Star System Sol - Planet Earth: The Blue Planet
Earth is also called the blue planet because 74% of its surface is covered by water. Water is essential to all life on Earth and is the most abundant liquid found on Earth. Earth is teeming with a rich variety of life, indeed wherever there is liquid water life exists on Earth - from ice fish which live in the cold Southern Ocean and have a body temperature of about minus two degrees C (some algae live in Antarctic rocks at -15 degrees C), to heat-loving bacteria that thrive in temperatures in excess of 100 degrees C (the current record holder thrives at 121 degrees C) near volcanic vents on the sea floor. From bacteria that thrive in atmospheric clouds to bacteria that live two miles underground inside rocks. From microscopic plankton that float on the sea's surface, to bizarre life-forms that live more than 11 000 metres beneath the ocean's surface, in the deepest trenches, at pressures in excess of 1100 atmospheres.Major Tourist Attractions
Structure of the
diagram above shows a section through the Earth (to scale). Moving
from the outside in we have the atmosphere, shown here are the
troposphere and stratosphere only, which contain the Earth's clouds
and weather systems seen as a thin blue line (the atmosphere extends
far above this, but as a thin cloudless sphere that is scarcely
visible from space). Beneath this is the Earth's crust, another thin layer (varies
from 5-70 km in thickness) is the Earth's rocky crust which contains
the continents and holds the oceans on its surface. The crust is
thinnest beneath the oceans (5-12 km) and thickest beneath mountains
(up to 70 km).
Beneath the crust is the mantle, which is divided into the upper mantle (about 200 km thick, shown as the dark red region) which is further divided into an outer region of about 50 km depth, which together with the crust forms the rocky lithosphere. Deeper down is the asthenosphere where rocks are near to their melting point and where magma, which erupts from volcanoes as lava, is formed. The asthenosphere is a convective layer, where hot material rises and cooler material sinks as heat is transported from the hot lower mantle to the cooler crust.
Note, that although magma is generated here, the rocks are not liquid as many suggest, because the pressure is too high. Rather the asthenosphere consists of solid red-hot rocks that flow, rather like hot plastic. Lava is liquid because when this hot rock reaches the surface, the sudden drop in pressure allows the hot rock to liquefy.
Beneath the upper mantle is the lower mantle, which forms the bulk of the Earth, shown in orange-red in the diagram. The total thickness of the mantle is some 2885 km. The nature of the lower mantle is not well understood.
Beneath the lower mantle is the outer liquid core, shown in light grey, which is thought to be 85% liquid iron alloyed with sulphur and oxygen with some nickel.
At the center of the Earth is the solid inner core (1216 km radius) which is mostly solid iron, but may contain some nickel. The pressure at the Earth's center is estimated to be 4 million times atmospheric pressure and may be over 4000 degrees C in temperature! To describe the core as solid iron is a bit misleading, since at the enormous temperatures and pressures found at the center of the Earth, iron becomes a very different material than the form we are more familiar with.
Why is the Earth layered (differentiated)?
The Earth formed from smaller particles that came together under the force of gravity (a process called accretion), in the early disc of material that surrounded the newborn Sun. Much of the gravitational energy is converted into heat during the process, so that the mass of accumulating material would have melted. This allowed the heavier materials, such as iron and nickel to sink down to the core, whilst the lighter materials, such as the silicates that make up most of the crust, floated on top of the melt and eventually cooled and solidified.
Why is the Earth's core so hot?
The Earth is hot because it was originally molten when it formed, but also because it has its own internal heat source which prevents the core from cooling down as it replaces the heat lost to outer space. This heat source is the natural radioactivity of the Earth's rocks. The radiation produced gets largely converted into heat.
How do we know what the Earth is like inside?
The chemical nature of volcanic rocks tells us about the upper mantle (lithosphere), but it is the analysis of Earth tremours and quakes that tells us most. When there is an earthquake or tremour, waves of vibration (seismic waves) travel throughout the Earth as it rings like a giant bell. Analysis of the characteristics of these waves tell us when they crossed boundaries and when they passed through liquid or solid. The crust-mantle boundary is called the Mohorovicic discontinuity (Moho) and the mantle-core boundary is the Gutenberg discontinuity. A discontinuity is a boundary where properties suddenly change, and the presence of these discontinuities reveal that the Earth is indeed layered. Nuclear test explosions can also generate useful vibrations to test finer details of the models. Finally, theory based upon the known laws of physics can be used to make predictions, and then the agreement or disagreement between observation and theory can be examined more closely and the theory refined.
The Earth's Dynamo
As the Earth rotates, its crust and mantle rotate at a different rate than the solid inner core. As a result, the liquid outer core is caught between two spherical surfaces that rotate at different rates and this sets up turbulence in the outer core, that is the fluid flow becomes chaotic and disorderly. Experiments have shown that when a liquid metal is under such turbulence, that it can generate a magnetic field similar in many ways to that of the Earth. It is therefore hypothesizes that turbulence in the liquid metallic outer core due to the differential rotation of the mantle and inner core generates the Earth's magnetic field. This is reasonable, since we know that magnetic fields arise when positive or negative electric charges are in motion. For example, the flow of electricity through a wire generates a magnetic field around the wire. Liquid metals contain positive and negative charges and so the flow of a liquid metal is expected to generate a magnetic field. The Earth is rather like a giant bar magnet, with a magnetic North pole close to the geographic North pole and a magnetic South pole close to the geographic South pole. (The magnetic and geographic poles do not exactly coincide as the magnetic poles slowly drift as the magnetic North and South poles periodically reverse over thousands of years. The geographic poles coincide with the Earth's rotation axis). The magnetic field of the Earth stretches out into space as the magnetosphere.
Do not confuse the Earth's magnetic field with its gravitational field. Magnetism is produced by moving electric charges within the earth, whilst gravity is generated by energy of any kind. It is often said that gravity is produced by mass, since more massive objects tend to have stronger gravitational fields than less massive objects, however, the famous equation E = mc2 tells us that energy (E) equals mass (m, strictly relativistic mass) times the speed of light (c) squared, so energy behaves as if it has mass and therefore generates a gravitational field. All forms of energy contribute to the gravitational field, the Earth's mass, its heat energy, its pressure, its viscosity, its magnetic field and gravity itself all generate the Earth's gravitational field. The gravitational field keeps you on the ground, the magnetic field makes your compass needle point to magnetic North.
(Tech note: Electric and magnetic fields always go hand-in-hand, since according to Einstein's theory of Special Relativity a magnetic field and an electric field are essentially the same phenomenon as seen from different inertial frames).
A dynamo is a power generator that converts rotatory mechanical energy into an electric current, and an electric current generates a magnetic field. Thus, the Earth contains a giant dynamo that existed long before human engineers discovered the same principle and used it to generate electricity in power plants.
Bot has written almost an entire textbook, with lots of pics, on the geology and physical geography of the Earth, including chapters about rivers, the water cycle, the atmosphere and weather, climate change, natural hazards and more, but this material is copyrighted (though not published) which unfortunately means that I can not share it with you here.
Above: The Plaque placed on the side of the Pioneer space probe. Designed by Carl Sagan and Frank Drake. Artwork prepared by Linda Salzman Sagan. Photograph by NASA Ames Resarch Center (NASA-ARC).
human race is by far the most technological species on Earth and has
given itself the title 'Homo sapiens' which can be
interpreted to mean 'wise man', despite the fact that humans are
familiar with the saying: 'Only a fool thinks he is wise'! Perhaps a
better translation would be 'thinking man'. Human beings are
mammals, belonging to the animal kingdom (one of the main taxonomic
groupings of life on Earth).
and Rees (1972) in volume 2 of Plant and Animal Biology (Pitman
Publishing) have summed up the physical adaptations of the human
being quite well as follows:
"Man is a very generalized animal, conspicuous for his lack of specialized features; he is not well adapted for any particular environmental niche. Whereas other animals have evolved a host of delicate adaptations which enable them to fit perfectly into a particular habitat, man has become generalized, fitting no environment perfectly but able to adapt himself to a great variety of surroundings with success. He is able to live in all climates from extreme cold to extreme heat. He is not well adapted to running, jumping, swimming, or climbing, but can achieve a fair degree of proficiency in all of them"
human beings have various notable physical adaptations that they
themselves may overlook since they take them for granted. (Humans
are often self-deprecating in assessing their own physical
attributes). These include the versatile hands which act as
multi-purpose manipulators, capable of powerful grip but also
precision dexterity with an opposable finger and thumb.
evolved from a taxonomic group of bipedal hominids, along with the
great apes with whom they share a related ancestry. The great apes,
however, reverted to quadrupedal locomotion and some degree of
arboreal life, necessitating the development of powerful upper
bodies and the lack of precision grip, the hands being developed
instead for walking and power grip for use in tree climbing. Humans
have remained bipedal and so generally have lighter upper
musculoskeletal systems but retaining powerful hind legs (at least
in active individuals). The foot is well-adapted for efficient
locomotion. Many morphological and anatomical features of human
beings can be explained by neotenization or the retention
of fetal/juvenile characteristics by the adult animal.
The relationships between humans and apes becomes more apparent when
comparing humans to infant apes, including the lack of body hair,
the proportions of the skull and anatomy of the genitalia. (Some
features of the juvenile ape, however, such as the upright posture
are perhaps better explained through ancestral inheritance, however,
with quadrupedalism being a secondary addition).
have medium-speed locomotion compared to their mammalian cousins
(achieving sprint speeds of up to about 10 m/s) and can achieve high
levels of physical strength with training, with the muscle and
skeletal systems responding readily to appropriate exercise. humans
do seem to have a naturally high ability to become good
long-distance runners with training, which possibly reflects the
hunting methods their ancestors employed (and a few still do) of
tracking and chasing fast animals to the point of exhaustion. They
also have good maneuverability and generally make good throwers,
again reflecting the skills they used in hunting and in warfare.
This large degree of movement has compromised physical strength,
however, with the shoulder joint developed mainly to possess a wide
range of movement (sacrificing strength in doing so) and long limbs
(gaining some speed and power but again by sacrificing strength due
to unfavorable leverage).
The bipedal locomotion of humans may lack speed, but is very energy efficient. Human respiration can only supply enough oxygen for about 50% of the mitochondria of the muscle system. The mitochondria power cells by generating ATP, the universal energy currency of life on Earth, from respiration. this is because bipedal locomotion does not require much exertion from the upper body, whereas quadrupedal mammals must supply twice as much oxygen necessitating a more extensive cardiorespiratory system. Bipedal locomotion also frees breathing movements from the stride-cycle to a large extent, since the chest is no longer having to support the body's weight during breathing. Thus, bipedal locomotion is very energy efficient and humans in general can walk and run over long distances. Bipedal locomotion has freed the hands to carry and use items.
human specialization, which sets them off from other mammals, is
their ability to develop and utilize complex language. The voicebox
(larynx) has no obvious specializations compared to the voiceboxes
of other mammals, and is a complex system of articulating cartilages
and bone which allow the vibratory vocal cords to be carefully
adjusted. Singing is a popular and effective means of communication
among humans and individuals deemed to have good quality voices are
generally held in high regard. However, facial structures have
become highly specialized for communication, both in shaping sounds
from the vocal cords and also in facial gestures (visual
communication). This has necessitated a shortening of facial
structures, so that the face is quire flat compared to that of other
mammals. This shortening is also thought to assist the energy
efficiency of locomotion since this species lacks a counterbalancing
tail. This facial shortening has resulted in smaller teeth, however,
and although this is no longer considered a degenerative adaptation
to the use of fire to cook and soften food, the latter process may
have facilitated this transition, facilitating speech. Problems with
tooth overcrowding are a modern problem resulting from urbanization
and appear to be developmental issues (rather than genetic
evolution) possibly due to high levels of pollution.
(The urban lifestyle that makes heavy use of the eyes for close-up work, such as reading from computer screens, gives rise to high incidences of 'school myopia'. Although some individuals are more genetically predisposed to this condition, it is apparently not due to 'bad genes' but rather to bad environment).
Humans have adapted to have very broad omnivorous diets, although their guts are primarily adapted for meat digestion. This has enabled them to utilize a wide range of habitats and may have been key to surviving past ice ages.
are gregarious, forming loose societies with strained
relationships. Humans have diplontic life-cycles, in which
a diploid stage is multicellular, producing haploid single-celled
gametes. (Humans are heterogameteic - the gametes are
morphologically distinct and designated male spermatozoa and
female oocytes.) Gamete synthesis has evolved to introduce genetic
diversity into the gametes (due to a process called meiosis) and
hence into the offspring that result from fertilization (fusion of
one male and one female gamete to form a diploid zygote). Siblings
share a considerable amount, but not all, of their genetic
This sexual mode of reproduction, whilst ensuring some genetic diversity between individuals, creates limited kin selection according to the selfish gene perspective. According to this model, genes that favor the survival not only of themselves but also of copies of themselves will be naturally selected for. An individual's close kin share more genes in common with that individual, so evolution is expected to favor those who show compassion to their closest kin. If an individual sacrifices itself and in so doing saves multiple copies of its own genes then it has increased its evolutionary fitness. As individuals become more distantly related, altruism becomes weaker or less frequent. Genetic kinship is estimated based on physical appearance, proximity, language and culture. This has resulted, in the evolution of strong tribal instincts in the human species.
speaking, when comparing humans to their mammalian cousins it is
apparent that human behavior is in large part at least,
genetically determined. Tribal instincts extend to cultural
traditions which give each in-group a sense of shared origin and
destiny, through such devices as mythical story-telling and
cultural codes often affiliated with religion. Religion
is, put simply, the peculiar belief in unseen supernatural powers
that are alleged to mediate on behalf of the tribe or in-group and
to the detriment of out-groups. It is often accompanied by
mythical origins of the tribe and always elevates the importance
of the tribe above all others. These narratives are often
circular, self-contradictory and devoid of empirical evidence and
not supported by rational or critical thinking, but are
nevertheless widely upheld due to the strength of tribal
instincts. In modern societies, the 'tribe' may no longer consist
of a close genetic unit and cultural traditions may instead come
to define the in-group.
aspect of tribal culture and/or religion is to designate a set of
rules. These rules are sometimes sensible, but often times
ritualistic and generally meaningless instructions about what an
individual can and can not eat, for example, and are often cruel,
but nevertheless they serve the biological function of aiding the
identification of members of one in-group from another. This
assists in-group cohesion and kin selection according to the
selfish gene principle. Tribalism may have worked to a greater
extent when tribes occupied their own discrete territories, but as
tribes expand and mix it almost invariably results in tension, war
and horrific acts of cruelty. it is generally devoid of genuine
ethics and morality.
Historically, most humans were very short-lived, indeed most died during birth or in infancy. This has pressured human development to ensure likely reproduction at the first viable opportunity and young individuals, in particular, have very strong reproductive drives. A full-lifespan for a human being is typically around 70 - 80 years. Lifespan has been optimized by evolution (according to computer simulations) such that once past a certain age the human body undergoes programmed senescence. Efforts to extend the human lifespan have so far been largely unsuccessful, although more individuals get to live out their lifespans due to a technological reduction in infant mortality.
Undoubtedly, a key to
human's ability to out-compete other organisms on Earth, is the
relative sophistication of their methods of communication. Both
vocal and scribed forms of communication are of central
importance. The mammalian voicebox (larynx) is an
intricate device consisting chiefly of articulating cartilages
and the muscles that operate them (as previously mentioned). It
is equipped with a pair of vibrating cords (rather like
rectangular drum heads) the length and tension of which is
controlled by the muscles and cartilages of the voicebox to
generate different frequencies of sound. Although the human
voicebox has no obvious adaptations to distinguish it from those
of other mammals, humans have evolved the ability to shape their
mouths in a wide variety of ways to allow the formation of a
variety of syllables, assisted by a complex variety of facial
muscles and the fore-shortening of the face and muzzle. Their
brains have also evolved the ability to encode/decode a rich
variety of sounds. Singing is a particularly popular
form of communication, but usually left to those individuals who
train their voices. Singing, often accompanied by music, is well
suited to convey emotional information in addition to the
Most humans are very economical organisms, with behavioral traits optimized for genetic survival. Most of them devote little energy to critical thinking. Their programmed traits can make them extraordinarily easy to manipulate, especially when their tribal instincts are exploited. Human history is cluttered with atrocities. In many cases these atrocities are catalyzed by psychopaths. There is a relatively high incidence of psychopathy among humans. Psychopaths lack empathy due to impairment in certain modes of learning. In a tribal and warlike species, psychopaths may make useful warriors, but their tendency to crave power over others combined with their poor judgement of human behavior results in frequent wars and societal collapse. In particular, psychopaths appear to lack empathy due to an impaired ability to learn from the mistakes of others and most psychopaths ultimately fail in society, but the more successful psychopaths often drag society down with them. This has greatly hampered the human species from progressing in any meaningful way.
The pros and cons of psychopathy are weighed against one-another in evolution, resulting in about 10% of human beings being high on the psychopathy spectrum, or at least on several of its key traits, such as lack of empathy. Thus, although usually an evolutionary disadvantage, psychopathy can benefit an individual in certain situations and so is not completely selected against. This analysis drawing on Games Theory can be extended to other human traits. It can, for example, explain to a large extent the existence of social cheats and criminals as well as social and religious ideas of 'justice' such as tit-for-tat (e.g. 'an eye for an eye') a strategy that evolution has selected for.In conclusion, human behavior is well explained by evolutionary theory.
on the oak tree forest to learn about life on Earth in the Bio/Nature
Click on the image below to explore the scientific principles behind Earth's cities - their location, growth, structure and distribution:
Structure of the Earth