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October 26 and 28:Chapter 9 - Planetary Geology

of Earth and the other Terrestrial Worlds

Time to start looking at www.sci.angelo.edu/phys1302/termexam3.pdf

8.4 Evolution of the Solar System

• Our goals for learning– What happened to the leftovers?

Wh did t id d t– Where did asteroids and comets come from?

– How do we explain “exceptions”?

– How do we explain Earth’s moon?

– How “typical” is our solar system?

What happened to the leftovers?

Asteroids, Comets & KBOs

• Leftovers from the accretion process• Rocky asteroids inside frost line• Primordial KBOs outside the Frost Line and away from

strong gravity of jovian planets• Icy interstellar debris (comets) beyond in Oort Cloud

Heavy Bombardment• Leftover

planetesimals bombard other objects in late jstages of solar system formation– Earth

– Mars

– Uranus

– Venus?

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Late Heavy BombardmentLate Heavy Bombardment

300 million year delay. Why?300 million year delay. Why?

Origin of Earth’s Water?• Shouldn’t be here!

• Water must have come to Earth by way of icy planetesimals from

Some H2O released fromhydrated rocks

planetesimals from outer solar system

Primordial CO2

atmosphere

DRY!

Too hot at time of Too hot at time of heavy bombardmentheavy bombardment

• Gravity attracts high mass (high-density) material to center

• Lower-density material is displaced

DifferentiationDifferentiation

pto surface– Atmospheric gases– Silicate rocks– Nickel/Iron core

• Material ends up segregated by density

Major Impacts• Surfaces must be cooled and solidified to leave

a trace of major impacts → differentiation

M C t l Di h t• e.g. Mars Crustal Dichotomy

Mars Crustal Dichotomy

Like Earth & Venus, initiallydense CO2 atmosphere

Origin of Earth’s Magnetic Field

• Differentiation must be essentially complete

• Molten convecting Nickel/Iron core

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Captured Moons

• Unusual moons of some planets are likely captured planetesimals or actual KBOs

Phobos & Demios @ MarsPhobos & Demios @ Mars

Triton @ NeptuneTriton @ Neptune

Earth’s Moon?

•Apollo and Luna samples

prove Earth and Moon

are chemically related

• So why didn’t they form

as one larger planet

during accretion period?

• Answer: The Moon was

created later as part of

a heavy bombardment

event.

Giant Impact HypothesisMarsMars--sized impact on early Earthsized impact on early Earth

Stripped matter goes in to orbitStripped matter goes in to orbit

Then accreted into MoonThen accreted into Moon

Most interesting?

• Entire Uranian system (planet, rings & moons) are all knocked on h i idtheir sides

• Likely from a megaimpact before the planetesimal period

• Venus flipped almost 180°

8.5 Age of the Solar System

• Our goals for learning– How does radioactivity reveal an

bjobject’s age?

– Ultimate Question: When did the planets form?• ergo…How long have they been evolving?

Radiation

• An exact definition of the meaning of this word…

• ra di a tion [rey-dee-ey-shuhn] –noun – the complete process in which energy is emitted by one

body, transmitted through an intervening medium or space, and absorbed by another body.

• All “light” is radiation. The key is “what’s the λ”?

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Radioactive Materials are ubiquitous

• The concept of Metastability

• Very heavy elements are unstable – they decay.

• The decay process generates radiation. – e.g. radioactive elements

• The decay process happens at a very specific rate.– e.g. there is a “clock” in the rocks

Background Radiation

295 natural background+63 anthropogenic~265 mRems

-vs.-

Standard medicalX-ray <200 mRems

Most natural radon isfrom radioactive decayof igneous rocks

Radioactive Decay

Count the Count the number ofnumber ofparent andparent and

daughter atomsdaughter atoms

Radioactive Decay• Many isotopes

decay into other nuclei

A h lf lif i h• A half-life is the time for half of the parent atoms in a substance to decay into a daughter product

What are the results?What are the results?• Radiometric dating tells us that oldest Earth rocks are

4.4 billion years old• Radiometric dating tells us that oldest Moon rocks are

4 4 billion years old4.4 billion years old• Radiometric dating tells us that oldest meteorites are

4.55 billion years old (younger ones are mostly SNCs from Mars).

• Radiometric dating tells us that cometary material is up to 4.6 billion years old

• Planets probably formed 4.5 billion years ago

The Stardust MissionThe Stardust Mission

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Not to hot, not too cold, not too new and not too old…• If our solar system

is a book that isis a book, that is the story.

• What are the questions?

http://www.lpi.usra.edu/education/timeline/

How “typical”? See Fig. 8.13• Formation of planets

in solar nebula seems inevitable

• Physics should be i luniversal

• Major variables are: Total mass of cloud Composition of cloud Angular Momentum Random interactions

within solar system

http://www.sci.angelo.edu/phys1302/universe_timeline.pdf