JA

8.1 Earth Perspective

8.1 The Global Perspective

Learning Objectives

  • By the end of this section, you will be able to:

    • Describe Earth as a planet, including its size, composition, and orbit.

    • Explain the significance of Earth's ability to support liquid water and sustain life.

    • Illustrate and summarize the basic properties of Earth as outlined in Table 8.1.

Physical Characteristics of Earth

  • Earth is identified as a medium-size planet with a diameter of approximately 12,760 kilometers (Figure 8.2).

    • It belongs to the category of inner or terrestrial planets.

    • Its primary composition consists of heavy elements such as iron, silicon, and oxygen.

    • This composition starkly contrasts with the Sun and stars, which are enriched in light elements like hydrogen and helium.

  • Earth’s orbit is near circular, allowing it to maintain a stable environment.

    • The planetary conditions are just right to sustain liquid water, which is essential for life.

  • Notable facts about Earth:

    • It is the only planet that is neither too hot nor too cold but termed “just right” for life development.

Table 8.1: Basic Properties of Earth

Property

Measurement

Semimajor axis

1.00 AU

Period

1.00 year

Mass

Diameter

12,756 km

Radius

6378 km

Escape velocity

11.2 km/s

Rotational period

23 h 56 m 4 s

Surface area

Density

5.514 g/cm³

Atmospheric pressure

1.00 bar

Earth’s Interior

  • Accessing and studying the interior of Earth poses certain challenges since it must be analyzed indirectly.

    • The only tangible experience we have is with the outermost layer, known as the Earth's crust, which is a thin veneer several kilometers thick.

    • Intriguingly, scientists have noted that our understanding of Earth's inner layers is less comprehensive than our comprehension of the surfaces of Venus and Mars.

Composition of Earth

  • Earth predominantly consists of metal and silicate rock; most of which is solid but includes molten sections.

  • The structure within Earth's interior has been extensively studied using seismic waves generated by earthquakes or explosions.

    • Seismic waves function similarly to sound waves through a struck bell, altering in frequency based on the materials present in Earth's interior.

    • By monitoring seismic waves at varied locations, insights about the structure and layers of the Earth can be gathered. Some waves travel along the surface, while others penetrate deeper into the planet.

  • Seismic studies have successfully indicated several distinct layers within the Earth, as visually represented in Figure 8.3.

  • As seismic waves traverse different materials, they bend (or refract), allowing certain seismic stations to detect them while others fall into “shadows”. This process helps scientists construct models of Earth’s interior.

    • Analogous to medical ultrasound imaging, seismic imaging provides valuable information regarding the internal structure.

Layers of the Earth

  • Crust:

    • The outer layer that we are most familiar with (Figure 8.4).

    • Composed of oceanic and continental crust:

      • Oceanic Crust:

      • Covers 55% of Earth’s total surface area and generally lies below oceanic waters.

      • Thickness averages about 6 kilometers and consists of volcanic rocks known as basalt.

        • Basalt is formed from cooled volcanic lava, primarily made of silicon, oxygen, iron, aluminum, and magnesium.

      • Continental Crust:

      • Accounts for 45% of Earth’s surface, some parts of which are submerged.

      • Thickness ranges from 20 to 70 kilometers, mainly composed of granite, another volcanic class of silicates.

        • Granite is also comprised of silicon and oxygen, similar to basalt but with differing properties.

    • Both crustal types have a density of approximately 3 g/cm³, compared to water's density of 1 g/cm³.

    • Significantly, the crust contributes only about 0.3% of Earth’s total mass.

  • Mantle:

    • This layer extends from the base of the crust down to approximately 2900 kilometers depth.

    • Although largely solid, the mantle can deform and flow slowly under extreme temperature and pressure conditions, with density increasing from 3.5 g/cm³ to over 5 g/cm³ due to the major compressive forces of the overlying material.

    • Occasionally, upper mantle material is expelled during volcanic eruptions, thereby allowing scientists to analyze its chemical makeup.

  • Core:

    • Beyond the mantle lies the dense metallic core, reaching depths of around 7000 kilometers, significantly larger than Mercury.

    • Divided into:

      • Outer Core: Liquid state.

      • Inner Core: Likely solid, with a diameter of about 2400 kilometers.

    • Contains high concentrations of iron, nickel, and sulfur compressed to a considerable density.

  • The arrangement of Earth’s layers based on density signifies differentiation, which describes the process by which a planet sorts its major components according to density. This suggests that Earth’s interior was once sufficiently warm to melt, allowing denser metals to sink to the core.

Evidence of Differentiation

  • The differentiation process is evidenced by comparing Earth’s bulk density (5.5 g/cm³) against surface material density (3 g/cm³), suggesting denser materials reside in the core.

Earth's Magnetic Field and Magnetosphere

  • Observations of Earth’s magnetic behavior imply that it possesses a magnetic characteristic similar to a giant bar magnet aligned with Earth's rotational poles.

  • The magnetic field is produced by the movement of materials within the *liquid metallic core.

    • As the liquid metal flows, it establishes circulating electric currents that generate the magnetic field.

Attributes of the Magnetic Field

  • Earth’s magnetic field extends into surrounding space and has the ability to trap charged particles within its influence. This phenomenon leads to the formation of the magnetosphere, defined as the region where Earth’s magnetic field prevails over the weaker interplanetary magnetic field originating from the Sun (Figure 8.5).

  • The magnetosphere's characteristics include:

    • Charged particles primarily sourced from solar wind, which consists of material escaping the Sun’s hot surface.

    • This inflow of particles contributes to Earth’s magnetic field while also influencing its orientation in relation to the Sun.

    • The magnetosphere typically extends approximately 60,000 kilometers, or 10 Earth radii, towards the Sun. In contrast, the field may stretch as far as the Moon’s orbit or even further in the opposite direction from the Sun.

Figures and Illustrations

  • Figure 8.2: Blue Marble - A notable image of Earth from space captured by Apollo 17 astronauts, showcasing a full view of Earth.

  • Figure 8.3: Visual representation of Earth’s interior structure as revealed through seismic studies, including divisions into the crust, mantle, and outer/inner cores.

  • Figure 8.4: A computer-generated image depicting Earth’s crust based on satellite imagery and ocean floor radar mapping.

  • Figure 8.5: A cross-sectional view of Earth's magnetosphere depicting the magnetic influences affecting charged particles from the solar wind.