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Comparing the Terrestrial Worlds

Reading: Chapter 13, Celestial profile cards (Appendix C)

Terrestrial worlds have similar origins but have evolved very differently. Why are there no Martians, Venusians, Mercurians or Lunatics ?

Comparing the Terrestrial Worlds

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Earth’s Interior •  Core: Highest density;

nickel and iron •  Mantle: Moderate

density; silicon, oxygen, etc.

•  Crust: Lowest density; granite, basalt, etc.

•  A planet’s outer layer of cool, rigid rock is called the lithosphere. It “floats” on the warmer, softer rock that lies beneath

•  Vibrations that travel through Earth’s interior tell us what Earth is like on the inside

Seismic Waves How do we know what’s inside a

planet?

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Seismic Waves How do we know what’s inside a

planet? •  P waves push matter

back and forth •  S waves shake matter

side to side •  P waves go through

Earth’s core but S waves do not

•  We conclude that Earth’s core must have a liquid outer layer

Reason for layers: Differentiation

•  Gravity pulls high-density material to center

•  Lower-density material rises to surface

•  Material ends up separated by density

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Thought Question What is necessary for differentiation to occur in a

planet?

a) It must have metal and rock in it b) It must be a mix of materials of different density c) Material inside must be able to flow d) All of the above e) b and c

Heating of Interior •  Accretion and

differentiation when planets were young

•  Radioactive decay is most important heat source today

•  Interior heat drives geological activity

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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

Terrestrial Planet Interiors •  Applying what we have learned about Earth’s

interior to other planets tells us what their interiors are probably like

•  The terrestrial worlds are made up of rock and metal.

•  They are all differentiated: – Rocky, low-density crusts – High-density metal cores – Mantles composed of dense rock between

the cores and crusts

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Role of Size in Heating and Cooling

•  Smaller worlds cool off faster and harden earlier •  Mercury and the Moon are now geologically “dead” since

their interiors have cooled down.

Sources of Magnetic Fields •  A world can have

a magnetic field if charged particles are moving inside

•  3 requirements: – Molten interior – Convection – Moderately rapid

rotation

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Processes that Shape Planetary Surfaces

•  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 solar system formed

•  All planets were equally impacted

•  Craters are about 10 times wider than object that made them

•  Small craters greatly outnumber large ones

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Impact Craters

Meteor Crater (Arizona)

Tycho (Moon)

Standard crater: Mars

History of Cratering

•  Most cratering happened in first billion years

•  A surface with many craters has not changed much in 3 billion years

•  Erosion can erase craters. Thus fewer craters indicate geological activity

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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

•  Volcanism also releases gases from interior into atmosphere: Outgassing

Iceland

Volcano on Io

Tectonics

•  Convection of the mantle creates stresses in the crust called tectonic forces

•  Compression forces make mountain ranges •  Valley can form where crust is pulled apart •  Earth’s continents slide around on separate plates of

crust: plate tectonics

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Erosion •  Weather-driven processes that break

down or transport rock •  Processes that cause erosion include

glaciers, rivers, wind

•  Erosion not only wears down features, it also builds them: sand dunes, river deltas sedimentary rock

canyons carved by rivers

valleys shaped by glaciers Sand blown by wind

Comparison of Planetary Surfaces •  Mercury & the Moon

•  heavily cratered {scars from the heavy bombardment} •  some volcanic plains

•  Venus •  volcanoes and bizarre bulges

•  Mars •  volcanoes and canyons •  apparently dry riverbeds {evidence for running

water?}

•  Earth •  all of the above plus liquid water and life

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Geology of Earth •  most active geology •  volcanoes & tectonics

•  ongoing plate tectonics •  moderate atmosphere

•  N2 O2 H2O •  H2O exists in liquid state

•  rampant erosion •  few craters

•  life

Continental Motion

•  Earth’s crust is divided into moving sections called plates

•  Idea of continental drift was inspired by puzzle-like fit of continental plates

•  Mantle material erupts where seafloor spreads

•  Motion of continents can be measured with GPS

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Surface Features

Earthquakes, volcanism, and mountain building are linked to motions of the crust and the location of plate boundaries.

Surface Features •  Himalayas are forming

from a collision between plates

•  Red Sea is forming where plates are pulling apart

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Rifts, Faults, Earthquakes •  San Andreas

fault in California is a plate boundary

•  Motion of plates causes earthquakes

Hot Spots

•  Hawaiian islands have formed where plate is moving over volcanic hot spot

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Earth’s Atmosphere

•  Consists mostly of molecular nitrogen (N2) and oxygen (O2)

•  Most of Earth’s gas is < 10 km from surface, but a small fraction extends to >100 km

Thought Question Why is the sky blue?

a) The sky reflects light from the oceans. b) Oxygen atoms are blue. c) Nitrogen atoms are blue. d) Air molecules scatter blue light more than red light. e) Air molecules absorb red light.

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Thought Question Why is the sky blue?

a) The sky reflects light from the oceans. b) Oxygen atoms are blue. c) Nitrogen atoms are blue. d) Air molecules scatter blue light more than red light. e) Air molecules absorb red light.

Why the sky is blue •  Atmosphere scatters

blue light from Sun, making it appear to come from different directions

•  Sunsets are red because red light scatters less

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Magnetospheres •  The Sun ejects a stream of charged particles, called the

solar wind. •  it is mostly electrons, protons, and Helium nuclei

•  Earth’s magnetic field attracts and diverts these charged particles to its magnetic poles. This protective “bubble” is called the magnetosphere •  the particles spiral along magnetic field lines and

emit light •  this causes the aurora (aka northern & southern

lights) •  Other terrestrial worlds have no strong magnetic fields

Earth’s Magnetosphere

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Greenhouse Effect

•  Visible Sunlight passes through a planet’s atmosphere.

•  Some of this light is absorbed by the planet’s surface.

•  Planet re-emits this energy (heat) as infrared (IR) light.

•  IR light is “trapped” by the atmosphere. its return to space is slowed

•  The overall surface temperature is thus higher than if there were no atmosphere at all.

•  Key to Greenhouse Effect…gases which absorb IR light effectively: •  water [H2O] •  carbon dioxide [CO2] •  methane [CH4]

•  These are molecules which rotate and vibrate easily. •  they re-emit IR light in a

random direction •  The more greenhouse gases

which are present, the greater the amount of surface warming.

Greenhouse Gases

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Earth’s Water and CO2 •  Earth’s temperature

remained cool enough for liquid oceans to form

•  Oceans dissolve atmospheric CO2, enabling carbon to be trapped in rocks

Nitrogen and Oxygen •  Most of Earth’s

carbon and oxygen is in rocks, leaving a mostly nitrogen atmosphere

•  Plants release some oxygen from CO2 into atmosphere

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Ozone and the Stratosphere •  Ultraviolet light can

break up O2 molecules, allowing ozone (O3) to form

•  Without plants to release O2, there would be no ozone in stratosphere to absorb UV light

Carbon Dioxide Cycle 1.  Atmospheric CO2 dissolves

in rainwater 2.  Rain erodes minerals which

flow into ocean 3.  Minerals combine with

carbon to make rocks on ocean floor

4.  Subduction carries carbonate rocks down into mantle

5.  Rock melt in mantle and outgas CO2 back into atmosphere through volcanoes

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Earth’s Thermostat

•  Cooling allows CO2 to build up in atmosphere •  Heating causes rain to reduce CO2 in atmosphere

Dangers of Human Activity

•  Human-made CFCs in atmosphere destroy ozone, reducing protection from UV radiation

•  Human use of fossil fuels produces greenhouse gases that can cause global warming

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Global Warming •  Earth’s average

temperature has increased by 0.5°C in past 50 years

•  Concentration of CO2 is rising rapidly

•  An unchecked rise in greenhouse gases is causing global warming