Structure and Composition of the Sun and Earth
The Sun
Structure of the Sun
Core: The Sun’s core is where nuclear fusion occurs due to high temperatures. It is the power generator of the Sun.
Radiative Zone: In this zone, energy-carrying particles (photons) try to escape, but their journey can last 100,000 years. This is because these photons are continuously absorbed and re-emitted in different directions.
Convective Zone: This zone experiences the phenomenon of convection. Columns of hot gas rise to the surface, cool, and then descend again.
Photosphere: This is a thin layer, about 300 km thick, which is the part of the Sun that we see as its surface. Light and heat radiate into space from here. The temperature is about 5,000°C. Dark spots and bright faculae appear in the photosphere. Faculae are regions around the spots with a higher temperature than the normal photosphere and are related to the Sun’s magnetic fields.
Chromosphere: This layer can only be seen during a total solar eclipse. It is red, has very low density, and a high temperature of about half a million degrees. It consists of rarefied gases and contains very strong magnetic fields.
Corona: This is a large layer with high temperatures and very low density. It consists of rarefied gases and gigantic magnetic fields that change shape from hour to hour. This layer offers a breathtaking view during a total solar eclipse.
Solar Activity
Sunspots: Sunspots are dark central areas (umbra) surrounded by a lighter region called the penumbra. They are dark because they are cooler than the surrounding photosphere.
Sunspots are the location of strong magnetic fields. The reason why they are cooler is not yet fully understood, but one possibility is that the magnetic field in the spots suppresses convection beneath them.
Sunspots usually grow and last from several days to several months. Observations of sunspots first revealed that the Sun rotates with a period of 27 days (as seen from Earth).
The number of sunspots on the Sun is not constant and changes over an 11-year period known as the solar cycle. Solar activity is directly related to this cycle.
Solar Prominences: Solar prominences are huge jets of hot gas expelled from the Sun’s surface, extending thousands of miles. The largest prominences can last for several months.
The Sun’s magnetic field diverts some of the prominences, forming a giant arch. They occur in the chromosphere, which has a temperature of about 100,000 degrees.
Prominences are spectacular phenomena. They appear at the edge of the Sun as billowing clouds that rise high into the atmosphere and lower corona. They consist of clouds of material at a lower temperature and higher density than their surroundings.
Temperatures in the central part of a prominence are approximately one-hundredth of the temperature of the corona, while its density is about 100 times that of the ambient corona. Therefore, the gas pressure inside a prominence is approximately equal to that of its surroundings.
The Solar Wind: The solar wind is a stream of charged particles, mainly protons and electrons, escaping from the Sun’s outer atmosphere at high speed and penetrating the solar system.
Some of these charged particles become trapped in Earth’s magnetic field, spiraling along the lines of force from one magnetic pole to the other. The northern and southern lights (auroras) result from the interactions of these particles with air molecules.
The solar wind speed is about 400 kilometers per second near Earth’s orbit. The point where the solar wind meets the interstellar medium is called the heliopause and is the theoretical limit of the Solar System. It is located about 100 AU from the Sun. The space within the boundary of the heliopause, containing the Sun and the solar system, is called the heliosphere.
The Earth
Formation and Early History
Formation of the Earth: The Earth was formed about 4.65 billion years ago, along with the entire solar system. Although the oldest rocks on Earth are no more than 4 billion years old, meteorites, which geologically correspond to the Earth’s core, date back to about 4.5 billion years. The crystallization of the core and the precursor bodies of meteorites are believed to have occurred simultaneously, some 150 million years after the Earth and the Solar System formed.
After condensing from cosmic dust and gas by gravitational attraction, the Earth was nearly homogeneous and quite cold. But the continued contraction of materials and the radioactivity of some heavier elements caused it to heat up.
Then, it began to melt under the influence of gravity, leading to differentiation between the crust, mantle, and core. Lighter silicates moved upwards to form the crust and mantle, while heavier elements, especially iron and nickel, sank towards the center to form the core.
Simultaneously, volcanic eruptions released volatile gases and vapors. Some were trapped by Earth’s gravity and formed the early atmosphere, while condensed water vapor formed the first oceans.
Terrestrial Magnetism
Earth’s Magnetic Field: Terrestrial magnetism means that the Earth behaves like a giant magnet. The English physicist William Gilbert was the first to propose this in 1600, although the effects of terrestrial magnetism had been used much earlier in primitive compasses.
The Earth is surrounded by a strong magnetic field, as if the planet had a huge magnet inside with its south pole near the geographic North Pole and vice versa. Parallel to the geographic poles, the Earth’s magnetic poles are called the magnetic north pole and the magnetic south pole, although their actual magnetic polarity is opposite to what their names suggest.
The magnetic north pole is currently located near the west coast of Bathurst Island in the Northwest Territories of Canada. The magnetic south pole is at the edge of the Antarctic continent in Terre Adélie.
The positions of the magnetic poles are not constant and have shown significant changes over the years. Variations in the Earth’s magnetic field include a shift in the direction of the field caused by the displacement of the poles. This is a periodic variation that repeats every 960 years. There is also a smaller annual variation.
Structure of the Earth
From the outside in, the Earth can be divided into five parts:
Atmosphere: The gaseous envelope surrounding the planet’s solid body. It has a thickness of more than 1,100 km, but half of its mass is concentrated in the lowest 5.6 km.
Hydrosphere: It consists mainly of oceans, but strictly speaking, it includes all water surfaces worldwide, including inland seas, lakes, rivers, and groundwater. The average depth of the oceans is 3,794 m, more than five times the average height of continents.
Lithosphere: Consisting mainly of the Earth’s crust, it extends to a depth of 100 km. The rocks of the lithosphere have an average density of 2.7 times that of water and are composed almost entirely of 11 elements, which together comprise 99.5% of its mass. The most abundant is oxygen, followed by silicon, aluminum, iron, calcium, sodium, potassium, magnesium, titanium, hydrogen, and phosphorus. In addition, 11 other elements are present in amounts less than 0.1%: carbon, manganese, sulfur, barium, chlorine, chromium, fluorine, zirconium, nickel, strontium, and vanadium. The elements are present in the lithosphere almost entirely in the form of compounds, rarely in their free state.
The lithosphere comprises two layers, the crust and the upper mantle, which are divided into about a dozen rigid plates. The upper mantle is separated from the crust by a seismic discontinuity, the Mohorovičić discontinuity, and from the lower mantle by a weak zone known as the asthenosphere. The plastic and partially molten rocks in the 100-km-thick asthenosphere allow the continents to move across the Earth’s surface and oceans to open and close.
Mantle: It extends from the base of the crust to a depth of about 2,900 km. Except in the asthenosphere, it is solid, and its density, which increases with depth, ranges from 3.3 to 6. The upper mantle is composed of iron and magnesium silicates such as olivine, and the lower mantle consists of a mixed oxide of magnesium, iron, and silicon.
Core: It has an outer layer about 2,225 km thick with an average relative density of 10. This layer is probably rigid, and its outer surface has depressions and peaks. In contrast, the inner core, with a radius of about 1,275 km, is solid. Both core layers are composed of iron with a small percentage of nickel and other elements. Temperatures in the inner core can reach 6,650°C, and its average density is 13.
The inner core continuously radiates intense heat outwards through the various concentric layers that form the solid part of the planet. The source of this heat is the energy released by the decay of uranium and other radioactive elements. Convection currents within the mantle transfer most of the Earth’s thermal energy to the surface.
The Atmosphere
Early Atmosphere
Formation of the Atmosphere: The mixture of gases that forms the current air has evolved over 4.5 billion years. The early atmosphere must have been composed solely of volcanic emissions, i.e., water vapor, carbon dioxide, sulfur dioxide, and nitrogen, with no free oxygen.
To achieve the transformation to the present atmosphere, a series of processes had to occur. One was condensation. As it cooled, most of the water vapor of volcanic origin condensed, giving rise to the ancient oceans. There were also chemical reactions. Some of the carbon dioxide reacted with crustal rocks to form carbonates, some of which dissolved in the new oceans.
Later, when primitive life capable of photosynthesis evolved, it began producing oxygen. About 570 million years ago, the oxygen content of the atmosphere and oceans increased enough to allow the existence of marine life. Later, some 400 million years ago, the atmosphere contained enough oxygen to permit the evolution of land animals capable of breathing air.
Structure of the Atmosphere
The atmosphere is divided into several layers:
Troposphere: The troposphere reaches an upper boundary (tropopause) located at an altitude of 9 km at the poles and 18 km at the equator. It experiences significant vertical and horizontal movements of air masses (winds) and contains a relative abundance of water. It is the region of clouds and weather: rain, wind, temperature changes, etc., and the layer of most interest to ecology. The temperature decreases with increasing altitude, reaching -70°C at its upper limit.
Stratosphere: The stratosphere begins at the tropopause and reaches an upper limit (stratopause) at 50 km altitude. The temperature trend reverses and increases to about 0°C at the stratopause. There is almost no vertical air movement, but horizontal winds frequently reach speeds of up to 200 km/h, facilitating the rapid global dissemination of any substance that reaches the stratosphere. For example, this is the case with CFCs that destroy ozone. In this part of the atmosphere, between 30 and 50 km, ozone is crucial because it absorbs harmful shortwave radiation.
Mesosphere: The mesosphere, which extends between 50 and 80 km in altitude, contains only about 0.1% of the total mass of the air. It is important for the ionization and chemical reactions that occur within it. The decrease in temperature combined with the low air density in the mesosphere leads to the formation of turbulence and atmospheric waves that operate on very large spatial and temporal scales. The mesosphere is the region where spacecraft returning to Earth begin to feel the structure of the wind, not just air resistance.
Ionosphere: The ionosphere extends from an altitude of 80 km above the Earth’s surface up to 640 km or more. At these altitudes, the air is extremely thin. When particles in the atmosphere are ionized by ultraviolet radiation, they tend to remain ionized due to the minimal collisions that occur between ions. The ionosphere has a significant influence on the propagation of radio signals. A portion of the energy radiated by a transmitter towards the ionosphere is absorbed by the ionized air, and the rest is refracted or deflected back to the Earth’s surface. This latter effect allows the reception of radio signals at distances much greater than would be possible with waves traveling along the Earth’s surface.
Exosphere: The region beyond the ionosphere is called the exosphere and extends up to 9,600 km, which is the outer limit of the atmosphere. It extends beyond the magnetosphere, the space around Earth where the planet’s magnetic field dominates the interplanetary magnetic field.
The Oceans
Origin of Sea Salt
From the volcanic chains located on the ocean floor, lavas emerge containing many of the components of seawater: chlorine, sodium, bromine, iodine, carbon, and nitrogen, which gradually turn into salts. Additionally, rivers carry salts and minerals they encounter during their journey across continents. In the oceans, strong solar radiation evaporates water, causing salts to accumulate over time. In seawater, along with a large number of dissolved chemical elements, there are gases and nutrients essential for ocean life.
The overall salinity of the oceans is 35 parts per thousand (35‰). This means that in 1,000 grams (1 kilogram) of seawater, 35 grams are salts.