The science of comparative planetology seeks to find similarities and differences amongst the various planets. Naturally, the Earth has become the benchmark for such comparisons.
Basic Properties of the Earth |
|
|---|---|
| Semi-Major Axis | 1.00 AU |
| Revolution Period | 1.00 Yr |
| Diameter | 12,756 km |
| Mass | 5.98 X 1024 kg |
| Density | 5.5 gm/cm3 |
| Uncompressed Density | 4.5 gm/cm3 |
| Surface Gravity | 9.8 m/s2 |
| Escape Velocity | 11 km/s |
| Rotation Period | 23h 56m |
| Surface Area | 5.1 X 108 km2 |
| Atmospheric Pressure | 1.00 bar |
| Atmospheric Composition | N2 (78%), O2 (21%) |
Although the Earth rotates on its axis and revolves about the sun in the Copernican model of the solar system, for nearly 200 years after the time of Copernicus there was no direct proof of these motions. There now exist several kinds of evidence of Earth's rotation and revolution.
For several centuries after the time of Copernicus, astronomers tried to verify the Earth's revolution by detecting stellar parallax. In one of those attempts in the 1720s, two British astronomers, Molyneux and Bradley, discovered the aberration of starlight. This is the shift in the apparent directions of all the stars due to the motion of the Earth. Molyneux and Bradley found that the stars appeared to shift back and forth by a total of about 40" in an annual cycle. To observe these stars their telescope had to be pointed at angles up to 20" from vertical. Bradley correctly reasoned that the shifting direction was due to the changing direction of the motion of the Earth as it orbited the sun.
This is analogous to the shift in the angle at which an umbrella must be held in order to keep a person dry in a vertically falling rain. If the person is stationary, the umbrella must be held vertical. But, when the person begins to walk the umbrella must be tilted forward at an angle that is proportional to the speed with which the person walks.
tan 20" = vearth/c
vearth = 30 km/s
In 1851 Jean Foucault used a large pendulum suspended from a low-friction pivot point to demonstrate that the Earth rotates on its axis. When the pendulum is set swinging, Newton's first law demands that it continue to swing in the same plane. As the Earth rotates "under" the pendulum, it results in an apparent shift in its swing plane, completing a complete cycle in 23 hours 56 minutes - one sidereal day - at the poles. The pendulum takes increasingly longer to complete one cycle at lower latitudes.
In accelerating reference frames fictitious forces appear. For example, when a car rounds a curve there is a fictitious force, usually called a centrifugal force in this case, which seems to cause an object to move toward the outside of the curve. In fact there is no force on the object, as it moves in a straight line in accordance with Newton's first law while the reference frame of the car turns under it.
Because the rotating Earth is an accelerated reference frame, we experience similar fictitious forces. For example the weight of an object on the rotating Earth (other than at the poles) is less than it would be on a non-rotating Earth due to the inertial effects of the centrifugal force. In fact the Earth itself is subject to the same inertial effects causing its shape to deviate from perfectly spherical. (0.3% in the Earth's case) The resulting oblate shape of the Earth is typical of all rotating planets, with the amount of deformation depending on size, rate of rotation, and type and distribution of matter composing the planet.
Another fictitious force generated by the Earth's rotation is the Coriolis effect, which causes the paths of moving objects, winds, water currents, etc. to curve to the right in the northern hemisphere and to the left in the southern hemisphere. Consider, for example, a projectile fire due north from a point on the equator. In addition to its northward velocity it also possesses the 1675 km/hr eastward motion of a point on the equator. But 10o north of the equator a point on the surface of the Earth is only moving eastward at 1650 km/hr, so the projectile is moving 25 km/hr faster to the east than the terrain below it. Plotting the path of the projectile on a map would result in an eastward curvature of its path.
For example, in the northern hemisphere, as air moves in toward the center of a low pressure region it will be deflected to the right, resulting in a counter-clockwise rotation. This is easily seen in hurricanes and in the direction of winds surrounding low pressure regions. Conversely, rotation around a high pressure region is clockwise in the northern hemisphere. In the southern hemisphere the rotational senses are reversed.
The Earth is a terrestrial planet, composed mainly of rock and metal. Only the first few kilometers of the Earth's crust has been directly examined. Our knowledge of the bulk properties of the Earth has been obtained indirectly.
Surface rocks have densities in the range of 2.5-3.0 gm/cm3, but the overall density of the Earth is 5.5 gm/cm3. Thus, the density of the interior must be much higher, and its composition is probably very different from that of the crust. However, it is possible that the extreme pressure of the overlying material compresses the material in the interior to high densities, even though the composition of the material is similar to that near the surface. Experimental tests show that rocky material is not sufficiently compressible to account for these higher densities. This leads to the concept of uncompressed density, which is the density an average piece of a planet would have if not subject to high pressures.
In the case of the Earth the uncompressed density is 4.5 gm/cm3, and so the interior must include high density material in addition to rock. Since iron is the most abundant metallic element on the cosmic scale, it is assumed that the interior of the Earth is enriched in iron as well as (possibly) other metals.
Nearly all our knowledge about the interior structure of the Earth has been gained by analyzing the propagation of seismic waves. On Earth seismic waves are generated by earthquakes which result from the slippage between crustal plates. As internal forces drive crustal plates, adjacent plates come into contact with each other in various ways (see below). As a result stresses build up until the stress is released in a large slippage, the associated release of large amounts of energy serves as the source of the resulting earthquake.
Some of these waves travel on the Earth's surface, while others propagate through the interior. Although it is the surface rolling-type wave which causes the greatest damage to buildings, etc. it is the interior waves that are of interest here. Some of these waves travel on the Earth's surface, while others propagate through the interior. The interior waves are known as P waves and S waves. P waves are longitudinal waves and are therefore able to travel through solids and fluids, while S waves are shear waves and so only able to move through solid material.
Therefore, by studying the propagation of seismic waves out from an earthquake's epicenter it is possible to infer the state of the various portions of the Earth's interior. The speed of S waves and P waves differ from each other, and also each varies with the density of the material it is traveling through, so a careful study of the arrival times of various waves from an earthquake at some remote location can provide information about the density profile of the Earth.
For a current
plot of earthquakes see
http://www.iris.edu/seismon/
As a result of this difference in the ability to propagate through different types of material, as well as the refraction which occurs with a change of density (and therefore temperature), so-called "shadow zones" are produced. The geometry of these shadow zones allows for the determination of the locations of various boundaries in the Earth's interior.
See simulations of seismic wave propagation at http://epsc.wustl.edu/~saadia/page2.html.
Based on this type of detailed information the following picture of the Earth's interior emerges. A thin solid crust exists of thickness approximately 100 km, although thinner in the ocean basin regions and thicker in the continental landmass regions. Moving down through the crust from the surface, the temperature is found to increase at about 20-30 K per kilometer of depth. Yet this rate cannot continue or all rock below a depth of about 50 km would be molten. Thus the source of the temperature increase near the surface must be due to the presence of radioactive materials in the upper crust. This outer region is also known as the lithosphere.
Beneath the lithosphere is the asthenosphere (or zone of weakness) extending to a depth of around 300 km. The material here is plastic, that is, it behaves as a solid when subjected to a sudden impulse, but will flow like a fluid when subjected to continual strain over a long period of time. The crustal plates float on this plastic region and very slow currents in the outer core, lower mantle and asthenosphere drive the tectonic processes of the crust. Note that the crust is distinguished from the mantle by composition, while the lithosphere is distinguished from the asthenosphere by degree of plasticity.
The lower mantle extends down to about 2800-2900 km, at which point an abrupt change occurs. P waves no longer propagate through this region, so it must be liquid. However, seismic studies show that at even greater depths the inner core again becomes solid, probably due to the extremely high pressures involved. The outer core has a radius of about 3500 km, although its surface is not perfectly spherical. The inner core has a radius of about 1200 km. Temperatures in the core reach around 7000 K (~surface temperature of sun), and pressures reach to about 3.6 million times atmospheric pressure.
Additional evidence for the existence of a liquid portion of the Earth's interior comes from the existence of the Earth's magnetic field. The Earth's magnetic field resembles that of an ordinary bar magnet. The magnetic poles are not coincident with the rotational poles; neither does the magnetic axis pass through the Earth's center. Investigation of orientation of magnetic domains in the ocean floor indicate that the Earth's magnetic field direction wanders and occasionally reverses polarity. (~100 times in the past 50 million years)
The dynamo theory proposes that the magnetic field arises from the rotation of the Earth's liquid metallic (conducting) core. This rotating conductor acts as a giant dynamo, and the turbulent motions of the liquid core result in the fluctuations of the field.
The Earth's field extends far out into the surrounding space. This region, known as the magnetosphere extends out to nearly 10 Earth radii in the direction of the sun, and may extend well beyond the orbit of the moon in the direction away from the sun. Electrically charged particles are affected by the magnetic field. Electrons, protons, and other electrically charged particles in the solar wind spiral around the magnetic field lines becoming trapped in two doughnut-shaped regions which surround the Earth - the van Allen belts. When these charged particles encounter the Earth's atmosphere near the poles the atmospheric molecules are excited and as a result emit light as the aurora borealis and aurora australis.
For a good overview of Earth structure and tectonics see
http://www.seismo.unr.edu/ftp/pub/louie/class/100/100-earthquakes.html
Processes which in some way modify the crust of the Earth are classified as endogenic or exogenic. The source of an endogenic process is internal, while the source of an exogenic process is external. For example, volcanism is an endogenic process, while impacts of meteorites would be an example of exogenic processes. The relative importance of these two types of processes largely determines the type of surface features that will be found on a planet. Impact craters are produced on the Earth, just as they are on the moon or Mercury. These craters are not evident due to the high level of endogenic processes and the presence of weathering and erosion.
Internal processes which result in the compression or expansion of the crust are referred to as tectonic. Alfred Wegener proposed in the early 20th century the hypothesis of continenetal drift - the idea that the continents had moved apart to assume their present positions. This idea was based on detailed comparisons of the geology of the east and west Atlantic ocean shores, and on similarities in the fossil record of Africa and South America. No feasible explanation for a mechanism by which the continents could move through the basaltic crust was available.
With the development of the theory of plate tectonics it is recognized that the dozen or so major crustal plates that make up the lithosphere can move relative to each other, driven by slow convection currents within the mantle. First evidence for this theory was found in the 1960s when geologists found evidence of sea floor spreading on either side of the Mid-Atlantic Ridge at the rate of a few meters per century. At this rate the entire Atlantic Ocean would have formed over a period of 100 million years, in agreement with Wegener's fossil findings.
Crustal plates move apart from each other in regions known as rift zones. This splitting is driven by upwelling convection currents in the mantle. Most rift zones are found in oceans where the crust is thinner, and therefore more fragile, but some exist on land (central African rift).
About 60,000 km of active rift zones have been identified. The average separation rate is about 4 cm per year, so the amount of new sea floor added each year is about (6 X 107 m) X (4 X 10-2 m) = 2.4 X 106 m2 = 2.4 km2. Since the total sea floor area is about 260 million km2, the average age of the oceanic crust is about 100 million years. This is very recent in geologic terms!
To compensate this spreading apart of plates in rift zones there must be regions where plates collide. This can happen in several ways. When one plate slides under another the region is a subduction zone. Continental plates are generally not subducted, but oceanic crust is relatively easily forced into the upper mantle. Subduction zones are marked by oceanic trenches (Japan Trench), earthquake-prone regions, and volcanoes.
Some plates will slide parallel to their boundaries along much of their lengths. Such areas are marked by cracks or faults, and in these fault zones one plate will move several meters per century - comparable to rift zone spreading rates. Typically this motion is not uniform, however. Stresses build up and then are released when violent slippages occur, resulting in earthquakes. The longer the interval between earthquakes, the larger the energy release when it finally does occur. For example, near Parkfield, CA the San Andreas falt has been slipping about every 22 years, with an average motion of about 1 m. However, the average interval between major quakes in the Los Angeles region is about 140 years, with an average motion of about 7 m. The last major slippage in this area was in 1857. Thus the area is due for a major earthquake of serious magnitude.
Whe two continental plates collide, neither can be subducted. As a result the crust buckles and folds with some rock being forced below the surface, and other folds being thrust upward. By this process most of the mountain ranges on Earth were formed. For example, the Himalayas are still in the process of being formed as the Indian plate is forced against the Eurasian plate. [Note that the sharp peaks and ridges are actually due to erosion by water and ice, not tectonic processes.]
Volcanoes mark locations where magma rises to near the surface of the Earth. This occurs at rift zones, subduction zones, and (occasionally) in regions of mountain building. Another location for volcanism is over mantle hot spots where heat wells up from the Earth's interior. The Hawaiian islands, for example, are the result of the motion of the Pacific plate over a mantle hot spot, with each major island in the chain a few million years older than the next.
Shield volcanoes result from gradual build-up of successive lava flows (Mauna Kea). Conical volcanoes result from the fall-back of violent eruptions of lava (Mt. Fuji, Mt. Vesuvius, Mt. St. Helens). Flood basalts result when very fluid lava is erupted rapidly (Columbia River flood basalts in Washington state).
Sculpting effect of water and ice - even in arid regions. Primary role of wind is transporting sand and dust produced by weathering. These sediments cover much of the igneous rock of the ocean basins and continental masses, eventually being formed into sedimentary rock.
The pressure exerted by the atmosphere at the Earth's surface is equivalent to the weight of 1.03 kg over each square centimeter, or 10.3 metric tons per square meter. The total mass of the atmosphere is then about 5 X 1015 metric tons. As described in the last chapter, the atmosphere gradually thins merging with the extremely thin gases of the magnetosphere at several hundred kilometers altitude.
Troposphere: 0-10 km. Convection currents. Clouds. Temperature drops to -50oC at the upper limit (tropopause).
Stratosphere:
10-80 km. Temperature uniformly cold (~-50oC), but rises slightly due to
absorption of solar radiation by an ozone layer (~50-65 km). CFCs. From 65-80
km the temperature drops again to ~-50oC.
See http://science.nasa.gov/newhome/headlines/essd06oct99_1.htm for additional recent information.
Thermosphere: 80-500 km. Temperature rises reaching ~1000oC at 500 km.
Exosphere: >400 km.
Ionosphere: The region of the thermosphere and exosphere where molecules of oxygen and nitrogen break up into individual atoms. UV radiation from the sun ionizes these atoms.
| Nitrogen (N2) | 78.1% |
| Oxygen (O2) | 21.0% |
| Argon (Ar) | 0.93% |
| Carbon Dioxide (CO2) | 0.03% |
| Neon (Ne) | 0.002% |
In addition to trace amounts of other gases, variable amounts of water vapor and dust particles are also found. Nitrogen, argon, and neon are relatively inert. Oxygen and carbon dioxide important for animal and plant life.
Note that on a warmer Earth some (or all) of the ocean water would be in vapor form contributing up to 300 bars of atmospheric pressure. In addition some carbon dioxide would be released from carbonate rocks in the Earth's crust, contributing up to 70 bars of atmospheric pressure (compared with the current 0.0005 bar). So a warmer Earth would have an atmosphere dominated by water vapor and carbon dioxide, with a pressure of as much as 400 bars.
How the Earth first acquired an atmosphere is not known for certain. Three possibilities exist: (1) formed with the Earth from the solar nebula, (2) released from the Earth's interior by volcanism subsequent to the Earth's formation, or (3) result from the impacts of comets or other icy materials from the outer solar system. The cometary hypothesis is currently favored although all three may have been involved to an extent.
Weather results from the evaporation and condensation of water, and convection in the troposphere. The energy powering the processes is absorbed solar radiation. As water is evaporated it stores large amounts of energy, which can later be released through condensation. On a non-rotating planet the circulation patterns would be dominated by rising warm air near the equator and sinking cooler air near the poles. On Earth this convection is modified by the Coriolis force producing cyclonic weather systems which largely dominate the weather in the temperate latitudes.
Periodically occuring during the past million years. Probably caused by changes in the tilt of the Earth's rotation axis due to gravitational influences of other planets.
Terrestrial life is (with great probability) the only life in the solar system. On larger scales the question remains very much open. Interaction between the Earth and its lifeforms has played an important role in the development of the planet.
At the molecular level, all life on Earth is similar. The DNA and RNA fundamental to all life are in all cases right-handed helices, and all based upon the same basic genetic code. Thus there is apparently a single common ancestor for all life forms.
The constant reforming of the Earth's crust has erased the record of the beginnings of life on Earth, yet some conclusion may be drawn. During the first several hundred million years of its existence impacts of fragments left over from the formation of the solar system would have destroyed any early life. By 3.6 billion years ago there was abundant life in the oceans.
Conditions at the time of the appearance of the first forms of life can be inferred from laboratory experiments. For example, to produce the type of organic molecules that serve as the building blocks of living organisms, an oxygen rich atmosphere like that found today could not have led to the necessary chemical reactions. However, in an atmosphere with no free oxygen, and the presence of abundant water it is easy to produce a wide variety of the molecules of interest, including amino acids and simple proteins. Thus the atmosphere must have evolved over time.
In the absence of life, oxygen in the atmosphere combines with surface rock to produce oxides. Prior to the onset of life the atmosphere was probably dominated by carbon dioxide, much like the atmospheres of Mars and Venus today.
In addition to an absence of oxygen, formation of organic compounds requires hydrogen-rich compounds like methane and ammonia. Either the early atmosphere contained these compounds, or they were deposited by cometary bombardment. (Compounds like methane, hydrogen cyanide and alcohol are abundant in comets.)
If the early atmosphere did contain such reduced compounds, they would not last long. In the absence of an ozone shield, UV light from the sun would break such compounds apart, and once released, the hydrogen from materials like methane and ammonia would have escaped the Earth, preventing re-formation.
NH3 --> N2 and CH4 --> CO
While photosynthetic plants existed in the seas over 3 billion years ago, studies of ancient rock show that there was no substantial oxygen in the atmosphere until at least 2 billion years ago. During the intervening time the oxygen was apparently removed by reactions with the crust as quickly as it formed.