Earth: Composition, Structure, and Atmosphere

Posted on Jan 25, 2025 in Geology

Earth, the third planet from the Sun (150 million km), is not a perfect sphere due to its rotation. Its equatorial radius is 21 km longer than its polar radius, with an average radius of 6368 km. The Earth’s mass is approximately 6 x 1024 kg, and its density is 5.5 g/cm3.

Origin of Earth

The Earth’s origin is linked to the formation of the solar system. It began with a cloud of gas and dust, the remnants of material expelled by supernovae. A perturbation in this cloud, possibly from a supernova, caused a concentration of material in one area. Gravitational force then acted on the dust and gas, increasing its mass and gravitational pull. This process led to the cloud contracting and rotating, with the rotation speed increasing as the gravitational contraction intensified. Rings formed and separated from the central disc, eventually becoming the planets, while the central part became the Sun. The temperature of the forming planet increased due to the attraction. The initial atmosphere, composed of H, He, HN3, CH4, and CO2, was lost due to high temperatures and solar wind.

Earth’s Interior

The Earth’s interior is characterized by a gradual increase in temperature, pressure, and density with depth.

  • Crust: The rigid outer layer of the Earth, with a thickness of about 20 km. The oceanic crust is thinner than the continental crust.
  • Mantle: Constitutes 83% of the Earth’s volume, with a thickness of 2900 km. The boundary between the crust and mantle is called the Mohorovičić discontinuity (approximately 50 km deep). The mantle is divided into the upper and lower mantle, separated by the asthenosphere (around 100 km deep).
  • Core: A sphere with a radius of 3486 km, primarily composed of iron and nickel. It consists of a liquid outer core (2270 km radius) and a solid inner core (1216 km radius). The outer core is liquid, while the inner core behaves like a solid under seismic wave pressure.

The Hydrosphere

Unlike other planets, Earth has a hydrosphere, a mass of water in constant motion. Water evaporates from the oceans into the atmosphere. Oceans cover 71% of the Earth’s surface.

The Atmosphere

The atmosphere provides us with oxygen, shields us from intense heat and harmful UV radiation from the sun. It began forming around 4.6 billion years ago, with the birth of the Earth. The initial atmosphere was composed of CO2, N, H2, and CO, without oxygen. About 2.5 billion years ago, photosynthetic life began introducing oxygen and ozone into the atmosphere.

Atmospheric Gases:

  • Nitrogen (78%)
  • Oxygen (20%)
  • Argon (0.93%)
  • CO2 (0.033%)

The atmosphere is layered: the troposphere (up to 10 km) has decreasing temperature, the stratosphere (10-30 km) has constant temperature, the quimiosphere has decreasing temperature, and the ionosphere has increasing temperature up to 80 km. Between 80 and 90 km, the temperature is constant, and above 90 km, it decreases.

Lithosphere

The Earth’s surface is primarily divided into continents and ocean basins. The coastline is not the limit between these two regions, as the continental shelf extends seaward from the coastline.

Oceans

Ocean basins feature prominent oceanic ridges. The primary elements found in the ocean are chlorine, sodium, magnesium, and calcium.