The purpose of this report is to describe the most important features of Earth from a scientific point of view. After investigating the Earth system, four components are identified. They are namely atmosphere, biosphere, hydrosphere and geosphere. Different components are interconnected so that no single part of the system can work without any other. This report will focus on the composition, operation and evolution of different components as well as an Earth system as a whole. The report will begin by describing the four spheres one by one, followed by a conclusive overview of how the four spheres work together.

Hydrosphere and the hydrologic cycle

Earth is a blue planet with a wide surface coverage of water. It is approximated that 75% of the Earth surface, which equivalents to about 361 million m2, is covered by ocean. The hydrosphere is important as lives cannot exist without water.

Composition of hydrosphere

The hydrosphere is composed of all of the water on or near the earth. The total stock of it is approximately 1400 million km3 (Bronstert et al. 2005). This includes all forms of water in the oceans, rivers, lakes, and even the moisture in the air. Ninety-seven percent of the earth’s water is in the oceans while the remaining three percent is fresh water for which three-quarters of the fresh water is solid and exists in ice sheets.

The major reservoirs in the hydrologic cycle are surface water, groundwater and glacier (Bronstert et al. 2005).

Water cycle

The water cycle, also known as the hydrological cycle, describes the continuous movement of water on, above and below the surface of the Earth. It includes the processes of evaporation, condensation, precipitation, infiltration, runoff, and subsurface flow.

The Sun is the main energy to drive the whole hydrological cycle. Water takes up heat and evaporates as water vapor into the air. Water can be released out from plants through evapotranspiration. Ice and snow can change to gaseous form by sublimation. Water vapor is transferred by air to different lattitudes. They condense and fall as precipitation in the form of rain, snow, hail and sleet. The water can be stored in solid form as ice caps and glaciers for thousands of years. Most water falls back into the oceans or onto land as rain. The water flows over the ground is known as surface runoff and part of it flows into rivers. Much of it soaks into the ground as infiltration. Runoff and groundwater are stored as freshwater in lakes. Over time, the water returns to the ocean, where our water cycle started.

Hydrological cycle also involves the exchange of heat and contributes to temperature changes. For example, water takes up energy as latent heat when evaporates and thus lower the surrounding temperature. When it condenses, latent heat is released and warms the environment.

By transferring water from one reservoir to another, the hydrological cycle purifies water through infiltration, replenishes the land with freshwater, and transports minerals to different parts of the globe. Erosion and sedimentation reshape the geological features of the Earth. Moreover, the hydrological cycle helps in maintain life and ecosystems on Earth.

Evolution through history

The hydrosphere has been changed and evolved over the geological time. The amount and distribution pattern of precipitation, salinity of water, glacial pattern and the quality of freshwater have all been changed. Some are natural evolution but some are altered by human.

Precipitation

Salinity

Glacial retreat

Glacial retreat is also an example of a changing water cycle, where the supply of water to glaciers from precipitation cannot keep up with the loss of water from melting and sublimation. Glacial retreat since 1850 has been extensive.

Pollution and scarcity

Freshwater resources have been severely polluted by human especially since the industrial revolution. Human activities like agriculture and industry discharge enormous untreated contaminants into freshwater system through rivers and ground water. This leads to scarcity of freshwater to human in some regions.

Geosphere

The outer layers of the Earth are composed of lithosphere and asthenosphere. The lithosphere is the rigid outermost part consists of the crust and the portion of the upper mantle. The asthenosphere is the weaker and deeper part of the upper mantle which flows more easily.

The lithosphere is underlain by the asthenosphere. The boundary between them is defined by a difference in response to stress: the lithosphere remains rigid for very long periods of geologic time in which it deforms elastically and through brittle failure, while the asthenosphere deforms viscously and accommodates strain through plastic deformation.

Plate Tectonics

The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which ride on the fluid-like asthenosphere. Tectonic plates contain both oceanic lithosphere and continental lithosphere with its own kind of crust on top. There are seven, some say eight, major plates and many minor plates on Earth. There are three kinds of plate boundaries namely convergent, divergent and transform. Convergent boundary is where two plates collide; Divergent plate boundary is where plates move apart with each other; Transform boundary is where plates slide past each other. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries.

Lithosphere’s properties of bigger strength and lower density allow tectonic plates to float and move on asthenosphere. Lateral density variations in the mantle result in convection.

The relative motion between the plates is accommodated by seafloor spreading and the creation of new plates at oceanic ridges, subduction of the surface plates at ocean trenches, and strike-slip motion at transform faults which allows plate motion without creating or removing surface plates. (Landuyt, William 2009)

Evolution

It is believed that the present continents once formed a single land mass Pangea. This supercontinent existed during the late Paleozoic and early Mesozoic eras, forming about 300 million years ago and beginning to rift around 200 million years ago.

In the Early Jurassic at 175 Ma, Pangea was begun to separate and form two supercontinents, which were Gondwana and Laurasia.

Gondwana included most of the landmasses in today’s Southern Hemisphere, including Antarctica, South America, Africa, Madagascar and the Australian continent, as well as the Arabian Peninsula and the Indian subcontinent, which have now moved entirely into the Northern Hemisphere.

The plates move slowly, leading to the positions of continents and oceans today by collision and separation. The break-up of Pangea still continues today. Therefore, the distribution of continents and ocean on Earth is very likely to be changed gradually in the future.

Atmosphere

The atmosphere of Earth is a layer of gases retained by Earth’s gravity surrounding the planet. It can absorb ultraviolet solar radiation, warm the surface through greenhouse effect, and reducing temperature extremes between day and night. The importance of atmosphere is to sustain life on Earth.

Structure

The atmosphere can be divided it into several layers with different rate of change in temperature with height and composition. The Earth’s atmosphere consists, from the ground up, of the troposphere, stratosphere, mesosphere, thermosphere, exosphere and also the magnetosphere.

Variation of properties through the layers of the atmosphere

Figure 1 cr: http://www.aerospaceweb.org/question/atmosphere/q0090.shtml

Composition nowadays

The atmospheric composition on Earth is largely governed by the by-products of the very life that it sustains. Earth’s atmosphere contains roughly (by molar content/volume) 78.08% nitrogen, 20.95% oxygen, a variable amount (average around 1.247%) water vapor, 0.93% argon, 0.038% carbon dioxide, and traces of hydrogen, helium, and other “noble” gases.

Additionally, among all layers of Earth atmosphere, only the troposphere is found to be suitable for terrestrial plants and terrestrial animals.

Evolution

The earliest atmosphere was mainly consisted of hydrogen. Moreover, it was likely that there were simple hydrides, especially methane, ammonia and water vapor. The atmosphere then evolved after some time, containing lots of nitrogen and carbon dioxide as well as inert gases. Outgassing from volcanoes contributed to the evolution of atmosphere at this stage. Besides, large asteroids bombarded the Earth, also producing gases took part in the evolution. Furthermore, much carbon dioxide exhalations were dissolved in ocean made up heavy rainfall.

Nitrogen was the major component of the atmosphere 3.4 billion years ago. An influence of life has to be taken into account rather soon in the history of the atmosphere, since hints of early life forms are to be found as early as 3.5 billion years ago.

Oxygen began to develop in atmosphere in the late Archaean eon about 2.7 billion years ago. Photosynthesizing algae as stromatolite is believed to contribute to it.

Free oxygen did not exist until about 1.7 billion years ago. The evidence of it is the existence of the red beds and the end of the banded iron formations. Iron was oxidized by oxygen and the oxygen content did not get high until the huge amount of iron had been oxidized. This marks a change from a reducing atmosphere to an oxidizing atmosphere.

The accretion of continents about 3.5 billion years ago added plate tectonics, constantly rearranging the continents and also shaping long-term climate evolution by allowing the transfer of carbon dioxide to large land-based carbonate stores.

There was a peak 280 million years ago, when the amount of oxygen was about 30%, much higher than today. The process of plants emitting oxygen and the volcanoes effect on sulphur affect the amount of oxygen. The break down of pyrite rocks cause sulphur to be added to the oceans. Volcanos cause this sulphur to be oxidized, reducing the amount of oxygen in the atmosphere. Nevertheless, volcanos also emit carbon dioxide which can be converted into oxygen by plants.

Recent: Air pollution and increasing greenhouse gases

Air pollution is the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to organisms into the atmosphere. Human activities emitted huge amount of greenhouse gases such as carbon dioxides and methane into the atmosphere. It is believed that the anthropogenic alteration of the atmospheric gases causes global warming on Earth. In addition, the increase in CFCs usage by human is also believed ozone to be the reason for ozone depletion in the stratosphere.

Biosphere

It is estimated that the biosphere have begun to evolve 3.5 billion years ago. The biosphere is the global sum of all ecosystems. It can also be called the zone of life on Earth, a closed and self-regulating system. The biosphere is the global ecological system integrating all living beings and their relationships, including their interaction with the elements of the lithosphere, hydrosphere, and atmosphere.

The biosphere is divided into a number of biomes, inhabited by broadly similar flora and fauna. On land, biomes are separated primarily by latitude. Terrestrial biomes lying within the Arctic and Antarctic Circles are relatively barren of plant and animal life, while most of the more populous biomes lie near the equator. Terrestrial organisms in temperate and Arctic biomes have relatively small amounts of total biomass, smaller energy budgets, and display prominent adaptations to cold, including world-spanning migrations, social adaptations, homeothermy, estivation and multiple layers of insulation.

Evolution

Origins of life

There is a lot of research on the origin of life. One of the idea is that the beginning of life may have included self-replicating molecules such as RNA and the assembly of simple cells.

Evolution of life

Prokaryotes inhabited the Earth from approximately 3—4 billion years ago.[255][256] No obvious changes in morphology or cellular organisation occurred in these organisms over the next few billion years.[257] The eukaryotic cells emerged between 1.6 â€” 2.7 billion years ago. The next major change in cell structure came when bacteria were engulfed by eukaryotic cells, in a cooperative association called endosymbiosis.[258][259] The engulfed bacteria and the host cell then underwent co-evolution, with the bacteria evolving into either mitochondria or hydrogenosomes.[260] Another engulfment of cyanobacterial-like organisms led to the formation of chloroplasts in algae and plants.[261]

The history of life was that of the unicellular eukaryotes, prokaryotes and archaea until about 610 million years ago when multicellular organisms began to appear in the oceans in the Ediacaran period.[255][262] The evolution of multicellularity occurred in multiple independent events, in organisms as diverse as sponges, brown algae, cyanobacteria, slime moulds and myxobacteria.[263]

Soon after the emergence of these first multicellular organisms, a remarkable amount of biological diversity appeared over approximately 10 million years, in an event called the Cambrian explosion. Here, the majority of types of modern animals appeared in the fossil record, as well as unique lineages that subsequently became extinct.[264] Various triggers for the Cambrian explosion have been proposed, including the accumulation of oxygen in the atmosphere from photosynthesis.[265]

About 500 million years ago, plants and fungi colonised the land and were soon followed by arthropods and other animals.[266] Insects were particularly successful and even today make up the majority of animal species.[267] Amphibians first appeared around 364 million years ago, followed by early amniotes and birds around 155 million years ago (both from “reptile”-like lineages), mammals around 129 million years ago, homininae around 10 million years ago and modern humans around 250,000 years ago.[268][269][270] However, despite the evolution of these large animals, smaller organisms similar to the types that evolved early in this process continue to be highly successful and dominate the Earth, with the majority of both biomass and species being prokaryotes.[151]

3.6 billion years of simple cells (prokaryotes),

3.4 billion years of stromatolites demonstrating photosynthesis,

2 billion years of complex cells (eukaryotes),

1 billion years of multicellular life,

600 million years of simple animals,

570 million years of arthropods (ancestors of insects, arachnids and crustaceans),

550 million years of complex animals,

500 million years of fish and proto-amphibians,

475 million years of land plants,

400 million years of insects and seeds,

360 million years of amphibians,

300 million years of reptiles,

200 million years of mammals,

150 million years of birds,

130 million years of flowers,

65 million years since the dinosaurs died out,

2.5 million years since the appearance of the genus Homo,

200,000 years of anatomically modern humans,

25,000 years since the disappearance of Neanderthal traits from the fossil record.

13,000 years since the disappearance of Homo floresiensis from the fossil record.

Conclusion

The characteristics of the four components (atmosphere, biosphere, hydrosphere and geosphere) of the Earth system have been summarized by discussing each of their composition and evolution history. The four spheres are linked to and interact with each other to sustain the Earth. The planet Earth has a history of 4.6 billion years. Lots of things have been changed and evolved since its formation. The evolution is going to continue.

Landuyt,William, I.,II. (2009). The generation of plate tectonics on a planet. Yale University). ProQuest Dissertations and Theses, , 195. Retrieved from http://search.proquest.com/docview/305041644?accountid=14548. (305041644))

Axel Bronstert, Jesus Carrera, Pavel Kabat, Sabine Lutkemeier.(2005).Coupled Models for the Hydrological: CycleIntegrating Atmosphere, Biosphere, and Pedosphere. Springer Berlin Heidelberg