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Astronomy 2021A/B

Astronomy Lecture #4 Search for Life in the Universe  Midterm 25% - starts at 7 PM  40 Multiple Choice  8:15 Lecture #5 begins  TA Monday 1-2, Tuesday 2-3 PAB 49  Or PB 312  Prof – Wed 12:30-2:30 The Earth From Space Earth: The La-Z Boy of the Terrestrial Worlds  Earth’s moon is dry and airless  Mars – next best chance for life Geology  “ A study of earth” but we mean study of processes and features that shape our world o Three meanings to focus on:  Volcanism  Plate tectonics  Magnetic field Earth has a long history  We can study rocks and fossils  Contain info about when they were formed  The geological record  Arrival, birth and evolution of life on earth  Understand when they formed and what Earth was like when they formed Types of rocks: Igneous, Metamorphic, and Sedimentary  Igneous o Solidified rock o Pure coming from heart of the world  Metamorphic o Rock that has gone through transformations  Sedimentary o Useful for revealing history o Layers  All go through a cycle  Any type of rock is made from different minerals (crystals of certain elements) like silicates or carbonates  A silicate would be like quartz, carbonate is like graphite or diamond  Any of these types of rocks could be diff versions of these o Rock types tell us how it was made, teaches us the history of Earth Sedimentary Strata  Diff layers of sediment  River deposit some material (minerals) and deposits them in lake head  Over time keeps laying new layers  End up with diff layers  If river carves through the sedimentary layer, you can see it and reveal the layers  We can find things in the layers – like fossils  We can figure out the layers age  So when things appeared o Few diff layers, two diff hills o Will have two layers the same = the same age o Same process, same age o At the top, most recent o At the bottom, first layer that was made (ancient stuff) o This is crucial because we want to determine the age  A rock found above a layer that’s 100 million yrs old and below a layer that’s 10 million years old, must be between 10 and 100 million years old  And we never found fossil that didn’t match (like a dinosaur bone found in a 10 million year old layer) How to analyze rock  Beyond just type: o Mineralogical analysis – formation temp, pressure o Chemical analysis – what’s it made of o Isotropic analysis – age  Radiometric dating o Radioactive decay transforms elements into other elements on a well- known timescale o Example: potassium-40 decays into Argon-40  A proton turns into a neutron  Potassium has one more proton – it changes to neutron  One atom – can never measure age of this atom  But can measure age of rock In radioactive decay, some isotopes in a rock turn into other nuclei  Half-life: time it takes for half of the nuclei (atoms) in a substance decay o Or turn into e.g. Argon-40 o 1.2 billion years is the half life for potassium turning into Argon  Clock starts ticking when the rock solidifies Decaying Rock  When rock first solidified, it had no argon in it  Argon is a gas  But it did have potassium-40  After 1.25 billion years, half the potassium in the rock had turned into argon o Equal amounts potassium and argon in the rock  After 2.5 billion years half the remaining potassium has turned into argon We date rocks by measuring their composition of different isotopes  Half lives can be measure in Earth labs  Can guess where it started from – geochemistry  Make good guesses about original rock composition from o Geochemistry o Nuclear physics  Look at relative proportions of isotopes, use half-lives to derive age  Compare to other things like tree rings (really) or models of star lives, or artifacts with dates on them  Overall uncertainty of < 1-2% What the rocks tell us about fossils  Dinosaur bones  Petrified stones  200 million yr old impressions of snails  40 million yr old leaf  Tusks of entire mammoth  Dinosaur tracks o Leaf still retains organic material o All of these fossils are rocks What is a Fossil?  Dead organism buried  Leaves a hollow  Eventually filled by minerals
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