Summary lecture notes, Section 1 of the course (Chapters 1, 2, 3, and 4).

These notes are merely to high light the points we focused on during class from these chapters. They are a supplement to, not a replacement of, the text book.

DO NOT (YET) PRINT OUT THESE NOTES UNTIL WE HAVE COMPLETED THIS SECTION, SINCE THEY ARE BEING UPDATED AS WE GO THROUGH THE MATERIAL.

A tip: have a look at the various appendices in the book. You will find  a refresher on scientific notation (useful since we are dealing with large or small numbers with many zeros!), a review of some basic math skills you may find helpful, tables with properties of planets,  the periodic table of the elements, and selected astrobiology websites that provide interesting reading and ideas.

Intro - Some wonders of astronomy

What we'd like to find
Maybe not a bad place to start
We look around these objects
This one is easier to find far away
And so is this one
While these can only be found locally
There is a lot of places to look
And a lot of interesting stuff along the way
More here
Wow, a nursery here
Of course we should not only think of our home town
Or how about here?
It does get crowded out there

Chapter 1

What are we searching for?

Apparently, 50% of the people in the US think we have already been visited by aliens. However, we can not go by beliefs; in this course we ask this question scientifically. And scientifically there is no compelling evidence this is the case.

So what do we look for? Well, we'll take anything, from intelligent life to the most primitive bacteria as evidence, but it has to be solid evidence. One thing we need to think about is how to define life. Any ideas?

The scientific considerations:

- Conditions elsewhere in the solar system are not hospitable to advanced life but we have not searched very deeply for primitive life forms.

- We have a very good handle on how many stars suitable to sustain life-bearing planets are out there. We know how long those stars can shine and that it is long enough to develop life.

- We have found that planetary systems around other stars are common enough that we can predict more confidently that there will be planets suitable for life out there. From our solar system we have a pretty good idea what it requires for a planet to be "habitable", and we can identify zones around other stars where conditions would allow for habitable planets.

- From biology, we see that life can evolve over time from primitive to more advanced forms and that it may arise spontaneously. We have evidence for microbial life on Earth dating back 3 to 4 billion years, indicating that life formed early in Earth's history. Thus, it could have happened elsewhere like it did on Earth.

- We have found life on Earth in extreme environments in conditions thought impossible to sustain life, yet it exists; once formed, life seems very capable of surviving extreme conditons (note I didn't say advanced life..).

The nature of the search

"Locally" in our solar system, we can send spacecraft to study in detail the surfaces and atmospheres of the terrestrial planets. We can put landers on the surface and possibly bring back samples to look for life forms. Eventually, we will be able to put astronauts there to explore in more detail.

On larger scales, we can explore distant planets and search for terrestrial analogs with future instrumentation. We can detect signatures of the planets' atmospheres and look for evidence of life (molecular oxygen, water, etc).

We can listen passively for signals from other planets that might prove they have civilizations on them.

With our current understanding of physics, interstellar travel remains very difficult to foresee; we will discuss the prospects and problems.

The most likely places in the Solar System

Planet Mars.

The next planet out from the Sun after Earth is Mars. It orbits the Sun in a little less than 2 years, has seasons like Earth, but is smaller and with much less dense atmosphere. Some pictures:

Mars, overview picture The big valley running down the middle, Valles Marineris, would span the entire width of the US and is quite a big bigger than the Grand Canyon!
 
Mars has polar ice caps and enormous (dormant) volcanoes

The surface of Mars in one of the Viking lander spots

Some of Jupiter's moons.
The planet Jupiter is 5 times further away from Sun than Earth. It is the first of the 4 giant gaseous planets in our solar system. Jupiter itself is an unlikely place to find life (no solid surface, nasty atmospheric composition) but some of its large moons offer promise. Here are a few snapshots.

Jupiter with red spot and shadow of one of its moons

The giant red spot in closeup. Is it bigger than the Earth.

Jupiter's moon Europa is a possible place where life may exist under the ice

Here is another look.

Odds for life outside our solar system, some key discoveries:

 Chapter 2

The key features of this chapter include a discussion of how we obtained our understanding of the solar system over time, and a discussion of how science works. While scientists do not always meet this expectation, in essence, science is a systematic way of inquiry about our world.

It assumes that we can explain phenomena by a restricted set of principles that do not require invoking magic, mysteries, miracles, etc. It assumes there is some sort of logic to how things behave and it tries to discover what that logic is. It tries to simplify the world by discovering the basic principles that operate in many circumstances and that describe phenomena in many different places. Such principles are only considered to be valid or useful if they can make predictions that can be tested, and/or if they help explain past experiments and data. One of the miracles of this process of inquiry has been the realization of the tight connection between our world and mathematics; nature does behave according to mathematical laws, and mathematics is one of the principle tools by which science can operate and advance.

Examples where science has discovered unifying principles:

- mechanics and laws of motion
- gravity
- building blocks of all elements, including molecules of life
- conservation of mass/energy

You have all seen how science operates. We discussed it again in class. Here I want to emphasize the most important point that many people do not seem to understand. What is a "theory" in science? In every day life we use "theory" as something "dismissive", something that is closer to speculation than truth. In science, when one talks about a particular "theory", it refers a very well established and thought-out system of natural laws that explains a lot of observational data. Even today, the difference between these two uses of the word "theory" is not understood, e.g. when people dismiss Darwin's theory of evolution as "just theory".  What others may think of as theory in every day life is what a scientist would call an "guess" at worst, an "educated guess" if it has some plausibility or thinking behind it, or a "hypothesis" if had some scientific thinking behind it to back up the idea. None of these comes close to the meaning of a scientific theory. Even scientists can be guilty in that they may refer to a new framework to explain some phenomena as a "theory" when it hasn't really met the requirements yet to call it that.

A scientific theory is not absolute truth, in that it can be and is being modified as new data come available. However, it is understood what was wrong with the old theory that makes it necessary to replace it. It is not a total new start and rarely a dismissing of the entire old theory. The new theory will build on the old one, and may enlarge the scope of the old theory, or it may replace it while recognizing the strengths and weaknesses of the old theory or theories.  A good example is Einstein's theories of relativity that superceeded Newton's theory of motions and gravity; relativity is radically different in concepts, yet Newton's theory still works fine to send rockets to Mars or describe how airplanes fly, it is just that in certain limits it fails.

There are cases where a new theory is so different that it is seen as a "paradigm" shift, a radical new view of how things happen. Examples of these include the realization that the Earth is very old and that most geological chances happen very slowly, Darwin's theory of evolution, Einstein's relativity, and the discovery of the expansion of the universe which led to the Big Bang theory. Even in these cases though, the older theories have played an important role in laying a foundation and providing guidance for ever more detailed and challenging experiments or observations that ultimately led to their demise.

We will come back to Darwin's theory of evolution in this course. It is a scientific theory, with lots of evidence to back it up.

The following phenomena are distinctly non-scientific "theories" or claims:

- contacts with the deceased
- UFOs
- astrology, horoscopes
- bending or moving matter with the mind
- intelligent design or creationism

They are non-scientific for various reasons. The first 4 do not stand up to scientific scrutiny; the experiments cannot be replicated in controlled conditions or the claims for what was seen or experienced cannot be verified or can be explained in other ways. The 4th one listed falls under "magician's tricks" according to those who have seriously studied the claims. For a good debunking of the questionable see the amazing Randi.

Some characteristics of the non-scientific approach include:

- the answer is assumed to be known and data or theories are
  developed to fit in the existing frame work;
- assumptions or beliefs are adopted that are a principal part
  of particular viewpoint or approach adopted, yet they not
  subject to scientific test or inquiry;
- they rely on the sentimental appeal to convince many
  people there is something to it (e.g. seances, horoscopes,
  magic).
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The key steps in developing our understanding of the solar system and universe

Let us know move to the early discoveries on the solar system leading to the laws of planetary motion and ultimately Newton's laws of motion and law of gravity.

- Early Greek models (Ptolemy and the work of others) of the solar system, which had the Earth at the center and the Sun, Moon, planets and "fixed stars" orbiting Earth. These are called "geocentric models". The Greeks were among the first to use scientific principles to develop models. That is, they tested their predictions against observations of the positions of objects in the sky. They were not right, but very brilliant in many ways and they did not have telescopes to improve their observations. These theories were developed from 500 to 100 B.C.

The Greeks made several important contributions to science:

a. They developed a tradition of thinking and reasoning independent of pre-conceived beliefs; they made thereby the first real attempt to understand nature on its own terms.

b. They developed and applied mathematics, in particular geometry, to interpret and analyze nature (e.g. the motions of objects in the sky).

c. They saw the power of reasoning from observations. So, they realized that if observations contradicted their models, they would have to improve the models. Case in point: retrograde motion of planets.

A model of the sky and the objects in it, needs to explain several phenomena that can be easily observed with the naked eye:

1. daily rising and setting of sun, moon, stars, and planets
2. phases of the moon
3. montly motion of moon with respect to the stars
4. annual motion of the sun with respect to the stars
5. eclipses of the sun and eclipses of the moon
6. motion of planets with respect to the stars
7. Apparently fixed position of the stars on the sky with respect to each other.

The last point leads to dividing the sky into "constellations" which are groupings of stars that suggest some symbol or shape and therefore was given a name. We now know that the stars in one constellation are not at all physically connected with each other, but that it is just a chance superposition in the sky (the actual distances to the stars are all different).

Here are a few constellations you can look for:

Orion
Big Dipper, Cassiopeia, the star Polaris, and the Northern lights

The Greeks had all these observational clues to help them develop their model of the universe (remember, this was more than 1500 years before telescopes were invented):

What happens during eclipses?
- eclipses of the Sun ("solar eclipse")
- eclipse of the Moon ("lunar eclipse")

Total solar eclipses are quite spectacular (picture1, picture2, picture3).

A critical concept to understand is "retrograde motion of planets, in particular Mars".  The simple idea of "nested spheres" on which the Sun, Moon, stars and planets orbit around Earth, does not work very well for the planets. The Greeks introduced "epicycles" to explain the motion of planets, and stuck to circular shapes for all orbits.

Some useful links:
retrograde motion of Mars
Greek explanation for retrograde motion: epicycles

The Greeks also made the critical discovery that the Earth is round, from several lines of reasoning. So, no, it was not Columbus who discovered that, although his and others' voyages helped prove the point.


Their "final" model is called the Ptolomaic Model for the solar system, here is a sketch of what that looked like: Ptolomaic model

Check: visualize how the Ptolemaic model can account for all 7 observations we have listed above.

Ptolemy's model was not perfectly in agreement with the observed positions of planets, but it survived for centuries without dispute. Europe was still in the dark ages, observational tools to get much better data were lacking, and the Christian Church surely advocated a view in which Earth was the center of the universe.

- Nevertheless, without new data, Copernicus (1473-1543) developed the "heliocentric model", which showed the plausibility of a model in which we, with the other planets, orbit the Sun. The Moon does orbit Earth, and together we orbit the Sun. Copernicus published his revolutionary theory in 1543 A.D. The Greeks had thought about such a model, but ruled it out for good scientific reasons. Copernicus had no proof or better data his model was right, but that soon followed through observations of the planet Mars by Tycho Brahe (1546-1601), and a correct model of planetary orbits by Johannes Kepler (1571-1630).  Another key figure was Galileo (1564-1642), make sure to study in the book what he contributed.  Copernicus's model had the correct explanation for retrograde motion, but still the wrong shape for the planetary orbits (circles instead of ellipses).

the Copernican explanation for retrograde motion in the heliocentric universe model

Check: Visualize how all 7 observed points listed above are explained with the Copernican model.

Kepler derived the correct laws for planetary motion and the correct shape of the orbits, but did not have an explanation for why the orbits were as they are. He did show conclusively that the Sun is at the center of the solar system. Kepler discovered the 3 laws of planetary motion:

 Figure for Kepler's first law
Figure for Kepler's second law
Figure demonstrating accuracy of Kepler's third law

Newton (1642-1727) was a genius who discovered the laws of motion and the law of gravity, and together these two sets of laws explained the motions of planets and other objects in the Universe. We still rely today on Newton for most of our general understanding of the laws of motion and the concept of force and acceleration.

Briefly, Newton's laws of motion:

1. An object that is at rest will stay at rest unless acted upon by a net force. An object that is moving at constant velocity will continue to move at that constant velocity unless acted upon by a net force.

2. Force equals mass times acceleration (hence acceleration equals force divided by mass).

With acceleration we mean the rate of change in velocity with time. This change can be in direction (e.g. for an object in circular motion at constant speed we do need a force to keep changing its direction of motion) and/or in magnitude (increasing or decreasing speed).

3. When an object, A, is exerting a force on another object, B, object B exerts an equally large but opposite force on object A (action = reaction law).

These three simple laws have enormous implications for our understanding of motions and interactions among objects. They define the concept of "inertia", the tendency of objects with mass to not change their speed when they are in motion, and to stay at rest when they are at rest. They describe how much force (and energy) is required to accelerate objects (very relevant to e.g. every day driving, flying airplanes, shooting missiles or guns, rocket flight,  etc).

Newton also discovered the "law of gravity", which describes how strong the force of gravity is between two objects, and in which direction it acts.

Force of gravity between two objects with mass M1 and M2 is equal to a constant times the product of the masses divided by their distance squared:

Fg = G * M1 *M2 / R2

Together with the laws of motion, the law of gravity can be used to describe how objects move under gravity; hence how the planets move as they orbit the Sun, and why they move the way they do. Newton's laws produce Kepler's laws; he provided the true explanation for Kepler's results.

Some examples to discuss:

- action = reaction in collisions
- why do objects fall at same rate?
- relevance of Kepler's and Newton's laws for discovering
  extra-solar planets
- inertial mass and gravitational mass
- being weightless in space

Chapter 3

 This is challenging material, since it covers so much ground (or space rather). Some key concepts:
Distances to objects are very large, scales in the universe are beyond anything we can imagine on earth.

Here is a scale comparison:

Imagine the Sun were the size of an orange (10 cm diameter). That makes:



Let's put it another way. Since the distances are so big, astronomers use "light years" as a unit of distance. A light year is the distance light travels in 1 one year in a vacuum. The speed of light is 300,000 km per sec, or about 190,000 miles per second (7 times around Earth in one second!).
A light year then is:

365 days x 24 hours/day x 3600 seconds/hour x 300,000 km/second = 9.5x1012 km.

Using light travel time as our distance measure, we can now more easily express the actual distances in the solar system and universe:
An easy way to remember this is, that if you picture the Sun to have the size of an orange, the universe that we can observe today spans about the size of the solar system. That doesn't mean it couldn't be bigger, it just means that light from more distant objects could not have reached us yet in the time the universe has been in existence.

The box on page 6 in your book gives a glossary of some common astronomical definitions of the objects we distinguish.

Important concept: Look Back Time
Since the light we see from distant galaxies came to us traveling at the speed of light (duh), we actually see distant objects the way they were in the past; our view of the Moon is about 1 second old, our view of the Sun 8 minutes, our view of Pluto 5+ hours, and our view of the Milky Way center about 25,000 years! Likewise, the most distant galaxy we observe we see now the way it was 12.5 billion years ago, when the universe was only 1 billion years old.

Conclusion 1: we do history as well as science!

Conclusion 2: interstellar travel is an enormous challenge!

Conclusion 3: interstellar communication can be a slow process! (makes postal service look pretty good!)

On the plus side, the large size of the universe implies there are literally billions and billions of galaxies in the universe, each containing billions of stars that may have planets like our Solar System, so the chances that life developed elsewhere are quite good from that perspective.


Make sure you review the material in the book on the structure of matter. Important concepts you should understand are:
If any of these is not clear to you, of course ask about it in class!

Big Bang cosmology, the key evidence.
Two-dimensional analog of an expanding universe with no center: the surface of a sphere that is blown up. The distances between all points gets bigger over time, and this is true for any point. There is no center on the surface, all observers on the surface would see the same features.

Einstein showed that matter and energy are equivalent (the famous equation E = M c2). Thus, in the early universe matter was created from energy (light), and even today in high energy physics experiments we can observe the change of light into matter and vice versa. As matter was created from energy in the early universe, gravity began to accumulate the matter to form new objects, since gravity is caused by matter (mass). The first stars formed, and galaxies began to form. We can still observe stars forming in the Milky Way and in other galaxies today; when they form, invariably a disk of material is formed around them from which it is likely that planets are forming. The universe is about 13 billion years old, whereas our Sun, Earth and Moon have all been shown to be about 4.5 billion years old. If the Sun had formed immediately after the Big Bang, we would not have had the heavy elements (carbon, oxygen, silicon, iron, etc etc) required to make the planets as we know them.

In answer to a question: how do we measure what the composition of the early universe was: we can observe the chemical composition of the oldest stars we can find, by analyzing the light they emit, using our big telescope. A spectrum is the detailed breakup of a stars light into all its colors or wavelengths. This is the solar spectrum:
The spectrum of the Sun
All the black lines you see in the solar spectrum come from certain chemical elements. They are like fingerprints that we can use to measure the chemical composition of the Sun. If you get a spectrum of a very old star, you will not find this multitude of lines, but the spectrum is dominated by hydrogen and helium lines only. We can also observe the helium abundance in other stars. We always find, no matter how many stars we look at, that all stars have a lot of helium in them, never less than 23-24%. That implies the helium existed before the star was formed, and Big Bang models predict that helium abundance. It also implies that stars make the other elements through nuclear fusion and when they explode as supernova. We can also study this process by studying many stars and see how their chemical composition is as a function of age, or what materials are brought up to the surface of a star that were made inside the nucleus. Finally, in exploding stars we the formation of heavy elements through the radiation they emit.

Some concluding comments of this first section:

The book goes into more detail on nuclear fusion. The key is that stars are very hot in the center, so hot that matter is compressed into other matter (hydrogen gas in particular) and in that process hydrogen is converted into helium and a lot of energy is released since some mass is converted into energy (the helium that forms has less mass than the 4 hydrogen nuclei (protons) that went into it). Einstein predicted that mass and energy are equivalent, and can be converted into one another. The mass difference between the 4 hydrogen nuclei and the helium nucleus is only 0.7%, yet this is enough to produce enormous amounts of energy and allow the sun to shine for 10 billion years. The energy is equal to (mass difference) * (speed of light squared), the famous equation E= mc2. Billions of hydrogen nuclei are converted into helium and energy all the time in the sun.

The authors discuss how galaxies, stars and planets can form from contracting gas clouds. Make sure you understand the difference between a "galaxy", a "star" and a "planet".

In general, planets orbit around stars, and stars are so massive that they generate their own visible and other light through a process called "nuclear fusion" in the core. A planet has not enough mass to have nuclear fusion in its center (it is not hot enough there). We will see later-on that there are intermediate objects that astronomers call "brown dwarfs" which are stars that are too massive to be called a planet yet did not have enough mass to sustain nuclear fusion of hydrogen gas in their cores.

We observe contracting gas clouds and new stars forming in many places in our Milky Way. The most famous place perhaps is in the Orion Nebula, which you can see on a dark night with no city lights and no moon light with the naked eye (and spectacularly well with binoculars) in the sword of the constellation Orion. We see dust disks around new stars that are forming, and it is likely that the dust disks are the first stage towards formation of planets.

The process of planet formation is discussed in some detail. We distinguish rocky planets (we call them "terrestrial planets" in the solar system) from "jovian planets" which are like Jupiter, hence large gas spheres. The terrestrial planets are found closer to the Sun than the Jovian planets, which has likely to do with how they formed. We will get back to differences between the two.


Chapter 4 - Geology of Earth: the Earth is old

We will focus not on details of the Earth's history, but the chances of finding life elsewhere. Still, there are some key points about Earth's history and geology that are very critical to understand.The text book will give you lots of details. As always, use these notes as a guide to focus your reading and study of certain topics. Also, always ask questions that you may have.

Key aspects in untangling the history of the Earth:

- formation and destruction of mountains and rocks.
- age and properties of Moon; the Moon's surface is old and cratered.
A picture of the lunar surface
This suggests that the Earth had many impacts too in the past (fewer today, but not zero.)
- changes to Earth's surface and atmosphere over time.
- How constant a source of light (energy) is the Sun? How long can the Sun shine? Sun and planets formed together.
- Fossil record as it traces biological evolution and climatic and geological changes on Earth

Concepts: volcanic rock versus sedimentary rock
                 the fossil record
                 erosion
                 plate tectonics
                 radio active dating of rocks
                 catastrophism and uniformitarianism

 GEOLOGICAL HISTORY OF EARTH

The challenge in dating the Earth's crust (and hence the Earth itself) is that due to erosion, volcanism, and plate tectonics, the Earth's surface is continuously changing. Contrast this with the Moon, where the lack of atmosphere and liquid water implies an ever unchanging surface. Lunar rocks have been brought back by the astronauts and dated in the same way as the Earth's rocks; the oldest lunar rocks are indeed over 4 billions years old. See this link for details on the lunar geology. We have also found rocks from Mars on Earth (how can that be?) and have measured their ages.

Here is a discussion of the history of dating the age of the Earth:
Changing Views of the History of the Earth

There are many places on Earth where the effects of plate tectonics and erosion have exposed the older rock layers, showing a gradual build-up of layer upon layer, with the oldest layers at the bottom. The Grand Canyon is a famous example, but we can see similar features in many mountain chains. Note that the age of the rocks is usually much larger than the age of the mountain! Such geological features have been used in the past by e.g. James Hutton to argue for an old Earth. Likewise, Charles Lyell has been advocating an old Earth.

The age of rocks is determined by radio-active dating.

The principle behind this is that rocks contain radio-active elements that over time decay into other elements. A radio-active element is a particular form of certain atoms that are not stable; the nucleus of the atom disintegrates into other atoms. Isotopes are often unstable nuclei. An isotope is a form of the nucleus of an element that has a different number of neutrons in it compared to the normal form of that element. E.g.  14C(arbon) has 6 protons and 8 neutrons in the nucleus, while the stable form 12C(carbon) has 6 protons and 6 neutrons.  By studying the relative amounts of the radio-active materials and their end products, we can find the time the rock was formed. Rocks form through three processes: igneous rock is formed from molten materials that cools down (think of volcanoes), sedimentary rocks are formed by gradual deposition of materials on the surface, metamorphic rocks are structurally transformed by high heat and pressure (so they do not cool down from molton lava, but are transformed deeper in the crust due to the conditions there). Once a rock is formed, its composition is fixed and so then the radio-active clock starts ticking and causes the changes in composition over time.

See information in book on radio-metric dating methods, different types of rocks, plate tectonics, etc.

Some key points:

- radiometric or radioactive dating is the key point in dating rocks. Make sure you understand the basic principle, and the concept of "half-life".

- There are many radio-active elements with different half lives. For dating "young events" we have e.g. the 14C isotope with a half-life of 5730 years. This decays into 14Na (sodium) + electron. Then there is 26Aluminum with a half-life of 700,000 yrs, and 238Uranium with 4.47 billion years. The 238Uranium decays into 208Pb (lead).
How much of a radio active substance is left after 3 half lives have past? (0%, 12.5%, 50%, or 75%)?

- radio active dating is complex and not all rocks can be dated individually. However, the overall picture emerging is very conclusive:

The oldest rocks on earth are about 4 billion years old. Meteorites from space are 4.5 billion years old, giving very consistent numbers. Very detailed physics models of the sun confirm that the sun is 4.5 billion years old (we know more about the sun's interior than the earth's, from studies of solar oscillations, observations of solar neutrinos, and chemical analysis of the outer solar atmosphere).

- The early thoughts centered on CATASTROPHISM. This was inspired by bibical story of Noah's flood and other disasters, and experience of major eruptions or earth quakes that had large effects. However, much of geological evolution happens slowly, a view advocated by Hutton, Lyle, and others, which is called UNIFORMATARIANISM.

The pendulum swings back and forth to some extent, in that we do realize now that many catrastophic changes can occur in short time, for example the creation of the Channeled Scab lands in the NW US. See here for the channeled scablands in Washington state. Likewise, it is possible that in biblical times a major flood did occur in the Middle East. However, there is no evidence of any world-wide flooding of the entire Earth.

Also, mountains are made over long times, by uplifting and collisions of the major plates on earth, or occasional volcanic eruptions. Accurate position measurements across the continents can now measure how much mountains rise every year, e.g. in the Himalaya's, due to plate tectonics.

Most canyons, like the Grand Canyon have been created over millions of years as the rivers cut through the various rock layers. The oldest rocks at the bottom layers of the canyon are of course much older than the canyon itself; it didn't take the Colorado River billions of years to carve the canyon. But it did expose the oldest rock layers which are about 2 billion years old.

Here is a great summary of the geology and age of the Grand Canyon:
The geology of the Grand Canyon (with pictures!)

PLATE TECTONICS

It was Alfred Wegener who proposed in the 1930s that the Earth's continents are drifting apart, based on the similarity in coast lines between Africa and South-America, both in shape and geological features found there. This idea, which was not accepted for many years, is now solidly confirmed through observations of the actual movements of the plates. It accounts for the occurrence of earth quakes and volcanoes, the rise and subduction of plates, the presence of earth encompassing long zones of volcanic or earth quake activity ("ring of fire"), the creation of mountain chains, etc. Read the web based materials and text book to familiarize yourself with this radical idea.

The ultimate "engine" which provides the energy to move continents is the interior heat of the earth, combined with the Earth's rotation, which create convective motions that bring hot material up from the interior to the mantle, and which drive motions of the mantle. Make sure you understand what convection is.

 Some aspects critical to developing and sustaining life on Earth:


An atmosphere can form around a planet due to "outgassing"; originally methane and CO2 gas trapped in rocks at time of formation is released when the rocks are heated or melt inside Earth. Volcanoes are critical. Early bacteria and eventual evolutoin of plant life led to the creation of an oxygen rich atmosphere which is now critical for life as we know it.

Can a planet maintain an atmosphere? The air can escape into space if the planet's gravity is not strong enough and/or the air's temperature is too high. E.g. light elements such as He can escape the Earth's gravitational field at prevailing temperatures since they move faster at a given temperature than heavier elements such as oxygen molecules.
 
The "greenhouse effect" is critical to regulate the difference between day/night temperature on Earth and to maintain a higher temperature than the Earth would have without an atmosphere. The greenhouse effect is the property of the atmosphere that makes it act like a greenhouse (or a parked car): light can get in, heat generated inside cannot get out and so the interior warms up. The atmosphere contains "greenhouse gasses" such as carbon-dioxide (CO2) and water (H2O) that help it act as the glass walls of a greenhouse, in that these molecules do not transmit the infrared light (heat) emitted by the Earth's surface so it cannot escape back into space. The result is a warmer planet. This is a good thing.

Excess emission of "greenhouse gasses", which comes from e.g. burning of fossil fuels, can lead to global warming, which is a global increase in the average temperature on Earth. This may or may not lead to disastrous consequences, as we are about to find out (perhaps...) This may be a bad thing. We don't want to turn into Venus.  Remember, fossil fuels: oil, gas, coal, are all dead plant life and in the burning process we release large amounts of carbondioxide into the atmosphere.

Biggest risk: a run-away greenhouse effect, whereby some warming of Earth leads to emission of more water vapor and carbon-dioxide into the atmosphere, which leads to a yet warmer Earth, which leads to more water and carbon-dioxide into the atmosphere, etc. Note that the oceans and carbonate rocks (e.g. lime stone) contain far more carbon-dioxide now than our atmosphere; we don't want all that released into the atmosphere. The oceans act like a sort of thermostat in possibly helping to regulate carbon-dioxide content of the atmosphere. We don't understand this balance very well.

Discussion point: can growing more trees stop global warming (since the trees will absorb the carbon-dioxide and store it as they grow).

Key point: global warming is not the same as the "ozone hole problem".