Earth is roughly spherical, 13,000 km in diameter. But the deepest wells have only drilled down about 10 km at the deepest.

Transcription

1 The Earth Earth is roughly spherical, 13,000 km in diameter. But the deepest wells have only drilled down about 10 km at the deepest. So how do we tell anything about the interior? We need some probe of the interior Start with what we can tell from the outside Must first realize that the bulk of the Earth is rock, not water as you might expect from a view of the surface. Rocks on the surface are mostly silicates. We will look at density, ρ = M V. We will often compare to water which has a density of 1gm/cc. Well, what about the density of the whole Earth? We know the mass of the Earth. How?? We know the volume of the Earth. How?? So we can calculate the density of the Earth. It is about 5.5 gm/cc. But the density of silicate rocks varies from 2-3 gm/cc. Obviously something else is in the interior with substantially more density. Iron is a likely candidate. 1

2 Need some way to see into the inside of the Earth Let s talk about waves (Longitudinal and Transverse) Liquids and Gases cannot support Transverse sound waves If we could only send sound waves through the Earth, we could image it like a Sonogram But where will we find a source of sound waves to do this? Earthquakes! Earthquakes Two kinds of wave motion set up in the Earth due to Earthquakes P-waves (Pressure, Primary) Longitudinal S-waves (Shear, Secondary) Transverse which cannot go through liquids Changes in the density of the rocks affect how the wave travels So you study lots and lots (and lots) of earthquakes, and look for patterns. 2

3 Temperature of the Earth s interior 3

4 How do we know this? We study metals under pressure and temperature and see how fast waves move through them. Why is the inner core solid when the outer is liquid? PRESSURE! Without all the pressure the Earth would only have a density of about 4.6x water. Our model of the Earth has it looking like the highest density stuff is at the center, lowest density on the outside We call this DIFFERENTIATION, where dense materials sink to the bottom, lighter materials rise up. Clearly seen in liquids (Drop a rock and styrofoam into a pool-what happens?) The conjecture is the Earth s interior was once mostly liquid, allowing differentiation to occur. What supplied the heat to keep rock molten? Bombardment and Radioactivity! Earth s size has helped to keep it hot (Radiation losses go as 1/Radius) Age of the Earth How old is the Earth? How would you tell? Have to use something that correlates with time, obviously, and believe that what it tells you is related to reality (Realism!) Several techniques have been used. Tree rings Silt layers due to annual floods Coral reef layers 4

5 Age of the Earth Obviously these can only count back to certain times Can we find something that isn t tied to processes defined by Earth? Radioactive Dating A hot button issue... Radioactivity Quick and dirty description of radioactivity Take some radioactive material like 235 U, 40 K, 12 C. Amount of radioactivity is proportional to the amount of radioactive stuff N t N 1 Fraction of Material Remaining Time (arbitrary Units) N(t) = N o e t/τ τ is the time constant which is unique to every radioactive material. It is related to the famous (and contentous) half life, or the time it takes for 1/2 of the material to decay by t 1/2 = τ ln(2). How do we measure half lives in excess of the time of measurement, like for 5

6 238 U: t 1/2 = 4.5 billion years 87 Rb: t 1/2 = 52 billion years 40 K: t 1/2 = 1.3 billion years 14 C: t 1/2 = 5730 years We simply measure how much of the material we have, and then measure how radioactive it is. Then we use the simple relationship N t = N τ It is quite simple, but it depends on assumptions. The question is always which do you hold to or not, and why. How does this tie into dating rocks and stuff? 14 C dating can only date things that were once alive up to 60,000 years ago. (Assumptions...) 40 K dating. Molten rocks with certain fraction of 40 K. No argon to begin with. One decay chain is 40 K 40 Ar (β + decay). So we measure the fraction of argon to potassium 40 in the rock. (Argon is trapped in rock) Typically look at several radioactive species to see if they agree. Issue of uncertainty Earth rocks date to about 3.9 billion years for the oldest. Belief is that crust recycling limits maximum age. Why? Age of meteorites and moon rocks all imply 4.5 billion years one, and with the assumption of common origin, that would put the Earth at the same age. So how old is the sun? How long till the suns runs out? How do we know? Atmosphere of the Earth 6

7 Ionosphere, 1/millionth atm Mesosphere, 1/1000 atm Stratosphere, 1/100 atm Troposphere-weather, convection Atmosphere of the Earth The troposphere, where weather is driven by Convection. Atmosphere is about 78% nitrogen, 21% oxygen, 0.9% argon, 0.03% carbon dioxide, and % water vapor. These last two raise the temperature of the Earth by about 40 C! This ties into the Greenhouse Effect. What is your opinion about Global Warming? The problem with predictions is we have only three real data points. Earth, Mars, and Venus. Earth has only 0.03% carbon dioxide. Without it, average temperatures would be below freezing! Mars has 100% carbon dioxide, but overall levels are too small to have any effect. 7

8 Venus is essentially 100% CO 2, with lots of gas, and its surface temperature is higher than the melting temperature of Lead! No question that CO 2 levels are increasing... The question is what effect it will have. But remember this, when you burn 1 gallon of gas, you produce 30 lbs (!) of carbon dioxide... Studies of early Earth atmosphere: CO 2 levels dramatically higher than now! Connection to the Sun and models of stellar dynamics, as well to Plate Tectonics Plate Tectonics What is the meaning of the greek tekton? A builder (Used to describe Jesus) A theory to describe how the continents came to be. Relatively new theory. First hints were made in 1596 by a Dutch map maker who saw that the west coast of Africa seemed to fit the east coast of South America. First proposal (Continental Drift) made by Wegener in

9 Initial response to theory? Very negative! What could move such a large land mass? Wegener died while trying to prove it in What evidence was there to support it? Not much until 1950 s. Studies of the ocean floor. Huge mountain ranges down there! Topographic map of a segment of the Paci c Mid-Oceanic Ridge. "Warm" colors (yellow to red) indicate the ridge rising above the sea oor, and the "cool" colors (green to blue) represent lower elevations. (Imagery courtesy of Stacey Tighe, Univ of Rhode Island.) Need to detect submarines leads to studies of the magnetic field in the ocean floor. What they found was a puzzle 9

10 Not just limited to the Atlantic Ocean! This is off the coast of Washington State Other evidence of Plate Tectonics? Earthquakes and the Ring of Fire 10

11 Nowadays GPS keeps track of the motion of the different plates But what drives the motion of the plates? CONVECTION 11

12 What does Plate Tectonics do for you? Maintains the level of CO 2 Recycles minerals Produces Habitat Continents! Earth s magnetic field Notice the North pole is actually a magnetic South pole! Strength of the Earth s field is about 400 times less than a refrigerator magnet. What does it do for you? Protects you from the mean old Solar Wind 12

13 Without the magnetic field, life would not be around here for long. Mars and Venus have very low magnetic fields. What produces the magnetic field? Earth s rotation and the liquid outer core. Best models imply the need for sulfur in the mix. Not an easy process to understand. Presently supercomputers are used to model the field. Rare Earth Spin Rate (Too fast not enough weather, too slow, too much variation) Right zone for water but only because of our present situation If all water froze, would take long time to recover. If all water turned to steam, would take long also. Rare Earth CO 2 levels have tracked the increasing power levels of the Sun Only about several hundred million years left on that. Gravity of Earth holds onto N 2, O 2, CO 2, but not CH 4 or NH 3 From Rare Earth... 13