Chapter 7 Notes: Jessica Horn

Chapter 7: Earth and the Terrestrial Worlds

• Mercury
• Craters, smooth plains, cliffs
• Venus
• Volcanoes, few craters
• Mars
• Some craters, volcanoes, riverbeds?
• Moon
• Craters, smooth plains
• Earth
• Volcanoes, craters, mountains, riverbeds

Earth's Interior

• Core: Highest density; nickel and iron
• Mantle: Moderate density; silicon, oxygen, etc.
• Crust: Lowest density; granite, basalt, etc.

Terrestrial Planet Interiors

• Applying what we have learned about Earth's interior to other planets tells us what their interiors are probably like.

Differentiation

• Gravity pulls high-density material to center
• Lower-density material rises to surface
• Material ends up separated by density

Lithosphere

• A planet's outer layer of cool, rigid rock is called the lithosphere
• It "floats" on the warmer, softer rock that lies beneath 

Strength of Rock

• Rock stretches when pulled slowly but breaks when pulled rapidly 
• The gravity of a large world pulls slowly on its rocky content, shaping the world into a sphere

Heat Drives Geological Activity

• Convection: Hot rock rises, cool rock falls
• One convection cycle takes 100 million years on Earth

Sources of Internal Heat

• Gravitational potential energy of accreting planetesimals 
• Differentiation
• Radioactivity

Heating of Interior over Time

• Accretion and differentiation when planets were young
• Radioactive decay is most important heat source today

Cooling of Interior

• Convection transports heat as hot material rises and cool material falls
• Conduction transfers heat from hot material to cool material
• Radiation sends energy into space

Role of Size

• Smaller worlds cool off faster and harden earlier
• The Moon and Mercury are now geologically "dead"

Surface Area-to-Volume Ratio

• Heat content depends on volume
• Loss of heat through radiation depends on surface area
• Time to cool depends on surface area divided by volume
• Larger object have a smaller ratio and cool more slowly

Planetary Magnetic Field

• Moving charged particles create magnetic fields 
• A planet's interior can create magnetic fields if its core is electrically conducting, converting, and rotating

Earth's Magnetosphere

• Earth's magnetic field protects us from charged particles from the Sun
• The charged particles can create aurorae ("northern lights")

Geological Processes

• Impact cratering: Impacts by asteroids or comets
• Volcanism: Eruption of molten rock onto surface
• Tectonics: Disruption of a planet's surface by internal stresses
• Erosion: Surface changes made by wind, water, or ice

Impact Cratering

• Most cratering happened soon after the solar system formed
• Craters are about 10 times wider than the objects that made them
• Small craters greatly outnumber large ones

Volcanism

• Volcanism happens when molten rock (magma) finds a path through lithosphere to the surface
• Molten rock is called lava after it reaches the surface

Outgassing

• Volcanism also releases gases from Earth's interior into the atmosphere

Tectonics

• Convection of the mantle creates stresses in the crust called tectonic forces
• Compression forces make mountain ranges
• A valley can form where the crust is pulled apart

Plate Tectonics on Earth

• Earth's continents side around on separate plates of crust

Erosion

• Erosion is a blanket term for weather-driven processes that break don or transport rock
• Processes that cause erosion include: grlaciers, rivers, wind

Erosion by Water

• The Colorado River continues to carve the Grand Canyon

Erosion by Ice

• Glaciers carved the Yosemite Valley

Erosion by Wind

• Wind wears away rock and builds up sand dunes

Erosional Debris

• Erosion can create new features by depositing debris

Radiation Protection

• All X-ray light is absorbed very high in the atmosphere
• Ultraviolet light is absorbed by ozone (O3)

Greenhouse Effect

• Certain molecules let sunlight through but trap escaping infrared photons

A Greenhouse Gas

• Any gas that absorbs infrared 
• Greenhouse gas: molecules with two different types of elements
• Not a greenhouse gas: molecules with one or two atoms of the same element 

Greenhouse Effect: Bad?

• Because of the greenhouse effect, Earth is much warmer than it would be without an atmosphere..but so is Venus.

Why is the Sky Blue?

• Atmosphere scatters blue light from the Sun, making it appear to come from different directions
• Sunsets are red because less of the red light from the Sun is scattered 

Moon

• Some volcanic activity 3 billion years ago must have flooded lunar craters, creating lunar maria
• The Moon is now geologically dead

Cratering of Mercury

• Mercury has a mixture of heavily cratered and smooth regions like the Moon
• The smooth regions are likely ancient lava flows
• The Caloris Basin is the largest impact crater on Mercury
• Region opposite the Caloris Basis is jumbled from seismic energy of impact

Tectonics on Mercury

• Long cliffs indicate that Mercury shrank early in its history

Mars vs Earth

• 50% Earth's radius, 10% Earth's mass
• 1.5 AU from the Sun
• Axis tilt about the same as Earth
• Similar rotation period
• Thin CO2 atmosphere: little greenhouse
• Main difference: Mars is SMALLER

Seasons on Mars

• Seasons on Mars are more extreme in the southern hemisphere because of its elliptical orbit

Storms on Mars

• Seasonal winds on Mars can drive huge dust storms

Climate Change on Mars

• Mars has not had widespread surface water for 3 billion years
• The greenhouse effect probably kept the surface warmer before that
• Somehow Mars lost most of its atmosphere 
• Magnetic field may have preserved early Martian atmosphere
• Solar wind may have stripped atmosphere after field decreased because of interior cooling

Cratering on Venus

• Impact craters, but fewer than Moon, Mercury, Mars

Volcanoes on Venus

• Many volcanoes

Tectonics on Venus

• Fractured and contorted surface indicates tectonic stresses

Erosion on Venus

• Photos of rocks taken by lander show little erosion

Does Venus have plate tectonics?

• Most of Earth's major geological features can be attributed to plate tectonics, which gradually remakes Earth's surface
• Venus does not appear to have plate tectonics, but its entire surface seems o have been "repaved" 750 million years ago

Why is Venus so hot?

• The greenhouse effect on Venus keeps its surface temperature at 470 degrees Celsius 

Atmosphere of Venus

• Venus has a very thick carbon dioxide atmosphere with a surface pressure 90 times that of Earth

Greenhouse Effect on Venus

• Thick carbon dioxide atmosphere produces an extremely strong greenhouse effect
• Earth escapes this fate because most of its carbon and water are in rocks and oceans

Atmosphere of Venus

• Reflective clouds contain droplets of sulfuric acid
• The upper atmosphere has fast winds that remain unexplained

Runaway Greenhouse Effect

• The runaway greenhouse effect would account for why Venus has so little water

What unique features of Earth are important for life?

1. Surface liquid water
2. Atmospheric oxygen
3. Plate tectonics
4. Climate stability

Continental Motion

• Motion of continents can be measured with GPS
• Idea of continental drift was inspired by puzzle-like fit of continents
• Mantle material erupts where seafloor spreads

Seafloor Recycling

• Seafloor is recycled through a process known as subduction

Plate Motions

• Measurements of plate motions tell us past and future layout of continents

Carbon Dioxide Cycle

1. Atmospheric CO2 dissolves in rainwater
2. Rain erodes minerals that flow into the ocean
3. Minerals combine with carbon to make rocks on ocean floor
4. Subduction carries carbonate rocks down into the mantle
5. Rock melts in mantle and outgases CO2 back into atmosphere through volcanoes

Long-Term Climate Change

• Changes in Earth's axis tilt might lead to ice ages
• Widespread ice tends to lower global temperatures by increasing Earth's reflectivity
• CO2 from outgassing will build up if oceans are frozen, ultimately raising global temperatures again

Dangers of Human Activity

• Human made CFCs in the atmosphere destroy ozone, reducing protection from UV radiation
• Human activity is driving many other species to extinction
• Human use of fossil fuels produces greenhouse gases that can cause global warming

Global Warming

• Earth's average temp. has increased by .5 degrees Celsius in the past 50 years
• The concentration of CO2 is rising rapidly
• An unchecked rise in greenhouse gases will eventually lead to global warming

What makes a planet habitable?

• Located at an optimal distance from the Sun for liquid water to exist
• Large enough for geological activity to release and retain water and atmosphere

Planetary Density

• Earth is habitable because it is large enough to remain geologically active, and it is at the right distance from the Sun so oceans could form