© 2004 Pearson Education Inc., publishing as Addison-Wesley 14. Planet Earth: Seen in a New Light Loren Acton (1936 – ) U.S. astronaut Looking outward

© 2004 Pearson Education Inc., publishing as Addison-Wesley

14. Planet Earth: Seen in a New Light

Loren Acton (1936 – )Loren Acton (1936 – )U.S. astronautU.S. astronaut

Looking outward to the blackness of space, Looking outward to the blackness of space, sprinkled with the glory of a universe of sprinkled with the glory of a universe of lights, I saw majesty – but no welcome. lights, I saw majesty – but no welcome. Below was a welcoming planet. There, Below was a welcoming planet. There, contained in the thin, moving incredibly contained in the thin, moving incredibly fragile shell of the biosphere is everything fragile shell of the biosphere is everything that is dear to you, all the human drama and that is dear to you, all the human drama and comedy. That’s where life is; that’s where comedy. That’s where life is; that’s where all the good stuff is.all the good stuff is.


14.1 How is Earth Different?

• Describe five major ways in which Earth differs from other worlds in our solar system.

Our goals for learning:


What Features are Unique to Earth among the Planets?

• plate tectonics• only planet with a surface shaped by this type of tectonics

• atmospheric Oxygen• only planet with significant Oxygen in its atmosphere

• surface liquid water• only planet where temperature & pressure allow surface water

to be stable as a liquid

• climate stability– differs from Venus & Mars in having a relatively stable

climate

• life– only world known to have life; it certainly has the most

abundant & diverse life in the Solar System


14.2 Our Unique Geology

• Describe the conveyor-like action of plate tectonics.

• Why does the Earth have two types of crust?• How does plate tectonics explain mountain

ranges, earthquakes, and island chains such as Hawaii?

• How has plate tectonics changed the Earth’s appearance through time?



Plate Tectonics• Earth’s lithosphere is fractured into more than a dozen plates.

• These plates “float” on top of the Earth’s mantle.

• Convection in the mantle cause the plates (and continents) to move.


Earth’s Crust

• Seafloor crust• high-density basalt

• young (< 200 million yrs)

• 5 to 10 km thick

• Continental crust• low-density (e.g. granite)

• older, as much as 4 billion yrs old

• 20 to 70 km thick

Earth has two types of crust:


Plate Tectonics• New basaltic crust emerges at mid-ocean ridges.

• it is pushed away in both directions (seafloor spread)

• Seafloor crust is pushed under continental crust at ocean trenches.• this is called subduction, the seafloor crust is partially molten• the less dense lava erupts through volcanoes to form new continental crust


Geological Features of Plate Tectonics• Continents are shaped by:

• volcanism• stresses from plate tectonics • erosion

• Subduction zones cause:• volcanic eruptions which form

mountain ranges• islands to be scraped off seafloor

plates onto continents

• When two continental plates collide• one can not subduct under the other

• crust is pushed up to form mountain ranges

• e.g. the Himalayas


Geological Features of Plate Tectonics

• When two continental plates pull apart• a rift valley is created

• mantle convection causes eruption of basalt from valley floor

• a new zone of seafloor spreading is created

• e.g Arabian peninsula detached from Africa to form the Red Sea

• When two plates slip sideways against each other• rough grinding of plates builds up

pressure along the crack between them

• this crack is called a fault

• pressure eventually breaks, causing a sudden shift, or earthquake


Geological Features of Plate Tectonics

• Not all volcanoes occur near plate boundaries.• a plume of hot mantel rock can

rise within a plate• we call this a hot spot

• These hot spot volcanoes form islands• as a plate moves over a hot

spot, a chain of islands, like Hawaii, is formed

• without plate tectonics moving the plate, we would have a huge volcano like Olympus Mons on Mars


Earth’s Appearance: Past, Present, & Future• Plates (and continents) move at 2 cm/year.

• that’s 2,000 km in 100 million years

• We can project the motion of continents into the past or future.• 200 million years ago all continents were connected into one supercontinent

• Total continental area has increased with time as new continental crust has formed.


14.3 Our Unique Atmosphere and Oceans

• Where did all the water in our oceans come from?

• Where is all of Earth’s outgassed carbon dioxide?

• Why does Earth’s atmosphere have oxygen and a stratosphere?



The Uniqueness of Earth’s Atmosphere

• Outgassing from volcanoes on Venus, Earth, & Mars released the same gasses:• primarily water (H2O), Carbon dioxide (CO2), and Nitrogen (N2)

• So why did Earth’s atmosphere end up so different?• why did Earth retain most of its H2O – enough to form oceans?

• why does Earth have so little CO2 in its atmosphere, when it should have outgassed just as much CO2 as Venus?

• why does Earth have so much more Oxygen (O2) than Venus & Mars?

• why does Earth have an ultraviolet-absorbing stratosphere?


The Uniqueness of Earth’s Atmosphere • Earth’s H2O condensed because of the temperature.

• oceans formed, in which the CO2 gas dissolved

• chemical reactions bound the C of CO2 into rocks like limestone

• low level of atmospheric CO2 causes moderate greenhouse effect

• temperatures on Earth remain where H2O can be a liquid CO2

• There was once liquid H2O on Mars and maybe Venus.

• before CO2 could dissolve out, temperatures fell/rose so that

• oceans boiled away on Venus and froze out on Mars

• Earth’s O2 was not outgassed by volcanoes.

• O2 is a highly reactive chemical

• it would disappear in a few million years if not replenished

• no geologic process creates O2


The Uniqueness of Earth’s Atmosphere

• Earth’s O2 was created through the evolution of life.

• plants & microorganism release O2 via photosynthesis

• they convert CO2 into O2

• In the upper atmosphere, O2 in converted into ozone (O3).• via chemical processed involving Solar ultraviolet light

• O3 absorbs Solar UV photons which heats the stratosphere

• Venus & Mars lack plant & microbial life.• so they have no O2 in their atmospheres and no stratospheres


14.4 Climate Regulation and the CO2 Cycle

• What is the carbon dioxide cycle?

• How does the CO2 cycle regulate the climate?

• Is the Earth’s climate perfectly stable?



The CO2 Cycle


The CO2 Cycle is a Feedback Mechanism which Regulates Earth’s Climate


Stability of Earth’s Climate• Plate tectonics causes the relative stability of Earth’s climate.

• plate tectonics makes the CO2 cycle work

• it takes about 40,000 years for the CO2 cycle to restore balance

• There have been temporary episodes of extreme cooling and heating in Earth’s history.• these ice ages & hothouse period have their own feedback mechanisms


14.5 Life on Earth

• When did life arise on Earth?

• Describe a scenario by which chemicals on the early Earth might have made living organisms.

• How did life diversify?

• How did life create our oxygen atmosphere?

• What was the Cambrian explosion?



Studying Past Life• We know the history of life on Earth by studying the fossil record.

• fossils are more difficult to find as we look back to earlier epochs• more organisms which lacked skeletons leave fewer fossils

• erosion erases much old fossil evidence

• subduction destroys fossils carries deep beneath Earth’s surface

• we have found fossils of large & small animals, plants, microorganisms

• the fossil record goes back 3.5 billion years• deficit of 13C, a sign of life, in rocks as old as 3.85 billion years


Origin of Life• All known organisms:

• build proteins from same subset of amino acids

• use ATP to store energy in cells

• use DNA molecules to transmit genes

• All organisms share same genetic code…sequence of chemical bases

• Organisms have similar genes.

• Indicates that all living organisms share a common ancestor.

• Life on Earth is:• divided into three major groupings

• plants & animals are just two tiny branches


Origin of Life• We have no direct evidence of when or how life began.• We have a plausible scenario of how chemistry begat biology:

• chemicals found on Earth, “sparked” by lighting, can form complex organic molecules naturally

• RNA can form, and if some of it becomes self-replicating, it can lead to…• DNA and full, self-replicating organisms.

• Alternatively, this process could have begun on Mars or Venus.• life could have been transported to Earth via meteorites

• organic molecules have been found in meteorites


Evolution of Life

• The oceans were full of single-celled life 3.5 billion years ago.• conditions on land were too inhospitable due to lack of O3 in atmosphere

• Organism’s DNA is reproduced, but can change due to copying errors or external factors • a change in the base sequence of an organism’s DNA is called a mutation

• Some mutations are lethal, others make the cell better able to survive.• those organisms which are better at adapting to their environment… thrive

• this process is called natural selection, first proposed by Charles Darwin

• Natural selection helps some species to dominate, and creates entirely new species from older ones.• life on Earth rapidly diversified

• Some 2 billion years ago, life was still confined to the oceans.


The Rise of Oxygen

• Single-celled organisms called cyanobacteria appear to have created O2 via photosynthesis as early as 3.5 billion years ago.

• the first O2 produced was absorbed into rocks via chemical reactions

• it was not until about 2 billion years ago than the rocks were saturated with O2 and so it began to accumulate in the atmosphere

• fossil record indicates current levels of O2 were reached 200 million yrs ago

• The build-up of O2 in the atmosphere permitted:• the evolution of oxygen-dependent animals

• the formation of O3 in the stratosphere, making it safe for life to move out onto dry land

• Plants first appeared on land some 475 million years ago.

• Animals soon followed…


The Rise of Humans• Some 540 million years ago, most of the life on Earth was still

one-celled and tiny.

• For the next 40 million years, until 500 million years ago:• there was a dramatic increase in the number of species, especially animals

• animals diversified into all the basic body plans which we find today

• this incredible diversification of species is called the Cambrian explosion

• The first humans appeared a few million years ago.• humans civilization is 10,000 years old, industrial society 200 years old

• Although latecomers to the scene, humans are the most successful species to survive on Earth

• Human population has grown exponentially

• How will this affect our host – the Earth?


14.6 Our Future Survival: Lessons from Other Worlds

• Why is global warming a significant threat to our future?

• What is ozone depletion and why is it dangerous?

• In what sense are we now causing a mass extinction?



Global Warming• Venus is a searing example of what can happen to a planet with

large amounts of CO2 in its atmosphere.

• Human activity has added CO2 and other greenhouse gases to Earth’s atmosphere, through burning of fossil fuels.• could the greenhouse effect raise Earth’s average surface temperature?

• Measurements have shown:• amount of CO2 in atmosphere

has increased over past 50 yrs

• over the same period, Earth’s average surface temperature has risen by 0.5º C

• Understanding of greenhouse effect is sound.• increase in greenhouse gases

cause warming in long-term


Global Warming

• These observable facts individually do not prove that human activity has caused global warming to date.

• Computer models contain unknowns.• there could be negative feedback processes which cool the Earth

• there could be positive feedback processes which further heat Earth

• it is difficult to predict short-term consequences

• our studies of Venus (& Earth) make the long-term warming more certain

• Consequences of global warming• weather would distribute the temperature changes

• increased evaporation from oceans would cause more intense storms

• polar ice would melt, increasing sea levels and flooding coastal regions

• effect on ecosystems is much harder to predict


Ozone Depletion

• O3 in the stratosphere shields Earth’s surface from Solar UV

• A depletion of O3 was observed• ozone hole over Antarctica

• discovered in mid-1980’s

• appears in Antarctic spring

• The apparent cause of the ozone hole is a man-made chemical, CFC• used as a refrigerant, CFC is inert and rises to the stratosphere

• Solar UV photons break down CFCs and create Cl – Chlorine gas

• Cl serves as a catalyst in destroying O3

• reaction goes faster at low temperatures … like over Antarctica

• Mars is an example of no ozone, UV photons sterilize the surface.• increased UV on Earth would increase cancer rates and genetic mutations

1979

1998


Mass Extinctions

• Evolution of species normally occurs gradually.• one species goes extinct per century

• Evolution can receive a jolt during a mass extinction.• historically, this has been caused by an impact

• even species not directly killed by the impact will soon go extinct due to lack of food and changes in the ecosystem

• species at the top of the food chain are most susceptible

• Human activity in recent times have driven many more species to extinction.• could we be undergoing an episode of mass extinction now?

• what will the consequences be for humans?


What have we learned?• Describe five major ways in which Earth differs from other

worlds in our solar system.• Plate tectonics, atmospheric oxygen, surface liquid

water, climate stability, life.


What have we learned?• Describe the conveyor-like action of plate tectonics.

• New crust emerges at mid-ocean ridges, leading to seafloor spreading. Subduction at trenches sends seafloor crust plunging back into the mantle.

• Why does the Earth have two types of crust? • Dense seafloor crust emerges at mid-ocean ridges and

remains on the surface only until it subducts under continental crust. Near subduction zones, the seafloor crust begins to melt, releasing a lower-density lava which erupts from volcanoes to make new continental crust.


What have we learned?• How does plate tectonics explain mountain ranges,

earthquakes, and island chains such as Hawaii? • Mountain ranges are built up by volcanic eruptions near subduction

zones or by rocks bulging up where continental plates collide. Earthquakes occur along plate boundaries, including those where two plates are sliding past one another, sometimes getting stuck and allowing pressure to build until they lurch violently in an earthquake. The Hawaiian islands are a chain of volcanic islands still forming as a plate moves over a mantle hot spot.

• How has plate tectonics changed the Earth’s appearance through time? • About 200 million years ago, the continents were all together in one

large “supercontinent.” Plate tectonics has moved continents around for much if not all of Earth’s history. The total continental area also increases with time as new continental crust is made.


What have we learned?• Where did all the water in our oceans come from?

• Water vapor trapped in Earth’s interior after formation was outgassed by volcanoes and condensed to make rain, accumulating into the oceans.

• Where is all the Earth’s outgassed carbon dioxide? • The vast majority of it is locked up in carbonate rock, formed by

chemical reactions after carbon dioxide dissolves in the oceans.

• Why does the Earth’s atmosphere have oxygen and a stratosphere? • Life releases oxygen from carbon dioxide through

photosynthesis. Atmospheric oxygen enables ozone to form and absorb ultraviolet light in the stratosphere.


What have we learned?• What is the carbon dioxide cycle?

• Carbon dioxide dissolves in the oceans and reacts with silicate minerals to make carbonate rock. Plate tectonics carries the carbonate rocks to subduction zones, where they plunge into the mantle. The carbonate rock then melts, releasing its carbon dioxide into the atmosphere through volcanic eruptions.

• How does the carbon dioxide cycle regulate the climate? • Through feedback processes, it tends to counter act any

warming by removing carbon dioxide from the atmosphere and counteract cooling by adding carbon dioxide to the atmosphere.


What have we learned?• Is the Earth’s climate perfectly stable?

• No. Ice ages and perhaps even snowball Earth episodes have occurred in the past. In the future, the warming Sun will eventually lead to a runaway greenhouse effect.

• When did life arise on Earth? • At least 3.5 billion years ago, and perhaps much earlier than that.

• Describe a scenario by which chemicals on the early Earth might have made living organisms. • Chemicals known to be present on Earth react naturally to make

complex organic molecules, including chains of RNA. Some of the RNA may have become self-replicating, enabling chemical competition that could ultimately have led to DNA and full, self-replicating organisms.


What have we learned?• How did life diversify?

• Once living organisms with DNA existed, they evolved as mutations changed their DNA and natural selection allowed organisms that were better adapted to their environment to survive and multiply.


What have we learned?• How did life create our oxygen atmosphere?

• Cyanobacteria began releasing oxygen through photosynthesis, but for at least a billion years chemical reactions with surface rock removed the oxygen as fast as it was made. Once the surface rock was saturated with oxygen, the gas began to build up in the atmosphere.

• What was the Cambrian explosion? • An explosion in the diversity of life, especially animal

life, that took place over about 40 million years, beginning about 540 million years ago.


What have we learned?• Why is global warming a significant threat to our future?

• There is no doubt that human activity is increasing the concentration of carbon dioxide in the atmosphere, which should tend to strengthen the greenhouse effect and warm the Earth. The latest computer models of the climate are consistent with observed warming, and predict further warming in the future. Warming can change local climates, raise sea level, alter ocean currents, and alter ecosystems in ways that we cannot fully anticipate.

• What is ozone depletion and why is it dangerous? • Ozone depletion is a reduction in stratospheric ozone caused by

human-made chemicals known as CFCs. It allows more ultraviolet light to reach the surface, increasing mutation rates among living organisms.


What have we learned?• In what sense are we now causing a mass extinction?

• At current rates, in this century we may drive so many species extinct that it will look like one of the great mass extinctions of geological history.

Documents

© 2004 Pearson Education Inc., publishing as Addison-Wesley 14. Planet Earth: Seen in a New Light Loren Acton (1936 – ) U.S. astronaut Looking outward