🪐Intro to Astronomy Unit 8 – Earth as a Planet

Earth's Place in the Solar System

• Third planet from the Sun in our solar system
• Located in the habitable zone where liquid water can exist on the surface
• Orbits the Sun at an average distance of 1 astronomical unit (AU), which is approximately 149.6 million kilometers
• Part of the inner solar system along with Mercury, Venus, and Mars
• Formed approximately 4.54 billion years ago from the solar nebula
• Slightly larger than Venus, which is sometimes referred to as Earth's "sister planet"
• Only planet in the solar system known to support life as we know it

Physical Characteristics of Earth

• Oblate spheroid shape with a slight bulge at the equator due to its rotation
• Equatorial diameter of 12,756 km and polar diameter of 12,714 km
• Surface area of approximately 510 million square kilometers
• Highest point is Mount Everest at 8,848 meters above sea level
• Lowest point is the Mariana Trench at 11,034 meters below sea level
• Covered by 71% water and 29% land
  • Oceans hold approximately 97% of Earth's water
  • Ice caps and glaciers hold approximately 2% of Earth's water
• Average density of 5.51 grams per cubic centimeter

Earth's Structure and Composition

• Divided into three main layers: crust, mantle, and core
• Crust is the outermost layer with an average thickness of 30-50 km
  • Oceanic crust is thinner (5-10 km) and composed primarily of basalt
  • Continental crust is thicker (30-50 km) and composed primarily of granite
• Mantle is the middle layer, extending from the base of the crust to a depth of about 2,900 km
  • Composed of silicate rocks rich in iron and magnesium
  • Upper mantle is solid, while the lower mantle is partially molten
• Core is the innermost layer, extending from the base of the mantle to the center of the Earth
  • Outer core is liquid and composed primarily of iron and nickel
  • Inner core is solid and composed primarily of iron and nickel
• Lithosphere is the rigid outer layer that includes the crust and uppermost mantle
• Asthenosphere is the partially molten layer beneath the lithosphere that allows for plate tectonic movement

Earth's Atmosphere and Magnetosphere

• Atmosphere is a layer of gases surrounding the Earth, held in place by gravity
• Composed primarily of nitrogen (78%) and oxygen (21%), with trace amounts of other gases
• Divided into five layers: troposphere, stratosphere, mesosphere, thermosphere, and exosphere
  • Troposphere is the lowest layer where most weather phenomena occur
  • Stratosphere contains the ozone layer, which absorbs harmful UV radiation from the Sun
• Atmospheric pressure decreases with altitude, with sea level pressure averaging 1013.25 millibars
• Magnetosphere is the region of space surrounding Earth that is dominated by its magnetic field
  • Generated by the motion of molten iron in the outer core (geodynamo)
  • Protects Earth from harmful solar radiation and charged particles
  • Responsible for the formation of the Van Allen radiation belts

Earth's Rotation and Orbital Motion

• Earth rotates on its axis once every 24 hours (sidereal day)
  • Rotation is responsible for the day-night cycle
  • Earth's rotation is gradually slowing down due to tidal interactions with the Moon
• Earth orbits the Sun once every 365.24 days (sidereal year)
  • Orbit is elliptical with an eccentricity of 0.0167
  • Perihelion (closest to Sun) occurs in January at a distance of 147.1 million km
  • Aphelion (farthest from Sun) occurs in July at a distance of 152.1 million km
• Earth's axis is tilted 23.5 degrees relative to its orbital plane
  • Axial tilt is responsible for the seasons as Earth orbits the Sun
  • Summer solstice, winter solstice, and equinoxes are determined by the orientation of Earth's axis relative to the Sun

Earth's Geological Processes

• Plate tectonics is the theory that Earth's lithosphere is divided into plates that move relative to each other
  • Convergent boundaries occur where plates collide, resulting in subduction or mountain building
  • Divergent boundaries occur where plates move apart, resulting in seafloor spreading and rift valleys
  • Transform boundaries occur where plates slide past each other, resulting in earthquakes
• Volcanism is the process by which molten rock (magma) and gases are expelled from Earth's interior
  • Volcanoes can be shield, composite, or cinder cone types
  • Volcanic eruptions can be effusive (lava flows) or explosive (pyroclastic material)
• Earthquakes are sudden releases of energy in Earth's crust, causing seismic waves
  • Caused by movement along faults or by volcanic activity
  • Measured using the moment magnitude scale
• Weathering is the breakdown of rocks and minerals at Earth's surface
  • Physical weathering involves mechanical breakdown (frost wedging, abrasion)
  • Chemical weathering involves chemical reactions (dissolution, oxidation)
• Erosion is the transport of weathered material by water, wind, ice, or gravity
  • Leads to the formation of sedimentary rocks through deposition and lithification

Earth's Unique Features for Life

• Presence of liquid water on the surface, which is essential for life as we know it
  • Water has unique properties (high specific heat, cohesion, adhesion) that make it crucial for life
  • Hydrologic cycle involves the continuous movement of water through evaporation, precipitation, and runoff
• Moderate surface temperatures allow for the existence of liquid water and the development of complex life forms
  • Greenhouse effect, caused by atmospheric gases like carbon dioxide and water vapor, helps maintain suitable temperatures
  • Milankovitch cycles, involving variations in Earth's orbit and axial tilt, influence long-term climate patterns
• Ozone layer in the stratosphere protects life from harmful UV radiation
• Magnetic field shields Earth from solar wind and cosmic radiation
• Plate tectonics and the rock cycle recycle elements and minerals necessary for life
• Presence of a large moon (Luna) stabilizes Earth's axial tilt and influences tides
• Biodiversity, with millions of species adapted to various ecosystems, demonstrates the success of life on Earth

Observing Earth from Space

• Satellites provide valuable data for studying Earth's systems and processes
  • Weather satellites (GOES, NOAA) monitor atmospheric conditions and help predict weather patterns
  • Landsat satellites provide high-resolution images of Earth's surface for monitoring land use and environmental changes
  • Oceanographic satellites (TOPEX/Poseidon, Jason) measure sea surface height and study ocean circulation patterns
• Earth observation programs, like NASA's Earth Science Division, use satellite data to study global climate change, natural disasters, and human impacts on the environment
• Remote sensing techniques allow for the analysis of Earth's surface and atmosphere
  • Multispectral imaging captures data in multiple wavelengths to study vegetation, soil, and water properties
  • Radar imaging (SAR) penetrates clouds and can detect surface deformation related to earthquakes and volcanoes
• International Space Station (ISS) serves as a platform for Earth observation experiments and research
• Astronaut photography provides unique perspectives on Earth's features and phenomena
  • Visible effects of human activities (city lights, deforestation, pollution)
  • Natural wonders (aurora, noctilucent clouds, geological features)