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Astronomy 141
Life in the Universe
Prof. Scott Gaudi

Lecture 2: History of the Earth


Key Ideas

We have a reasonably accurate timeline of the history of:

The Universe:
The Solar System
The Earth
The Moon
Life

There were several important events for biology:

Formation of the Elements of Life
Formation of the solar system.
Formation of the Earth
Formation of the atmosphere
Formation of the oceans
Foramtion of the moon
Impact history
Conclusions from Timeline:
Life formed about as soon as it could.
Life may have arisen more than once!


Age of the Universe

How do we measure its age?
--Expansion rate of Universe.
--Ages of oldest stars
--Ages of white dwarfs
--Radioactive dating


Age of the Universe

All give a consistent answer

Age = 13.7 +/- 0.2 billion years


Looking back in time

We can use astronomical observations to look back in time and reconstruct the history of the Universe.

Examples:
--Cosmic Microwave Background
--Hubble Ultra Deep Field (shows us first Galaxies)


History of the Universe

3 minutes
--Simple elements form
300,000 years
--Universe becomes transparent
300 million years
--First stars (and more complex elements)
9 billion years
--Solar System forms
9-10 billion years
--Life on Earth
13.7 billion years
--Intelligent Life


Origin of the Elements

Immediately after the Big Bang

The Universe was very hot.
The Universe was filled with quarks (building blocks of protons and neutrons) and other elementary particles.
Universe cooled quickly as it expanded.
After about 10-6 seconds, protons and neutrons formed.
After three minutes, protons and neutrons became to form simple elements. -> "Big Band Nucleosynthesis"

The elements formed during Big Bang Nucleosynthesis are:

Hydrogen (1 proton) and its isotopes (Deuterium, Tritium)
Helium (2 protons) and its isotopes
Lithium (3 protons)
Beryllium (4 protons)
No heavier elements were formed.
No Carbon, Oxygen, and other elements of life.

All elements heavier than Beryllium were formed well after the big bang.
The elements of life were produced in Stellar Nucleosynthesis: these elements were created by sunlike stars at the ends of their life.
We are stardust!
As a consequence, life cannot arise before the second generation of stars, several million years after the big bang.


Overview of the Formation of the Solar System

--The solar systems was formed from a disk of gas and dust orbiting around the protosun.
--Dust grains in this disk settle to the midplane of the disk, where they combine to form planetesimals: the building blocks of planets.
--These planetesimals attract each other via gravity to form planets.
--The final configuration is a coplanar system of planets all orbiting the sun in the same direction.
--Planet construction is messy: the leftover building blocks of planets remain in the solar system and occasionally impact the planets: these are asteroids, meteoroids, etc.

Implications of this formation model:

Planets are composed of dust; this dust is composed of the elements produced in stars.
->As a result, many generations of stars are likely required before planets are formed in order to have sufficient material to make planets.
Planets are initially hot.
->The energy of the planetisimal's motion is converted to heat.
Leftover debris from formation impacts the planets over the age of the solar system.
->More debris at first.
->Gradual decrease in the amount of debis.


Age of the Solar System

Some meteorites are thought to be primordial
Most primitive are chondrites

Chondrites contain chondrules

Chondrules: nuggets of material that melted and cooled quickly
--Once were very hot (remnants of solar system formation?)
--Composition very similar to the Sun (not planets)

How old are Chondrites?


Radiometric Ages

Radiometric ages allow us to date a sample of material.


Age of the Solar System

Radiometric Age:

Age of Solar System = 4.55 +/- 0.01 billion years


Age of the Earth

Much harder - Earth is its own censor
Oldest rocks we know about are Zircons
--Zirconium Slicate - ZrSiO4
--Very hardy element (resists destruction, melting)
--Radiometric dating gives 3.9-4.4 billion years
--Have isotopic Oxygen ratios that imply existence of water


Age of the Moon

The Moon was thought to have formed from a large impact from a Mars-sized body, which removed a large part of the outer shell of the Earth. This material then recollected to form the Moon.
This Moon formation mechanism implies the Earth is older than the Moon. At least 100 Myr older:

Moon is composed of stuff much like the mantle of the Earth.
Earth must have differentiated prior to moon-forming impact.
Differentiation takes about 100 Myr.
The age of moon rocks from radiometric dating is about 4.4 Gyr, so the Earth must be at least 100 Myr older than this, or older than 4.5 Gyr.


History of the Earth

Bombardment occured for the first ~1 Gyr, the Hadean ("Hellish") Era


Signs of Life

How do we recognize ancient life?
Fossils
--Not very hardy: single-celled, microscopic organisms
--At least 3.0 and maybe 3.5 billion years old
Stromatolites
--Layered rocks
--Colonies of microbes
--3.5 billion years old, photosynthesis
Isotopic Evidence
--12C versus 13C
--3.85 billion years old!


Sterilizing Impacts

Late heavy bombardment lasted ~1 billion years
An impact of ~500 km would sterilize the planet
Last such impact, 3.8-4.2 billion years ago


Early Earth History

Time = 0: Earth formed (4.6 Gyrs ago)
Time = 0-100 Myr: Earth differentiated
Time = 100 Myr: moon-forming impact
Time = 200 Myr: oceans and atmosphere formed
Time = 300-800 Myr: last sterilizing impact
Time = 1000 Myr: end of heavy bombardment
Time = 700-1000 Myr, first evidence for life


One Important Conclusion, and One Very Interesting Possibility

Life arose about as soon as it could.

Life may have arisen more than once!


Timeline of the Universe

A lot of time between first stars and the Sun.

Not much time between formation of the Earth and life.

A lot of time between life and intelligent life.


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

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