Earth Facts

The place we call home, Earth is the third rock from the sun and the only planet with known life on it – and lots of it too!

Earth is the third planet from the Sun and the fifth largest in our solar system. It is the only planet that we know of that can house living things, from plants to animals to humans – even though it has not always been able to do so.

Planet Profile

orbit: 149,600,000 km (1.00 AU) from Sun
diameter: 12,756.3 km
mass: 5.972e24 kg

History of Earth

Earth is the only planet whose English name does not derive from Greek/Roman mythology. The name derives from Old English and Germanic. There are, of course, hundreds of other names for the planet in other languages. In Roman Mythology, the goddess of the Earth was Tellus – the fertile soil (Greek: Gaiaterra mater – Mother Earth).

It was not until the time of Copernicus (the sixteenth century) that it was understood that the Earth is just another planet.

Earth, of course, can be studied without the aid of spacecraft. Nevertheless it was not until the twentieth century that we had maps of the entire planet. Pictures of the planet taken from space are of considerable importance; for example, they are an enormous help in weather prediction and especially in tracking and predicting hurricanes. And they are extraordinarily beautiful.

The Earth is divided into several layers which have distinct chemical and seismic properties (depths in km):

0- 40 Crust
40- 400 Upper mantle
400- 650 Transition region
650-2700 Lower mantle
2700-2890 D” layer
2890-5150 Outer core
5150-6378 Inner core

The crust varies considerably in thickness, it is thinner under the oceans, thicker under the continents. The inner core and crust are solid; the outer core and mantle layers are plastic or semi-fluid. The various layers are separated by discontinuities which are evident in seismic data; the best known of these is the Mohorovicic discontinuity between the crust and upper mantle.

Most of the mass of the Earth is in the mantle, most of the rest in the core; the part we inhabit is a tiny fraction of the whole (values below x10^24 kilograms):

atmosphere = 0.0000051
oceans = 0.0014
crust = 0.026
mantle = 4.043
outer core = 1.835
inner core = 0.09675

The core is probably composed mostly of iron (or nickel/iron) though it is possible that some lighter elements may be present, too. Temperatures at the center of the core may be as high as 7500 K, hotter than the surface of the Sun. The lower mantle is probably mostly silicon, magnesium and oxygen with some iron, calcium and aluminum. The upper mantle is mostly olivene and pyroxene (iron/magnesium silicates), calcium and aluminum. We know most of this only from seismic techniques; samples from the upper mantle arrive at the surface as lava from volcanoes but the majority of the Earth is inaccessible. The crust is primarily quartz (silicon dioxide) and other silicates like feldspar. Taken as a whole, the Earth’s chemical composition (by mass) is:

34.6% Iron
29.5% Oxygen
15.2% Silicon
12.7% Magnesium
2.4% Nickel
1.9% Sulfur
0.05% Titanium

The Earth is the densest major body in the solar system.

The other terrestrial planets probably have similar structures and compositions with some differences: the Moon has at most a small core; Mercury has an extra large core (relative to its diameter); the mantles of Mars and the Moon are much thicker; the Moon and Mercury may not have chemically distinct crusts; Earth may be the only one with distinct inner and outer cores. Note, however, that our knowledge of planetary interiors is mostly theoretical even for the Earth.

Unlike the other terrestrial planets, Earth’s crust is divided into several separate solid plates which float around independently on top of the hot mantle below. The theory that describes this is known as plate tectonics. It is characterized by two major processes: spreading and subduction. Spreading occurs when two plates move away from each other and new crust is created by upwelling magma from below. Subduction occurs when two plates collide and the edge of one dives beneath the other and ends up being destroyed in the mantle. There is also transverse motion at some plate boundaries (i.e. the San Andreas Fault in California) and collisions between continental plates (i.e. India/Eurasia). There are (at present) eight major plates:

  • North American Plate – North America, western North Atlantic and Greenland
  • South American Plate – South America and western South Atlantic
  • Antarctic Plate – Antarctica and the “Southern Ocean”
  • Eurasian Plate – eastern North Atlantic, Europe and Asia except for India
  • African Plate – Africa, eastern South Atlantic and western Indian Ocean
  • Indian-Australian Plate – India, Australia, New Zealand and most of Indian Ocean
  • Nazca Plate – eastern Pacific Ocean adjacent to South America
  • Pacific Plate – most of the Pacific Ocean (and the southern coast of California!)

There are also twenty or more small plates such as the Arabian, Cocos, and Philippine Plates. Earthquakes are much more common at the plate boundaries. Plotting their locations makes it easy to see the plate boundaries.

The Earth’s surface is very young. In the relatively short (by astronomical standards) period of 500,000,000 years or so erosion and tectonic processes destroy and recreate most of the Earth’s surface and thereby eliminate almost all traces of earlier geologic surface history (such as impact craters). Thus the very early history of the Earth has mostly been erased. The Earth is 4.5 to 4.6 billion years old, but the oldest known rocks are about 4 billion years old and rocks older than 3 billion years are rare. The oldest fossils of living organisms are less than 3.9 billion years old. There is no record of the critical period when life was first getting started.

71 Percent of the Earth’s surface is covered with water. Earth is the only planet on which water can exist in liquid form on the surface (though there may be liquid ethane or methane on Titan’s surface and liquid water beneath the surface of Europa). Liquid water is, of course, essential for life as we know it. The heat capacity of the oceans is also very important in keeping the Earth’s temperature relatively stable. Liquid water is also responsible for most of the erosion and weathering of the Earth’s continents, a process unique in the solar system today (though it may have occurred on Mars in the past).

The Earth’s atmosphere is 77% nitrogen, 21% oxygen, with traces of argon, carbon dioxide and water. There was probably a very much larger amount of carbon dioxide in the Earth’s atmosphere when the Earth was first formed, but it has since been almost all incorporated into carbonate rocks and to a lesser extent dissolved into the oceans and consumed by living plants. Plate tectonics and biological processes now maintain a continual flow of carbon dioxide from the atmosphere to these various “sinks” and back again. The tiny amount of carbon dioxide resident in the atmosphere at any time is extremely important to the maintenance of the Earth’s surface temperature via the greenhouse effect. The greenhouse effect raises the average surface temperature about 35 degrees C above what it would otherwise be (from a frigid -21 C to a comfortable +14 C); without it the oceans would freeze and life as we know it would be impossible. (Water vapor is also an important greenhouse gas.)

The presence of free oxygen is quite remarkable from a chemical point of view. Oxygen is a very reactive gas and under “normal” circumstances would quickly combine with other elements. The oxygen in Earth’s atmosphere is produced and maintained by biological processes. Without life there would be no free oxygen.

The interaction of the Earth and the Moon slows the Earth’s rotation by about 2 milliseconds per century. Current research indicates that about 900 million years ago there were 481 18-hour days in a year.

Earth has a modest magnetic field produced by electric currents in the outer core. The interaction of the solar wind, the Earth’s magnetic field and the Earth’s upper atmosphere causes the auroras (see the Interplanetary Medium). Irregularities in these factors cause the magnetic poles to move and even reverse relative to the surface; the geomagnetic north pole is currently located in northern Canada. (The “geomagnetic north pole” is the position on the Earth’s surface directly above the south pole of the Earth’s field.)

The Earth’s magnetic field and its interaction with the solar wind also produce the Van Allen radiation belts, a pair of doughnut shaped rings of ionized gas (or plasma) trapped in orbit around the Earth. The outer belt stretches from 19,000 km in altitude to 41,000 km; the inner belt lies between 13,000 km and 7,600 km in altitude.

Interesting Facts about Earth

  • Earth is the third planet from the Sun. Falling behind Mercury and Venus and in front of Mars.
  • Earth is the only planet in our Solar System that can support life. This is because it is so well protected by its atmosphere, has water on its surface and breathable oxygen levels.
  • Earth is the only planet with water in liquid form on its surface. This takes up 70% of the planet.
  • Earth takes just over 365 days to rotate around the Sun. 365 and a quarter to be exact. That’s why a year is usually 365 days, and every four years we have a leap year with 366 days in it. This extra day is always on the 29th of February.
  • The earth takes 24 hours to turn on its axis. This is why we spend half of the day with daylight when our part of the earth is facing the Sun. And the other half in darkness when we are facing away from it.
  • The Earth is travelling through space at roughly 67000 miles (107,826 kilometres) per hour.
  • The Earth is tilted to a 23.5-degree angle, and this is what gives us seasons. This makes the Earth point in a particular direction as it goes around the Sun meaning throughout a year parts of
  • Earth are different distances away from the Sun.
  • The North Pole always points in the same direction. When it points towards the Sun, it is Summer in the Northern Hemisphere and Winter in the Southern Hemisphere. When it is pointing away from the Sun, this reverses meaning it is the Winter in the Nothern Hemisphere and Summer in the Southern Hemisphere.
  • Earth is the only planet with a breathable level of oxygen – 21%. The rest of the Earths atmosphere is made up of 78% nitrogen with trace amounts of other gases such as carbon dioxide.
  • The reason Earth has plenty of oxygen when other planets don’t is because of the massive amounts of plant life. The plants use up the carbon dioxide during photosynthesis.
  • Earth is the only planet not named after a mythical being. Instead, its name comes from the old English word “Ertha” meaning ground or soil.
  • Planet Earth formed around 4.54 billion years ago. However, it was not how it is today, and life would not have been possible for a very long time. The Earth has been continuously changing since it was formed and is still changing today.
  • It is believed the first life on Earth began in the oceans.
  • Ancient astronomers believed Earth to be the centre of the Solar System and that all other planets orbited around it. In 1543 a Sun-Centered model of the Solar System was published which showed the Sun to be the centre of the Solar system.
  • The Earth has its moon which it is tidally locked to. It rotates in the same amount of time it takes to orbit, so the same side of the moon is always facing the Earth.
  • The Earths rotation is slowing down over time. This is happening very slowly but in around 104 million years a day on Earth will go from being 24 hours to 25 hours.
  • The Earth has a diameter of 7928 miles (12760 kilometres). That is the distance measured around the Equator, which is the line around Earth dividing the northern and southern hemispheres.
  • A very long time ago the Earths water was actually under the surface before volcanic activity brought it to the surface. A recent discovery has shown large volumes of water – enough to fill our oceans three times – are still near the Earth’s core.
  • The highest point on Earth is Mount Everest which is 5.45 miles (8.8km) high.
  • Earth has a lot fewer craters than the surface of most other planets. This is because the planet has lots of volcanoes and earthquakes which cause the highs and lows of the surface to sink or be worn down over time.
  • The Earth has an Ozone Layer. This is a shell that protects the Earth from a lot of the Sun’s harmful ultraviolet rays. These would be damaging to animals, humans and plant life. Life could not continue on Earth without the Ozone Layer.
  • Earth has a very powerful magnetic field. This protects the atmosphere of the planet from solar winds. These solar winds would otherwise strip away our precious ozone layer. It is thought that the lack of atmosphere on Mars is because of the loss of its magnetic field.
  • The Earth’s atmosphere is about 100km thick. The part of this that we can live in, the troposphere, is only about 14km thick. Compare that to the radius of the Earth which is nearly 6,500km.
  • If the Earth were the size of a bowling ball, then its liveable atmosphere would be the thickness of the varnish on its surface. The atmosphere is of vital importance to us. We could not survive without it.
  • Without carbon dioxide, there could be no plant life on Earth. Without the oxygen produced by plants, there could be no animal life. Plants were around (in one form or another) around a billion years before any animal life came into being.
  • Without ozone to absorb harmful ultraviolet rays from the sun, we could not survive.
  • The Earths sky is blue, and this is caused by light from the Sun passing through our atmosphere. It contains all the colours of the visible spectrum ROYGBIV. When this light passes through gas molecules the colours with a longer wavelength (ROYG) more or less pass straight through. Colours with a shorter wavelength (BIV) are absorbed by gas molecules and then reemitted but in any direction. This process is called Rayleigh scattering.

Earth’s Satellite

Earth has only one natural satellite, the Moon. But

  • thousands of small artificial satellites have also been placed in orbit around the Earth.
  • Asteroids 3753 Cruithne and 2002 AA29 have complicated orbital relationships with the Earth; they’re not really moons, the term “companion” is being used. It is somewhat similar to the situation with Saturn’s moons Janus and Epimetheus.
  • Lilith doesn’t exist but it’s an interesting story.
           Distance  Radius    Mass
Satellite  (000 km)   (km)     (kg)
---------  --------  ------  -------
Moon       384         1738  7.35e22

More about Earth and the Moon

Open Issues

  • Our knowledge of the interior of the Earth is derived almost entirely from highly indirect evidence. How can we get more information?
  • How fast is Earth’s core growing?
  • Despite substantial increases in the solar “constant” the average temperature on the surface of the Earth has remained very stable for several billion years. The best theory is that this has been accomplished by varying the amount of carbon dioxide in the atmosphere to regulate the greenhouse effect. But just how is this done? The Gaia Hypothesis asserts that the biosphere actively regulates it. More detailed information about Venus and Mars may provide clues.
  • How much more carbon dioxide can we dump into the Earth’s atmosphere before it ends up like Venus?