Planet Earth

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Earth: Vital Statistics

• Mean radius 6371 km (3981 miles)
• Average distance from Sun 149.6 million km (93 million miles)
• Closest to Sun (perihelion) 147.1 million km (91.5 million miles) about Jan. 3
• Farthest from Sun (aphelion) 152.1 million km (94.5 million) miles about July 4
• Rotates on axis in 24 hrs., revolves around Sun in 365.2564 days.
• Diameter: through equator 12756.26 km (7928 miles); through poles 12713.5 km (7899 miles)
• Difference is due to centrifugal force of earth's rotation (at equator, centrifugal force is about 3/1000 as much as Earth's gravity)
• Surface area: 510 million square km (198 million) square miles, of which about a quarter is land.
• Mass: 5.97x10²⁴ kg or 6,000,000,000,000,000,000,000 (6x10²¹) metric tons
• Of the Earth's mass:
  • about 1/3 is in the core
  • 2/3 in the mantle
  • 4/1000 in the crust
  • 2/10,000 in the oceans
  • 1/1,000,000 in the atmosphere
  • 1/100,000 in ice caps

What Makes the Earth an Unusual Planet

• Life (based on carbon--over 3 million species)
• Atmosphere (79% nitrogen, 20% oxygen, 1% argon, variable amounts of water vapor, carbon dioxide)
• Presence of Large Amounts of Liquid Water
• Very Active Geologically
  • Active erosion by water
  • Plate tectonics--metamorphism, deformation, granitic rocks
  • Few impact craters visible
• Strong Magnetic Field for Such a Small Planet
• Has an Unusually Large Satellite in Proportion to Its Size

Overview of Earth Systems

The three main components of the Earth are the atmosphere, its gaseous envelope, the hydrosphere, the surface coating of water, and of course, the solid earth. All three are subdivided into subsystems. The atmosphere and hydrosphere get their energy mostly from the Sun, and the solid earth gets its energy from internal heat, some of which is produced by radioactive decay and some is left over from the formation of the earth. A tiny amount of energy also comes from gravitational interactions between the Earth, the Moon, and the Sun.

The atmosphere is driven by unequal solar heating: unequal heating of land and water, unequal heating between day and night, and unequal seasonal heating. The hydrosphere is partly driven by winds, which drive ocean currents, but the hydrosphere is also the earth's principal heat storage system. Evaporation of water from the hydrosphere, its transport by the atmosphere, its eventual condensation as rain or snow, and its eventual return to the oceans make up the hydrologic cycle.

The earth's internal heat causes hot material to rise and cool material to sink in the earth's interior, and this movement causes large slabs of the outer rigid crust of the earth to move around.

The hydrologic cycle modifies the surface of the earth. Water breaks rocks down chemically and mechanically, a process called weathering. Water flowing on the surface carries loose material with it, a process called erosion.

Plate tectonics drives geologic processes like mountain building, earthquakes, and volcanism. It also contributes to heating and melting of rocks.

Crustal movements uplift or lower the crust. Meanwhile weathering and erosion wear down mountains and transport the debris to lower areas. Between the two sets of processes, the earth's landscapes are created - and destroyed. Today's landscape is the modified remains of yesterday's landscapes, and landscapes of millions of years ago are lost beyond recovery.

The destruction of rocks by surface processes and their modification by subterranean heat and pressure results in a constant recycling of rocks called the rock cycle. Igneous rocks, the result of melting, are eventually exposed on the surface by uplift and erosion. They are broken down by weathering, and the debris transported and deposited to make sedimentary rocks. These in turn can be buried, changed by heat and pressure, and become metamorphic rocks. Some can even be heated to the point of melting and creating a new generation of igneous rocks.

Finally, extraterrestrial disturbances can disrupt earth systems. Large impacts create dramatic geologic effects near the impact, but also can cause global climatic and environmental effects. If a star within a few light years of the earth were to go supernova, the radiation effects on earth's life could be dramatic. Close approaches of other stars to the Sun can cause distant comets to drop into the inner solar system, possibly increasing the risk of impacts on Earth.

Earth's Subsystems

The diagram above shows the main components of the earth:

Solid Earth

The solid earth consists of concentric shells that differ chemically and mechanically. The middle shell, the mantle, can be defined differently depending on whether we use chemical or physical definitions.

The chemically defined shells are:

• The crust, ranging from 5 km thick under the oceans to 40 km thick under the continents and up to 80 km thick under mountain ranges. The crust is rich in elements like potassium that don't fit easily in the minerals that make up the deep earth. The rocks of the crust are the result of accumulating these elements over time. The crust makes up about 0.3% of the Earth's mass.
• The mantle, made of iron and magnesium silicates. The mantle makes up 2/3 of the mass and 5/6 of the volume of the earth. When we're talking about the chemistry of the earth, we generally use this definition of the mantle.
• The core, mostly iron and nickel. The core is very nearly the size of the planet Mars, a bit over half the diameter of the Earth. It makes up 1/6 of the volume of the earth and 1/3 of its mass.

The mechanically defined shells are:

• A thin outer layer, the lithosphere, is about 100 km thick. In comparison to the whole earth, it is about as thick as the skin on an apple. The lithosphere consists of the crust and some of the upper mantle. This part of the lithosphere is often called lithospheric mantle. The lithosphere is what makes up the tectonic plates.
• Mechanically, the mantle begins with the asthenosphere, a thin, weak, partially molten layer. The asthenosphere is the lubricating layer over which the plates slide. The remainder of the mantle is solid but flows. It consists of several layers that probably mark where minerals are compressed into denser forms by the great pressure.
• The core is the same as the chemically defined core. Temperatures in the core probably rival the surface of the Sun, but because of the enormous pressure, the outer core is liquid and the inner core (a bit smaller than the Moon or about a quarter of the earth's diameter) is solid. Currents in the outer core probably generate earth's magnetic field.

Biosphere

The domain of life, from several kilometers deep in the lithosphere to 10 km or so above the surface.

Hydrosphere

The zone of liquid water on the earth, dominated by the oceans but also including lakes and rivers and liquid underground water.

Cryosphere

The zone of frozen water on the earth, including the Antarctic and Greenland ice caps, glaciers, and permanently frozen ground (permafrost). The cryosphere is often considered part of the hydrosphere.

Atmosphere

• The bottom 10 km or so is the troposphere, from a Greek word for "turning." The sun warms the surface, but warm air rises, and as it rises, it expands and cools. Then the cool air sinks. This constant churning creates the weather.
• Above 10 km or so, the air is too thin for warm air from the surface to continue rising. The next layer, the stratosphere, is stable and stratified, with cold air at the base and warm air above. Because it is so stable, commercial aircraft fly at the lower boundary of the stratosphere. Military aircraft and the supersonic airliner the Concorde fly within the stratosphere. Near the top of the stratosphere, solar ultraviolet causes oxygen to form ozone (O₃). Ozone absorbs solar ultraviolet and helps protect the surface.
• From 50-80 km, the temperature of the atmosphere again begins to fall. This layer is the mesosphere. At about 80 km we find the coldest temperatures in the atmosphere, about -95 C. In the mesosphere, solar radiation and particles create electrically charged atoms and this screen of charged atoms (the ionosphere) absorbs radio signals. However, at night most of the charged atoms recombine with stray electrons and the remaining charged atoms settle into fairly smooth layers that reflect radio signals. This is why at night you can often pick up AM stations a thousand miles or more away.
• Above 80 km temperatures again increase. The air is so thin that atoms are accelerated by sunlight and solar particles. They are moving very fast, so their temperature is very high, but there are so few atoms that a spacecraft passing through this layer, the thermosphere, feels no appreciable heat. The thermosphere tapers off into space, but traces of atmosphere extend 1000 km or more above the earth.

Convection in the Earth

Convection is the transportation of heat by moving hot or cold material from place to place. Convection works because warm material is light and rises, while cool material is denser and sinks. As long as there is a temperature difference with depth, there will be a cycle of rising and sinking material. A lava lamp is a perfect illustration of convection - if geophysicists, astronomers, and other scientists who teach about convection had their way, lava lamps would never go out of style.

In reality, there is a continuous cycle. Material at the base of the mantle becomes hot and rises. As it rises, it expands and cools, and near the surface, heat leaks through the crust and escapes. Cooled mantle material begins sinking and descends to the bottom of the mantle, to be heated again.

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Created 30 July 2008, Last Update 22 September 2008

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