A Short Look at Earth History
Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green
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Formation of Sun
- Formation of Universe: 13 billion years ago
- Formation of Galaxy: 11 billion Years ago
- Formation of Solar System: 4.6 billion years ago
- 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 1000 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")
- What we've seen so far is biased because big planets orbiting rapidly
around their parent stars are easiest to spot.
Formation of Earth
- Planets formed by accretion of smaller objects. Each accretion was an impact
- Very tiny objects in space hold together by atomic forces
- Objects kilometers across hold together by gravity
- How do objects the size of a refrigerator hold together? Why don't they
break up by impact as fast as they form?
- As planets get bigger, gravity gets stronger, impacts get more violent
- Big impacts throw out ejecta, trap heat
- Magma oceans result because so much heat builds up in the crust.
- Formation of core early in earth history as iron sinks. This event
liberated a lot of energy that further heated the early earth.
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.
- For a long time there were three equally unsatisfactory theories:
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
Finally in the 1980's a new theory appeared: Collision:
- 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 governs climate: 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?
- Earliest life 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.
- Snowball Earth: About 800 million years ago, the earth's climate
regulators nearly broke down and the earth mostly or entirely froze over. What survived and how?
- Cambrian "Explosion:" About 550 million years ago we see a rapid
proliferation of fossils due to the appearance of animals with shells. Life
was abundant before then but lacked hard parts. Nevertheless, it left trails
and tracks. The term "explosion" is inaccurate since the earliest fossil
organisms were few in number and biodiversity grew steadily for over 100
Permian Mass Extinction
- Many Brachiopods
- Many Tree Ferns, Scale Trees
- Worst in Earth History - 90% of species
Cretaceous Mass Extinction
- Moderately Bad - only 60% of species
- Three others as bad or worse
- Climate Change
- Disease? Virtually impossible because survivors have resistance.
- Sea Level Change
- Competing Organisms? Exotic species can cause havoc but extinction because
a new species out-competes an established one just doesn't happen.
- 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
- Not really a "cycle"
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Page Created 10 October 2003, Last Update
02 September 2011