A Short Look at Earth History

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Formation of Sun

Planets made of same material as Sun, minus elements that remain mostly in gases
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

Over 1000 extrasolar planets known
Barely can detect Jupiter-size planets, don't yet have technology to see small planets
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.

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.

It's very hard to account for the Moon:

Co-creation with Earth?

Fission?

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.

Capture?

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.

May have been several moon-forming events
1000-km impactors can melt crust
100-km impactors create temporary atmosphere of vaporized rock, vaporize oceans
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 planets
To have life on Earth, we need Moon?
- Stabilizes changes in earth's axis tilt

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.

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

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 million years.

Climate Change
Disease? Virtually impossible because survivors have resistance.
Mountain-building
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.
Over-specialization
Volcanism
Meteor Impact

Page Created 10 October 2003, Last Update 02 September 2011