Physical Science
Physical Science

Physical Science

Lead Author(s): Trisha Jackson

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A tour of the physical sciences, this text starts with the beginning of the universe, the origin of the solar system, and the Earth and the Moon, setting the stage for an overview of the history of science, and leading seamlessly into Physics and Chemistry. From here, a natural progression into the Earth Sciences commences, including the Atmosphere and Geosphere. Finally, a capstone chapter on climate, soils, and agriculture brings the class together, wrapping up on a relatable note.

Origins of Our World

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Figure 1.1 Earthrise from the Moon captured in 2015 by NASA's Lunar Reconnaissance Orbiter (LRO) .


Physical Science explains the most basic aspects of our physical world, from the physics of gravitational attraction to the chemistry of water.  To launch our exploration of the physical world, we start with the beginnings of our corner of the Universe, including the formation of our solar system, Sun, Earth, and Moon, and how the atmosphere and oceans of the Earth formed.


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  • Understand the origin of our solar system.
  • Explain how the moon formed.
  • Explain how the Earth's oceans and early atmosphere formed.
  • Describe how Earth's modern atmosphere formed.
  • Understand the significance of the Montreal Protocol, the ozone layer, and greenhouse gases.


Origin of the Solar System

The story of our solar system begins with a swirling dust cloud full of the ingredients for our sun, planets, and the other bits that make up our neighborhood of the universe.  This dust cloud, or solar nebula, comes from the remnants of a star that went supernova, or exploded and ejected its mass. Interestingly, the energy from a nearby supernova injects energy into the solar nebula to start a spinning motion.

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As the solar nebula spins, the material becomes more organized into a spiral shape and begins the process of contraction, where the nebula becomes smaller and denser due to gravity. Eventually, at the center of the contracting spiral, a young star, or protostar forms.  


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Figure 1.2. This artist's conception shows a protostar surrounded by a swirling protoplanetary disk. Dust and gas coalesces to create planetesimals that collide and grow to eventually form planets.


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Spiraling around the young star is a protoplanetary disc, swirling material that organizes into planetesimals, or young planets. As the protostar and its protoplanetary disc continues to spin,  gravitational attraction causes accretion, which is the clumping and collision of matter that results in the increasing size of planetesimals.

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At this stage we have increasingly larger planetesimals and denser protostar, all driven by gravity.  With more matter, mostly hydrogen and helium,  coalescing in the protostar, the temperature and pressure builds.  Over tens of millions of years, the temperature and pressure reach a critical point that initiates a fusion reaction of hydrogen, and the sun forms.

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The sun has formed and young planetesimals have undergone accretion and collisions, becoming larger masses.  Jupiter, the largest planet in our solar system, has formed toward the outer edge of the solar system, and begins to migrate toward the sun.  Other planets orbits and objects such as meteors and comets are dramatically impacted by Jupiter's movement.  They begin a slow migration in their orbits, adjusting to Jupiter's and the other planets' positions.  (Watch the following video starting at 2:44 for more detail.)

As shown in the previous video, Jupiter's enormous gravity hurls asteroids and comets toward the sun, which causes some of them to strike the planets, a violent period in our solar system's formation known as bombardment.  Because many of these asteroids struck the inner planets, they became heavily cratered, as we see evidence of on the surface of the moon.

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Figure 1.3. Crater Daedalus photographed from the Apollo 11 spacecraft in lunar orbit. The crater has a diameter of about 50 miles and is surrounded by rugged terrain which is typical of the surface of the farside of the moon.


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Figure 1.4. This photograph is the first taken of the Earth from the vicinity of the moon, taken on August 23, 1966 from the Lunar Orbiter I.


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Q1.06 Bombardment Discussion

Earth bears very little evidence of the period of bombardment, but Mercury, Venus, Mars, and the Moon all show a multitude of craters from the increase in collisions. What processes on Earth explain this difference?


Origin of the Moon

Until recently, there were many ideas about how the moon came to be, but there was little evidence to make a strong case for any of them.  Armed with moon rocks and computer models, scientists finally solved the mystery of the moon's origin.  Watch the following video starting at 19:44 for details about how the mystery was solved.  Take note of the suite of evidence that finally pointed out the best model to explain its existence.

Q1.07 Evidence of the Moon's Origin

Select ALL types of evidence that were used by scientists to determine the origin of the moon.

A

Composition of moon rocks

B

Supercomputer simulation

C

Composition of asteroids

D

Photographs of the moon


Origin of Earth's Atmosphere

The previous discussion of the birth of our solar system, Earth, and moon describes a violent beginning.   The Earth formed 4.6 billion years ago and as it cooled, gases were expelled out of the interior of our planet such as hydrogen (H2), water vapor (H2O), methane (CH4), and carbon oxides (CO2 and CO).   This cooling led to the emergence of Earth's early atmosphere  in the first 500 million years, although the atmospheric composition, or the gases that make up the atmosphere, have changed over time.  

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Figure 1.5. An artist's depiction of what a young Earth would have looked like.

Up until about 3.5 billion years ago, the gases listed above, along with nitrogen (N2), made up Earth's atmosphere.   Note that oxygen (O2), which supports much of the life on Earth, was absent during this ancient time.  Atmospheric oxygen didn't occur until microorganisms started releasing it as they grew.  A chance mutation by cyanobacteria allowed this early form of life to absorb sunlight to make food, a process known as photosynthesis.  A by-product of photosynthesis is the release of oxygen gas.  As this cyanobacteria thrived, the concentration of oxygen in Earth's atmosphere increased.


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Life on Earth dramatically altered the composition of the Earth's atmosphere and the atmosphere, in turn, changed the composition of life on Earth.  For instance, three billion years ago, the sun was only 70% as bright as it is today.  Earth should have been a very cold place, but because the atmosphere contained greenhouse gases, or gases that absorb heat, the Earth remained warm.  At that time on Earth, there were much higher concentrations of methane (CH4) and carbon dioxide (CO2), two potent greenhouse gases. 

Q1.10 Atmosphere and Life

Why didn't the dimmer Sun three billion years ago cause Earth to be much cooler?

A

Because the Earth was much closer to the Sun, which made up for it's dimness.

B

Because the greenhouse gases in the atmophere absorbed heat, keeping the planet warm.

C

Because there was an increase in photosynthesis, which releases heat.


Throughout Earth's history, this interplay between the atmosphere and life on Earth has continued.  The presence of oxygen in the atmosphere promotes life processes on Earth, but it also plays  a role in protecting living creatures from harmful ultraviolet radiation.  High in the atmosphere, some oxygen molecules (O2) absorb ultraviolet (UV) radiation from the sun and split to form single oxygen atoms.  These single atoms combine with O2 molecules to form ozone, O3, which are extraordinarily effective at absorbing UV rays.  The presence of the ozone layer, where ozone molecules are more concentrated in the atmosphere, has provided a shield from UV radiation, allowing life to evolve on Earth.

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Figure 1.6. Human-produced chemicals harming the ozone layer created an ozone hole over the north pole and south pole (shown above). These chemicals were being used until the Montreal Protocol was signed, banning them and allowing ozone levels to stabilize. According to this 2015 NASA article, the ozone layer will continue to recover.


In the early 1970s, scientists recognized that certain human-produced chemicals were harming Earth's protective ozone layer.  Because of this information, governments around the world began working together to negotiate a solution to this global problem.  In 1987, these countries signed the Montreal Protocol, an international treaty banning the harmful chemicals to protect the ozone layer. 

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Life emerged in the early oceans.  It accelerated with the onset of photosynthesis, which released oxygen into the atmosphere.  Life was restricted to the ocean until about 600 million years ago, when there was enough ozone to protect organisms living on the land.


Origin of Earth's Oceans

As we have been discussing, after Earth formed, gases were emitted from the interior of the young planet and it began to cool.  Recall that one of the gases emitted during this early period was water vapor.  As the planet cooled, the water vapor formed clouds and eventually it began to rain.  Scientists estimate that it rained for thousands of years, filling up ocean basins.  The remaining surface water on Earth likely came from space in the form of comets, snowballs of frozen rock and ice.

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Figure 1.7. Halley's Comet, pictured above, one of the earliest recorded comets. It takes 76 years for it to make one pass around the sun.

Recall that a later stage in the formation of our solar system was bombardment, when orbits of comets and meteors from the outer solar system were disrupted by the movement of Jupiter and the other outer  planets.  Many comets and meteors were thrust into the inner solar system, colliding with Earth, the Moon, and the other inner planets.  These objects that collided with Earth brought ice that became part of our planet's vast oceans. 

Recall that a later stage in the formation of our solar system was bombardment, when orbits of comets and meteors from the outer solar system were disrupted by the movement of Jupiter and the other outer  planets.  Many comets and meteors were thrust into the inner solar system, colliding with Earth, the Moon, and the other inner planets.  These objects that collided with Earth brought ice that became part of our planet's vast oceans. 

Q1.12 Origin of Earth's Oceans

Select the two sources of Earth's oceans.

A

gases from the interior of the Earth (including water vapor) which cooled and fell to the surface as rain.

B

the Moon attracted water molecules to the Earth and also controls the ocean tides.

C

meteors and comets bombarding our young planet.

D

alien spaceships brought giant balloons full of water through space to Earth.


Summary Questions

Q1.13 Stages of Solar System Formation

List the stages of solar system formation in the order they occurred.

Premise
Response
1

Stage 1

A

bombardment

2

Stage 2

B

solar nebula

3

Stage 3

C

contraction

4

Stage 4

D

accretion

5

Stage 5

E

protostar

6

Stage 6

F

supernova

7

Stage 7

G

planetesimals

8

Stage 8

H

migration

Q1.14 Moon Origin

Which of the following hypotheses has been accepted as best explaining the moon's origin?

A

The Earth's gravity captured a rogue planet that became the moon.

B

The Earth and Moon formed together as a double system.

C

The early Earth collided with another planet and the debris coalesced to form the moon.

D

The early Earth spun so fast that it expelled a piece of its mass, which became the Moon.

Q1.15 Origin of the Atmosphere and Oceans

Match the following terms regarding the atmosphere with the correct response.

Premise
Response
1

Early atmosphere

A

A molecule that contains three oxygen atoms and makes up a protective layer in our atmosphere.

2

Modern atmosphere

B

Contains a much higher concentration of oxygen.

3

Ozone

C

Composed of high concentrations of greenhouse gases.

4

Montreal Protocol

D

An agreement signed in 1987 to bad ozone-depleting chemicals.

Q1.16 Origin of the Atmosphere and Oceans

Match the following terms regarding the atmosphere with the correct response.

Premise
Response
1

Photosynthesis

A

Cyanobacteria used this to make food from sunlight which releases oxygen as a byproduct.

2

Greenhouse gases

B

Gas from the interior of Earth that cooled and fell as rain to form oceans.

3

Comets

C

Methane and carbon dioxide

4

Water vapor

D

Icy rock that is a source of water on Earth.

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accretion - The clumping and collision of matter that results in the increasing size of planetesimals.

atmospheric composition - The gases that make up the atmosphere.

bombardment - A period during our solar system's formation during which orbital migrations resulted in the peppering of inner planets by asteroids and comets.

comets- Bodies of rock and ice that orbit the sun.

contraction - A stage in solar system formation duriing which the spiraling nebula begins to tighten into a smaller, denser shape.

greenhouse gases - Gases that absorb heat.

migration - Alteration of a planet's orbit in response to interactions with other bodies. 

Montreal Protocol - An international treaty banning the harmful chemicals to protect the ozone layer. 

ozone - A molecule composed of three bonded oxygen atoms.  Ozone near the surface of Earth damages life while ozone in Earth's statosphere protects life from ultraviolet radiation.

ozone layer - A zone in Earth's atmosphere (in the stratosphere) that contains relatively high concentrations of ozone, a gas that absorbs life-harming ultraviolet radiation.

photosynthesis - A chemical reaction utilized by plants to absorb sunlight to make food.

planetesimals - A body of material that has organized into a young planet in orbit around a protostar.

protostar - A contracting mass of dust and gas representing an early stage in star formation.

solar nebula - A swirling disc of dust and gases surrounding a young star.

supernova - A star expelling most of its mass in a brilliant explosion.

Summary

The birth of our solar system began when a nebula of dust and gases starting spinning.  As the cloud of material became more organized, most of it coalesced into the center to form a protostar that would become our Sun.  The leftover material gathered together to form planetesimals.  Eventually, the young protostar gathered enough matter to create the pressure and temperature required to start nuclear fusion, and our Sun was born.  The planets continued to accrete material and adjust their orbits.  One planet collided with Earth, a collision that led to the formation of our moon.  During  orbital migration of the planets, comets and meteors were thrown into the inner solar system, bombarding the rocky planets and moons.  Our moon's surface still holds a record of this period of bombardment with its cratered surface.  

Once bombardment subsided, Earth began to cool, vulcanism slowed, and water condensed and rained down to form oceans.  About half of the oceans' water also came from comets during the period of bombardment.  The cooler Earth and presence of liquid water created ideal conditions for life to start.  Simple microbes inhabited the ocean.  A chance mutation allowed cyanobacteria to conduct photosynthesis, which added significant amounts of oxygen to the atmosphere, a chemical reaction that led to the composition of the atmosphere we have today. 

Today part of our atmosphere, the ozone layer, protects life Earth from harmful radiation from the Sun. Human activities damaged the ozone layer, which were curbed due to the Montreal Protocol, a victory in coordinating  a global effort to heal a hole in the ozone layer.  The Earth's atmosphere also contains greenhouse gases, that help the Earth stay warm.  However, human activities have enhanced the concentration of these gases, adding heat to the Earth system.

Now that we have explored the origins of our world, we have set the foundation for understanding the universe.  The story of the origins of our solar system provides a general idea about how other solar systems formed.  On the other hand, unique conditions within our solar system led to the formation of the moon, and the oceans and atmosphere on Earth where life began.  By understanding our solar system, we can expand this knowledge to our understanding of the universe.

In the next chapter, we explore our Sun in particular and stars in general: how they form, their source of energy, their life cycles, and how some of them become black holes.  In addition, we learn about the tools of astronomy including telescopes and spectroscopy, which helps us gather information about distant objects in the universe. Finally, we come back to our star, the Sun to learn about how it affects life on Earth.


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1.1 Image courtesy of  NASA/Goddard/Arizona State University in the public domain. 

1.2 Image courtesy of NASA in the public domain. Artist: Lars Buchhave. 

1.3 Image courtesy of NASA in the public domain.

1.4 Image courtesy of NASA in the public domain.

1.5 Image courtesy of NASA in the public domain.

1.6 Image courtesy of NASA in the public domain.

1.7 Image courtesy of Halley Multicolor Camera Team, Giotto Project, ESA under the CC BY-SA 3.0 IGO.  


A swirling disc of dust and gases surrounding a young star.
A star expelling most of its mass in a brilliant explosion.
A stage in solar system formation duriing which the spiraling nebula begins to tighten into a smaller, denser shape.
A contracting mass of dust and gas representing an early stage in star formation.
A body of material that has organized into a young planet in orbit around a protostar.
The clumping and collision of matter that results in the increasing size of planetesimals.
Alteration of a planet's orbit in response to interactions with other bodies.
A period during our solar system's formation during which orbital migrations resulted in the peppering of inner planets by asteroids and comets.
The gases that make up the atmosphere.
A chemical reaction utilized by plants to absorb sunlight to make food.
Gases that absorb heat.
A molecule composed of three bonded oxygen atoms. Ozone near the surface of Earth damages life while ozone in Earth's statosphere protects life from ultraviolet radiation.
A zone in Earth's atmosphere (in the stratosphere) that contains relatively high concentrations of ozone, a gas that absorbs life-harming ultraviolet radiation.
An international treaty banning the harmful chemicals to protect the ozone layer.
Bodies of rock and ice that orbit the sun.