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Astronomy 141
Life in the Universe
Prof. Scott Gaudi

Lecture 4: Earth's Global Environment and Its Impact on Biology


Key Ideas

Key Ideas
There are both local and global drivers of biological evolution and longevity

Global drivers come in many different forms
--Geophysical processes
--Climate variations
--Impacts and Extinctions

Geophysical processes that affect biology:
--The differentiated structure of the Earth
--Heat in the Earth drives convection
--Convection drives plate tectonics

Greenhouse effect and the CO2 cycle
Climate changes in Earth's history:
--Evidence of ice ages, their cause, and Earth's recoverability
--Evidence for global warming and the implications for longevity

Impacts have been a continuing threat to life throughout Earth's history

There is ample evidence for mass extinctions


Earth's Delicate Climate


Structure of the Earth

Differentiated
--Dense things sink

Inner Core
--Solid
--Ni and Fe
Outer Core
--Liquid
Mantle
--Si and O
Crust
--Granite, basalt


Internal Heat and Convection

The Earth is Hot!
Latent heat from accretion
--Differentiation
--Radioactivity
Drives convection


Plates and Plate Tectonics

Two kinds of crust
Plates float on the mantle, driven by the convective motions of the mantle
Material gets recycled into the Earth via subduction


Greenhouse Effect

Why is the Earth so warm?
Should be just below freezing.

Water, Carbon Dioxide, Methane all absorb and re-radiate infrared radiation
This radiation therefore gets trapped under a "blanket"


Carbon Dioxide Cycle

Plate Tectonics, Subduction, Volcanism, Precipitation, and the Oceans make up the carbon dioxide Cycle.

Without the oceans, all the Earth's carbon dioxide would be in the atmosphere (and it would be very hot!).


Global Warming

Adding CO2 faster than the cycle can adjust.


Impacts and Extinctions


Time History of Earth Bombardment

Time History of Earth Bombardment
Initially impacts declined rapidly
Impact rate has been constant for ~2-3 billion years

Near-Earth Objects

Asteroids and comets
Orbits bring them close to Earth
Probably ~1000-2000 larger than one kilometer
On unstable orbits but continually supplied by asteroid belt
Discovered by many groups (Spaceguard)


The Continuing Impact Threat

Close Call
--March 18, 2004 very close approach of asteroid 2004 FH
--about 30 meters in diameter
--passed only one-tenth of the distance to the Moon.
--detected only three days before closest approach

Direct Hit

--1908 in Tunguska
--seismic activity recorded 1000 km away
--much more powerful than Hiroshima
--about 40 m in diameter
--About once per century

Other Direct Hits

--Meteor crater in Arizona
--50,000 years ago
--about 50 m in diameter
--many other craters known


K-T Boundary

This layer of sediments divides the Cretaceous and Tertiary periods (K is the abbreviation for Cretaceous)
Contains an unusual concentration of iridium (100 times normal)
Common element in asteroids and chondrites
Enormous fraction of planet and animal species went extinct
Goodbye dinosaurs!

Asteroid Hypothesis

~10 km diameter asteroid
2 million times more powerful than most powerful atomic bomb
would've done very bad things


Evidence for Extinctions

There is evidence for many extinction events, but their origins are not clear.


See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.

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