Earth’s most outstanding colour is blue, hence it has been nicknamed the blue planet. However, it is not the only blue planet in our solar system; Neptune is also blue, but not for the same reason that Earth is blue. Neptune appears uniformly blue because of the 1% methane in its atmosphere which absorbs the red end of the visible light spectrum, reflecting back the blue end of the spectrum thus making it appear blue to us.
The blue of the earth is due to the amount of water that the planet contains which covers about 70% of the earth’s surface. In addition to the blue, we can easily see the green of the forests, especially the Amazon, and the brown of the land, especially the Sahara Desert. The solid surface layer of the earth is called the lithosphere (“litos” means rock in Greek) which corresponds to 29% of the earth’s surface. Finally, we can also detect some white patches here and there, moving all over the planet. This indicates that the earth has an atmosphere, that is, it is surrounded by gas and water vapor present in the clouds.
The biosphere, the habitat or vital space of life, is the sum and interaction of these three spheres: atmosphere, hydrosphere, and lithosphere. However, beneath the surface of the earth, as indicated in the diagram above, there are three other elements called crust, mantle and core that have some influence on life, and remind us how Earth was before the appearance of life.
The Atmosphere
The earth’s atmosphere is a relatively thin gaseous envelope composed primarily of nitrogen (N2: 78%) and oxygen (O2: 21%), with small amounts of other gases such as water vapour (H2O) and carbon dioxide (CO2). In addition to water in vapour form, there are also clouds of liquid water and ice.
Conventionally, it is understood that the atmosphere is about 480 kilometers thick although there is no definite upper limit. Its density decreases with altitude, becoming thinner and thinner until it merges with the outer space that surrounds all the planets. Ninety percent of the atmosphere, however, is within the first 30 kilometers from the earth’s surface.
The further we move away from the surface of the earth, the more we’ll notice the decrease in the air density and air pressure, and in the force of gravity. The decrease in temperature with altitude is seen in the first layer of the atmosphere (but follows a more complicated profile in the upper layers). Without being airborne, we can experience this even at the earth’s surface if we rise from the Dead Sea, the lowest point on the planet at 420 meters below sea level, or climb to Mount Everest the highest point on the planet at 8,848 km above sea level. The variation in temperature can be between positive 50 degrees Celsius on the shores of the Dead Sea to negative 50 on top of Mount Everest.
If Earth was the size of a soccer ball, the habitable layer would be as thin as a sheet of paper. At the altitude of 11 km, the temperature is at minus 50 degrees Celsius – furthermore, even though there is enough oxygen for the engines of an airplane to operate, there is not enough for a human being to live; this is why the climbers of Mount Everest must use supplemental oxygen.
The earth’s gravitational pull also decreases with altitude, but it does not disappear completely. It has an average value of 9.8 m/s2, being stronger at the poles because of the shorter distance to the center of the earth and weaker at the equator due to the equatorial bulge. An object must overcome this force in order to rise through the atmosphere. However, the decrease in gravity is not as drastic as we might think and it continues to be felt even beyond the atmosphere.
The International Space Station (ISS), for instance, orbits at an average altitude of 400 km, and at that height gravity is still almost 90% as strong as at the surface of the earth. Like an airplane, if it did not move at a certain speed in a straight line, it would end up falling to Earth. The extraordinary speed at which the ISS travels, at 27,600 km/hr, makes it counteract the earth’s gravitational force, giving the astronauts who live in it the experience of weightlessness or constant free fall. Therefore, the televised videos of the astronauts floating in air inside the ISS is not due to an absence of gravity as many think.
The same force of gravity that keeps us anchored to Earth is what keeps the thin layer of air that envelops our planet from escaping into the outer space. Of the other terrestrial planets aside from Earth, only Venus has an atmosphere because it is only slightly smaller than Earth (95% of Earth’s diameter and 80% of Earth’s mass).
Both Mercury and Mars are too small for their gravitational force to maintain an atmosphere clinging to them. Atmosphere not only supplies us with the air we breathe but it also protects us from harmful ultraviolet radiation from the sun, keeps us warm by virtue of the greenhouse effect, and reduces the temperature extremes between night and day as seen in planets without an atmosphere.
The earth’s atmosphere can be divided into five layers based on its temperature profile with increasing altitude, starting from the earth’s surface:
- Troposphere (0 to 12 km) – It is here in the lowest part of the atmosphere that climate change occurs and temperature tends to decrease with altitude.
- Stratosphere (12 to 50 km) – It is here that the ozone layer that protects us from the harmful UV rays is found and causes temperature to rise with increasing altitude; it is in this layer that the airplanes fly to avoid climate disturbances.
- Mesosphere (50 to 80 km) – It is where most of the meteorites burn out when they enter the atmosphere. Temperatures drop with increasing altitude and the coldest place on Earth is found here with an average temperature of minus 85 C.
- Thermosphere (80 to 700 km) – Air in this layer is very rarefied, that is, thin and temperature gradually increases with height, from 200 C to 500 C reaching as high as 1500 C at the top region of this layer. Because the air is very thin the molecules with high energy are very far apart so that heat transfer would not be efficient and hence it would not feel as hot as 1500 C would feel on the surface of the Earth. It is analogous to sticking one’s hand rapidly in and out of a hot oven. The ISS orbits in this layer.
- Exosphere (700 to 10,000 km) – This is the layer where most of the earth’s satellites orbit; the air is even thinner than in the thermosphere so that atoms and molecules may travel hundreds of kilometers without colliding with one another. Temperature can range from 0 to over 1700 C, colder at night and much hotter during the day.
The Van Allen Belt
It is usually described as two donut-shaped variable zones high above the earth’s atmosphere comprised of highly energetic charged particles that have been captured and trapped by the earth’s magnetic field. The inner belt can typically be found between 6,000 to 12,000 km above the earth’s surface and the outer belt between 25,000 to 45,000 km.
They act like a shield that protects our atmosphere from damaging energetic charged particles from solar wind and cosmic rays. Although its existence was proved in 1958, it is only since 2012 that more detailed knowledge of these belts has been collected by the Van Allen probes sent out by NASA. It is because of the high radiation in these belts that many people think that humans never left the earth’s low orbit and therefore never went to the moon.
The Hydrosphere
The hydrosphere is the total amount of free water found on the planet that is not contained or confined chemically or physically in the minerals of the earth’s crust or in the living beings, both plants and animals. In other words, water that is on the surface of the earth like oceans and ice caps, in the underground as in groundwater and wells, and in the air as cloud. The hydrosphere occupies most of the earth’s surface, more than 70% of the total area of the planet. The volume of the hydrosphere is about 1.4 billion cubic kilometers.
The oceans make up most of the hydrosphere, about 96.5% of its total volume. Considering this fact, our planet should have been called Water and not Earth. The oceans and the seas serve not only as water reservoirs but also as heat sinks as they control the energy regime on earth’s surface, producing the necessary conditions for life.
Almost 97.5% of the water in the world is saltwater while fresh water makes up only 2.5% and of this, 69% is retained in the glaciers, 30% in the fresh groundwater and only 0.3% is the freshwater that we can use found in lakes and rivers.
The Lithosphere
The interior of the earth is composed of three main layers: the crust, about 1% of the earth’s volume, thinner under the oceans than under the continents, the mantle, 84% of the earth’s volume and is divided into an upper and a lower layer, and the core, 15% of the total volume of the planet composed of an inner core and an outer core.
Lithosphere is therefore comprised of the relatively cool and rigid rock combination of the earth’s crust and the upper part of the upper mantle. Now, below the lithosphere the temperature is believed to be about 1000 C, resulting in a weak, less rigid zone which seems to be continuously moving. This motion creates stress in the rigid rock layers and forces the plates of the lithosphere to move against each other. The movement of the lithospheric plates is known as plate tectonics, and this motion manifests at the earth’s surface as earthquakes and continental drift.
The seven most important tectonic plates or huge slabs of lithosphere are: the African Plate, the Antarctic Plate, the Eurasian Plate, the Indo-Australian Plate, the North American Plate, the Pacific Plate and the South American Plate. The shifting or collision between these plates produces high mountains like the Himalayas, and causes volcanoes and earthquakes.
Continuing the journey to the center of the earth, we find below the mantle, the earth’s core which is separated into two layers. The outer core is comprised of liquid iron and nickel with temperature varying from 4,000 to 5,000 C, and is responsible for the earth’s magnetic field. Under the liquid outer core is a solid inner core about 70% the size of our moon composed mainly of iron and at 5,700 C is as hot as the surface of the sun.
It seems to be a contradiction that the solid iron inner core is surrounded by liquid iron and nickel at such an extreme temperature. This is because the pressure inside the planet is so great that the atoms are forced to concentrate near one another, as with matter in the solid state, and the extreme pressure does not allow dispersion that characterizes atoms in the liquid state.
The average distance to the centre of the earth at the equator is 6,731 km, with the crust being 35 km thick, the mantle 2,855 km, and the core 3,481 km. Although the core is thicker than the mantle, it actually forms only 15% of the earth’s volume, with the mantle occupying 84% and the crust a mere 1%.
The Biosphere
Biosphere is the combination of the three factors described above: the atmosphere, the hydrosphere and the lithosphere. These three factors interact with one another, allowing life. The area of this interaction is very small – the upper layers of the atmosphere, the bottom of the sea (hydrosphere) and the bowels of the Earth (lithosphere) are not part of this equation. Someone once said that if we were to compare Earth with an apple, the biosphere will correspond to its skin.
The biosphere has therefore about 10 km of altitude in the atmosphere, most of the clouds are formed between the 2,000 m and 6,000 m; about 1 km of depth in the oceans, with some exceptions. Beneath this depth, the environment is very dark and cold. On firm land, only first 2 meters in depth are fertile soil, favorable for agriculture. Below the two meters, organic matter is scarce.
The Five Main Biogeochemical Cycles of Life
It is within this very small zone, where the lithosphere, the atmosphere and the hydrosphere meet and form the biosphere, that life is possible. The elements that make up these three spaces interact through five fundamental cycles: the water cycle, the oxygen cycle, the carbon cycle, the sulfur cycle and the nitrogen cycle.
We could understand these cycles as inorganic matter that temporarily transforms or is assimilated by organic matter to favor or permit life and then returns to its inorganic state.
These cycles are carried out individually, but also in interdependency with each other, for the efficient maintenance of the ecosystem. Living beings need many chemical elements and these must be available at the right time, in the right quantity, and in the right concentration relative to one another. This is the essence and the importance of these biogeochemical cycles.
In the oceans there are unicellular algae that release a sulfide compound which oxidizes in the atmosphere, producing condensation nuclei that are needed in the formation of clouds that transport water and sulfur to the land.
The green plants, both on land and in the sea, use carbon dioxide with water and sunlight to produce sugar. A byproduct of this reaction is oxygen, and it is for this reason that we have free oxygen in the atmosphere. Here we see the interaction or interdependency of three different cycles: water, carbon and oxygen. Without this interaction life would not be possible. Without the carbon and the water cycle none of this would take place and life as we know it would not exist.
Life on Earth depends on a constant recycling of vital and non-vital elements. Nature has always recycled because the elements are always the same elements. The water that now forms our body may have already been at the bottom of the sea, on top of a cloud, in the trunk of a tree or even in the body of a dinosaur. Nature has always recycled; human beings are only now beginning to realize that they cannot continue to have the use and throw away mentality; that they have to recycle because this is crucial to the life of the planet.
The Water Cycle
The water cycle is the principal biogeochemical cycle. It happens by the process of evaporation of surface water (rivers, oceans, lakes, etc.) on planet Earth and also by the transpiration of living beings, both plants and animals.
This cycle begins with evaporation, the phenomenon that occurs when water vapor rises through the atmosphere forming clouds. When the clouds become saturated and upon reaching very high altitudes where it is cooler, condensation of the water back into the liquid form takes place, which will fall and return to the surface in the form of rain or snow. This cycle is known as the short water cycle.
The long water cycle is when water passes through the bodies of living beings before returning back to the environment. The water is absorbed from the soil by the roots of the plants, being used in photosynthesis or passed to other beings through the food chain. In this way, water will return to the atmosphere or to the earth through breathing, sweating, feces and urine.
The water cycle is extremely important for the maintenance of life on the planet. It is from this that climate change occurs, the development of living beings and the functioning of rivers, oceans and lakes.
The Nitrogen Cycle
Nitrogen is a very important element for living beings for it is part of the composition of the two organic molecules fundamental to life: proteins and nuclei acids. The nitrogen cycle is the process by which this element circulates through soil and plants, from the action or utilization of this element by living organisms.
The air we breathe is composed of 78% nitrogen in gaseous form, this being its largest reservoir. In addition to the atmosphere, it is also possible to find nitrogen in oceans, in organic matter and also in soil.
Although nitrogen is in the atmosphere, there are some bacteria that have the ability to fix nitrogen from the atmosphere into the soil, releasing it in the form of ammonia molecules. Other bacteria have the function of converting this ammonia molecule into nitrates, and it is in this form that plants absorb nitrogen from the soil through their roots. There are also bacteria that can fix nitrogen into the nodules of the roots of plants, especially in the legumes.
Plant-eating animals absorb nitrogen by feeding on these plants. The return of nitrogen into the atmosphere occurs through other bacteria known as denitrifiers which transform the soil nitrate into nitrogen gas that returns to the atmosphere, thus closing the nitrogen cycle.
The Oxygen Cycle
Oxygen is an element present in diverse chemical compositions essential for the maintenance of life, such as in carbon dioxide (CO2) and water (H2O), which makes it possible for photosynthesis to take place in plants. In addition, it is an element necessary for human respiration, that is, indispensable for our survival.
The oxygen cycle is linked to these two phenomena: photosynthesis and respiration. In the process of photosynthesis plants and algae use carbon dioxide, water and sunlight to produce energy for their growth, releasing oxygen as a waste product; we human beings take in oxygen by the process of respiration to produce energy releasing carbon dioxide as a waste product, this therefore means that respiration and photosynthesis go together.
The Carbon Cycle
Carbon is used by living beings as a raw material in the synthesis of organic compounds through photosynthesis. It is oxidized in the process of respiration resulting in the release of carbon dioxide into the environment. The CO2 contributes, as we have seen, to the planet’s thermal balance, by avoiding the loss of heat to space by trapping it in the atmosphere, keeping the Earth warm enough to sustain life. However, the decomposition and burning of fossil fuels (coal and oil) also release CO2 into the environment.
The increase in the atmospheric CO2 aggravates the greenhouse effect that can lead to the thawing of the polar icecaps, with consequent increase in sea level and flooding of coastal cities.
The Sulfur Cycle
Sulfur is a yellowish substance found in the soil, which burns easily. It enters into the production of sulfuric acid, a substance widely used in fertilizers, dyes and explosives (gunpowder, matchsticks, etc.). Sulfur is found in sedimentary rocks, volcanic rocks, coals, natural gas, etc.
Sulfur is essential to life, it is part of protein molecules vital to our body. About 140 g of sulfur can be found on an average in a human body. Nature recycles sulfur whenever an animal or plant dies. When they decompose and rot, substances known as sulfates combine with water and are absorbed by the roots of plants. Animals acquire sulfur by eating plants or other animals.
Conclusion
Life occurs in the biosphere which is comprised of three parts: atmosphere – hydrosphere – lithosphere. Each of these parts contains organic and inorganic elements that through complex physical and chemical processes react with each other and with the elements in the other two parts in an eternal and interdependent recycling motion. As Lavoisier very well observed, in Nature, nothing is lost, nothing is created, everything self transforms.
Fr. Jorge Amaro, IMC