The Earth

[1.1] THE EARTH

The field of stars appeared to be fixed. The Sun, the Moon, and the planets moved through this field in predictable fashions. Every now and then the Moon would pass in front of the Sun, causing a "solar eclipse", and every now and then the Earth would cast a shadow on the the Moon, causing a "lunar eclipse".

Eclipses were baffling and frightening for a long time, but early astronomers soon learned they were predictable and all part of the celestial machinery. However, on occasion a ghostly comet would pass through the neat machinery of the skies, often causing consternation.

Some cultures thought the Earth was flat, but seafarers could see that mountains on distant shores rose up from the horizon as they approached, indicating that the Earth might well be spherical.

The ancient Greeks were among the first to seriously consider this idea, as part of their efforts in "natural philosophy" that would be the seeds of modern science. With the establishment of Greek colonies and the later conquests of Alexander the Great, the knowledge obtained by the Greeks was spread around the eastern Mediterranean, forming a "Hellenic" culture that was to prove very influential.

In the third century BC, the Hellenic scientist Eratosthenes of Alexandria, in Egypt, calculated a fairly accurate value of the diameter of the Earth, now known to be about 12,750 kilometers. He measured the length of of the shadow of a stick of known length at noon on midsummer's day in Alexandria, and compared to the shadow of a stick of the same length in lands to the north.

A century later, the Hellenic scientist Hipparchus calculated a rough, but not wildly off, distance to the Moon from a lunar eclipse. However, while the Moon and Sun were sometimes thought to be other bodies in the Universe comparable to but different from the Earth, there was no general belief that the skies were an infinite space containing other Suns and other worlds.

The skies were a sphere flecked with points of light, and the planets were other points of light, moving on concentric arrangements of transparent spheres. The Sun was on a sphere below them, and the Moon below the sphere of the Sun. The Earth was at the center of this arrangement of spheres.

In the 16th century this notion began to waver. The Polish astronomer Niklas Koppernigk, or Nicholas Copernicus, published a book on his deathbed that suggested the Earth orbited the Sun, not the reverse. Later in the century the German astronomer Johannes Kepler refined Koppernigk's ideas, modifying the circular orbits proposed by Koppernigk into elliptical orbits that accounted better for the motions of the planets.

The real breakthrough was the introduction of the telescope early in the 17th century. Scientists quickly turned it to the skies and were able to see that the planets were in fact other worlds, in some cases with visible moons of their own. Soon, there was no doubt that the Earth was just one of the planets, and in fact hardly the biggest of them.

In modern times, we know the Earth to have a diameter of about 12,750 kilometers. The planet is slightly oblate, with an equatorial diameter about 42 kilometers greater than its polar diameters. The Earth orbits the Sun at a mean distance of 149,503,000 kilometers, a distance that is referred to as an "astronomical unit (AU)", and is used as a yardstick for distances to other planets.

The length of the day on the Earth is of course 24 hours by definition, and there are about 365 and a quarter days in a year. The Earth's axis of rotation is inclined by 23.4 degrees to the plane of its orbit around the Sun. This, along with the slight eccentricity of the Earth's orbit, accounts for the Earth's seasons, with the tilt axis causing the Sun to be low in the sky in each hemisphere in alternating cycles during the year. The Earth's spin axis also has a slow "precession", tracing out a circle in the heavens once every 40,000 years.

The mass of the Earth is about 6 x 10^21 tonnes, an unimaginable value, and in considering the planets it is generally more useful to define this value as an "Earth mass (Me)" and to compare the mass of the larger worlds relative to this mass. The Earth is, incidentally, the densest of all the planets, even though it is far from the biggest. This is because the compression of a planet's own mass increases its density as the planet becomes larger, and the Earth is the biggest of the rocky planets. The bulk of the larger planets is composed of the light elements hydrogen and helium rather than dense rock.

The acceleration of gravity at the surface of the Earth is 9.81 meters per second squared, a value which is defined as the "G" or "gee" and is also used as a standard of comparison.

Without the Earth's biosystems, there would be much more carbon dioxide in the atmosphere, and little or no oxygen, which is reactive and tends to be absorbed into mineral oxides. Plant life converts the carbon dioxide into atmospheric oxygen and carbon compounds that make up plant structures, which in turn sustain the animal life of the planet. The carbon dioxide is also absorbed into carbonate rocks and, to a lesser extent, absorbed in the oceans.

The presence of oxygen not only sustains animal life on the Earth, at the upper "stratospheric" levels of the atmosphere it is converted down by the Sun's rays into ozone (O3), which absorbs solar ultraviolet radiation that would damage organisms living on the Earth's surface.

The levels of carbon dioxide have a very strong influence on climate. The carbon dioxide in the atmosphere tends to trap heat from the Sun, and so it is sometimes referred to as a "greenhouse gas". If there was no life on Earth, the high levels of carbon dioxide in the atmosphere would make the Earth a hot planet.

At present, the average temperature of the Earth is about 35 degrees Celsius. If the current levels of carbon dioxide were to increase, as is generally believed to be occurring at present because of human burning of hydrocarbon fuels, the Earth would logically be expected to heat up, though by how much and how quickly is energetically debated. Were the levels of carbon dioxide to fall, the Earth might cool, resulting in another "ice age", similar to those that have come and gone over the past few million years.

Trace gases such as methane and ammonia are even more potent greenhouse gases than carbon dioxide, and variations in their concentrations could also have significant effect on climate.

In fact, the Earth's weather and climate system is extremely complicated and subtle. Atmospheric circulation is driven by solar heating and influenced by the Earth's rotation and surface features. Temperatures tend to be high near the equators, and frigid near the poles, which are covered by deep caps of water ice. The weather effects result in storms that can feature high winds, strong rains, and lightning, ranging up in scale to the huge oceanic cyclonic storms known as "hurricanes" or "typhoons".

About 70.8% of the surface is covered with water, mostly in the Earth's oceans, which on the average are about 3,800 meters deep. The oceans act as a huge "thermal reservoir" and ocean currents are another major influence on global climate.

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     iron:       34.6%
     oxygen:     29.5%
     silicon:    15.2%
     magnesium:  12.7%
     nickel       2.4%
     sulfur       1.9%
     titanium     0.05%
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The lithosphere is divided into about a dozen "tectonic plates", which are rigid in themselves but can move relative to each other, driven from one side by upwellings of rocky materials from the "mid-ocean ridges" of undersea volcanoes, and pushed slowly back down into the Earth at the other side into "oceanic trenches".

There are a number of layers below the lithosphere, each having somewhat different compositions and seismic properties. The layers become increasingly hot with depth and are plastic or semi-fluid, down to a a solid core. The heat is mostly generated by the decay of radioactive isotopes contained in the mantle and the core.

The layers are arranged as follows from the top down:

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    lithosphere:        0 to 40 kilometers
    upper mantle:       40 to 400 kilometers
    transition region:  400 to 650 kilometers
    lower mantle:       650 to 2,700 kilometers
    D" layer:           2,700 to 2,890 kilometers
    outer core:         2,890 to 5,150 kilometers
    inner core:         5,150 to 6,378 kilometers (to the center)
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The magnetic field of the Earth has interesting interactions with the flow of particles from the Sun, known as the "solar wind". It funnels the solar wind down to the poles, resulting in "auroras", the ghostly veils of light seen at high latitudes during strong solar activity. It also traps solar particles in a pair of concentric, doughnut-shaped belts, known as the "Van Allen radiation belts". The inner belt extends from 7,600 to 13,000 kilometers above the surface of the Earth, while the outer belt stretches from 19,000 to 41,000 kilometers.

The Moon's rotation period matches its orbital period, and so it keeps one side always facing the Earth. This neat synchronization is due to the differential gravitational force, or "tides", of the Earth on the Moon, which slowed down its rotation to this stable state.

The Moon of course similarly causes tides on Earth that are slowly lengthening the Earth's day. More visibly, they cause the Earth's oceans to rise and fall by a number of meters on a daily cycle.

The Sun also has a tidal effect on the Earth's oceans, though one that is less significant than that caused by the Moon. However, at some times the two tidal effects will work together, causing high "spring tides", named because the waters "spring up", not because they happen in the spring. If the two work against each other, they cause low "neap tides".

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[1.2] THE HISTORY OF THE EARTH

The process of "accretion" began about 4.65 billion years ago. The early solar system was a collection of a multitude of small objects that collided with each other to form larger ones. As the objects grew bigger, the collisions grew more violent. The process of building up the planets took about 150 million years, and impacts remained common for hundreds of millions of years after than.

Bodies such as the Moon, with their craters and huge volcanic plains, still show vivid evidence of this period of "planetary bombardment." However, the Earth does not. Between weather and plate tectonics, the Earth's surface is continually renewed, and the surface of the planet shows few features less than a half-billion years old.

Life seems to have risen surprisingly quickly after the formation of the Earth. The oldest fossil organisms, of single-celled "blue-green algae", are about 3.2 billion years old, and given the difficulty of forming such fossils such organisms were very likely around well before that.

The early atmosphere of the planet was mostly carbon dioxide and nitrogen. As the Sun was substantially less bright billions of years ago, the high concentrations of carbon dioxide probably helped trap heat to keep the Earth from freezing over. The Earth's oxygen atmosphere appears to have begun its formation somewhat abruptly a little over two billion years ago, and reached its current composition about half a billion years later.

During the time of the formation of the Earth's modern atmosphere and for almost a billion years after that, the only life on Earth was in the form of single-celled organisms. To be sure, during this long period of time, these single-celled organisms evolved considerably, most significantly in the development of "eukaryotic" cells, which have a nucleus, as opposed to much more venerable "prokaryotic" cells, which were much simpler.

Multicellular organisms based on eukaryotic cells began to emerge about 570 million years ago, they began to undergo a explosion of different forms that was incredibly rapid compared to the billions of years before that time when single-celled organisms ruled the Earth.

Geologists and paleontologists have divided this time into a series of geologic "eras", "periods", and "epochs" that are strongly associated with increasingly complicated organisms:

    date         era        period          epoch          lifeforms
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    570,000,000  Paleozoic  Cambrian                       shellfish
    500,000,000             Ordovician                     fish
    435,000,000             Silurian                       land plants
    410,000,000             Devonian                       amphibians, insects
    360,000,000             Carboniferous  Mississippian   fern forests
    330,000,000                            Pennsylvanian   reptiles 
    290,000,000             Permian
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    240,000,000  Mesozoic   Triassic                       age of dinosaurs
    205,000,000             Jurassic                       
    138,000,000             Cretaceous
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     65,000,000  Cenozoic   Tertiary       Paleocene       age of mammals
     54,000,000                            Eocene
     38,000,000                            Oligocene
     24,000,000                            Miocene
      5,000,000                            Pliocene
      1,500,000             Quaternary     Pleistocene     rise of humans
         10,000                            Holocene        human civilization
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The greatest of the mass extinctions, at the end of the Permian period 240,000,000 years ago, led to the disappearance of at least 80% of marine species, and even the extinction of large numbers of insect species, which went through most of these cataclysms with relatively little trouble. The cause of the Permian extinction remains mysterious, though it may have been due to climate change and an outburst of volcanic eruptions.

However, the end of the Cretaceous period, 65 million years ago, was marked by the impact of a large asteroid in what is now the Yucatan peninsula that threw up a huge cloud of dust and debris, causing a "global winter" that is now generally believed to have done much to end the predominance of the dinosaurs.

There is no doubt that the Yucatan impact occurred and that the extinction of the dinosaurs occurred at that time, but there is still some debate as to the linkage of the two. Although such large impacts have occurred periodically through the Earth's history, there is no persuasive evidence that any other mass extinctions were caused by impacts. In fact, the Yucatan impact may have simply been the final blow to a world order that was already in decline. The Earth has been on a cooling trend for about the last 100 million years, and the dinosaurs may not have survived much longer even if the impact hadn't happened.

Human activities that have increased the concentrations of greenhouse gases in the atmosphere may be reversing this cooling trend. Improved climate data collection and modeling has become a high priority.

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[1.3] THE EARTH IN THE SOLAR SYSTEM

The following table gives coarse data on the planets, with diameters, masses, and distances relative to the Earth:

    planet    distance   diameter     mass     moons  
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    Mercury    0.39 AU      0.38    0.055          - 
    Venus      0.72 AU      0.95    0.815          -
    Mars       1.52 AU      0.53    0.107          2
    Jupiter    5.20 AU     11.2   317.8          >16
    Saturn     9.54 AU      9.41   95.2          >17
    Uranus    19.18 AU      4.01   14.5          >15
    Neptune   30.06 AU      3.88   17.1           >8
    Pluto     39.44 AU      0.18    0.002          1
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The smaller bodies of the solar system consist of the moons, the asteroids, and the comets. There are seven moons with diameters greater than 2,500 kilometers, listed below in order of size:

    moon        primary      diameter
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    Ganymede    Jupiter      5,268 km
    Titan       Saturn       5,150 km
    Callisto    Jupiter      4,806 km
 
    Io          Jupiter      3,630 km
    Moon        Earth        3,476 km
    Europa      Jupiter      3,120 km
    Triton      Neptune      2,705 km
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There are ten more moons with diameters greater than 1,000 kilometers; 18 more with diameters greater than a hundred kilometers; and a large number of moons smaller than a hundred kilometers, with the number increasing all the time as new technology allows the discovery of smaller and smaller bodies. Most of the smaller moons are irregular lumps rather than spheres.

The asteroids are small, rocky objects, mostly accumulated in the "asteroid belt", which ranges from 2.1 to 3.3 AU. The largest such "main belt" asteroid, Ceres, is only about 933 kilometers across, while the second largest, Pallas, is not much more than half that size, only 533 kilometers across. There are also some "near Earth asteroids" that orbit through the inner solar system, with a few of them intimidatingly crossing the orbit of Earth, raising the possibility of a disastrous impact at some time in the future.

The comets are even smaller, balls of ice maybe a few tens of kilometers across. They generally have elliptical orbits, most apparently originating from the "Kuiper belt" of comets outside the orbit of Neptune, but every rare now and then one falls in from the much more distant "Oort Cloud" that surrounds the solar system. Comets can be very spectacular as they approach the Sun, generating a cloudlike "coma" and a great "tail" that sweeps through the night sky.

Knowledge of these worlds and objects has increased greatly in the last 50 years, due to visits by spacecraft and refinements in Earth-based astronomy. The following chapters in this document detail what is known about them.

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[1.4] EARTH STATISTICS

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   mean distance from Sun              149.6 x 10^6 kilometers
   orbital period (sidereal)           365.26 days
   orbital eccentricity                0.017
   orbital inclination                 0 degrees
   equatorial diameter                 12,756 km (3.67 Moon)
   mean density (relative to water)    5.52
   escape speed                        11.2 kilometers per second
   rotation period                     1 day
   oblateness                          1/298
   inclination of equator              23.4 degrees
   albedo                              0.37
   max surface temperature             58 degrees Celsius
   atmosphere (major constituents )    N2, O2, A, clouds of H20
   atmospheric pressure at surface     1 atmosphere
   number of known moons               1
 
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See also: Posters