[1.0] Introduction: The Earth

v2.0.0 / chapter 1 of 20 / 01 jul 16 / greg goebel

* Of all the planets in the solar system, the Earth is so familiar that it almost seems uninteresting. However, it is a unique world in the solar system, a "bioplanet" covered with a network of living organisms, and also makes a good starting point for the discussion of the other worlds in the solar system. This chapter provides a short description of the Earth and its place in the solar system.

Earth from Apollo 17



* Humans have always been aware that they lived on a world they called, in different languages, "Earth", but they were often ignorant of what the Earth really was. There was the ground underneath and the vault of the heavens above, containing the Sun, the moon, the stars, and the five bright points of light that wandered through the fields of stars, the planets Mercury, Venus, Mars, Jupiter, and Saturn.

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 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 without warning through the neat machinery of the skies, often causing public 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 an influential "Hellenic" culture.

In the third century BCE, 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 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 incorrect, 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 worlds, much less other Suns. 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, better known as Nicholas Copernicus, published a book on his deathbed that cautiously 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 described the motions of the planets more accurately.

The real breakthrough was the introduction of the telescope early in the 17th century. Scholar quickly turned it to the skies and were able to see that the planets were in fact other worlds, in some cases with 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.

* Over the next few centuries, astronomers discovered new planets and learned more about them, while explorers went to all corners of the world, with a wide range of scientists, from geographers to biologists, geologists to atmospheric scientists, acquiring a better vision of our planet from these explorations.

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 diameter. 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", its axis of tilt tracing out a circle in the heavens once every 25,800 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 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, not 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 to gravities of the other planets.

* The Earth's atmosphere is composed of about 77% nitrogen, 21% oxygen, with minor quantities of argon, carbon dioxide, and water, plus trace quantities of other gases such as methane and nitrous oxides. The atmospheric chemistry of the Earth is deviously subtle and closely related to the biological activity on the planet. Without the Earth's biosystems, there would be much more carbon dioxide in the atmosphere, and little or no oxygen, since oxygen 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, by 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 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 were 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 15 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. 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 much 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. 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 salt-water 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. Only about 3% of the Earth's water is fresh, with about two-thirds of that locked up in polar icecaps, and the remaining third in fresh-water lakes and rivers.

* The elemental composition of the Earth is as follows:


    iron:       34.6%
    oxygen:     29.5%
    silicon:    15.2%
    magnesium:  12.7%
    nickel       2.4%
    sulfur       1.9%
    titanium     0.05%

The Earth is geologically active, with obvious volcanic activity that also contributes to climate, and has a well-differentiated structure. It is covered with a cold, solid layer known as the "crust" or "lithosphere", which is roughly 40 kilometers thick, though the thickness varies considerably from place to place.

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". This phenomenon is known as "continental drift". It was regarded as a preposterous notion well into the 20th century, but precise measurements of the positions of the continents do show they are moving as predicted.

Traditionally, the lithosphere has been seen as the topmost of a set of layers, with each layer having somewhat different compositions and seismic properties. The layers become increasingly hot with depth and are plastic or semi-fluid, down to a solid core where the temperatures run at about 5,000 degrees Celsius. The heat is left over from the formation of the planet, boosted by the decay of radioactive isotopes contained in the mantle and the core. The layers are arranged as follows from the top down:


   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)

There is an increasing awareness that this "onion" model may be oversimplified, that the interior of the Earth is a more complicated place than has been thought. It is known that the core is iron-rich, and electric currents set up in the fluid outer core give the Earth a significant magnetic field. Interestingly, the magnetic poles of the Earth are not aligned with the Earth's axis of rotation and drift perceptibly year by year. Even more interestingly, the polarity of the Earth's magnetic field also seems to reverse or "flip over" abruptly, in geological terms at least, on a periodic basis, the normal period being about 250,000 years, with the flip taking several thousand years.

One of the initial proofs of continental drift was that materials pushed up from the mid-ocean ranges, which contained magnetic components whose polarity lined up with the Earth's magnetic field and were then "frozen" in position when the molten materials solidified, showed reversals in their own magnetic polarity that could be traced to match the times of the reversals of the Earth's magnetic poles. It's actually been 780,000 years since the last flip, and it seems likely a flip will occur sometime in the "near" future -- that is, sometime over the next few thousand years. The instability of the Earth's magnetic field seems to be due to turbulent patterns of flow in the fluid outer core, which may be related to the fact that the core spins slightly faster than the outer shell of the Earth, advancing maybe a quarter to a half of a degree per year. Incidentally, the magnetic field produced by the outer core remains mostly trapped there; only about 1% of the magnetic energy escapes to provide the Earth's external magnetic field.

The magnetic field of the Earth has interesting interactions with the flow of particles from the Sun, known as the "solar wind". The magnetic field 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 Earth has a single large Moon, of similar general geological composition as the Earth, but airless and lifeless. The Moon has a diameter of 3,480 kilometers, is 81 times less massive than the Earth, and orbits the Earth at a distance of 384,403 kilometers once in a little more than 27 days.

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".



* The Earth has been estimated from radioactive dating to be about 4.5 billion years old. According to a theory originally suggested by the Russian geophysicist Otto Schmidt in 1944 that has since become widely accepted, the Earth was created from a "primordial nebula" of gas and dust that collapsed under its own gravitational attraction into the Sun, the planets, and the smaller bodies of the solar system.

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 that. 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 continental drift, 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. Since 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 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 of bacteria, which were much simpler and smaller.

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 certain orders of organisms:

ages of the Earth

Shellfish appeared in the Cambrian, with the later periods of the Paleozoic seeing, more or less consecutively, the emergence of fish; land plants; amphibians and insects; fern forests; and reptiles. The Mesozoic was the age of the "great reptiles", with the emergence of modern plants; while the Cenozoic saw the predominance of mammals and birds, though both had appeared relatively early in the Mesozoic. The Pleistocene saw the emergence of humans, with the rise of civilizations in the Holocene. That's a rather simplified notion of the emergence of life, suggesting some sort of scale of progress, which is misleading: even today, most of the biomass of the Earth is in the form of bacteria, with some species of bacteria even found hundreds of meters underground, thriving under high temperatures and pressures.

* The transitions between periods of geological history are often marked by "mass extinctions", where large numbers of species suddenly disappear from the fossil record.

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 other 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 definitely 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 took place and that the extinction of the dinosaurs occurred at approximately that time, but there is still some debate as to the linkage of the two, history, there is no persuasive evidence that any other mass extinctions were caused by impacts. In fact, the Yucatan impact may have simply been another, major blow to a world order that was already in decline. The Earth has been on a cooling trend for the previous 100 million years, with volcanic events causing major climate fluctuations late in the scene, and the dinosaurs might not have survived much longer even if the impact hadn't happened.

The cooling trend has continued. About three million years ago, the Earth entered a period of ice ages, shifting between warm and cold climates on a 40,000 year basis. The ice ages may have been introduced by some rearrangement of the continents due to plate tectonics that modified ocean currents, or had some other major effect on climate; concentrations of greenhouse gases may have also had an effect on climate, with the current period of warming clearly linked to human emissions of such gases.



* There are eight known planets in the solar system including the Earth, and countless numbers of small bodies. The first four planets -- Mercury, Venus, Earth, and Mars -- are basically balls of rock like the Earth, while the next four -- Jupiter, Saturn, Uranus, and Neptune -- are "gas giants", worlds composed mostly of hydrogen and helium, with a rocky core and traces of other elements. Beyond Neptune, there is a system of small bodies, the best-known being Pluto, which was once considered to be a planet but was "demoted" after similar bodies were found in the outer 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  

   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          >49
   Saturn     9.54 AU      9.41   95.2          >59
   Uranus    19.18 AU      4.01   14.5          >27
   Neptune   30.06 AU      3.88   17.1          >13

All the planets more or less orbit in the same plane around the Sun, known as the "ecliptic" since it is the plane in the Earth's sky in which eclipses can occur.

Solar System scale diagram

The smaller bodies of the solar system consist of the moons, the asteroids, the comets, and the "Kuiper Belt Objects (KBOs)", which include Pluto. There are seven moons with diameters greater than 2,500 kilometers, listed below in order of size:

   moon        primary      diameter

   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

While the Earth's Moon is composed almost entirely of rock, the other major moons are cold worlds that are composed of mixtures of rock and ices in varying proportions.

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 detection of smaller and smaller bodies. Most of the smaller moons are irregular lumps, since their gravitational forces aren't strong enough to pull them into 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 -- and sets of asteroids trapped in the orbits of Jupiter and Neptune.

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.

The KBOs range in size up to Pluto, with a diameter of 2,345 kilometers, about two-thirds the diameter of Earth's Moon. Other KBOs have been discovered that are about as big, but their size is still being calibrated. Of course, most of the KBOs are smaller, with the number of KBOs over a hundred kilometers in diameter estimated to be over 100,000.

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.



* Statistics for the Earth:


  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 (reflectivity)               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