A Short Look at Earth History
Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green
Formation of Sun
- Formation of Universe: 13 billion years
- Formation of Galaxy: 11 billion Years
- Formation of Solar System: 4.6 billion years
- Sun is probably a third generation star
- Probably takes 10-100 million years for planets to form
Formation of Planets
- Planets made of same material as Sun, minus elements that remain mostly in
- Inner Rocky Planets: iron and magnesium silicates
- Outer gas giants and moons: water ice
- If a protoplanet gets big enough, it can hold everything (Jupiter, Saturn)
- Very far from sun: methane, ammonia, nitrogen ice
Our Solar System is Not Typical
- Over 3000 extrasolar planets known
- Barely can detect Jupiter-size planets, don't yet have technology to see
- Many have very eccentric orbits
- Some have gas giants very close in to sun ("hot Jupiters")
Formation of Earth
- Planets formed by accretion of smaller objects = impact
- Very tiny objects hold together by atomic forces
- Objects kilometers across hold together by gravity
- How do objects the size of a refrigerator hold together?
- As planets get bigger, gravity gets stronger, impacts get more violent
- Big impacts throw out ejecta, trap heat
- Magma ocean
- Formation of core early in earth history as iron sinks
Formation of Moon
It's very hard to account for the Moon:
- Very big compared to its parent planet
- Orbits nearly in plane of earth's orbit, not over equator.
Co-creation with Earth?
- Doesn't match earth's composition
- Should orbit in earth's equatorial plane like most moons do.
- Rapidly spinning early earth became unstable and split, flinging off a
part of its crust
- Before plate tectonics, some thought the Pacific Ocean was the scar
- Should orbit in earth's equatorial plane
- Can't fling something into orbit. It will either escape or come back down.
- Can explain why moon orbits in earth's orbital plane
- Extremely improbable: moon would have to pass earth at exactly right
distance and speed
- Capture is very improbable, a collision takes no finesse at all
- Can explain why moon orbits in earth's orbital plane
- Can explain why moon's composition differs from earth
- Moon rocks suggest moon formed in a hotter part of the solar system than
earth. Elements that evaporate easily are less abundant on moon than on earth.
- Models of solar system evolution suggest that hundreds of Moon-Mars sized
proto-planets formed and collided to make major planets. Early solar system
was violent beyond belief.
- Impact would have melted most of earth and moon. Earth would have been
incandescent for about 10,000 years.
Unstable Early Earth
- May have been several moon-forming events
- 1000-km impactors can melt crust
- 100-km impactors create temporary atmosphere of vaporized rock, vaporize
- Life not possible until large impacts cease
- To have life on Earth, we need Jupiter?
- Sweeps up debris and reduces impacts
- Huge mass, right location and circular orbit stabilize orbits of other
- To have life on Earth, we need Moon?
- Stabilizes changes in earth's axis tilt
Conditions on Early Earth
- Oldest existing earth materials are 4.1 billion years old
- Oldest rocks are 3.9 billion years old
- Oxygen-poor atmosphere (present oxygen is created by life)
- Faint Early Sun: stellar evolution models suggest early sun was perhaps 30
per cent less bright than present
- Evidence for liquid water from the git-go
- Atmosphere and sun must have evolved in tandem to keep earth in liquid
water temperature range
- Carbonate-Silicate Cycle: life not essential but liquid water is.
What Is It?
- "Life is what dies when you stomp on it"--Dave Barry
- A self-replicating chemical system
How Did It Originate?
- Certainly not as complex as anything now alive
- Lots of candidates for first self-replicators
- Role of minerals as catalysts and templates?
- Simplest organisms are extremophiles, can tolerate very hot, very cold,
chemically extreme conditions, no oxygen.
- At least since 3 billion years, probably much earlier
Major Events in the History of Life
- Oxygen levels in atmosphere. Carbon dioxide plus water plus solar energy
--> sugars, starches, etc. plus waste oxygen. As oxygen increased in
atmosphere, eventually organisms developed a way to utilize oxygen, reverse
the reaction, and tap into the stored solar energy.
- Sex: Who Needs It? Sex is a great tool for genetic diversity, but
biologically sex is a mystery. Asexual reproduction is very efficient, and how
exactly did this business of swapping nuclear material first originate? (Not
that anyone's complaining!)
- We are a team: mitochondria (energy producing structures in cells) have
their own DNA, probably were originally separate organisms. Also most of our
cells are not us, but commensal bacteria.
- Snowball Earth: what survived and how?
- Cambrian "Explosion"
- Many Brachiopods
- Many Tree Ferns, Scale Trees
- Worst in Earth History - 90% of species
- Moderately Bad - only 60% of species
- Three others as bad or worse
- Climate Change
- Sea Level Change
- Competing Organisms
- Meteor Impact
- Very early earth may have had thin, unstable crust
- Large areas of continental crust by 2.5 billion years
- Plate tectonics since at least two billion years
- Wilson Cycle
- Not much room for continents to move
- Repeated cycles of assembly into supercontinents followed by dispersion.
- The farther back we go, the more pieces are missing
- Rodinia 800 million years
- Pangaea 250 million years
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Page Created 10 October 2003, Last Update