Universe Today
In addition to being the birthplace of humanity and the cradle of human civilization, Earth is the only known planet in our Solar System that is capable of sustaining life. As a terrestrial planet, Earth is located within the Inner Solar System between between Venus and Mars (which are also terrestrial planets). This place Earth in a prime location with regards to our Sun's Habitable Zone.
Earth has a number of nicknames, including the Blue Planet, Gaia, Terra, and "the world" - which reflects its centrality to the creation stories of every single human culture that has ever existed. But the most remarkable thing about our planet is its diversity. Not only are there an endless array of plants, animals, avians, insects and mammals, but they exist in every terrestrial environment. So how exactly did Earth come to be the fertile, life-giving place we all know and love?
moreSize, Mass, and Orbit:
With a mean radius of 6371 km and a mass of 5.97×1024 kg, Earth is the fifth largest and fifth most-massive planet in the Solar System. In essence, it is the largest terrestrial planet, but is smaller and less massive than any of the gas/ice giants of the Outer Solar System. And with a mean density of 5.514 g/cm³, it is the densest planet in the Solar System.
In terms of its orbit, Earth has a very minor eccentricity (approx. 0.0167) and ranges in its distance from the Sun from 147,095,000 km (0.983 AU) at perihelion to 151,930,000 km (1.015 AU) at aphelion. This works out to an average distance (aka. semi-major axis) of 149,598,261 km, which is the basis of a single Astronomical Unit (AU).
The Earth has an orbital period of 365.25 days, which is the equivalent of 1.000017 Julian years. This means that every four years (in what is known as a Leap Year), the Earth calendar must include an extra day. Though technically a full day is considered to be 24 hours long, our planet takes precisely 23h 56m and 4 s to complete a single sidereal rotation (0.997 Earth days).
Viewed from the celestial north pole, the motion of Earth and its axial rotation appear counterclockwise. From the vantage point above the north poles of both the Sun and Earth, Earth orbits the Sun in a counterclockwise direction.
Earth's axis is tilted 23.439281° away from the perpendicular of its orbital plane, which is responsible for producing seasonal variations on the planet's surface with a period of one tropical year (365.24 solar days). In addition to producing variations in terms of temperature, this also results in variations in the amount of sunlight a hemisphere receives during the course of a year.
Basically, when the North Pole is pointing towards the Sun, the northern hemisphere experiences summer and the southern hemisphere experiences winter. During the summer, the day lasts longer and the Sun climbs higher in the sky; while in winter, the climate becomes generally cooler, the days are shorter and the Sun appears lower in the sky.
Above the Arctic Circle, an extreme case is reached where there is no daylight at all for part of the year - up to six months at the North Pole itself, which is known as a "polar night". In the southern hemisphere, the situation is exactly reversed, with the South Pole experiencing a "midnight sun" - i.e. a day of 24 hours.
Earth's Structure and Composition:
The shape of Earth approximates that of an oblate spheroid, a sphere flattened along the axis from pole to pole such that there is a bulge around the equator. This bulge results from the rotation of Earth, and causes the diameter at the equator to be 43 kilometres (27 mi) larger than the pole-to-pole diameter.
Earth's interior structure, like that of other terrestrial planets, is differentiated between a metallic core and mantle composed of rock and silicate materials. However, unlike other terrestrial planets, it has a distinct inner core of solid material and a liquid outer core. This inner core has an estimated radius of 1,220 km, while the outer core extends beyond it to a radius of about 3,400 km.
Extending outwards from the core are the mantle and the crust. Earth's mantle extends to a depth of 2,890 km, making it the thickest layer of Earth. This layer is composed of silicate rocks that are rich in iron and magnesium relative to the overlying crust. Although solid, the high temperatures within the mantle cause the silicate material to be sufficiently ductile that it can flow on very long timescales.
The upper layer of the mantle is divided into the lithospheric mantle (aka. the lithosphere) and the asthenosphere. The former consists of the crust and the cold, rigid, top part of the upper mantle (which the tectonic plates are composed of) while the asthenosphere is the relatively low-viscosity layer on which the lithosphere rides.
The mechanically rigid lithosphere is broken into pieces called tectonic plates. These plates are rigid segments that move in relation to one another at one of three types of plate boundaries. These are known as convergent boundaries, at which two plates come together; divergent boundaries, at which two plates are pulled apart; and transform boundaries, in which two plates slide past one another laterally.
Interactions between these plates are what is responsible for earthquakes, volcanic activity (such as the "Pacific Ring of Fire"), mountain-building, and oceanic trench formation. As the tectonic plates migrate across the planet, the ocean floor is subducted under the leading edges of the plates at convergent boundaries. At the same time, the upwelling of mantle material at divergent boundaries creates mid-ocean ridges. The combination of these processes continually recycles the oceanic crust back into the mantle.
The seven major plates are the Pacific, North American, Eurasian, African, Antarctic, Indo-Australian, and South American. Other notable plates include the Arabian Plate, the Caribbean Plate, the Nazca Plate off the west coast of South America and the Scotia Plate in the southern Atlantic Ocean.
Earth's Surface Features:
Unlike other planet's in our Solar System, the majority of Earth's surface is covered in liquid water. In fact, about 70.8% of the surface - which works out to 361.132 million km² (139.43 million sq mi) - is covered by water, with much of the continental shelf below sea level. The remaining 148.94 million km² (57.5 million sq mi) is above sea level.
Whether it is underwater or above sea level, Earth's terrain varies greatly from place to place. The submerged surface has mountainous features, as well as undersea volcanoes, oceanic trenches, submarine canyons, oceanic plateaus and abyssal plains. The remaining portions of the surface are covered by mountains, deserts, plains, plateaus, and other landforms.
Over long periods known as geological time, the surface undergoes reshaping due to a combination of tectonic activity and erosion. Those features that are built up or altered by plate tectonics are subject to steady weathering and erosion from precipitation, flowing water, thermal cycles and chemical effects. Glaciation, coastal erosion, the build-up of coral reefs, and large meteorite impacts also act to reshape the landscape.
The continental crust is made up of three kind of lower-density rock material - igneous rock, sedimentary rock, and metaphormic rock. Igneous rock can be subdivided into granite and andesite (which are the most common) and basalt, a denser form of volcanic rock that is less common on the surface but accounts for the majority of the ocean floors.
Sedimentary rock, which makes up 75% of continental surfaces (though only 5% of the crust), is formed when accumulated sediment is buried and is compacted. Metaphormic rock is the result of igneos and/or sedimentary rock undergo transformation due to heat and pressure, and go on to form materials like gneiss, slate, marble, schist, and quartzite.
The elevation of the land surface varies from the lowest point of -418 m (at the Dead Sea) to the estimated maximum altitude of 8,848 m at the top of Mount Everest. The average height of land above sea level is 840 m. Commonly, the planet is divided between northern and southern hemisphere, though the somewhat arbitrary division between eastern and western hemispheres is also acknowledged. The Earth's land masses are also divided between the seven continents of Africa, Asia, Australia, Europe, North, South America and Antarctica.
The outermost layer of the Earth's surface (known as the pedosphere) is where soil exists, a combination of minerals and organic compounds. This layer exists as the interface between the lithosphere, atmosphere, hydrosphere (all watery surfaces in the world) and biosphere (where all terrestrial life exists).
The total amount of arable land makes up approximately 13.31% of the Earth's surface, with 4.71% supporting permanent crops. Close to 40% of Earth's land is used for cropland and pasture, or an estimated 1.3×107 km2 being used for cropland and 3.4×107 km2 for pastureland.
Earth's Atmosphere:
Earth's atmosphere is made up of five main layers - the Troposphere, the Stratosphere, the Mesosphere, the Thermosphere, and the Exosphere. As a rule, air pressure and density decrease the higher one goes into the atmosphere and the farther one is from the surface. However, the relationship between temperature and altitude is more complicated, and may even rise with altitude in some cases.
Earth's Average Temperature:
And given that Earth has an inclined axis (approximately 23.4° towards the Sun’s equator), the Northern and Southern Hemispheres of Earth are either tilted towards or away from the Sun during the summer and winter seasons, respectively. And given that equatorial regions of the Earth are closer to the Sun, and certain parts of the world experience more sunlight and less cloud cover, temperatures range widely across the planet.
However, not every region on the planet experiences four seasons. At the equator, the temperature is on average higher and the region does not experience cold and hot seasons in the same way the Northern and Southern Hemispheres do. This is because the amount of sunlight the reaches the equator changes very little during the course of the year.
The Moon and Near-Earth Asteroids:
Earth has only one orbiting satellite, The Moon. It's existence has been known of since prehistoric times, and it has played a major role in the mythological and astronomical traditions of all human cultures. A number of cultures saw it as a deity while others believed that its movements and phenomena associated with it could help predict worldly events.
In the modern era, the Moon has continued to serve as a focal point for astronomical and scientific research, as well as space exploration. In fact, the Moon is the only celestial body outside of Earth that humans have actually walked on. The first Moon landing took place on July 20th, 1969, and Neil Armstrong was the first person to set foot on the surface. Since that time, a total of 13 astronauts have been to the Moon, and the research that they carried out has been instrumental in helping us to learn about its composition and formation.
Thanks to examinations of Moon rocks that were brought back to Earth, the predominant theory states that the Moon was created roughly 4.5 billion years ago from a collision between Earth and a Mars-sized object (known as Theia). This collision created a massive cloud of debris that began circling our planet, which eventually coalesced to form the Moon we see today.
The Moon is one of the largest natural satellites in the Solar System and is the second-densest satellite of those whose densities are known (after Jupiter's satellite Io). It is also tidally locked with Earth, meaning that one side is constantly facing towards us while the other is facing away. The far side, known as the "Dark Side", remained unknown to humans until probes were sent to photograph it.
The helium and neon are the result of solar wind while the argon comes from the natural, radioactive decay of potassium in the Moon’s interior. There is also evidence of frozen water existing in permanently shadowed craters, and potentially below the soil itself. The water may have been blown in by the solar wind or deposited by comets.
The surface of the Moon is divided into different types of terrain. Maria are the flat plains, which is Latin for “seas", since ancient astronomers thought they were actual seas filled with water. Terre ("Earth") refers to the highlands, which appear lighter colored because they are closer to our own planet. There are also numerous mountainous regions on the Moon, and the surface is marred by many craters that are a result of impact by asteroids and other space debris.
Earth's Formation and Evolution:
Since the 18th century, the scientific consensus has been that the Earth and the entire Solar System was formed out of a cloud of nebulous material (aka. "Nebula Theory"). According to this theory, roughly 4.6 billion years ago, the entire Solar System was a circumstellar disk made up of gas, ice grains and dust. In time, most of this matter accumulated in the center and underwent a gravitational collapse, forming the Sun.
Earth's Habitability:
Any planet which boasts conditions that are amenable to the existence of life is deemed as being habitable. At present, Earth is the only known planet that is capable of supporting life. By studying Earth's climate, ecosystems, and the diverse nature of organisms that exist here, scientists have learned much about what conditions are needed in order for life to both develop and flourish in a planetary environment.
For one, a planet needs to have liquid water on its surface - i.e. an environment where complex molecules can assemble and interact. Second, it needs to be able to receive sufficient energy from its parent star in order to sustain metabolism. Third, it needs to be able to maintain an atmosphere that will shield organic life from harmful solar radiation.
Earth's distance from our Sun, which places it inside it's "Goldilocks Zone" (aka. "Habitable Zone"), ensures that it is neither too hot or cold. It is thus able to maintain liquid water on its surface, and its atmosphere (and magnetosphere) protect it from harmful radiation and solar rays. It's orbital eccentricity, rate of rotation, axial tilt, and geological history, all contribute to the current climatic conditions that contribute to the existence of life.
Venus, being on the inner edge of the Sun's habitable zone, is subject to a runaway greenhouse effect, where atmospheric pressure is too intense, and the concentrations of greenhouse gases and extreme heat make it hostile to life.
Mars, sitting on the outer edge of the zone, is too cold and has an atmosphere that is too thin to support life. While scientists are certain that Mars once had an atmosphere and warm, flowing water on its surface, this period ended an estimated 3.8 billion years ago.
History of Study:
Since ancient times, human beings have sought to explain the creation of Earth, the universe, and all life. The earliest known cases were unscientific in nature - taking the form of creation myths or religious fables involving the gods. However, between classical antiquity and the medieval period, several theories emerged about the origin of the Earth, its true shape, and its place in the cosmos.
To many ancient cultures, the Earth was personified as a deity - often as a "mother goddess" that was associated with fertility. Hence why many creation myths begin with a story in which the creation of the world involved an act of celestial procreation, where a goddess gave birth to all life.
With the rise of the Roman Empire and their adoption of Hellenistic astronomy, the view of a spherical Earth became widespread throughout the Mediterranean and Europe. This knowledge was preserved thanks to the monastic tradition and Scholasticism during the Middle Ages; however, astronomers continued to view the Earth as the center of the universe well into the 16th and 17th centuries.
The development of a geological view of Earth also emerged during Classical Antiquity. During the 4th century BCE, Aristotle observed the composition of the land and theorized that the Earth changes at a slow rate, and that these changes cannot be observed during a person's lifetime. This was the first evidence-based concept of geological time and the rate at which physical change happens on Earth.
In the 1st century CE, Pliny the Elder produced an extensive discussion of minerals and metals. In addition to properly identifying the origin of amber as a fossilized resin, based on the observations of insects trapped within some pieces, he also laid the basis of crystallography by recognizing the habit of diamonds to form into octahedrons.
By the early 11th century, The Persian astronomer and scholar Abu al-Rayhan al-Biruni conducted the first recorded study on the geology of India. In his encyclopedic work on India, entitled "Tarikh Al-Hind” (History of India), he hypothesized that the Indian subcontinent was once a sea.
By the 17th century, the term geology began to enter usage among scientists. There are two theories as to who coined the term, with one claiming it was Ulisse Aldrovandi (1522 - 1605) - an Italian naturalist - that made the first recorded use of the word. The second credits Mikkel Pederson Escholt (1600 - 1699), a Norwegian priest and scholar, who used the definition in his book 1657 work on Norway's geography ("Geologica Norvegica").
It was also during the 17th century that fossil evidence began to trigger a widespread debate about the true age of the Earth. During this time, theologians and scientists had been at odds about the age of the world, with the former insisting that it was 6,000 years old (based upon the Bible), while the latter believed it to be much older. However, the debate would soon be resolved in favor of the latter.
James Hutton, who is often viewed as the first modern geologist, is credited with ending the debate through the publications of paper titled Theory of the Earth to the Royal Society of Edinburgh in 1785. In this paper, he explained his theory that the Earth must be much older than previously thought in order to allow enough time for mountains to erode and for sediments to form new rocks at the bottom of the sea, which in turn were raised up to become dry land.
During the 18th century, opinion was divided between those who believed that rocks were deposited by the oceans during flooding events, and those that believed they were formed through heat and fire. In a two-volume study of his paper published in 1795, Hutton advanced the idea that some rocks are formed by volcanic heat while others are formed by sedimentation. These processes, he claimed, are ongoing and work over very long, very gradual time periods.
Earth's Future:
Given the immense time scales involved with Earth's formation, evolution, and eventual destruction, humanity is little more than a very recent development - a proverbial "flash in the pan", if you will. Nevertheless, given that it is where all terrestrial life as we know it originated, coupled with the fact that it is the only habitable planet known to us, Earth is likely to remain our spiritual and physical home for many eons to come.
One can only hope that by the time it does become uninhabitable, we will have long since gone extinct, or evolved to the point that we no longer have to worry about dying along with it. And in the meantime, we can only hope that our presence here on Earth doesn't ruin it.
We have many interesting articles about Earth here at Universe Today. Below is a list that covers the many different aspects of our planet and what we've come to know about. We hope you find what you are looking for: