A Short Look at Earth History

Formation of Sun Formation of Universe: 13 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 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 200 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

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 Sex: Who Needs It? We are a team: Mitochondria Snowball Earth: what survived and how? Cambrian "Explosion"

Mass Extinctions

Causes ? Climate Change Disease Mountain-building Sea Level Change Competing Organisms Over-specialization Volcanism Meteor Impact

Average Global Temperature

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 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

Earth History e=related