The Planet Earth
The Planet Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. It is an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all of Earth's water is contained in its global ocean, covering 70.8% of Earth's crust.
Physical Characteristics
Earth is rounded into an ellipsoid with a circumference of about 40,000 km. It is the densest planet in the Solar System. Of the four rocky planets, it is the largest and most massive. Earth is about eight light-minutes away from the Sun and orbits it, taking a year (about 365.25 days) to complete one revolution. Earth rotates around its own axis in slightly less than a day (in about 23 hours and 56 minutes).
Atmosphere
Earth has a dynamic atmosphere, which sustains Earth's surface conditions and protects it from most meteoroids and UV-light at entry. It has a composition of primarily nitrogen and oxygen. Water vapor is widely present in the atmosphere, forming clouds that cover most of the planet.
Geological Structure
At the planet's heart lies the inner core, a solid sphere of iron and nickel that's 759 miles wide and as hot as 9,800 degrees Fahrenheit. The inner core is surrounded by the outer core, a 1,400-mile-thick band of iron and nickel fluids. Beyond the outer core lies the mantle, a 1,800-mile-thick layer of viscous molten rock on which Earth's outermost layer, the crust, rests.
Hydrosphere
Water covers roughly 71% of Earth's surface, with most of that water located in our planet's oceans. The hydrosphere includes all liquid and frozen surface waters, groundwater held in soil and rock, and atmospheric water vapor.
Biosphere
The biosphere is the global sum of all ecosystems. It can also be termed the zone of life on Earth, a closed system, and largely self-regulating. The biosphere is postulated to have evolved, beginning with a process of biopoiesis (life created naturally from non-living matter, such as simple organic compounds) or biogenesis (life created from living matter), at least some 3.5 billion years ago.
Human Impact
Images of Earth taken from space, particularly during the Apollo program, have been credited with altering the way that people viewed the planet that they lived on, called the overview effect, emphasizing its beauty, uniqueness, and apparent fragility. This caused a realization of the scope of effects from human activity on Earth's environment.
Earth's Magnetic Field
Our planet’s churning liquid-metal core generates a magnetic field that shields us from most of the Sun’s harmful radiation. This magnetic field, along with the atmosphere, provides a blend of gas that animals and plants use to grow and reproduce.
Space Exploration
Astronauts who travel to space experience what is known as the overview effect upon seeing Earth from afar. They often return home feeling a deep responsibility to preserve and cherish our planet. We know of no other habitable worlds like Earth, nor do we currently have the technology to terraform a world like Mars to make it Earth-like.