Earth – A Tale of Spheres

Seen from space, Earth can be divided into a series of layers or zones that are referred to as spheres. The outermost of these layers is the magnetosphere, which is the region around Earth that is affected by its magnetic field. This field originates from a dynamo deep inside the planet and is mostly a dipole, characterized by a North Pole and a South Pole. Importantly, Earth’s magnetic field acts as a shield against the charged particles of the solar wind and the cosmic rays from supernova explosions. Still, some of these particles make it past the shield and are focused to the polar regions along magnetic field lines. When these particles interact with gas atoms in the upper atmosphere, they can result in spectacular phenomena such as aurorae.

The atmosphere is the envelope of gas that surrounds the Earth. It consists of ~ 78 % nitrogen (N2), ~ 21 % oxygen (O2) and a number of different traces gases, such as ozone (O3), carbon dioxide (CO2), methane (CH4) and water vapor (H2O). This mixture of gases is more commonly known as air. Both the air density and the air pressure gradually decrease with altitude and as a result, almost all gas molecules are found in the lowermost 50 km of the atmosphere. The air pressure is ~ 1 bar at Earth’s surface, but less than ~ 0.01 bar at an altitude of 30 km.

Approximately 30 % of Earth’s surface consists of continents and islands, while as much as 70 % is covered with water. The total mass of water on Earth, in the form of oceans, glaciers, lakes, rivers, groundwater and the atmosphere, is known as the hydrosphere. Notably, less than 3 % of all water on the planet is fresh water, and most of this fresh water is stored in glaciers and groundwater. The ice-covered regions of Earth’s surface are also known as the cryosphere.

The solid surface and interior of the Earth are referred to as the geosphere. Earth’s surface displays dramatic changes in topography that are related to a variety of processes, but unlike the surfaces of the Moon and Mars, it features relatively few meteorite impact craters. Both the continents and the ocean floor are characterized by steep mountains, extensive plains and deep valleys. The highest mountain above sea level is Mount Everest and the deepest part of the oceans is located in the Mariana Trench.

Perhaps the most striking feature of Earth is its ability to sustain life. Living organisms occur in many magnificent forms across a wide range of habitats and ecosystems, from barren deserts to lush rainforests. All life on Earth together constitutes the biosphere.

The boundaries between these so-called spheres are not always apparent, because the spheres interact in many ways. For example, the atmosphere plays a major role in the hydrological cycle, the exchange of water among the various reservoirs of the hydrosphere. In addition, some of the most impressive natural phenomena are related to the interplay between elements of the biosphere and the geosphere.

Book reference: Marshak, S. (2007). Earth: Portrait of a Planet: Third International Student Edition. WW Norton & Company.

Image: Earth as seen from space, also known as the Blue Marble. Credit: NASA/NOAA/Reto Stöckli.

Planetary Differentiation and the Development of the Earth-Moon System

After their initial formation, the planetesimals and protoplanets of the Solar System were characterized by a relatively homogeneous composition. However, they soon started to heat up and differentiate. This heat was generated mainly from two sources: further collisions with other celestial objects and the decay of radioactive elements. In planetesimals and protoplanets whose temperatures rose sufficiently to cause melting, denser metallic materials sank towards the center, while lighter rocky materials remained behind at the surface. During this process of differentiation, these objects developed a layered structure consisting of a core, mantle and crust, after which they eventually cooled and became mostly solidified. Under the effects of gravity, the irregular shapes of the protoplanets were molded into spheres.

Even after differentiation had taken place, the young planets were continuously bombarded by meteorites and as a result, their surfaces became scarred with large amounts of impact craters. Approximately 4.53 billion years ago, an especially intense collision occurred between the Earth and a small planet known as Theia. This collision caused Theia and a large part of the Earth to disintegrate, melt and be blasted into space, after which a debris ring formed around the Earth. The Moon was ultimately formed from the materials in this ring of debris. Scientists know this because Moon rocks have an age of roughly 4.53 billion years and because they are very similar in composition to rocks from Earth’s mantle.

Book reference: Marshak, S. (2007). Earth: Portrait of a Planet: Third International Student Edition. WW Norton & Company.

Image: Earth as seen from the surface of the Moon by the Lunar Reconnaissance Orbiter. Credit: NASA/Goddard/Arizona State University.

The Moon

The Moon is the celestial body that orbits the Earth. It has a mean radius of ~ 1737 km, a mean distance to Earth of ~ 385 thousand km and an orbital period of ~ 27.3 days. Earth’s satellite is thought to have formed approximately 4.5 billion years ago as a result of the impact between the proto-Earth and a celestial body called Theia, not long after the formation of the Solar System. The Moon has a differentiated structure and consists of a crust, mantle and core and its surface is scarred with many impact craters.

Information source: NASA

Image: Full Moon as seen from Earth. Credit: Gregory H. Revera, Wikimedia Commons.


Earth is the largest terrestrial planet in the Solar System, the third planet from the Sun and the only astronomical object known to sustain life. It has a mean radius of ~ 6371 km, a mean distance to the Sun of ~ 150 million km and an orbital period of ~ 365 days. Earth is thought to have formed approximately 4.55 billion years ago and has one natural satellite, the Moon. The planet has a differentiated structure consisting of a crust, mantle and core and features plate tectonics. Importantly, Earth harbors abundant liquid water at its surface, most of which can be found in its oceans. The planet is surrounded by an atmosphere that contains ~ 78 % nitrogen (N2), ~ 21 % oxygen (O2) and several other trace gases, including carbon dioxide (CO2) and water vapor (H2O).

Information source: NASA

Image: Earth as seen from space, also known as the Blue Marble. Credit: NASA/NOAA/Reto Stöckli.