Planetary Structure

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Queen's University
Physics, Engineering Physics and Astronomy
ASTR 101

Planetary Structure • Similar to how medical doctors try to observe the interior of a patient, there a variety of techniques to try remote sensing to view the interior of the Earth The Earth's Interior: • External Signs: Majority of surficial rocks are silicon rich however the density of Earth is greater than surface rocks  Greater density could be due to tighter packing of material near the center of the Earth or it suggests stratified layers of different rock composition  Can we assume the Earth is the same throughout? • Emissions: Volcanic material gives us information of rocks from hundreds of kilometers below the surface of the Earth  Although useful, these are from quite shallow depths considering Earth has a 6000km radius • Probing the Interior: Very deepest mines on penetrate a few hundred kilometers into Earth's crust  Despite a surgeon being able to open up a patient to look inside, we cannot dig deeper into Earth since the pressure of the surrounding rocks would collapse into the opening  Earth's magnetic field and how it changes provides a deeper understanding of continental drift  Ultrasound has become an important tool in geophysical studies in studying the Earth's interior Ultrasonics and Seismology: • Sound travels at different speeds through different materials which allows geophysicists to determine what is below by detonating small explosions  Earth's active geology produces Earthquakes which allow us to interpret the medium through which seismic waves pass through • Seismographs are instruments that measure and record vibrations of seismic waves  Body Waves (P and S Waves) travel through the earth whereas surface waves (L waves) travel along the surface  P waves cause "push and pull" vibrations parallel to the direction of travel and travel faster than S waves; can travel through any material (known as longitudinal waves)  S waves cause "shaking" vibrations perpendicular to the direction of travel and travel faster than L waves; can only travel through solids (known as transverse waves)  L waves can cause the surface to sway back and forth (Love waves) or act similar to ocean waves and cause circular movement (Raleigh waves)  Seismic wave velocity usually increases with depth within a given layer of material  At boundaries between materials, body waves can be reflected or refracted  When body waves are reflected/refracted at a boundary, some of the energy of one type is converted to waves of another type  For example, a P wave striking a boundary may produce reflect and refracted S waves, as well as reflected and refracted P waves • Velocity of seismic waves depends on density and elasticity of the material it travels through  Elasticity of material increases with depth as there is more pressure • Crust-Mantle Boundary (MOHO) was discovered by Mohorovicic who noticed that P- waves received at seismic stations further from an earthquake had greater velocities than those that arrived at nearby stations  Moho proposed that below 30-50km depth there is a boundary known as the Moho which contains a layer of very different material that better conducts P waves • Core-Mantle Boundary (Gutenberg Discontinutiy) discovered that at 103° (11 000km) from an earthquake, P waves die out completely and reappear at 142° (16,000km)  Known as the P-wave shadow zone, which suggested the Earth's core was made of different material than the mantle  P wave velocity is slower in the core; Calculated the core was at a depth of 2900km  S waves cannot travel through the core, suggesting that it is liquid; creates an S wave shadow zone past 103°, after which no direct S waves are received • Inner-Outer Core Boundary (Lehmann Discontinuity) found that some weak or unexpected waves would appear in shadow zones, which introduced an inner core where velocity of waves is greater  Suggest the inner core is a solid surrounded by a liquid outer core inner core suggested to be at 5100km depth • Important to remember that we do not know for certain that this model is correct as lab experiments do not account for how materials behave under such fantastic heat or pressure Plate Tectonics • Plates are carried over the mantle (which seems like solid rock however rocks are ductile materials) due to circulation currents caused by heat escaping from the core to outer space  Known as convection currents driven by the steady outflow of heat from Earth's interior  Analogy: Pot of soup has heat escaping from deep within to outer air, causing circulation of the soup (churning and rolling in on itself)  If Earth was smaller it would have lost its heat more quickly and become dormant like Mars, which demonstrates ancient tectonic activity  The moon cooled off so quickly tectonic motions did not have time to form • Earth's activity on a local scale (Example: volcanism) is driven by motions at a global scale (Plate Tectonics) Sources of Heat: • The presence of high pressure is not the source of heat in the Earth, however heat and pressure are closely related  When the Earth was formed, planetesimals collided forming a larger and larger planet  Conversion from kinetic energy (in fall) to thermal energy (on impact) yields heat  The buildup of material increased the pressure in the core • Radioactive elements heat the planets interior, specifically elements with a long half life such as Uranium (despite the r
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