How does CO₂ concentration relate to global temperature?

Higher CO₂ = higher average temperature.

Runaway Greenhouse Effect

When heat causes more greenhouse gas release, further increasing temperature.

If Venus’s CO₂ dropped by 100×, what would happen?

It would cool drastically.

What’s the dominant greenhouse gas on Earth today?

Water vapor.