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Chart of relative sizes of planets
Physical Orbital Atmosphere
physical characteristics
orbit characteristics
atmosphere characteristics
Surface
Inner Core
Core
Mantle

Earth is the most hospitable place for life in the whole Solar System. It is the third planet orbiting the Sun and has a mean distance from the sun of about 150 billion kilometers. This distance creates suitable temperatures for life and allows for liquid water. The Earth orbits the Sun for 365.24 days or one Earth year. Our day is determined by the Earth turning one rotation on its axis which takes 24 hours and 56 minutes. Since the axis is slightly tilted at 76.5 degrees to the orbital plane, the Sun warms the surface with varying intensity creating the seasons. Seen from space, the Earth looks like a pale blue ball covered with water and clouds. Keep reading to learn more about Earth's surface, atmosphere and exploration.

People often call our Earth the "The Water Planet" since water covers more than two thirds of its surface. The five oceans: Pacific, Atlantic, Indian, Arctic and the Southern surround the seven main continents. The highest peak on the Earth, Mount Everest in the Himalayas, rises 29,028 feet (8,848 km) above the sea level. The Earth's landscape is in constant change. While erosion and humans create small scale changes, plate tectonic activity causes the most significant and dramatic changes. The Earth's crust, located above the metallic core and silicate mantle, is not one unbroken layer like the rest of the solid planets. Instead it is composed of tectonic plates. Continental plates are thicker, lighter and more buoyant than the dense oceanic plates. Both types of plates move on top of the more fluid mantle below. Their movement causes earthquakes, volcanoes, and formation of new land and oceans. Earthquakes and volcanoes typically occur where the plates meet at subduction zones, with the more dense oceanic plates sinking under more buoyant continental plates. One very active area is called the Ring of Fire and is located at the boundaries of the Pacific Plate. When two continental plates collide, they have nowhere else to go, but upward creating mountains. In fact Mt. Everest and the Himalayas are continuing to grow higher because the Eurasian and Indo-Australian Plates are still colliding, pushing the mountains up higher and higher.
Living organisms influence the unique composition of our atmosphere, which is very different from the other planets. It contains 78% nitrogen and 21% oxygen. Other chemical elements like argon, carbon dioxide, neon, helium, hydrogen, methane, and ozone make up less than one percent. However, two of them play a key role. The ozone molecules, which contain three oxygen atoms, shield us from the harmful UV rays. Carbon dioxide causes the greenhouse effect, which currently keeps the Earth at temperatures suitable for life. Earth atmosphere is very dynamic and changes constantly. We see the changes in the seasons, which are due to Earth's slightly tilted axis. The seasonal changes influence the oceans, clouds, winds and cause damaging hurricanes.
Even though it is convenient to study our Earth directly, in recent years spacecraft has started to play a significant role in exploration. Satellites monitor the Earth every minute. Weather satellites study Earth's atmosphere. Data collected by these satellites help to create weather forecasts and track the formation and movement of hurricanes and typhoons. Other satellites search for underground minerals, oil and gas. Using remote sensing, satellites can find large scale features that are difficult to notice from Earth. Satellites can also help detect environmental damages, like the large oil spills and deforestation.
Currently the greenhouse effect on the Earth creates the perfect conditions for life. As we know carbon dioxide is the key element in this process. A billion years ago the Earth, Venus and Mars probably started their history in the same way. They all had plenty of carbon dioxide in their atmospheres and liquid water on the surface. However, on Venus the runaway greenhouse effect boiled away all the water and turned the planet into a baked desert. With Mars the opposite happened. All the water froze on Mars making it a frozen desert. Only on the Earth did everything work out just right to support life. The abundance of carbon dioxide kept the Earth warm supporting the formation of life. The first primitive living organisms appeared about 3.4 billion years ago. They started using carbon dioxide and producing oxygen. About 2 billion years ago, algae filled the oceans and consumed most of the carbon dioxide in the atmosphere. Why didn't the oceans freeze with the lack of carbon dioxide? Freezing did not occur because the Sun became warmer and brighter at the same time. Since then equilibrium has maintained the following two cycles:
  • The sun gets warmer causing water to evaporate and more rain to fall. The rain dissolves carbon dioxide reducing the amount in the atmosphere. The reduction of carbon dioxide weakens the greenhouse effect and the average temperature drops slightly.
  • The sun becomes cooler causing more water to freeze and reducing rainfall. Carbon dioxide levels rise with the reduction of rainfall. Increased carbon dioxide levels create a stronger greenhouse effect and temperatures rise slightly.
However, this fragile equilibrium could easily be destroyed by human activities. Global temperatures have increased about one degree during the last century and will rise another 3 to 9 degrees by the end of this century if we continue to contaminate the atmosphere with carbon dioxide and other chemicals. This will cause the level of the world oceans to rise changing the ocean currents. Such a global change could cause flooding, disease, fire, and even extinction of many species. We must reduce our consumption now and support efforts to stop global warming before it is too late and the changes are irreversible.
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Most of the craters on the Earth are volcanic and only a small fraction is from impacts. There several reason for this. First, because Earth is mostly water, meteoroids fall mainly in the oceans. Second, the small ones simply burn in the atmosphere. Third, erosion and human activities gradually erase the crater traces.

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Earth was heavily bombarded by solar system debris in its early days. Water came to the earth in this debris! Thanks to evaporation of this water, a significant amount of carbon dioxide was dissolved in the oceans and seas allowing for the first living organisms algae and cyanobacteria to evolve about 3.4 billion years ago. These organisms started to consume carbon dioxide and produce oxygen.

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Space is getting so crowded with satellites and space junk around the Earth that safety is a concern. There are currently more than 8,500 large object classified as space junk in orbit around the Earth! The best way to deal with this problem is to burn old satellites in the atmosphere.

Tectonic activity

The earthquakes and formation of mountains, volcanoes and continents that happen as a result of the motion of tectonic plates in relation to one another.

Tectonic plates

Both the continental and oceanic plates that make up the Earth's crust.

Continental plates

Regions of the Earth's outer rocky crust that are divided into huge sections that overlap continents and the shallow sea beds that surround them.

Oceanic plates

The large segments of the Earth's crust the mostly make up the floor of the world's oceans. These plates are commonly made up of solidified basaltic lava and float on top of the Earth's mantle.

Subduction zones

Regions in the Earth's crust where one tectonic plate is being driven underneath another tectonic plate.

Nitrogen

A colorless and odorless gas, and one of the most abundant elements in Earth's atmosphere, and the seventh most abundant element in the Universe.
Nitrogen is a byproduct of the fusion process that occurs in stars.

Oxygen

The third most abundant element in the universe and a key component of the air we breathe.

argon

A colorless and odorless element, it's the third most common gas in the Earth's atmosphere.

carbon dioxide

CO2
A colorless gas consisting of one carbon atom bound to two oxygen atoms.

Neon

A gas that though rare on Earth, is the fifth most common element in the universe.

helium

A colorless and oderless single-atom gas, it is the second most abundant element in the universe.

hydrogen

The most common element in the Universe and the main fuel of stars.

methane

CH4

A colorless, odorless gas and the main component of natural gas.

Ozone

A triatomic molecule, made up of three oxygen atoms.
Ozone is a large part of Earth's ozone layer, which absorbs most of the Sun's damaging ultraviolet light.

Ultraviolet

A type of raditation that's invisible to the human eye, since its wavelenghts are shorter than those of visbile light.
Ultraviolet light can be found in sunlight, and is responsible for sunburns when exposed to it for too long.

Greenhouse effect

The effect in which the gases of a planet's atmosphere, such as carbon dioxide and methane, allow sunlight to pass in and reflect back out, but trap most of the heat energy reflected from the surface of the planet, resulting in an increase in surface temperature.

atmosphere

The gaseous material surrounding planets; the air surrounding Earth.

Runaway greenhouse effect

A condition in which water vapor raises environmental temperatures, which in turn causes more water evaporation and continues the cycle.