EARTH - ALIVE PLANET

The largest of the inner planets, Earth is a gorgeous sight from space. Its swirling white clouds, green and brown continents, and blue oceans make it a sight to behold. Earth is the only planet we know of that has both solid and liquid on its surface. Our planet isn't the only one to have polar ice caps, but the caps on other planets are often made of other things, like the "dry ice" caps on Mars.

CRUST, MANTLE, AND CORE

Earth's surface is constantly changing due to the weather and erosion. The visible surface is part of Earth's crust. Oceans cover about 70 percent of the Earth's surface area. Beneath the crust are the upper and lower mantle, and the outer and inner core. The crust is rich in granite, basalt, and aluminum, the mantle is mostly magnesium and iron, and the core is made of molten nickel and iron oxides. Earth's crust is small compared to the layers underneath; the mantle is the largest part of the Earth, and the core makes up most of the rest. The theory of plate tectonics says that the continents are part of huge plates of rock which float on the molten mantle underneath. If two of the plates collide, the smaller of the two is often thrust under the larger. This process is called subduction. Also, when lava from volcanic eruptions spreads out over the ocean floor, it alters the ocean level of the floor in a process called sea-floor spreading.

ATMOSPHERE

Earth is the only planet known to have intelligent life, and this is largely thanks to Earth's atmosphere which is just right for supporting life as we know it for several reasons. Nitrogen makes up about 80 percent of the atmosphere's volume, and oxygen makes up most of the rest. Traces of carbon dioxide, water vapor, and ozone are also present. It turns out that those little traces of stuff are very important. They absorb infrared radiation and change atmospheric temperatures. The atmosphere is pretty uniform throughout the planet, except at high altitudes, where varies somewhat. The amount of water vapor is not uniform around the planet, though. Places closer to oceans and locations with certain temperatures have more water vapor.

OZONE LAYER

About 20 km above Earth's surface exists a concentration of the chemical ozone (O3). This "ozone layer" is formed when the sun's ultraviolet radiation breaks apart oxygen. Certain chemicals such as the dreaded chloroflourocarbons (CFCs) prevent this reaction from taking place, causing more UV radiation to shine down on the surface instead of breaking apart oxygen.

MAGNETIC FIELD AND IONOSPHERE

The Earth's poles are magnetic, as anyone who's ever used a compass can tell you. But you might not know that the magnetic field, or "magnetosphere," the poles create holds millions of tiny charged particles out in space, spiraling around in a radiation field. Earth also has a layer of electrons called the ionosphere, which are mostly unimportant except that they can interfere with radio signals from satellites or from radiowave astronomers.

THE MOON

Earth is often considered by astronomers to be in a binary planetary system with the Moon. The Moon isn't as big as Earth, of course (about one quarter the size and one eightieth the mass). However, when you consider that most moons are about a thousandth the size of the planets they orbit, it appears gigantic. The Moon is the only place humans have set foot other than Earth.