Transcript: Earth’s atmosphere is unique within the solar system mostly because of the nitrogen and oxygen that form the bulk of the Earth’s atmosphere: 75 percent nitrogen, 20 percent oxygen, plus carbon dioxide, argon, water vapor and other trace gasses.  The weather on the Earth is generated in the lowest part of the atmosphere, called the troposphere, at a distance of up to 10 miles from the Earth's surface or 50,000 feet.  The Earth's weather is caused primarily by convection patterns, the rising and falling of parcels of air caused by energy from the Sun either directly on to the atmosphere or Sun’s energy absorbed by the Earth and then reradiated into the atmosphere.  Convection plus the Earth's rotation gives the large scale weather patterns well known to all of us.  Earth’s atmosphere in the last two centuries has started to be altered in its chemical composition in subtle but not negligible ways, like human industrial activity.  The biggest consequences here are pollution which increases the components of nitrogen dioxide, sulfur dioxide, and sulfuric acid, the pollution of the ozone layer, the removal of a small isotope of oxygen called ozone leading to the increase in the ultraviolet radiation that reaches the Earth's surface, and deforestation which reduces the amount of oxygen and increases the carbon dioxide concentration in the atmosphere.

Earth's Atmosphere

Transcript: The ancient Greeks knew about loadstones.  These were curtain rocks which, when suspended in a fluid, would appear to line themselves in response to a mysterious force.  That mysterious force was magnetism, first understood through the experimentation of the physicist Michael Faraday.  Now we understand magnetism very well.  We understand that the Earth has a magnetic field.  The north pole of the Earth's relatively strong magnetic field is not perfectly aligned with the geographic North Pole.  Magnetic field is tethered in the iron and nickel solid core, and there's good evidence that occasionally throughout the Earth's history, not on a fixed periodic timescale but every 10 thousand to 100 thousand years, the Earth's core melts and the polarity of the Earth's magnetic field reverses.  The projection of the Earth's magnetic field up above the atmosphere into space also causes effects familiar to many people.  The interaction with the Earth's magnetic field and solar radiation and particles give rise to the aurora or the northern lights.

Magnetic Fields

Transcript: Cratering affects the evolution of planets.  The cratering history of the Earth has varied over its history.  Cratering was much stronger in the first half billion years when there was plenty of debris left over from the formation of the solar system.  When you look at the Moon we are looking at a mirror for the cosmic environment of the Earth.  The Moon shows evidence of many craters.  The Earth has apparently very few.  The difference is clear.  The Moon is not massive enough to have geological activity, so evidence of its craters is not been removed by tectonic motions.  The Moon is also too low in mass to retain an atmosphere, so erosion has not eradicated the evidence of cratering.  Earth has been subject to impacts from space over its history, and some of these are implicated in major changes in speciation and mass extinctions in the history of life on Earth.  But on the Earth because of the large amount of erosion and the large amount of geological activity, impact craters are difficult to discover and relatively rare.

Cratering

Transcript: Earth is continually sculpted by erosion.  Erosion refers to all processes generally caused by wind, rain, and moving water that break down rocks and transport them across the surface of the planet.  Erosion has a major effect on the way the Earth looks.  Tectonic forces can raise up a mountain chain in barely 100 million years, but erosion can destroy a mountain chain in about the same time.  Erosion is assisted by gravity, gravity causing erosion to move material generally from higher to lower places.  Gravity also plays a role in determining the maximum size of geological features on the Earth or any planet.  For example, we could imagine a mountain 10 times the size of Mt. Everest.  But should such a mountain exist, the pressure at its base would be such as to liquefy the rock, and so the mountain would eventually shift and settle to a smaller size.  Thus the maximum size of geological features on the Earth is limited by both erosion and gravity.

Erosion

Transcript: The convective motions of the liquid and semi-liquid rock deep within the Earth create enormous stresses on the lithospheric rock that sits just under the crust.  The lithosphere is not entirely solid, and it's not entirely liquid.  You can consider it like a material like putty, where if you pull it slowly it will stretch and deform; if you pull it quickly it might break.  Stresses on the lithosphere sometimes cause it to break or fracture.  The result is an earthquake.  Places where the lithosphere breaks allow channels by which material that is molten from the interior can emerge.  When the molten material is under the Earth’s surface it’s called magma.  When it appears on the Earth's surface its called lava.  This is the basis of volcanism.  Volcanoes occur where the lithosphere stretches or breaks or occasionally where it is thin.  The volcanic islands that define the Hawaiian chain are places where the lithosphere is relatively thin, and magma can punch its way out though the surface.  The volcanoes of the Earth trace exactly the plates of plate tectonics.

Volcanism

Transcript: The Earth is a geologically active place driven by energy release from radioactive decay deep in the interior of the Earth.  If you look at a map of the location of earthquakes and volcanoes, they trace out the geological plates exactly.  For example, volcanoes and earthquakes can be traced down the western coast of North and South America, across to the Asian continent, down the coast of Japan to Southeast Asia, and then across to New Zealand.  This is the Pacific Plate.  At the junctures of these plates geological activity occurs.  Examples of two plates that are moving apart are Iceland, the formation of new volcanic islands.  Examples of two plates moving alongside each other in a sheer pattern is California, the San Andres Fault.  Two plates in collision, the Himalayan Mountains.  Sometimes one plate will go under another, subduction; this is the cause of the raising of the Andes chain of mountains in South America.  All of these examples indicate that the Earth is a restless place, and many of the most dramatic geological features on the Earth are relatively young, only a few hundred million years old.

Geological Activity

Transcript: Every child who's stared at a map of the Earth has noticed that the continents seem to fit together like pieces of a jigsaw puzzle.  Could this just be chance that they look this way?  For many years scientists did not know.  In the early 1900's German geophysicist Alfred Wegener speculated that the continents had drifted over a long period of time to their present positions.  This was an outrageous hypothesis at the time.  Nobody could understand possibly how continent size plates of the lithosphere could move around the surface of the Earth.  Wegener was hounded, and his theory was not accepted.  Unfortunately for Wegener he was right, but the evidence to show that plate tectonics occur and that continental drift was real only happened after his death.  Now we have direct evidence that the continents move from very careful motion measurements that show that the continents move up to 10 centimeters per year.  And we also have indirect measurement from the spreading of the sea floor to see that the plates are actually drifting, and the continents are actually moving due to geological activity within the Earth.

Plate Tectonics

Transcript: There are three basic types of rocks found on the Earth.  The first are called igneous rocks.  These form from molten material that is cooled and solidified.  They are very common in the rocks of the Earth's surface.  Granite and basalt are two examples.  Earth is a restless place, and the second major type is called sedimentary rocks.  You might think that wind, sun, and rain can have little effect on something as hard as a rock, but acting over millions of years, erosion can take an enormous toll on a surface of a rock.  Wind and rain cause erosion, and erosion acting over millions of years breaks down rocks and turns it into small particulate matter that is washed by rain and rivers down to the ocean and into stream beds.  These layers, as they're deposited and compressed, create sedimentary rocks.  Two examples are sandstone and limestone.  The third major type of rocks are metamorphic rocks.  Earth is a restless place in its interior as well, and very few of Earth's rocks have remained unchanged for the history of the Earth.  Metamorphic processes occur when igneous or sedimentary rocks are modified by extreme heat or pressure or sometimes hot water in the presence of acidic solvents.  These materials fundamentally change the structure of the rock.  Marble is an example.

Rock Types

Transcript: The rocks of the Earth that we are most familiar with are those right at the surface of the crust.  The deepest mine we’ve every drilled on the Earth is only about 10 or 12 kilometers deep, doesn't even penetrate half the thickness of the crust.  Occasionally rocks from deeper down are brought to the surface of the Earth by convection or uplifting processes.  Most of the common low density minerals found in the crust are called feldspar, which is a form of molten lava that’s floated to the surface.  The rocks that include feldspar are basalt, which is the composition of a large part of the Earth and Moon’s crust, and granite.  Granite and basalt are actually low density materials, although you may not think this when you hold a lump of granite in your hand.  The material deep below the Earth’s crust is much denser because it's mostly metallic.  The crust has mostly silicates with a composition that’s nearly 50 percent oxygen and nearly 25 percent silicon and very small amounts of metals, eight percent aluminum, six percent iron, whereas the core, the iron-nickel core, both liquid and solid, occupy more than a half of the Earth’s radius and almost purely metallic consisting of an alloy of iron plus nickel.

Rocks of the Earth

Transcript: The earth has a number of layers, and its structure changes quite dramatically from the surface to the core.  The top layer is the rocky outer crust, about 30 kilometers thick.  Underneath the crust is the lithosphere, a semi-liquid rock layer that the crust floats on.  Below that is the thick mantle mostly made of rocks containing silicon and oxygen.  Inside that is the outer core, a liquid region of high pressure, high temperature iron and nickel.  At the very interior is the solid core primarily of iron and nickel.  This dramatic change in structure within the Earth is caused by the process of differentiation whereby heavier atoms or compounds sink to the center under the force of gravity because the Earth has been liquid or semi-liquid for much of its life.  This differentiation process is what happens in a smelter when the scum layer floats to the top and the heavy metal floats to the bottom.  You can simulate differentiation by filling a large jar with a mixture of ball bearings and marbles of same size.  If you stir the jar for long enough you will notice that the ball bearings will end up at the bottom while the marbles end up at the top.  This is an example of differentiation.

Layers of the Earth

Transcript: Radioactivity is the process by which we've estimated the age of the Earth.  It's also crucial in understanding how the Earth works and the source of geological activity.  The interior rocks of the Earth contain small concentrations of radioactive elements or compounds.  The radioactive decay process releases energy in the forms of particles and radiation.  This energy suffuses through the rock and melts the rock driving geological activity.  In any sufficiently massive planet, radioactive decay of the interior rocks causes geological activity.  Thus leads to a situation were the surface of a planet can be solid while parts of the interior are liquid.  In a sense radioactivity is the engine that drives geological activity.

Interior Structure

Transcript: With the radioactive decay process we can make an estimate for the age of the Earth.  However it's a difficult technique to apply reliably.  The technique only works if the parent and daughter atoms in the decay from one element to another or to an isotope are trapped in the same location.  So really radioactive decay only works as an age estimate after the last time that a rock solidified.  So we need to find rocks that are very old and have not melted or re-solidified.  Such rocks are very rare on the surface of the earth.  If you picked up a typical rock anywhere you might travel, it would only be one or two hundred million years old.  Geologists, however, have worked hard to find places, typically in the Antarctic plateau, in Greenland, and a few other places near Australia, where igneous rocks are preserved from the earliest phases of the Earth's existence.  The oldest rocks measured in this way give an age of about 4.0 billion years.  To this we add the age of the Earth when it was essentially molten to deduce an age of 4.5 or 4.6 billion years.  These age estimates on Earth were confirmed when we brought back moon rocks through the Apollo missions.  The moon does not have geological activity.  Moon rocks were measured to be 4.0 billion years in general with some chips as old as 4.4 or 4.5 billion years.

Earth's Oldest Rocks

Transcript: The age of the Earth is estimated via the fundamental physical process of radioactive decay.  It's possible to measure the age of things on the Earth by, say, counting tree rings, but the oldest ages are only several thousand years.  Geologists can use the sedimentation record of rock and ice to estimate ages, but the Earth is a restless place having resurfaced itself every hundred million years or so through out its history.  So it's hard to measure ages in more than millions of years with sedimentation.  Radioactive decay of the Earth's oldest rock gives us the most reliable estimate of the Earth's age.  We need a decay process whose half-life is nearly matched to the expected age of the Earth.  The favorite decay processes for this purpose are the decay of potassium-40 to argon-40 with a half-life of 1.25 billion years and the decay of uranium-538 by a series of processes to lead-206 with a half-life of 4.5 billion years.  Using these fundamental techniques geologists and astronomers have deduced that the age of the Earth is about 4.5 billions years.

Age of the Earth

Transcript: Radioactive carbon dating is the way we measure the age of formerly living things.  Every living thing takes in carbon either in the food that it eats or absorbing it from the carbon dioxide in the atmosphere.  Most of the carbon from the atmosphere is in the stable form carbon-12, but one in a million molecules of carbon in the atmosphere involve carbon-14, the radioactive form.  The radioactive form of carbon, carbon-14, decays with a half-life of 5700 years to nitrogen-14.  When any living thing dies it stops ingesting carbon, and so the amount of carbon-14 is fixed at a particular value.  The rate at which it changes to nitrogen-14 therefore gives the age of the object.  Carbon dating has been used to date the age of the oldest trees on Earth and has been used, for example, to show that the Shroud of Turin was a fake.  It's routinely used to measure the age of formerly living things.

Carbon Dating

Transcript: The physical process of radioactivity was discovered accidentally in the late nineteenth century by French physicist Antoine Becquerel.  He had left some uranium bearing minerals near a photographic plate in a drawer in his laboratory and came back to find the plate fogged, indicating that particles or emissions had gone from the radioactive material to the photographic plate.  Radioactivity involves the decay spontaneously of an element to another form, either a form of the same element with a different number of neutrons, which is an isotope, or an actual change of the number of protons, in which case it’s a new element.  The parent isotope is the starting point, and the daughter isotope is the end point.  The point about radioactivity that lets people calculate ages with it is the time taken for one half of the original atoms to decay to the daughter form is a fixed number determined purely by atomic processes; it's called the half-life.  The half-life of a radioactive decay process does not depend on any other physical condition, and so if we can measure the starting composition of parent and daughter molecules or atoms and the ending composition we can infer how long has past in the decay process.  This is the way that radioactivity is used to measure ages.

Radioactive Ages

Transcript: Earth, the third planet out from the Sun, is a special place.  We live here, and Earth is still the only place known in the universe to harbor life.  Related to that is the fact that Earth is the only planet in the solar system with an oxygen-rich atmosphere.  It’s the largest and densest of the inner terrestrial planets, about nine thousand miles in diameter, a smooth sphere.  If you could shrink the Earth down to the size of a billiard ball, it would feel as smooth as a billiard ball with a thin sheath-like atmosphere barely 50 kilometers thick.  The age of the Earth is inferred to be about 4.5 billions years from radioactive decay of rocks.

Earth

Transcript: Some of the general principals that relate planets to each other are to do with planetary geology.  For example, larger planets are more likely to have internal geological activity than smaller planets.  This is simply due to the increased mass of a planet increasing the amount of pressure in its interior and increasing the amount of energy released from radioactive elements deep within the core.  These radioactive decay processes release heat which melts the interior material which leads to geological activity.  During planet formation, another process kicks in called differentiation.  This is the method by which heavier elements and materials sink relative to lighter elements and materials.  So when planets form and they are still molten, the physical principal of differentiation will leave the metallic substances, pure elements and compounds, to gravitate toward the center while the lighter rocks end up in the mantel and the crust.

Planetary Geology

Transcript: One of the most exciting things about exploration of the solar system with spacecraft in the last few decades is the discovery that many of the objects in the solar system have their own peculiar characteristics that make them very interesting to study.  It's as if some of the moons and planets have their own personalities that we can learn through their physical properties.  Regardless of these distinctive features, one of the most important things to remember when studying the solar system is called comparative planetology, the idea that there are general features of the planets that can be explained by simple physical principles.  For instance, it's easy to see that the inner set of planets, Mercury, Venus, Earth, and Mars, are quite different in nature from the outer planets, Jupiter, Saturn, Uranus, and Neptune.  The inner planets are small, dense, and rocky.  The outer planets are large, gaseous, and have low density.  Comparative planetology uses physical principles to relate planets to each other and to give us ideas for expecting what we might find when we start to investigate other planetary systems around other stars.

Comparative Planetology

Transcript: Albert Einstein was the most famous scientist of the twentieth century and perhaps of all time.  The man who invented the theories of special and general relativity was an unconventional scientist who spent most of his career outside the mainstream.  His Greek teacher at high school famously said, “Einstein will never amount to much.” Einstein failed his college entrance exams twice and was only able to get admission to a teachers’ college.  Eventually he worked in the Berne patent office where he gestated his famous theory of relativity.  Einstein won the Nobel Prize in 1905 not for his famous theories of relativity but for his explanation of the photoelectric effect.  Even late in his career Einstein did not take part in the establishment process of science.  He was a loner for most of his life.  He became a cultural icon late in his career when he moved to the United States and worked at the Princeton Institute for Advanced Studies.  He was consulted by the president, Roosevelt, over the issue of the atomic bomb.  Einstein and a number of other scientists strongly petitioned that it not be used and not even be developed.  Einstein died as a cultural icon perhaps the only scientist recognizable to most members of the public.

Einstein

Transcript: In the physics of Isaac Newton, time and space are linear and absolute.  Einstein’s theories of relativity changed forever our notions of time and space.  In Einstein's special theory of relativity the equivalence of mass and energy and the idea that the speed of light is a fundamental constant means that time and space must be supple.  Time can slow down due to high speed travel near the velocity of light.  Masses can increase, and physical lengths can actually contract.  These effects are real and observed in physics labs every day.  In the general theory of relativity we have the idea that mass curves space.  We can see this because photons must have an equivalent mass, and so photons in the presence of another gravitating object must be affected by that object.  Mass curves light.

Time and Space

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7. Geology and Physics

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