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Formation of Sun • Formation of Universe: 13.7 billion years • Formation of Galaxy: 11 billion Years • Formation of Solar System: 4.6 billion years • Sun is probably a third generation star • Probably takes 10-100 million years for planets to form
Formation of Planets • 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
Our Solar System May Not Be Typical • Over 400 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")
Formation of Earth • Planets formed by accretion of smaller objects = impact • Very tiny objects hold together by atomic forces • Objects kilometers across hold together by gravity • How do objects the size of a refrigerator hold together? • As planets get bigger, gravity gets stronger, impacts get more violent • Big impacts throw out ejecta, trap heat • Magma ocean • Formation of core early in earth history as iron sinks
Formation of Moon • It's very hard to account for the Moon: • Very big compared to its parent planet • Orbits nearly in plane of earth's orbit, not over equator. • Co-creation with Earth? • Fission? • Capture?
Collision • 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 • Models of solar system evolution suggest that last stage is mega-collisions • Impact would have melted most of earth and moon • Earth would have been incandescent for about 10,000 years.
Unstable Early Earth • 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 • Stabilizes orbits of other planets • To have life on Earth, we need Moon? • Stabilizes changes in earth's axis tilt
Conditions on Early Earth • Oldest existing earth materials: 4.1 billion years old • Oldest rocks: 3.9 billion years old • Oxygen-poor atmosphere (present oxygen is created by life) • Faint Early Sun: perhaps 30 per cent less bright • Evidence for liquid water from very early on • Atmosphere and sun must have evolved in tandem • Carbonate-Silicate Cycle: life not essential but liquid water is.
Life • What Is It? • "Life is what dies when you stomp on it“ --Dave Barry • A self-replicating chemical system • Feedback (Homeostasis)
How Did Life Originate? • 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 • At least since 3 billion years, probably much earlier
Major Events in the History of Life • Oxygen levels in atmosphere • Plants release waste oxygen • Eventually organisms developed a way to utilize oxygen (6CO2 + 6H2O = C6H12O6 + 6O2) • Sex: Who Needs It? • We are a team: Mitochondria • Snowball Earth: what survived and how? • Cambrian "Explosion"
Causes ? • Climate Change • Disease • Mountain-building • Sea Level Change • Competing Organisms • Over-specialization • Volcanism • Meteor Impact
Plate Tectonics • Very early earth may have had thin, unstable crust • Large areas of continental crust existed by 2.5 billion years • Plate tectonics since at least two billion years • Wilson Cycle and Supercontinents • Supercontinents • Dispersal • Reassembly • Rodinia 800 million years • Pangaea 250 million years
Humans • Lumpers vs. Splitters • Australopithecus > 2 m.y. • Homo erectus 1-2 m.y. • Homo ergaster 1-2 m.y. • Homo habilis < 1 m.y. • Homo neanderthalensis 500,000-30,000 yr. • Homo sapiens<100,000 years