Earth, ThinkSpace 26220

Earth is a very special planet because it is unique in a lot of ways. It is the only planet covered mostly with water, the only planet with an atmosphere mostly made up of nitrogen and oxygen, the only body in the Solar System able to sustain life, and the only inner planet with a large satellite. It is also the largest of the inner planets, with the highest density of all the planets. Aged at about 4,600 million years, humans are fairly new to the scene on Earth. However, Earth is made perfectly for us: with just the right size, mass, and distance from the Sun to handle the heat.

General

The Earth could be summed up as a mass of water, since 71% of the Earth's surface is covered with it. Earth is the only planet on which water can exist in liquid form on the surface. It's strong magnetic field and its multi-layered atmosphere protect the water which is essential for life from space debris and solar radiation.

The Earth's magnetic field is produced by electric currents in the core. Magnetic poles are the result of irregularities in the electric currents, and these poles move relative to the surface. So, right now the north magnetic pole is located in northern Canada. The magnetic field (along with the solar wind) also produce the Van Allen incline3.GIF (2548 bytes)

radiation belts. These two belts are rings of ionized gas (plasma) trapped in orbit around the Earth. The outer belt extends from 19,000 to 41,00 km high, while the inner belt ranges from 13,000 to 7,600 km high.

Right now, the Earth has an inclination of 23°27' (right). However, over 40,000 years the incline is anywhere from 22°1' to 24°5'. The diameter of the Earth along the equator is 12, 750 km, but the Earth is not a perfect sphere (it's more like a pear).

The surface of the Earth is still young. Because of erosion (from our water) and tectonics (to relieve surface stress), it probably looks quite different than it did long ago. Our planet shows no major impact craters or other features from its earliest time (4.6 billion years ago). The oldest known rocks, for example, are around 4 billion years old, and the majority of the Earth's surface is probably only 500 million years old.

History
Earth gets its English name from Old English and Germanic, rather than mythology as the others do. In Roman Mythology, there is a goddess of Earth but she was called Tellus (the goddess of fertile soil). In Greek Mythology it was Gaia, or Mother Earth.

Earth was considered originally to be the center of the universe. However, Copernicus set us straight in the sixteenth century. We now realize the truth: Earth is just another planet rotating around the Sun.

The Earth has been around for a long time, but humans just recently arrived. If you thought of the Earth on a scale of a year, the Earth developed in January, the first life forms still around today appeared around November, and humans didn't arrive until December 31 at 11:59 p.m. Once we arrived, we didn't know much about Earth for a long time yet. Not until the end of the seventeenth century did we discover the other half of our world (at least the Europeans didn't), and not until we developed spacecraft were we able to obtain maps of the entire planet. Now pictures taken from space are very important because they help in weather prediction, especially in tracking and predicting hurricanes.

Composition
From what we can tell, Earth is the only planet with distinct outer and inner cores. However, our knowledge of the interior of planets, even our own, is rather limited. What we do know is that the Earth is divided into layers with special chemical and seismic properties: (as shown below)

Layer	Found (depth in km)	Characteristics
1. Crust	0-40	Solid, but varies in thickness: thinner under the oceans, thicker under continents. Primarily quartz and other silicates (like feldspar).
2. Upper Mantle	40-400	Plastic/semi-fluid consistency. Mostly consists of olivene and pyroxine (iron/magnesium silicates), calcium, and aluminum.
3. Transition Region	400-650	Found between the upper and lower mantles.
4. Lower Mantle	650-2700	Plastic/semi-fluid. Probably mostly silicon, magnesium, and oxygen with some iron, calcium, and aluminum.
5. D" Layer	650-2700	Discontinuities separate the various layers; found in seismic data.
6. Outer Core	2890-5150	Plastic/semi-fluid. Probably contains iron in it (see 'inner core').
7. Inner Core	5150-6378	Solid. Probably made of iron or a combo of iron and nickel, maybe with some lighter elements as well. Temperatures at the center of the core might reach as high as 7500 K, which is hotter than the surface of the Sun.

The information that we have about the inner layers of the Earth are mostly just guesses based on what we have found out by seismic testing. Samples from the upper mantle reach the surface as lava from volcanoes, but the majority of the Earth is not accessible for research (at least not yet). As a whole, Earth is (probably) composed of:

iron		34.6%
oxygen		29.5%
silicon		15.2%
magnesium		12.7%
nickel		2.4%
sulfur		1.9%
titanium		0.05%

The mass of the Earth is mostly concentrated in the mantle, with most of the remainder found in the core. What we inhabit is a very small fraction of the Earth.

DISTRIBUTION OF MASS:
atmosphere		= 0.0000051 x 10²⁴ kg
oceans		= 0.0014 x 10²⁴ kg
crust		= 0.026 x 10²⁴ kg
mantle		= 4.043 x 10²⁴ kg
outer core		= 1.835 x 10²⁴ kg
inner core		= 0.09675 x 10²⁴ kg

Atmosphere

Earth is also special because of the fact that it contains oxygen. The presence of free oxygen is remarkable from the chemical point of view. Oxygen is highly reactive and normally would quickly combine with other elements. The oxygen in our atmosphere is produced and maintained biologically.

So, what is the Earth made up of? The Earth's atmosphere is 77% nitrogen and 21% oxygen. The other 2% is argon, carbon dioxide, and water. Probably, there used to be much more carbon dioxide when Earth was first formed, but since then most of it has gone into the carbonate rocks, while some of it was dissolved into the oceans and consumed by plants. The carbon dioxide level is balanced by plate tectonics and biological processes, which keep it flowing back and forth (between the rocks, plants, and atmosphere). This is very important because it allows for the greenhouse effect. The carbon dioxide works in the atmosphere to raise the average surface temperatures about 35 degrees C above what it would be otherwise, keeping the oceans unfrozen and at the same time protecting us from the rays of the Sun.

Surface
Like it has been mentioned before, Earth is definitely a world of water. It turns out that the actual amount of land is only about the same as the amount of land on Mars. The rocks of Earth come in three types: sedimentary, metamorphic, and igneous. Another major feature of Earth is the mountain chains, many of which originate from the sea-bed. The Mid-Atlantic Ridge is one such mountain chain; it divides the Atlantic Ocean from north to south. The visible part of this chain is Iceland.

The Earth's surface is constantly changing. Erosion is always occurring, earthquakes shake the ground and move things, volcanoes erupt, and natural disasters (fire, flood, hurricanes, icebergs) clear out certain areas. Earthquakes occur because of plate tectonics (see below), volcanoes erupt because of hot magma that wells up from below the surface, and natural disasters occur for many different reasons.

From Space
When the Space Age started in 1957, of course one of the first reactions when in space was to look back to the Earth. What is that planet I'm living on? What does it look like from far away? This interest has continued since 1957. We've had weather satellites monitoring the atmosphere sine 1960, but didn't get any pictures of the entire Earth until 1968. These first pictures were taken with hand-held cameras by the Apollo astronauts.

The next step was to map the resources of the world. So, special satellites took pictures of the Earth in both visible and infra-red light. These pictures can help predict disasters, notice the steps leading up to a flood, and even indicate where mineral deposits may be expected. Such pictures are valuable to us and have become necessary to our prosperity.

Moon

The Moon is the Earth's only natural satellite. It rotates around us while we rotate around the Sun. The Moon is 384 km away, 3476 km across, and has a mass of 7.35 x 10²² kg. The Moon's rotation period is the same as its earthrise2.jpg (25455 bytes)

period of revolution: 27.3 days. This means that the Moon keeps the same face towards the Earth as it passes by. Obviously, the Moon is the closest body in the sky. So, it was the first place we ever visited (see "earthrise" from the Apollo 11, left). It is interesting to know that the interaction of the Earth and Moon slows the Earth's rotation by about two milliseconds per century. Because of this, 900 million years ago there were 481 18-hour days in a year.

Geography

This isn't a geography course, and you probably know generally what the world consists of. However, as a refresher: Earth is covered 2/3 by water. The last 1/3, the land, is divided into seven continents. The seven continents are Asia, Africa, Australia, Europe, North America, South America, and Antarctica. Each continent has several countries on it, except North America which only has two countries (U.S. and Canada) Antarctica is still not completely explored, but all the other continents are explored, mapped, and settled. There you have it: the geography of the world in a paragraph.

Plate Tectonics
Earth is special because it uses a system of plate tectonics. Basically, the crust is divided into several solid plates that float independently. When the plates move around, they create mountains, earthquakes, volcanic eruptions, and other natural occurrences. There are several processes involved with plate tectonics, especially: spreading (plates moving away from each other), subduction (one plate pushing on top of another plate's edge), and faults (sideways collisions)

Although there are nearly twenty small plates, the eight plates that cause most of this ruckus are:

Along the boundaries, the plates push against each other and earthquakes are much more common than in the middle of the plates. Plotting the faults shows the boundaries of the plates (see above). Plate boundaries are also common along mid-ocean ridges, deep-sea trenches, or active mountain belts.