GEOL 102 Lecture Notes - Lecture 12: Protoplanetary Disk, Gravitational Collapse, Planetesimal

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3 Mar 2017
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Geology 102 Lecture 12 Notes
The Early Earth
Origin of the Solar System
To understand the processes that formed our solar system, scientists often used
computer models and observations of other star systems
Uniformitarianism.. In space!
Nebula- A cloud of debris and gases (hydrogen, helium, etc) in space
Nebulae are where stars form
But first, how do nebulae form?
Nebulae
Form in a number of ways
Gravitational collapse
Supernova
Red Giant
Etc.
The Formation of Our Solar System
We think that the nebula where our star was born was formed from the remnants
of a supernova
The composition of the planets and other bodies in our solar system suggests this
origin
Heavy elements are formed during the violent collapse of the supernova
The dense cloud formed incorporating the remnants of the supernova
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The cloud had some rotational movement
Rotating inherent
Shockwave from another supernova
As the hot gases in the nebula cooled over time, the cloud started to contract
This would have increased the speed of rotation within the system conservation of
angular momentum
These rotational forces paired with the gravitational forces reshaped the cloud into
a disk
This “protoplanetary disk” had a dense inner core and a less dense outer rim
Collisions occur between grains within the disk
Some material joined the forming star at the core
Some particles collided with each other
The Formation of the Sun
The core does not have much angular momentum
Collapses under its own gravity
Large size and pressure initiates nuclear fusion of hydrogen and helium
The Formation of Planets
Further away from the core, grains continued to collide, forming larger and larger
bodies within the rotating disk
This accretion is driven by gravity
>1 km in diameter= planetesimals
These planetesimals start to arrange themselves into rings
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In large enough protoplanetary disks, these planetesimals will continue colliding
with one another forming small planets
The formation of the Earth
This is how the planets in our solar system, including formed ~4.6 billion years
ago
How do we know that?
The age of the Earth
Oldest rocks dated to 3.8 bya
Oldest detrital grains within other rocks (zircons) dated to 4.4 bya
But, Earth was partially molten during its early history
And Earth was (and is still) tectonically active
Old crust is altered and/or destroyed
The meteorites which sometimes fall to Earth provide us with the opportunity to
test other rocks in our solar system which have not undergone the same
destructive processes
Dates from these meteorites all cluster around 4.5- 4.6 billion years ago
The Early Earth
Hadean Eon= 4.6- 3.8 bya
Named for Hades
The Earth continued to grow by accretion was originally homogenous
Increasing size and density started to compact the early Earth, causing intense
heat
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Document Summary

To understand the processes that formed our solar system, scientists often used computer models and observations of other star systems. Nebula- a cloud of debris and gases (hydrogen, helium, etc) in space. We think that the nebula where our star was born was formed from the remnants of a supernova. The composition of the planets and other bodies in our solar system suggests this origin. Heavy elements are formed during the violent collapse of the supernova. The dense cloud formed incorporating the remnants of the supernova. As the hot gases in the nebula cooled over time, the cloud started to contract. This would have increased the speed of rotation within the system conservation of angular momentum. These rotational forces paired with the gravitational forces reshaped the cloud into a disk. This protoplanetary disk had a dense inner core and a less dense outer rim. Collisions occur between grains within the disk. Some material joined the forming star at the core.

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