Habitable Earth

Give some key conditions for habitability:

• Liquid water 

• Stable atmosphere 

• Sunlight + geothermal energy 

• Magnetic shielding from solar wind 

Describe the Faint Young Sun Paradox:

The Sun was 30% fainter 4bn years ago, but there is geological evidence for liquid water at the same time.

What is the explanation for the Faint Young Sun Paradox?

(Theory!) Elevated CO2 and CH4 levels warmed early Earth.

Sources included volcanic outgassing and methanogens. Also low O2 levels reduced CH4 breakdown.

Give a couple of negative feedback loops that helped to regulate CO2 levels:

• Carbon-silicate cycles increasing carbon storage via carbonates

• Methane haze shielding reducing UV radiation

Give a few alternative hypotheses that explain the Faint Young Sun Paradox:

• Lower albedo 

• Higher geothermal heat flux 

• Tidal heating contributions? (Moon formation) 

Describe the Snowball Earth:

• 700m years ago Earth’s climate rapidly cooled

• Ice covered the whole planet - possibly equatorial surface liquid water was around too

• An evolutionary bottleneck with opportunities

• Possible driver for multicellularity??

Give some evidence for the Snowball Earth:

• Glacial dropstones found in deep marine sediments

• Magnetic minerals aligned such that they must have been at equatorial latitudes when formed

Describe the silicate weathering feedback loop:

Weathered silicate rocks weathered by carbonic acid helps to form carbonates (e.g. shells) that draws CO2 back down into long carbon stores (tectonics).

How has the evolution of land plants increased global weathering rates?

Deep roots break apart rocks and increase soil formation, which can store carbon.