Astronomy 294
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 moon, impacts

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 seconds

--Elements form

300,000 years

--Universe becomes transparent

300 million years

--First stars

9 billion years

--Solar System forms

9-10 billion years

--Life on Earth

13.7 billion years

--Intelligent Life

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.

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

Moon rocks give a lower limit

--Radiometric dating gives 4.4 million years

Moon formation mechanism implies the Earth is older

Formation of the Moon

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

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!

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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