# Solar System, technical/Earth

School of Earth science > Earth
Earth is the third planet from the Sun, and the largest of the four terrestrial planets in the Solar System in terms of diameter, mass and density. It is also referred to as the World, the Blue Planet, and Terra.

## Earth-Moon system

"This is the first image of Earth ever taken from another planet that actually shows our home as a planetary disk. Because Earth and the Moon are closer to the Sun than Mars, they exhibit phases, just as the Moon, Venus, and Mercury do when viewed from Earth. As seen from Mars by MGS on 8 May 2003 at 13:00 GMT (6:00 AM PDT), Earth and the Moon appeared in the evening sky. The MOC Earth/Moon image has been specially processed to allow both Earth (with an apparent magnitude of -2.5) and the much darker Moon (with an apparent magnitude of +0.9) to be visible together. The bright area at the top of the image of Earth is cloud cover over central and eastern North America. Below that, a darker area includes Central America and the Gulf of Mexico. The bright feature near the center-right of the crescent Earth consists of clouds over northern South America. The image also shows the Earth-facing hemisphere of the Moon, since the Moon was on the far side of Earth as viewed from Mars. The slightly lighter tone of the lower portion of the image of the Moon results from the large and conspicuous ray system associated with the crater Tycho."

## Spheres

The Earth, like that of the other terrestrial planets, is chemically divided into layers or spheres. Two sets of spheres have been defined: one set above the surface and one set below. Besides the layers above and below the surface other spheres have been identified such as the hydrosphere, the biosphere and the magnetosphere.

## Above the surface

The Earth's atmosphere is a layer of gases surrounding the planet that is retained by the Earth's gravity. The fuzzy boundary between the atmosphere and outer space is called the exosphere. As we descend toward the surface, we pass through the ionosphere, which includes the exosphere and the thermosphere. Then we go through increasingly dense layers, the mesosphere, the stratosphere and finally the troposphere which contains the gaseous elements and water vapour that sustain life on the planetary surface. The average atmosperic pressure at sea level is 101.325 kPa (1013.25 mbar, or hPa) or 29.921 inches of mercury (inHg) or 760 millimeters (mmHg) of mercury. See International Standard Atmosphere for much more information about how atmospheric pressure is calculated and standardized.

## Atmospheric astronomy

Def. "[t]he lower levels of the atmosphere extending from [a solid or liquid] surface up to the tropopause ... characterized by convective air movements and a large vertical temperature change" is called a troposphere.
Def. "[t]he zone of transition between the troposphere and the stratosphere" is called a tropopause.
Def. "[t]he region ... where temperature increases along with the altitude ... [extending] from the tropopause ... to ... the mesosphere" is called a stratosphere.
Def. the region where "temperature decreases with increasing height" is called a mesosphere.
Def. "[t]he upper boundary of the mesosphere, which can be the coldest naturally occurring place [in the atmosphere of an astronomical object]" is called a mesopause.
Def. a layer within which "ultraviolet radiation (UV) causes ionization ... residual atmospheric gases sort into strata according to molecular mass ... temperatures increase with altitude ... [r]adiation causes ... this layer to become electrically charged" is called a thermosphere.

### Ionosphere

Upon reaching the top of the mesosphere, the temperature starts to rise, but air pressure continues to fall. This is the beginning of the ionosphere, a region dominated by chemical ions. Many of them are the same chemicals such as nitrogen and oxygen in the atmosphere below, but an ever increasing number are hydrogen ions (protons) and helium ions. These can be detected by an ion spectrometer. The process of ionization removes one or more electrons from a neutral atom to yield a variety of ions depending on the chemical element species and incidence of sufficient energy to remove the electrons.
"The ionosphere is a shell of electrons and electrically charged atoms and molecules that surrounds the Earth, stretching from a height of about 50 km to more than 1000 km."
"As a spacecraft travels through the solar system, a targeted radio signal sent back to Earth can be aimed through the ionosphere of a nearby planet. Plasma in the ionosphere causes small but detectable changes in the signal that allow scientists to learn about the upper atmosphere."

### Exosphere

Def. "a ... volume ... [with] molecules ... where the density is too low for them to behave as a gas by colliding with each other" is called an exosphere.
Into the exosphere or outer space, temperature rises from around 1,500°C (centigrade) to upwards of 100,000 K (kelvin).

## Climatology

Climatology "is the study of climate, scientifically defined as weather conditions averaged over a period of time, and is a branch of the atmospheric sciences."
"Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and numerous other meteorological elements in a given region over long periods of time."
The map at right shows the global long term monthly average surface air temperature for Earth.
The zones of Earth's climates, going from the equator upward (and downward) are Tropical, Dry, Moderate, Continental and Polar. There are subzones within these zones.

## Below the surface

Geology of the interior of Earth:
1. continental crust
2. oceanic crust
3. upper mantle
4. lower mantle
5. outer core
6. inner core
A: Mohorovičić discontinuity (Moho boundary)
B: Gutenberg Discontinuity
C: Lehmann discontinuity (inner core-outer core boundary)
The Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. The Inner Core of the earth is believed to be composed primarily of a nickel-iron alloy, with very small amounts of some other elements. The Outer Core of the earth is a liquid layer made of iron and nickel. [factual?]

## On and near the surface

On or near the surface of the Earth are oceans and landforms.

## Rocky objects

"In the latest map, as many as 20,000 previously unknown seamounts, between 1.5 and 2 kilometres high, pop into view scattered along relatively young sections of the sea floor."
"The gravity model of the North Atlantic ocean basin [above] reveals tectonic history in sharp detail. [The red] dots show the [locations] of past earthquakes of magnitude 5.5 or higher."

## Crater astronomy

In the image at left is an aerial view of the Barringer Meteor Crater about 69 km east of Flagstaff, Arizona USA. Although similar to the aerial view of the Soudan crater, the Barringer Meteor Crater appears angular at the farthest ends rather than round.
"Meteor Crater is a meteorite impact crater approximately 43 miles (69 km) east of Flagstaff, near Winslow in the northern Arizona desert of the United States. Because the US Department of the Interior Division of Names commonly recognizes names of natural features derived from the nearest post office, the feature acquired the name of "Meteor Crater" from the nearby post office named Meteor. The site was formerly known as the Canyon Diablo Crater, and fragments of the meteorite are officially called the Canyon Diablo Meteorite. Scientists refer to the crater as Barringer Crater in honor of Daniel Barringer, who was first to suggest that it was produced by meteorite impact."
From space the crater appears almost like a square. The image at right has a resolution of 2 meters per pixel, and illumination is from the right. Layers of exposed limestone and sandstone are visible just beneath the crater rim, as are large stone blocks excavated by the impact.
"The Holsinger meteorite is the largest discovered fragment of the meteorite that created Meteor Crater and it is exhibited in the crater visitor center." "The Canyon Diablo meteorite comprises many fragments of the asteroid that impacted at Barringer Crater (Meteor Crater), Arizona, USA. Meteorites have been found around the crater rim, and are named for nearby Canyon Diablo, which lies about three to four miles west of the crater." "There are fragments in the collections of museums around the world including the Field Museum of Natural History in Chicago. The biggest fragment ever found is the Holsinger Meteorite, weighing 639 kg, now on display in the Meteor Crater Visitor Center on the rim of the crater."
Occasionally, objects fall from the sky. When and where this occurs, depending on the energy dumped into the atmosphere and the impact on the crust of the Earth, life forms nearby hear it, feel the vibrations from it, and recoil if the intensity is too high.
But asteroid impacts, though rare, occur once in a while, over very large areas, at aperiodic intervals such as the Chicxulub crater. Most scientists agree that this impact is the cause of the Cretatious-Tertiary Extinction, 65 million years ago (Ma), that marked the sudden extinction of the dinosaurs and the majority of life then on Earth. This shaded relief image of Mexico's Yucatan Peninsula shows a subtle, but unmistakable, indication of the Chicxulub impact crater.
At right is a natural color photograph of the Aurora Borealis or northern lights and the Manicouagan Impact Crater reservoir (foreground) in Quebec, Canada. They are featured in this photograph taken by astronaut Donald R. Pettit, Expedition Six NASA ISS science officer, on board the International Space Station (ISS).