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October 26 and 28: Chapter 9 - Planetary Geology 8.4 Evolution of the Solar System of Earth and the other Terrestrial Worlds • Our goals for learning – What happened to the leftovers? – Where Wh did asteroids t id andd comets t come from? – How do we explain “exceptions”? – How do we explain Earth’s moon? – How “typical” is our solar system? Time to start looking at www.sci.angelo.edu/phys1302/termexam3.pdf 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 j in late stages of solar system formation – – – – Earth Mars Uranus Venus? 1 Late Heavy Bombardment Origin of Earth’s Water? Some H2O released from hydrated rocks • Shouldn’t be here! • Water must have come to Earth by way of icy planetesimals from outer solar system Primordial CO2 atmosphere 300 million year delay. Why? Differentiation • Gravity attracts high mass (high-density) material to center • Lower-density material is displaced p to surface Too hot at time of heavy bombardment DRY! Major Impacts • Surfaces must be cooled and solidified to leave a trace of major impacts → differentiation • e.g. Mars M C Crustal t l Dichotomy Di h t – Atmospheric gases – Silicate rocks – Nickel/Iron core • Material ends up segregated by density Mars Crustal Dichotomy Like Earth & Venus, initially dense CO2 atmosphere Origin of Earth’s Magnetic Field • Differentiation must be essentially complete • Molten convecting Nickel/Iron core 2 Captured Moons Earth’s Moon? •Apollo and Luna samples prove Earth and Moon are chemically related Phobos & Demios @ Mars • • So why didn’t they form as one larger planet during accretion period? Unusual moons of some planets are likely captured planetesimals or actual KBOs Triton @ Neptune Giant Impact Hypothesis • Answer: The Moon was created later as part of a heavy bombardment event. Most interesting? Mars--sized impact on early Earth Mars Stripped matter goes in to orbit Then accreted into Moon 8.5 Age of the Solar System • Entire Uranian system (planet, rings & moons) are all knocked on their h i sides id • Likely from a megaimpact before the planetesimal period • Venus flipped almost 180° Radiation • Our goals for learning – How does radioactivity reveal an object’s bj age? – Ultimate Question: When did the planets form? • ergo…How long have they been evolving? • An exact definition of the meaning of this word… • raڄdiڄaڄtion [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”? 3 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 medical X-ray <200 mRems Most natural radon is from radioactive decay of igneous rocks Radioactive Decay Radioactive Decay Count the number of parent and daughter atoms What are the results? • Many isotopes decay into other nuclei • A half-life h lf lif is i the h time for half of the parent atoms in a substance to decay into a daughter product The Stardust Mission • 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 old 4.4 • 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 4 Not to hot, not too cold, not too new and not too old… • If our solar system is a book, book that is the story. • What are the questions? How “typical”? See Fig. 8.13 • Formation of planets in solar nebula seems inevitable • Physics should be universal i l • Major variables are: http://www.lpi.usra.edu/education/timeline/ Total mass of cloud Composition of cloud Angular Momentum Random interactions within solar system http://www.sci.angelo.edu/phys1302/universe_timeline.pdf 5