Chapter 7

Earth and The Terrestrial Worlds

Principles of Comparative Planetology• Comparative Planetology is the study of the

solar system through examining and understanding the similarities and differences among the planets.

• Planetary Geology:

• The study of surface features and the processes that create them is called geology.

• Today, we speak of planetary geology, the extension of geology to include all the solid bodies in the solar system.

Viewing the Terrestrial Worlds• Spacecraft have visited and

photographed all of the terrestrial worlds. Some have even been landed on!

• Because surface geology depends largely on a planet’s interior, we must first look inside the terrestrial worlds.

Global views and surface close-upsVenus’ surface- atmosphere is not shown. Surface mapped from Megellan spacecraft radar data

• Surface Views of some of the terrestrial worlds.

• Venus, the Moon and Mars have all been landed on successfully by spacecraft from Earth.

Venus – Venera Missions (1961-1983)

Apollo Lunar Missions (1969-1972)Links

Mars Exploration Rover Mission: The Mission

Mars PathfinderMars Pathfinder Mission (1996-1997)

Inside the Terrestrial Worlds

• When subjected to sustained stress over millions to billions of years, rocky material slowly deforms and flows.

• Rock acts more like Silly PuddyTM , which stretches when you pull it slowly but breaks if you pull it sharply.

• The rocky terrestrial worlds became spherical because of rock’s ability to flow.

• When objects exceed about 500 km in diameter, gravity can overcome the strength of solid rock and make a world spherical

• Gravity also gives the terrestrial worlds similar internal structures.

• Distinct layers are formed by differentiation.

• Differentiation is the process by which gravity separates materials according to their density.

• This resulted in three layers of differing composition within each terrestrial planet.

• Core• Mantle• Crust

• Lithosphere: Outer layer of relatively rigid rock that encompasses the crust and the uppermost mantle.

• Heat flows from the hot interior to the cool exterior by conduction and convection.

• Condution: Heat transfer as a result of direct contact.

• Convection: Heat transfer by means of hot material expanding and rising and cool material contracting and sinking.

• A small region of rising and falling material is called a convection cell.

Shaping Planetary Surfaces

• Impact Cratering: the excavation of bowl-shaped depressions (impact craters) by asteroids or comets striking a planet’s surface.

• Volcanism: the eruption of molten rock, or lava, from a planet’s interior onto it’s surface.

• Tectonics: the disruption of a planet’s surface by internal stresses.

• Erosion: the wearing down or building up of geological features by wind, water, ice, and other phenomena of planetary weather.

There are four main geological processes

Impact Process

Impact Ejecta

Ejecta Blanket

Cratering

Volcanism

c) “Sticky” lava makes steep-sloped stratovolcanoes.

(Mount St. Helens)

Picture by US Geological Survey scientist, Austin Post, on May 18, 1980.

Tectonic Forces at work. Convection

Cells

Comparing Planetary Atmospheres

Atmospheric Structure

Visible Light: Warming the Surface and Coloring the Sky

Atmospheric gases scatter blue light more than they scatter red light.

Longer wavelength red light is more penetrating

Infrared Light: the Greenhouse Effect, and the Tropsosphere

• The Troposphere becomes warmer than it would if it had no greenhouse gases.

• Greenhouse gases include:

– CO2

– Water Vapor

The Greenhouse Effect

Temperatures of the Terrestrial Worlds

• Ultraviolet light is absorbed in the Stratosphere.

• X-Rays are absorbed in the Thermosphere and Exosphere.

The Magnetosphere

• The Magnetosphere blocks the Solar Wind

• This produces two regions where the charged particles get trapped – Van Allen Belts.

• The interaction of the charged particles from the solar wind near the poles, produces the:

– Aurora Borealis (Northern Lights)

– Aurora Australis (Southern Lights)

Aurora Borealis – Norhern Lights

Atmospheric Origins and Evolution

• Outgassing from Volcanic activity was most responsible for producing the earth’s early atmosphere. (Volcanoes give off H2O, CO2, N2, and sulfur compounds.

• As life developed, it too influenced the atmosphere of the Earth, allowing it to become what it is today. (e.g. plants give off O2 and consume CO2)

Many gases can escape from the planet if their thermal speed is greater than the escape speed of the planet.

Five Major Processes By Which Atmospheres Lose Gas.

A Tour of the Terrestrial Worlds

The Moon 1,738-km radius, 1.0AU from the Sun

Astronaut explores a small craterAn ancient lava river

Mercury (2,440-km radius, 0.39AU from the Sun)

Mars (3,397-km radius, 1.52 AU from the Sun)

Polar Ice Cap (Mars) Viking Orbiter

Dust Storm over northern ice cap, Mars Global Surveyor

Edge of polar ice cap showing layers of ice and dust.

Olympus Mons: – largest shield volcano in the solar system

Cratering, Volcanism and Tectonics

Heavy cratering in Southern Hemisphere

(Mars)

Valles Marineris

Martian outflow channels and flood planesAncient River beds Outflow channels indicate

catastrophic flooding

Water eroded crater

Gullies on a crater wall formed by water flows?

Venus (6,051-km radius, 0.72 AU from Sun)

Impact craters

on Venus

are rare

Fractured and

twisted crust

Shield Volcanoes are common

Earth (6, 378 km radius, 1.0 AU from the Sun)

Time-Line of Geologic Activity

End of Section