Formation of The Earth

Composition of the Sun

The Most Unusual ElementAdministratium (Ad)• No protons: Atomic Number Zero• One neutron• 27 Assistant neutrons• 137 Deputy assistant neutrons• 332 Associate neutrons• Detectable indirectly: slows down all

reactions it participates in

Composition of the Sun

• Abundance of Light Elements

• Rarity of Lithium, Beryllium, Boron

• Preference for Even Numbers

• Abundance peak at Iron, trailing off after

How Elements Form in Stars

• Sun: 4 H He• He + particle Mass 5 – Unstable• He + He Mass 8 – Unstable• He + He + He C• Add more He to make heavier elements• End of the line is iron for energy production• Atoms beyond Iron made in massive stars

What are Planets Made of?

• Same material as Sun• Minus the elements that remain mostly in

gases• We find this pattern in a certain class of

meteorites

Chondrites

Chondrite

The Earth’s Crust looks Very Different

Composition of the Crust

Hot or Cold?

• Up to 1940: Earth is hot inside, so must have formed hot

• 1940-1970: Earth need not have formed hot• 1970- Earth did form hot after all

Hot Early Earth?

• Lord Kelvin, 1862: estimate age of Earth from cooling.– Earth’s heat is left over from its formation– Heat travels outward by conduction– Earth is not producing heat

• Only one problem (actually three): Every one of Kelvin’s assumptions was wrong

Three Images of Early Earth

Chesley Bonestell’s Classic Image

Nuclear Processes

• Radioactivity (Becquerel, 1896)• Importance for Earth history:– Used for dating rocks– Explains sun’s energy output– Earth does produce heat

Maybe Earth Formed Cool?

• Planets formed by accretion of smaller bodies• Each impact produces heat• If rate is slow enough, heat can radiate away

as fast as it is produced

Earth Formed Hot After All

• Apollo samples: Moon had “magma ocean”• Better understanding of impact physics• Role of mega-impacts• Formation of core

Magma Ocean by Ron Hartmann

Craters and Planetary History

• Superposition• Crater Saturation• Crater Degradation

Superposition

Crater Saturation

Crater Degradation

Biggest and Oldest Crater on the Moon

Impact History

• Earliest records on Moon, Mars and Mercury: Intense Cratering

• As planets grow, their gravity increases. Impacts get more violent

• Debris from impacts buries hot rocks from earlier impacts

• Heat builds up• Magma Ocean

Basalt and

Anorthosite

How Do Planets Accrete?

• Tiny objects can be held together by welding, electrical forces, chemical interactions

• Big objects hang on to incoming material by gravity

• Things the size of a car are the mystery right now

Computer Studies

• Start with as many orbiting objects as your computer can handle

• Let them collide• Don’t get 8-10 nice, regular planets• Get 100’s of Moon and Mars-sized objects• These collide to make bigger planets• Violent beyond your wildest dreams

How Did the Moon Form?

• Co-Creation?• Fission?• Capture?

• 1985: Bill Kaufmann, Jay Melosh and others: Mega-Impact

Mega-Impacts:

As Usual, Gary Larson Gets There First

Computer Simulations by H.J. Melosh (University of Arizona)

Formation of the Moon

Formation of the Moon

View from the early Moon

Earth would have been as

hot as the Sun for 10,000

Years

Earth’s Atmospheres and Oceans• Primordial from accretion• Magma Ocean• Mega-Impacts (1000 km +)– Magma Ocean– Vaporized Rock (100’s years)– Steam

• Smaller Impacts (100 km +)– Vaporized Rock (Years)– Steam (Boil off Oceans)

Earth Finally Settles Down

• Origin of Atmosphere and Oceans?– Outgassing?– Impacts of comets?

• Early Atmosphere Probably Mostly CO2, and H2O

Bonestell: The Earth Cools

Bonestell: The Oceans Form

Hartmann: The First Moonrise

The Very Early Earth (Hadean)• Intense cratering on Moon (and

presumably Earth) ends about 4 billion years ago.

• Oldest earth material: 4 billion year old zircon from Australia.

• Oldest rocks: 3.9 billion years, NW Canada.• Minnesota River Valley rocks: 3.1 billion

years.• Can’t say much about processes• Liquid water from the git-go

The Faint Early Sun

• Sun 4 billion years ago was only 70% as bright as now.

• Would make average temperature of earth -15 F (-26 C)

• But earth has always had liquid water• Must have had denser atmosphere, greater

greenhouse effect.

The Archean• 3.0 – 2.5 billion years ago• About half of earth’s continental crust

forms• Granite, deep-water sediments and

volcanic rocks, deep crustal rocks• Were there mountains?• Did Plate Tectonics exist?

Molasse, Switzerland

Molasse and the High Alps

Archean Granite

Archean Pillow Lava

Archean Iron Formation

3.1 Billion Year Old Gneiss

Archean North America

Bear and Nain Provinces

Rae Block Collides

Penokean About to Begin

Penokean Orogeny and Churchill Collision

Wyoming Province Collision

Hearne Block Collides

Trans-Hudson Orogeny

Mazatzal Orogeny

Yavapai Orogeny

Midcontinent Rift Forms

Grenville Orogeny Complete

Present North America