Our home in the solar system, Earth is the 3rd planet from the Sun. Home to countless lifeforms, of which we humans are a part of, planet Earth has long undergone study by humans. Therefore it is only natural that we know most about our own planet.
The ancient Greeks had calculated the Earth's radius to be about 6900 km. Satellite measurements place that value at 6378 km. By employing Isaac Newton's laws of gravity and observing the effects of the Earth's gravitational field on nearby objects, we can calculate the mass of the Earth, which has been found to be approximately 6.0 x 10^24 kg. We divide the Earth's mass by its volume to find the average density to be 5500 kg/m^3.
We now know that the Earth can be divided into seven main regions : the magnetosphere, atmosphere, hydrosphere, the crust, mantle, outer core and inner core.
Approximately 75% of the Earth's surface is covered by water, of which only 2% is fresh water. The remaining 98% is found in the oceans.
Tides are the result of the gravitational influence of the Moon and the Sun on the Earth. The pull of the Moon and the Sun on the Earth produces a tidal bulge. In fact, two different tidal bulges are present – one pointing towards the Moon while the other points towards the Sun. Being a liquid, the water in seas undergo deformation and this results in tides being formed.
There are 2 low tides and 2 high tides each day. The wave height may vary from a few centimetres to many metres, depending on the time of the year and Earth's location in orbit. The highest tides are usually found at times when the Earth, Sun and the Moon are roughly lined up. This is when the gravitational forces of the Sun and the Moon reinforce one another, producing spring tides. When the Moon and the Sun are at right angles to Earth, the lowest tides, or reap tides, are formed.
The tidal effect of the Moon is slowing down the Earth's rotation. The tidal bulge does not point directly at the Moon. With the Moon pulling at the bulge while the Earth rotates, the length of the day increases at 0.002 seconds every 100 years. The Moon is also gradually spiraling away from the Earth at a rate of 4 cm per century. All these will continue until the Earth's rotation rate matches that of the Moon. By then, a month would have 47 days, while the Moon would be 550000 km (143% of the present distance) away!
What makes Earth such a remarkable planet is the fact that its oxygen-rich atmosphere is able to support life. The Earth's atmosphere is made up of nitrogen (78%), oxygen (21%), argon (0.9%), carbon dioxide (0.03%) and water vapour (variable). The atmosphere also acts as a protective blanket by blocking out UV and X-ray radiation, screening out most meteoroids and keeping the temperature on Earth at a habitable level.
The atmosphere can be divided into several zones. The portion of atmosphere below 15 km is the troposphere. Above it, extending to an altitude of nearly 100 km, is the stratosphere.
The troposphere is where weather phenomena occurs. Convection currents due to the heat from the Earth's surface set up circulation patterns that contribute to atmospheric heating and surface winds.
The ozone layer is found at an altitude of about 50 km, in the stratosphere. It plays an important role in absorbing the harmful UV radiation from outer space. Life would never have begun on Earth if there was no ozone layer.
Above the atmosphere lies the ionosphere, where ionization of molecules and atoms occur, forming ions. The energy required for this process is provided by bombardment of our upper atmosphere by X-rays and gamma rays from the Sun. Radio waves of certain wavelengths can also be reflected off the ionosphere.
THE GREENHOUSE EFFECT
Not all of the Sun's radiation is screened off, with mostly visible and infrared radiation reaching the Earth's surface. This heats up the land in the day. At night, the Earth's surface re-radiates much of the heat absorbed back into space. The greenhouse gases present in our atmosphere absorbs part of the re-radiated heat while the rest escapes back into space. This process is known as the greenhouse effect and is what makes temperatures on our planet conducive for life.
Sadly, carbon dioxide levels are increasing, and have risen by about 20% in the last century. The unnatural increase in greenhouse gases have resulted in global warming. The effects of global warming are already beginning to show (rise in sea levels, melting of polar ice caps, glaciers breaking free from the poles and drifting towards the warmer regions, climatic change in many parts of the world) and if left unchecked, may have adverse effects on our planet's climate.
The magnetosphere is the region around the Earth that is influenced by our planet's magnetic field. It was detected in the late 1950s by satelites put into space to study Earth. Later satelites have also found that the magnetosphere contains the Van Allen belts, one almost 3000 km and the other 20000 km above the Earth's surface, which are doughnut shaped zones of high-energy particles.
The magnetosphere is the result of the spin of the iron-nickel core deep inside Earth. Magnetic field lines run from the geographical south pole to the geographical north in a 3-dimensional fashion. The geographical north and south poles are roughly aligned with the spin axes of the Earth.
An interesting and beautiful phenomenon in the Earth's magnetosphere is the aurora. They are the result of collisions between charged particles that have escaped from the Van Allen belts, and atmospheric molecules. The charged particles excite the atmospheric molecules and when the atmospheric molecules fall back into their ground state, visible light is emitted. Each molecule or atom may fall back into their ground state via different paths, thus producing many different colours. It is known as aurora borealis (northern lights) in the north and aurora australis (southern lights) in the south.
THE EARTH'S INTERIOR
Geologists have studied the interior of the Earth through many means. Drilling allows them a view of our planet's Earth's interior to only 10 km in depth (as compared to a radius of 6500 km), as no equipment is yet able to withstand the pressure beyond that point.
Seismic waves are produced during and after earthquakes. There are 2 types of seismic waves that allow geologists to study the Earth's interior – primary (P-) and secondary (S-) waves. S-waves are absorbed by liquids while P-waves are refracted by them. The presence of S- and P-wave "shadow zones" (where no P- or S-waves are detected after a quake) after an earthquake indicate the presence of a liquid core. However, faint P-waves can still be detected in P-wave shadow zones. This is believed to indicate the presence of a solid inner core and a liquid outer core (the faint p-waves are weakened and refracted by the liquid outer core).
The Earth is geologically active. This can be seen from the volcanic eruptions and earthquakes that are occuring at specific zones around the world.
Volcanic activity and earthquakes are most common along plate boundaries. The Earth's crust is divided into many plates, all of which are slowly moving, pulled along by the liquid upper mantle, or asthenosphere. These plate motions are responsible for the various surface features – mountains like the Alps, Himalayas, Andes ranges are the result of converging plates while ocean trenches (The Marianas Trench) and rift valleys (The Great African Rift Valley) are due to diverging plates. This process of plate movement is popularly known as 'continental drift' and the study of it is termed 'plate tectonics'. The plates move very slowly – about a few centimetres a year. The plate movements are the result of convection currents in the liquid upper mantle. Warm mantle rock rises and spreads while some rocks cool and falls to lower levels in the asthenosphere to be reheated by the core. A large circulation pattern is established atop which rides the plates.
Continental Drift and Plate Tectonics
Alfred Wegener first proposed his idea of continental drift theory in 1912 to the skeptism of everyone. At that time, no one knew how the plates could move. The theory of continental drift was widely accepted only in the mid-1960s, when the mechanism for the movement of the plates were known.
A closer look at the world map shows that all the continents more or less fit together like the pieces of a jig-saw puzzle. This suggests that at one point of time in Earth's past, all the continents were actually part of a huge landmass (named Pangaea by Wegener). Rare fossils of already-extinct reptiles and dinosaurs were found at different continents separated by oceans. Scientists believe that this could only be possible if these continents had once been joined together. Carbon-dating of rocks near the Mid-Atlantic Ridge show that they are relatively younger while those further away are much older. All these serve to give claim to the theory of plate tectonics.