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The planet Earth

Living on the planet Earth, we certainly want to know more about it. During the lecture, students are invited to ask questions about the planet Earth. Here are some example questions. What is the size of Earth, radius, mass, volume etc?
How old is the Earth? What is meant by age of Earth?
How does the Earth form?
How does water form?
What is the gravity of Earth?
What is the structure of Earth?
Where is Earth in the Solar System?
What elements naturally exist as element and why?
How many elements are there on the planet Earth?
The Earth is estimated to be five trillion (5x109) years old; mean radius 6,371 km, mass 6x1024 kg, mass of oceans 1.4x1021 kg, mass of atmosphere 5.1x1018 kg, mean density 5.52x103 kg/m3, equatorial radius 6378.2 km, which is slightly longer than the polar radius 6356.2 km, mean radius 6371 km, and a volume 1.083x1021 m3. Due to a 23.5o inclination of the rotation axis away from the vertical axis of revolution, the temperate zone of Earth has four interesting seasons in a year.

Materials on the Planet Earth

The Earth has all kinds of material: Some of these materials are present in several states. We easily find examples of glass, crystal, liquid, gas, and plasma, each representing a state of material.

Many disciplines have developed for the study of Earth: geological sciences study the composition, structure, chemistry, and minerals; hydrologic sciences study the behavior of water on the planet; and atmospheric sciences study meteorology, climatology, and planetary atmosphere. Since ancient time, geometry and trigonometry have been developed to measure the vital data such as size, distances and locations. Thus we have some rather precise data about the planet. The information is often tabulated in many handbooks, and some have been provided here for your interest.

Structure of the Earth

Regarding the structure, the Composition of the Planets (1) gave an interesting comparison with respect to Planets Mercury, Venus, and Earth. The composition of the Earth Moon is also given. Inside the Earth gives more details. The picture is from this link. Further details are given below.

Structure of Earth provides some hint about its origin and evolutionand its somewhat erratic motion allows us to calculate positions of other stars and planets.

Aside from the atmosphere and oceans, the outer surface of Earth is a crust made up of mostly silicates, which has a simple chemical formula of SiO2. However, the silicates have many interesting forms and they appear as quartz, rose quartz, smoky quartz, amethyst, and other precious stones, the color of which are due to the presence of impurities. Other abundant oxides are Al2O3, TiO2, Fe2O3, CaCO3, etc. The Earth crust has all the stable chemical elements, but they form compounds with each other. Coal, diamond (carbon) and gold are some ]natural elements. During prehistoric times, people did find native silver and copper as collectibles, but these are depleted resource. Thickness of the Earth crust varies from some 30 to 50 km for continents to 5 km below the deep ocean floor. The crust has an average thickness of 15 km, and its density 3 g/mL

Many minerals are present in the earth crust, and we are interested in their formation and distribution whereas geologists, mining engineers and business people are interested in their exploitation and utilization. Table II summarizes some characters of the various shells of the planet earth, and the features of the shells are further described later.

Below the crust is a 3650-kilometer thick shell called mantle mostly made up of silicates. The second major compound in the upper mantle is aluminum oxide, Al2O3, whereas in the lower mantle is magnesium oxide, MgO. The density increases from 3.5x103 kg m-3 in the upper to 5.5 or 6x103 kg m-3 in the lower mantle, due to pressure increase with depth.

The Earth core is mostly iron and nickel, which ordinarily has a density of 7.9x103 kg m-3. The upper portion is mostly molten iron and nickel but the center is a solid iron and nickel core. The density increases from 12x103 kg m-3 in the molten shell to 13x103 kg m-3 or higher in the center core. The high density is again due to high pressure experienced by material in the center.

Between the inner core, outer core, mantles, and the crust are transition zones. Due to their differences in chemical composition and density, we describe them as different shells or layers. The methods used to determine the properties and thickness of these layers are interesting, and they can be found in books describing the planet Earth. Other properties such as magnetic field, gravitational field, mechanical motions, heat flow, continental movement, elemental and chemical distributions are also studied in specific fields.

In summary, the shell structure is given in a table form below:

Shell Structure of the Planet Earth

from surface
Shell name Major chemicals Density
/103kg m-3
0-15 Crust SiO2, Al2O3, MgO
TiO2, Fe2O3, FeO etc
3 30-50 km for continents
5 km under ocean floor
0.35 % of Earth mass
15-900 Upper mantle SiO2, Al2O3 3.5 Higher density than crust
900-2700 Lower mantle SiO2, MgO 5-6 The mantles:
84 % of Earth by volume
67 % of Earth by mass
2700-2900 Mantle core
SiO2, MgO, Fe 6 Transition region
2900-5000 Outer core Molten Fe and Ni 12-13 The cores:
15 % of Earth by volume
32 % of Earth by mass
5000-5150 Transition
Fe and Ni 13 Transition region
5150-6371 Inner core Solid Fe and Ni 13-14

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