Lecture 3 - The Early Earth
Lisa Tauxe

Reading: Chapters 1 and 2, Blue Planet

Making the building blocks for the Earth

In the last lecture, we discussed the first three minutes of the universe in which all of the hydrogen and most of the helium were created. Some time after that, matter condensed into giant clouds called nebulae perhaps similar to those seen by the Hubble Telescope here. Stars condensed in the nebulae and began to build helium from hydrogen atoms in a nuclear fusion process. Fusion of hydrogen to produce helium releases a huge amount of energy, which allows the process to continue.

Check out this link on stellar evolution .

When sufficient helium has been produced, the core of the star becomes massive enough to collapse, generating enough heat to begin "burning" of helium to produce carbon. If the star is big enough, temperatures can be reached that are high enough to create iron in the core of stars.

Elements heavier than iron take more energy to make than is released in the process, so iron production is the "end of the line" for nuclear fusion. Small stars never get this far anyway, but really big ones do. When a really big star starts making iron, the core becomes so heavy that the star collapses. Then, it either collapses further to become a "black hole" or explodes in a fantastic displays called supernovas.

Super novas

All the elements heavier than iron are made in an entirely different way than the slow buring in stellar nuclear reactors. They were made during the rapid explosion of super novas. When large stars have burned enough iron to become really dense, they collapse in upon themselves. If they are super massive, the star contracts into a black hole. Slightly smaller stars explode into incredible super novas like the famous hour-glass supernova.

Early Solar System

After some 5-10 billion years of stars forming and dying, a cloud of star dust formed and began to collapse. This so-called "solar nebula" was the womb of our solar system. The cosmic dust contained all the elements from which the sun and the planets are made and probably was a mixture of the exhaust from many supernova explosions. Here are some more examples of star forming areas in the universe: baby stars .

The cloud of gas and dust collapsed to disk and then into the sun and the planets. The early solar system was a a terribly violent place, as revealed by the scarred surface of the moon. Although collisions in the solar system still occur (meteroites, for example), there were a lot more collisions in the early solar system. One such collision is thought to have created the moon as shown here:

Creation of the moon from a collision of the earth with a  Mars sized body.

What you are supposed to see here is a large "impactor" slamming into the Earth. This kicks up a lot of dust. The heavy material (e.g. iron) falls back to the Earth and sinks to form the core. Lighter stuff (similar to the rocky part of the Earth) collects together to make the moon. After about a day, you see the Earth, with a heavy core and the "proto-moon" which is composed of "impactor " material.

After this collision, the Earth-moon system had the major features that we recognize today.

Structure of the Earth

Where is the Earth in the solar system:

Check out this fantastic link: Solar System Live

It shows where the state of the solar system NOW.

Some facts and figures

Earth Statistics (from here )
 Mass (kg) 5.976e+24 
 Mass (Earth = 1) 1.0000e+00 
 Equatorial radius (km) 6,378.14 
 Equatorial radius (Earth = 1) 1.0000e+00 
 Mean density (gm/cm^3) 5.515 
 Mean distance from the Sun (km) 149,600,000 
 Mean distance from the Sun (Earth = 1) 1.0000 
 Rotational period (days) 0.99727 
 Rotational period (hours) 23.9345 
 Orbital period (days) 365.256 
 Mean orbital velocity (km/sec) 29.79 
 Orbital eccentricity 0.0167 
 Tilt of axis (degrees) 23.45 
 Orbital inclination (degrees) 0.000 
 Equatorial escape velocity (km/sec) 11.18 
 Equatorial surface gravity (m/sec^2) 9.78 
 Visual geometric albedo 0.37 
 Mean surface temperature 15°C 
 Atmospheric pressure (bars) 1.013 
 Atmospheric composition


Composition of the Earth

The composition of the Earth has been figured out through a variety of methods. The deepest drill hole is about 12 km deep and cost over $100 million. Volcanoes bring up foreign rocks (known as xenoliths) from several hundred km depth. Since the Earth is some 12,000 km in radius, our knowledge of the upper 200 km may be completely irrelevant.

Nonetheless, we are fairly confident of we know what the Earth is made of because we can determine the chemistry of meteorites born of the same womb, hence can be reasonably expected to have the same chemistry. From analysis of these meteorites (and also the composition of the Sun reveals that the Earth is made up of Iron (35%), Oxygen (30%), Silicon (15%) Magnesium (13%) and traces of the other elements.

If you pick up your ordinary everyday average rock, however, you will immediately notice that it can't be 35% iron. In fact there is no where near that much iron in the crust in general. Looking at Figure 1.8 in the text, you will see that in fact the crust (which we can study directly) has only a little Iron (6%). This means that the iron must be "hidden".

From the orbits of satellites, we know that the Earth has a very dense core. We therefore surmise that the iron that the Earth was born with migrated at some point down to the core. Iron and a little nickel (which is also missing in the crust) together have the right density to explain the density structure of Earth.

Thus, the Earth is far from a homogeneous body but has differentiated into a metallic core (radius of about 6000 km) and a more rocky mantle. We walk about on the less dense froth ejected from the mantle known as the crust which varies in thickness from about 5km to about 60 km.

Earthquakes and the Earth's interior

Earth Structure

Most of what we know about the Earth comes from seismology, or the study of earthquakes. When the Earth breaks, it quakes and sometimes generates sound waves strong enough to blow houses down. There are special "listening" stations that record these sound waves called "seismometers". Large earthquakes can be "heard" the world over and these seismic messages have traveled through the Earth. Seismic waves tell us about the density structure of the Earth and what is liquid and what is solid. From these data we know that the Earth has a very thin outer crust (from 5 to 100km or so). Below this is a rocky mantle, composed of dense black rocks. About 3000 km we encounter a totally different world - a liquid iron core. At the very center of the Earth, the there is a solid iron core.

Gravity of the Earth

Measuring gravity also tells us something about the structure of the Earth. At first sight it might seem boring. gravity points pretty much down But! For one thing, the gravity of the earth tells us that it has a very dense core. Much denser than would be expected if it was made of rock. In fact, from this we know it is mostly iron and nickel.

Early workers expected to find a huge gravity anomaly from the attraction of big mountains, but found none. no big anomaly from mountains From this we know that mountains have low-density roots. But again! Looking at tiny deviations in the gravity field, workers such as your professor Dave Sandwell produced such pretty maps as this.

Earth's magnetic field

The magnetic field of the Earth is more or less like that that would be produced if there were a giant bar magnet at the center of the Earth. This image came from here.

One amazing fact is that the Earth's field sometimes switches polarity and compasses would point South! This happened last about 780,000 years ago and could happen again any millenia now. These facts confirm the existence of a fluid outer core made of pure metal. It is the motions in this core that produce the magnetic field.

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Lisa Tauxe