Earth

Earth Facts

• Earth is the third planet from the Sun and the fifth largest:
  • distance from Sun: 149,600,000 km (1.00 AU)
  • diameter: 12,756.3 km
  • mass: 5.976e24 kg

• 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.

• 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 (picture 5) of the entire planet. And 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, to this critic at least (picture 20).

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

  	     0-  40  Crust
  	    10- 400  Upper mantle
  	   400- 650  Transition region
  	   650-2700  Lower mantle
  	  2700-2890  D'' layer (sometimes included in the lower mantle)
  	  2890-5150  Outer core
  	  5150-6378  Inner core       

  The crust is thinner under the oceans, thicker under the continents. The inner core and crust are solid; the outer core and mantle layers are fluid.

• 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. 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 (picture 23):
  • 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-Austrailian Plate - India, Austrailia, 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.

• 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 been erased. The Earth is 4.5 to 4.6 billion years old, but the oldest known rocks are less than 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% 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). This 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 reponsible 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.

• 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. Current research indicates that there were 481 days in a year 900 million years ago.

• Earth has a modest magnetic field produced by electric currents in the 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).

Pictures

1. (above) Africa 404k gif

2. Galileo image of South America 1st flyby 224k gif

3. AVHRR mosaic 545k gif; 8811k gif; html

4. section of Earth from Clementine 82k gif

5. Map projected image of Earth 123k gif
6. Gibbous Earth with SW North America visible 127k gif
7. Mosiac of the Andes mountains in South America taken from Galileo 94k gif
8. Americas by GOES (9/21/86) 391k gif; 583k jpg
9. NOAA satellite's view of USA 322k gif
10. The easternhalf USA viewed by GOES satellite 306k gif
11. The great lakes of USA viewed by satellite 342k gif
12. The southeastern USA viewed by NOAA satellite 226k gif

13. Simpson desert in Austrailia 129k gif; 69k jpg

14. Pseudo color full disk image of pacific 316k gif
15. Image of the Gulf of Mexico 122k gif; 227k gif

16. Strait of Gibraltar 157k gif

17. Earth-Moon together from Galileo 2nd flyby 79K gif

18. San Francisco Bay infra-red from Shuttle 294k gif; 83k jpg

19. Space Station Mir over the Earth 111k gif
20. Earth from Apollo 11 52k gif; 32k jpg

21. USA at night 67k gif
22. Current weather maps html
23. Plate boundaries 171k gif
24. Earth Viewer html
25. AVHRR image browser html
26. Shuttle image browser html
27. ... more Earth images

Movies

1. Earth-Moon animation 382K fli; 373K pics; 140K quicktime
2. Clouds over West Europe, North Africa 379k mpeg 610k mpeg
3. Aurora Video 1300k AVI
4. Earth/Venus Rotation Movie 1000k AVI
5. Galileo Earth Encounter 14000k AVI
6. Clip 1 of Earth the Movie - Animation of Clouds 13000k AVI
7. Rotating relief globe of Earth 2200k MPEG

Earth's Satellite

           Distance  Radius    Mass
Satellite  (000 km)   (km)     (kg)
---------  --------  ------  -------
Moon       384         1738  7.35e22

More about Earth and the Moon

• from ASU
• from LANL
• Earth's Interior (Rosanna L. Hamilton at LANL)
• from JPL
• from RPIF
• from NSSDC
• from NASA Spacelink
• pictures of Terrestrial Clouds from space (LANL)
• text and pictures of Terrestrial Impact Craters (LANL)
• pictures of Terrestrial and Hawaiian Volcanoes (LANL)
• Volcano World
• Electric Volcano
• Volcanic Features of Hawaii and other Worlds (ExInEd Hypercard stack)
• Endeavour Views the Earth (ExInEd Hypercard stack)
• Comparing Earth and its Planetary Neighbors (ExInEd Hypercard stack)
• Virtual Geomorphology
• The Earth's Magnetosphere
• Dr. Cain's class notes
• Earth Viewer, an interactive map of the Earth
• The Great Globe Gallery
• Earth As Seen From Space (images and astronaut quotes; by Calvin Hamilton of LANL)

Open Issues

• Our knowledge of the interior of the Earth is derived almost entirely from highly indirect evidence. How can we get more information?
• 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?