Lecture 17: Earth s Interior The appearance of life led to further profound changes in the atmosphere about 3.5 x 10 9 years ago Plants produce free oxygen and remove carbon dioxide The Earth s surface is a giant terrarium, with interdependent species and ecosystems Are there other Earth-type planets in our galaxy? How could we detect them? Extrasolar Planets We can detect planets around other stars in several ways: Pulsar Planets planet Planet s orbit causes wiggles in the pulsar s pulsations We can detect Earth-mass planets using this technique Extrasolar Planets We can detect planets around other stars in several ways: Planetary Transits Planet s orbit causes periodic drops in stellar brightness We can detect Earth-mass planets using this technique 1
Extrasolar Planets We can detect planets around other stars in several ways: Doppler Shifts Planet s orbit causes Doppler shifts, indicating the star s velocity We can detect Jupiter-mass planets using this technique Example: Upsilon Andromeda Extrasolar Planets Planet mass (Jup) a (AU) e A 0.72 0.059 0.02 B 2 0.828 0.24 C 4 2.56 0.31 Jupiter-sized planets have been detected using Doppler spectroscopy Why don t these planets evaporate when so close to their star? Extrasolar Planets Comparison of planet-finding techniques: Habitable zone Earth-sized planets can be detected using the pulsar or transit methods Only the transit method can reveal planets in the habitable zone. 2
Earth s Interior Drills only penetrate the outer 10 km of the Earth s crust Therefore, in order to study the deep interior, we must use seismic waves generated by earthquakes There are two types of seismic waves: P (longitudinal) and S (transverse) P-WAVE S-WAVE Earth s Interior The P-waves are like sound waves, and they can propagate through liquids The S-waves are transverse, and therefore they cannot propagate through a liquid (they are absorbed) The observations of these waves indicate the presence of a liquid core deep inside the Earth shadow zones earthquake shadow zones 3
This explains why the average density of the Earth is 3 ρ = 6 g cm 4
Radioactive Dating Spontaneous breakup of heavy nuclei! Many heavy nuclei are unstable to decay via spontaneous fission (breakup) U 235 Pb 207 + energy + particles U 238 Pb 206 + energy + particles Th 232 Bi 209 + energy + particles This is called radioactivity Out of a given sample, ½ of the atoms will undergo fission during one half-life, t half Radioactive Dating Out of a given sample, ½ of the atoms will undergo fission during one half-life, t half Examples of half-lives: U 235 t half =713,000,000 years U 238 t half =4,500,000,000 years Th 232 t half =13,900,000,000 years 5
Radioactive Dating We can use radioactive dating to measure the age of a sample Example: U 238, with half-life t = 4.5 x 10 9 half years After a time period t = t half, only HALF of the U 238 remains U 238 Pb 206 t half t half t half ½ remains after t half 1/4 remains after 2 t half 1/8 remains after 3 t half Radioactive Dating If we started with pure Uranium, then the ratio amount of lead amount of uranium can be used to determine the age of the sample Using radioactive dating, the age of the Earth has been determined to be about 4.5 billion years Radioactive decay early in Earth s history released a lot of energy, which kept the Earth molten for 10 9 years! This allowed differentiation to take place the dense material (iron) sinks to the core of the planet Differentiation Radioactive decay allows differentiation to take place the dense material (iron) sinks to the core of the planet Some of the planets with lower masses cooled off too quickly for differentiation to be completed despite radioactive heating For example, Mars still has much iron at the surface, where it rusts and gives the planet its reddish appearance 6
Surface Activity Indicators of surface activity on the Earth include earthquakes and volcanoes Wind and water erosion wipes away evidence of activity Regions of current activity are NOT spread evenly across the planet In the 1960 s, it became clear that active regions define the edges of gigantic plates The plates move! The motion of the plates Causes earthquakes Shapes continents Creates mountain ranges Creates undersea trenches Surface Activity Typical plate speed is a few cm per year The plate motion is called Continental Drift, or Plate Tectonics This motion can be measured by observing distant quasars over 10 8 light-years away Note: The plates are not continents, but the continents and oceans ride on the plates Plates collide as a result of continental drift 7
Surface Activity Plates collide as a result of continental drift When plates collide, they keep moving! In a plate collision, one plate can be driven under the other, forming a subduction zone The lower plate melts in the subduction zone due to the high pressure and temperature 8
Seafloor Spreading Plates can also move apart, at mid-ocean ridges This process creates new seafloor 9
Earthquakes Plates can also slide along each other, causing earthquakes Earthquakes 10
Plate Collisions Plates collide as a result of continental drift When plates collide, they keep moving! The plate speeds do not seem to be slowing down, despite the friction of their motion What keeps the plates moving at a steady speed? What Drives the Plates The motion of the plates is driven by underlying convection in the mantle This picture provides a unified description of seafloor spreading, plate tectonics, and plate collisions 11
12