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.

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