SOLAR SYSTEM FAMILY-FACTS ABOUT EARTH, THE BLUE PLANET

  Earth is probably the only planet in our wide universe that speaks of something called 'life'. In spite of the fact that our universe is a place with infinite possibilities, it is Earth, whose surface is teeming with myriad species of flora and fauna. It is also here, where a species(who call themselves 'men') fought thousands of intellectual battles over the obvious shape of their home planet. Even today, after widespread scientific advancements there are some souls who still believe that the Earth is a flat land, placed above a giant tortoise, and has been created by a very special being, particularly for 'man'. But, science on the other hand aims to give explanations of the various natural phenomena, and is always open to questions, challenges and new ideas. Come, my fellow travelers, let's settle down and enlighten ourselves with certain facts about our home- The Planet Earth.

A Bit of Mythology- The word ''Earth'' is supposed to originate from the Latin word ''Terra'' meaning land, soil or ground. According to Roman mythology, goddess ''Tellus or Terra Mater'' is the personification of Earth, and in Norse Mythology, Earth has been personified as the mother of the legendary thunder god-Thor. However, it has been found that our planet does not have much mythological significance compared to other planets, and Earth has never been a major god or goddess.

Basic Facts: Earth is the third terrestrial planet in our solar system, located snuggly within the Sun's habitable zone, and receives an optimum amount of warmth required to sustain large bodies of liquid water and harbour life. It has also got a natural satellite, the moon. And finally this blue green, and quite an insignificant planet in the vast depths of interstellar space has got an innumerable species of plants, animals, microorganisms and a teeny tiny fraction of mumbling, jabbering human beings. 

A cute guy that inhabits Earth/Credits:Photo by form PxHere

Planetary Characteristics:

• Rotational & Orbital Characteristics: Earth completes its orbit around the Sun once in every 365 solar days(365 days, 5 hours, 48 minutes and a few seconds to be precise) with an orbital speed of 29.78km/s or 107,200 km/h. A solar day is defined as the time taken by our planet to spin(rotate) about its axis so that the Sun appears on the same position(meridian) again. The planet is situated within the Goldilocks(Habitable) Zone at an average distance of 150 million km from the Sun, with an orbital eccentricity of 0.016. Since Earth's orbit is a bit elliptical, so during aphelion the planet remains at a separation of 147 million km from the Sun, and at perihelion this separation becomes 149 million km. Earth has also got an axial tilt of 23.43° with respect to the axis of the orbital plane. This axial tilt is the reason for the formation of 'seasons' when different places receive different levels of solar radiation. Summer occurs in the Northern Hemisphere as the Tropic of Cancer faces the Sun, and winter occurs in the Southern Hemisphere as the Tropic of Capricorn faces away from the Sun.  

• Internal Structure: Earth is fairly big compared to other terrestrial planets of our solar system, with an equatorial radius of  6378 km. Its mean density is about 5.51 g/cc, with its core occupying 17% of the total volume. Since Earth is our home planet, so geologists have been able to determine its internal structure to much detail and accuracy. The subsequent layers are as follows: 

1. Crust-The Crust is the outermost layer, extending from 5-70 km towards the interior and is composed of Oxygen, Silicon, Aluminum, Iron, Calcium, Sodium, Potassium, Magnesium, Titanium and Phosphorous. The crust together with the next layer, the upper mantle(also called the Asthenosphere) forms the Lithosphere. The lithosphere is divided into a number of jigsaw puzzle like blocks called Tectonic Plates, that slide over the asthenosphere. There are seven major tectonic plates viz., Eurasian, African, Antarctic, North American, South American, Pacific and the Indo-Australian, with a bunch of several minor plates.
2. Mantle- The mantle being the thickest layer, extends to 2890 km towards the core and is primarily composed of silicate rock. It is again subdivided into the outer and the inner mantle. The mantle has a temperature ranging between 200-4000°C, and pressure ranging from some 24-136 gigapascals at the core-mantle boundary. Convection of molten rock occur in this region which ultimately drives the tectonic plates and also forms the global geo-magnetic field.
3. Core- The core extends from 2890-6378 km below the surface, and has a temperature ranging from 4000-6000°C. The core is extremely dense with a density of 13g/cc compared to 5.51g/cc of  mean density. It is also hotter than the Sun's surface and has a pressure of 135-330 gigapascals(1.3-3.3 million atmospheres, where 1 atmosphere is the pressure we experience at mean sea level). The core is also divided into the outer and the inner core.

• Surface Geology: Earth's surface features are in a process of continuous change due to tectonic activity and all sorts of weather phenomena. Earth's crust is covered by a very thin layer of soil called the ''Pedosphere'', where plants grow and flourish and various biochemical reactions take place. Circulation of magma and the movement of the tectonic plates give rise to mountains, rift valleys, oceanic trenches, island chains and continental landmasses. Once upon a time, Earth's surface was dominated by a single landmass called "Pangaea or Pangea'', which ultimately got broken up into smaller pieces and formed the seven continents that we inhabit today. As mentioned earlier, Earth's surface is being constantly reshaped by a variety of natural processes like volcanism, earthquakes, glaciation, erosion, weathering, biochemical processes and sometimes by deadly meteorite or an asteroid strike.  

• Surface Temperature: The global average surface temperature on Earth is nearly 15℃, with temperature extremes ranging from -89℃ in Antarctica and +56℃ in Death Valley, California, USA. However, the global temperature is predicted to rise by 1-3℃ by the end of the next century because of anthropogenic activities.

Typical of Earth's surface/Credits:Photo by form PxHere

• Atmosphere: Earth's atmosphere has been under constant modification since the dawn of the first simple form of micro-organisms. The cyanobacteria developed the Oxygen-Nitrogen atmosphere, which ultimately led to the proliferation of aerobic bacteria and other forms of micro-organisms. The newly formed O₂ transformed into Ozone(O₃), and Earth got its life giving O₃ layer. The ozone layer protects all lifeforms on Earth by filtering out the harmful ionizing radiation(UV Radiation) from the Sun. The atmospheric pressure at mean sea level is 101.325 kPa(14.96 psi) and is composed of nearly 78% Nitrogen, 20.9% Oxygen, 0.9% Argon with traces of Carbon Dioxide, Water Vapour, Neon, Krypton, Helium and Hydrogen. The atmosphere is again subdivided into subsequent layers, which are as follows-

1. Troposphere- Extends up to 10 km above sea-level. Almost all sorts of weather phenomena take place in this region.
2. Stratosphere & Ozone Layer-Extends up to 50 km above mean sea level. The ozone layer exists within the stratosphere, where it absorbs the incoming UV rays and traps the solar heat.
3. Mesosphere- Extends up to 85 km above mean sea level. Atmospheric pressure and temperature is extremely low. Smaller and moderate sized meteors typically burn up in this region.
4. Thermosphere- The thermosphere varies in altitude from 500-1000 km above mean sea level. Most of the incoming X-rays and UV rays from the Sun are absorbed in this region. The spectacular Aurora (Northern/Southern lights) occur in this region as the solar flares interact directly with the atmospheric gases.
5. Ionosphere-This region extends beyond 1000 km from mean sea level and slowly fades away with increasing rarefication of air. Here ionizing radiation from the Sun strip away electrons from their respective atoms and produce ions of various elements that exist in the atmosphere.
6. Exosphere & Magnetosphere- This region extends up to 10,000 km and slowly merges into space. Since Earth possesses a good magnetic field, the incoming solar radiation gets deflected and forms an envelope of charged particles around our planet, called the Van Allen Radiation Belt. 

• Hydrosphere & Climate: The hydrosphere includes all sorts of water bodies from streams, rivers, lakes, ponds and oceans. Earth is basically a water world with a total volume of 1386 million cubic kilometers or 326 million trillion gallons(1,260,000,000,000,000,000,000 litres) of all forms of water including ice, liquid water, ground water, water vapour and even clouds. However, almost 96% of this water is saline and unfit for human use, while the rest of fresh water is locked within permanent ice floors, glaciers, permafrost and groundwater. For our daily use we have only a tiny fraction(0.3%) of the global water content, and again nearly most of this 0.3% is unattainable. All this water, is part of a constant cycle called the Water Cycle or Hydrological Cycle. The hydrological cycle along with the atmosphere and variation of seasons controls global climate which is a unique feature of our planet. The atmosphere is a dynamic place with constant redistribution of thermal energy. Oceans produce ocean currents, the atmosphere generates rain bearing clouds which bring storms and heavy rainfall, and the winds carry hot or cold air from one place to another. The climate mechanism also plays an essential role in the survivability of all life on Earth.  

• Magnetic Field:  Earth acts like a giant magnetic dipole(a normal magnet having North and South poles) with a stable magnetic field generated by the convection currents within the core and the mantle. This magnetic field extends far out into space and forms the magnetosphere. The magnetosphere deflects solar plasma, and these interactions create some of the spectacular light shows in the Arctic and the Antarctic Circle. 

Components of Earth's Magnetosphere/Image Credits: !Original:NASAVector: Aaron Kaase, Medium69, Public domain, via Wikimedia Commons

• Natural Satellite(s): Earth has got a very good and beautiful natural satellite, called the ''Moon'' or ''Luna''. The moon is thought to have formed during the early days of Earth's formation. The moon has an equatorial diameter of 3478 km and shines from reflecting sunlight. The Earth-Moon system exerts tidal forces on each other, which results in the formation of ocean tides. The Moon is also believed to have played a significant role in changing Earth's geology and climate, which ultimately helped in the proliferation of life. 

Life & Biological Organisms: Earth has a wide variety of flora and fauna. It is estimated that the first biological organisms originated in the oceans, a billion years later after the planet's formation. These organisms were mostly single celled micro-organisms, called prokaryotes that made oxygen using sun-light and water. Afterwards the Great Oxidation Event occurred which led to widespread proliferation of multi-cellular and complex organisms, and finally these life forms evolved into the species that we see today. As mentioned earlier, life first appeared in deep oceanic trenches, slowly migrated to other parts of the world and finally the whole wide Earth was flooded with all sorts of living organisms viz., plants, animals, birds, insects, viruses, bacteria, protozoa and other cellular organisms.

                                                        But, life on Earth also suffered some catastrophic mass extinction events, with the recent one having occurred nearly 65 million years ago that wiped away the famous dinosaurs and other giant species. Until today, Earth has gone through five major mass extinctions viz., the Ordovician Extinction(nearly 445 million years ago), the Devonian Extinction(nearly 385-360 million years ago), the Permian Extinction(nearly 250 million years ago), the Triassic Extinction(nearly 200 million years ago) and finally the Cretaceous Extinction(nearly 65 million years ago). It is said that 99% of all species that walked on this planet are now extinct, like the dinosaurs, mammoths, dodo birds, and alike. However, life has a nasty habit of getting rebounded again and again. Apes came into existence after the last mega extinction event, and gradually evolved into today's human beings. Among all the biological beings, humans are the only living organisms with a heightened level of consciousness and a sense of their surrounding environment. 

Timeline of Earth/Credits:WoudloperDerivative work: Hardwigg, Public domain, via Wikimedia Commons

Observations & Explorations: 

• Observations: Earth is kept under constant observation with the help of some 2600+ artificial satellites launched by different nations. Some of those satellites are placed in LEO(Low Earth Orbit; altitude of 2000 km from surface), some in Geosynchronous Orbit(orbits that match with Earth's rotation about its axis; altitude of 35,000+ km from surface), while the rest are placed in Polar Orbits(orbit that passes above the two poles, North & South; altitude of 700-800 km from surface). The types of satellites that orbit Earth are as follows:

1. Astronomical Satellites- These are satellites that are used for astronomical observations, like taking high resolution images and other scientific data about distant stars, planets, and galaxies. Common examples would be the Hubble Space Telescope, Spitzer Space Telescope, Kepler Space Telescope, Chandra X-Ray Observatory and others.
2. Military Satellites- These are satellites that are put up by different countries primarily for the observation of international borders, military communications, tracking of certain military exercises, early response and warning systems against potential warheads and hostile troop movements.
3. Communication Satellites- As the name suggests, these are used for tele-communications. They receive and transmit signals to connect different parts of the world in just a tap. Some examples would be INSAT Satellites, GSAT Satellites, NATO Satellites, Starlink and others.
4. Earth Observation Satellites- These are non-military satellites used particularly for observing environmental impacts and changes across the globe that includes deforestation, floods, forest fire, and soil erosion. These are also used to map terrain and landforms across the globe and generate high resolution images of our planet for various purposes. Weather Satellites also fall under this category, which observes and predicts weather and climate phenomenon across the globe. Examples are GOES, NOAA Satellites, DSCOVR, INSAT Weather Satellites and similar others. 
5. Navigation Satellites- These are used for navigational purposes. Examples are GNSS, GPS, QZSS, IRNSS and others.
6. Space Stations- These are like hotels in space which are capable of supporting human crew for long periods of time. Space Stations are mainly built for the purpose of carrying out different scientific experiments and observations that are not possible on Earth's surface. Presently there's the International Space Station(ISS) situated at 400 km above surface. Former stations include the Skylab, Mir, Tiangong and Salyut.

• Explorations: Human beings, or rather early hominids appeared on Earth nearly 2 million years ago. But it was not until 70,00-100,000 years ago, when our distant ancestors-the Homo sapiens started to migrate away from the far interior of Africa and began settling on other parts of the world. And slowly they became settlers as they constructed homes for their family and themselves and again ventured out for long journeys. At one point men were confined within the boundaries of their local region, as they were too scared to wander away into the deep forests, scorching deserts and turbulent oceans. But this fear did not last long enough. With the rise of great empires and modern civilisations viz., the Phoenicians, Greeks, Romans, Egyptians, Chinese, Polynesians and countless others, our Earth started to look like a familiar place. This thirst for exploration passed into the modern age and from the mid 15-th century Earth started to witness some of man's greatest explorations as explorers like Vasco da Gama, Bartolomeu Dias, Christopher Columbus, Ponce de Leon and many others discovered new lands and frontiers. Finally from the 20th Century man learnt the art of flying and with a sudden boom of technological advancements he conquered the land, the air, the water and even space to some extent. Right now, there's hardly any land left where man hasn't set his foot.

Map of Earth showing artificial lighting from its night side/Credits:Photo by form PxHere     

Very Distant Future: Earth's near future is solely dependent upon human activities and their choices. Man's technological advancements, and a fruitless race towards establishing superiority has been continually pushing the planet's biosphere into a gradual ecological collapse. Drawing an estimate from our current activities it can be said that within a couple hundred years humanity is going to suffer from devastating effects of climate change. Humanity's greatest threats are loss of biodiversity and an ecological collapse, global terrorism, nuclear and biological weapons, starvation, and an exponential loss of morality. But there's always room for improvement if the human species can set aside their differences and work together for a common cause.

                                           However, the very far future of our planet does not depend upon the human species but on the Sun. In about 5-8 billion years from now the Sun would expand into a Red Giant Star, and might incinerate our planet, as the former sheds its outer layers. If humanity survives till that day, and becomes a galactic civilization, then they could watch a spectacular show from another star system. Who knows?

Some Quick Facts!