STUDY GUIDE
GRADE 11/12 | EARTH AND LIFE SCIENCE
UNIT 2
Why Life on Earth is Possible
In the previous unit, you have learned how the universe and the solar system began according
to different theories. How about Earth? How did it form? And why is it the only planet in the solar
system capable of sustaining life? These are the questions that you will answer in this unit.
Objectives
In this unit, the students will be able to
•• explain how Earth was formed according to accretion hypotheses;
•• explain why water is considered as the medium of life;
•• describe the sun as the major source of energy; and
•• describe the characteristics of Earth’s atmosphere.
Review
Review the origin of the solar system.
•• The nebular theory explains that the solar system originated from a nebula — a cloud made
up of dust and ionized particles.
•• The nebular theory describes an exploding star or supernova disrupting a nearby nebula.
This disruption created areas of high density. The denser the nebula became, the more heat
it produced.
•• At the center of the nebula, a ball of hydrogen was formed. The increase in temperature at
this center triggered nuclear fusion. This fusion resulted in the formation of the sun. As the
sun grew in size, the gases in the nebula formed a disk and spiraled more quickly around
its center. The particles that were not sucked up by the sun formed as rings. These rings of
particles collided with each other and stuck together to create larger, spherical bodies in a
process called accretion. They rotated and combined to form planets.
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Learn about It!
I.
The Origin of Planet Earth
•• There are two hypotheses on how the structure of Earth was formed: homogeneous and
heterogeneous accretion hypotheses.
•• According to homogeneous accretion hypothesis, the formation of Earth began
after the condensation of fine particles of the primitive nebula about 4.6 billion years
ago. When these particles accereted, they formed a homogeneous primordial Earth.
Thus, early Earth had a uniform solid composition. Its primary components were iron,
magnesium, nickel, silicates, and some radioactive elements such as uranium and
thorium.
«« Due to gravitational contraction and decay of radioactive elements, the temperature
of early Earth increased. Iron and nickel melted, and they sank towards the center
because of their high density. On the other hand, less dense silicates were displaced,
and they moved upwards.
«« According to the hypothesis, it took many years for iron and nickel to accumulate
and reach the center of about 4000 miles deep. During this time, Earth’s surface
experienced turmoil, violent earthquakes, continual volcanic eruptions, and covering
of the surface with flowing lava.
«« Eventually, iron and nickel accumulated as Earth’s core. As Earth cooled, a thin layer
of solid rock formed the crust including the continents and ocean basins. In between
the core and the crust is the mantle, which is made up of semi-molten silicate rocks
and other minerals.
Fig. 1 Homogeneous accretion hypothesis: (a) Earth began with a uniform composition;
(b) Gravitational contraction resulted in differentiation; (c) Earth’s layers
•• According to heterogeneous accretion hypothesis, the core has formed at the same
time as Earth. Therefore, early Earth had its basic layered structure with core, mantle,
and crust.
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«« According to this theory, as the nebula cooled down, its particles have condensed
depending on their condensation points. Oxides of aluminum and calcium condensed
first, followed by iron and nickel. When the nebula cooled further, the silicates
condensed.
«« The condensed particles collided with each other and accreted. The particles that
formed in the initial stage of condensation accreted first. Therefore, aluminum and
calcium oxides accreted first, followed by iron and nickel. These particles formed
Earth’s center or core.
«« The outermost layer is composed of silicates, as well as volatile particles including
water.
•• Homogeneous and heterogeneous accretion hypotheses have supporting evidence and
loopholes.
Table 1. Homogeneous vs. Heterogeneous Accretion Hypotheses
Homogeneous Accretion
Hypothesis
Heterogeneous Accretion
Hypothesis
Main Point
Earth accreted from materials
of the same composition after
condensation. Accretion was
followed by differentiation.
Earth accreted during
condensation, forming a
differentiated planet as it grew in
size.
Supporting
Statements
The homogeneous accretion
hypothesis provides a mechanism
that explains the presence of
volatile elements in the core. It
also provides an explanation of
the heat source for early mantle
melting (and formation of early
continents).
The heterogeneous accretion
hypothesis qualitatively explains
the density differences among
terrestrial planes (Mercury,
Venus, Earth, and Mars). Also,
it can explain the abundance of
elements such as osmium, iridium,
ruthenium, and rhodium in the
mantle.
Loopholes
The hypothesis cannot explain the
abundance of elements such as
osmium, iridium, ruthenium, and
rhodium in the mantle.
Accretion must be very fast (10
4
to 10 years for completion). This
rate does not coincide with the
occurrence of large impact craters.
Also, the abundances of iron,
calcium, titanium, and aluminum
do not coincide with what was
predicted by the theory.
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•• The more commonly accepted postulate is the homogeneous accretion hypothesis —
Most materials that formed early Earth homogeneously accreted after their complete
condensation. After the formation of early Earth, collisions with meteorites and comets
resulted in the presence of volatile elements on the surface.
•• The Earth is considered as a dynamic planet. It continuously changes ever since
its formation 4.6 billion years ago. Through time, several changes happened in the
geographic distribution of continents and composition of the atmosphere.
II.
Water: The Medium of Life
•• Life on Earth is possible because of water.
«« Water served as the medium of first lifeforms. This medium dissolved early Earth
molecules which reacted and formed more complex molecules.
«« Water has the right density, transition temperatures, and heat capacity that enable
existence and perpetuation of life.
ŠŠ Ice floats on the surface of liquid water because of lower density. Ice insulates the
underlying liquid and prevents the liquid from further freezing. If ice sinks when
frozen, then the surface of the liquid water will freeze and sink again until such
point that all water will be frozen, making chemical reactions impossible.
ŠŠ Water has high heat capacity, so it can store large amount of heat and serve as a
heat engine. The oceans and other bodies of water store heat given off by the sun.
This storage dictates Earth’s climate throughout the seasons. Without the water’s
capacity to store heat, the atmosphere will be extremely cold during winter and
extremely hot during summer.
ŠŠ Water is liquid at room temperature. As a liquid, it flows and allows transfer of
substances from the cell to its environment and vice versa.
•• The prevailing hypothesis on the origin of water on Earth suggests that water came from
comets that collided with Earth. In 2000, scientists investigating LINEAR S-4 comet had
discovered that water from the comet had the same isotopic composition as the water in
the seas. (Recall that isotopes are atoms with the same number of protons but different
number of neutrons.)
•• Other studies suggest that water was already present within Earth since formation as
volatiles trapped in magma, and manifested as liquid water during degassing after the
crust had formed. The truth may be a mixture of both theories.
•• At present, all water on Earth (water vapor, liquid water, and ice) comprise the
hydrosphere. Hydrosphere includes all bodies of water such as oceans, lakes, rivers, and
marshes. Clouds, snow, glacier, and rain are also part of the hydrosphere.
•• Hydrosphere is comprised of 97.5% saltwater and 2.5% freshwater.
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«« Saltwater or saline water comprises the oceans and seas. The ocean houses many
species of marine life and diverse mineral resources.
«« Freshwater accounts for only 2.5% of the total water on Earth. It can be present in the
form of rain and snow, and it can even be found in permanently frozen soil known as
permafrost. It can also be stored in rivers, streams, ponds, lakes, marshes, glaciers,
and polar caps.
•• In modern civilizations, water has a variety of uses.
«« Water is commonly used in irrigation of crops in the field of agriculture. About 70%
of global freshwater use is for agriculture. For domestic purposes, about 10% of
freshwater is used for drinking water and bathing. For industrial purposes, about 20%
of water is used globally.
«« Waterways provide cheap and easy transport for both people and goods. (In the past,
transportation through waterways was the standard for trade networks.)
«« The manufacturing and power generation industries rely heavily on water. For
instance, the production of a single car uses up to 110,000 liters of water. Power
companies also use steam to drive turbines in generators.
III.
Sun as the Main Source of Energy
•• The distance of Earth from the sun allows the planet to receive the right amount of solar
energy (the energy from the sun) to enable existence and maintenance of life.
•• The sun is the primary source of energy on Earth. This energy is required for almost
all processes that take place in Earth’s atmosphere, hydrosphere, lithosphere, and
biosphere.
«« Solar energy enables photosynthetic organisms such as plants to grow. This energy is
transferred to consumers that feed on photosynthetic organisms.
«« Solar energy warms the Earth. It is the driving force of weather and climate.
«« The sun’s energy is transferred across an empty space or vacuum to Earth’s surface
through radiation.
«« Radiation is the transfer of heat through electromagnetic waves. This heat is
essential in regulating Earth’s temperature.
•• Note that not all solar energy is absorbed by Earth. Some of it is reflected back to space
by the clouds, ice, and surface (land and water).
•• The Earth’s energy budget describes the relationship between the solar energy that
enters Earth’s system and the energy that is radiated back to space.
«« Keeping Earth’s energy budget ensures that the average temperature on Earth
remains stable and that life continues to exist.
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Fig. 2 Earth’s energy budget (Source: http://climate.ncsu.edu/edu/k12/.eeb)
•• The energy budget is determined by the characteristics of Earth’s surface.
«« About 30% of the solar energy that reaches the surface of Earth is reflected back to
space by the clouds and light-colored areas (deserts and areas covered with ice and
snow). The percentage of solar radiation that is reflected back into space is called
albedo.
«« The remaining 70% of the solar energy is absorbed by the atmosphere, land, and
oceans.
«« The absorbed energy drives wind and ocean currents. These currents distribute the
heat throughout the planet since more sunlight shines on equatorial areas than polar
areas.
•• There are certain factors that affect Earth’s energy budget.
«« Recall that light-colored areas enable the reflection of solar energy. Therefore, when
the size of these areas is altered, then energy balance is also affected.
«« Energy balance can also be affected when the amount of radiation received by Earth
from the sun changes. For instance, the changes in Earth’s orbit and axial tilt led to a
series of ice ages over the last million years.
«« Earth’s axial tilt also affects the amount of radiation coming from the Sun. Throughout
the year, the orientation of Earth toward the sun changes due to the 23.5° vertical
tilt of its axis, causing the position of the sun across the sky to wander at about 47°.
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This change has a direct effect on the intensity of insolation. Insolation is the amount
of solar radiation that reaches a given area. Simply put, it is the exposure to the
sun’s rays. For instance, if the Sun is located directly overhead, then the intensity of
insolation is higher as compared when the Sun’s altitude across the sky is about 45°
wherein the sun’s rays are spread over a larger area. This explains why areas within
or near the equator experience higher amounts of solar radiation, making the areas
warmer.
«« Lastly, the energy balance is affected by the presence of greenhouse gases such as
methane, nitrous oxide, ozone, water vapor, and carbon dioxide in the atmosphere.
These gases trap solar energy which should have been reflected back to space.
The increase of greenhouse gases would mean that more energy is trapped, and
some energy are re-emitted in all directions, thus, heating the Earth. This heating
phenomenon is known as the greenhouse effect.
Fig. 3 The greenhouse effect
•• The energy of the sun is released through ultraviolet, visible, and infrared radiation.
About 44% of the radiant energy emitted by the sun is in the form of visible light; 49% is
in the form of near-infrared, far-infrared, microwave, and radio waves; and the remaining
7% accounts for ultraviolet radiation.
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Fig. 4 The sun’s electromagnetic radiation
IV.
Earth’s Atmosphere
•• The atmosphere makes up all the gases on Earth. It is composed of 78.1% nitrogen,
20.9% oxygen, 0.9% argon, 350 ppm carbon dioxide, and other components. Water vapor
is also a significant component of the Earth’s atmosphere. Its concentration depends on
the variation in atmospheric pressure and temperature.
Table 2. Composition of the atmosphere
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•• The atmosphere has different layers – troposphere, stratosphere, mesosphere,
thermosphere, and exosphere.
«« The troposphere is the lowest layer that regulates weather and climate. It holds
nearly all water vapor in Earth’s atmosphere, and is characterized by relatively high
atmospheric pressure which allows high concentrations of oxygen ideal for life.
«« The stratosphere is where the ozone layer that protects the Earth from the Sun’s
harmful UV radiation is found.
«« The mesosphere protects the Earth from the impact of space debris. The debris
burns as a result of the frictional force between air and debris molecules.
«« The thermosphere regulates temperature and filters X-rays and some ultraviolet
radiation emitted by the sun.
«« The exosphere is the farthest layer which absorbs some radiation and protects the
layers underneath.
•• Considering the climate, the most important layer of the atmosphere is the boundary
layer composed of troposphere and stratosphere. The boundary layer is just next to
Earth’s surface. The energy transferred from the Earth’s surface in the form of conduction
or even moisture from evapotranspiration stays within this boundary and is not
transferred to higher atmosphere.
•• The atmosphere is crucial in enabling and maintaining life on Earth.
«« Without the atmosphere, Earth would look like the moon. There would be no life
forms existing on Earth.
«« Atmospheric gases, such as carbon dioxide and oxygen, are needed by organisms.
Carbon dioxide is used by photosynthetic organisms, such as plants and algae,
to convert the energy from the sun to usable energy through the process of
photosynthesis. On the other hand, oxygen is required by some living organisms
including humans for cellular respiration.
«« The ozone layer in the stratosphere is necessary in enabling life on Earth. (Ozone
is composed of three oxygen atoms.) Without this layer, harmful rays from the sun
would reach the surface of Earth and prevent most organisms from surviving.
→
Together with the oceans, the atmosphere keeps Earth’s temperature within the
suitable range for life forms.
«« The atmosphere is a crucial part of the water cycle. It serves as the reservoir of large
amounts of water.
•• The water cycle or hydrological cycle describes the movement of water from one area
to another by changing states―liquid to vapor to ice and back again. It is a never-ending
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cycle that has occurred for billions of years. All living things depends on this continuous
cycle.
«« One important process of this cycle is evaporation. Evaporation is the process of
converting liquid to gas. Water from the oceans, lakes, streams, rivers, and other
bodies of water undergo this process, and it becomes atmospheric water vapor. In
plants, instead of evaporation, the process of evapotranspiration takes place.
«« The water vapor in the atmosphere is stored in the form of clouds and moisture
(humidity). Cloud formation happens by converting water vapor to liquid form
through a process called condensation.
«« Precipitation is the process of releasing water from the clouds in the form of rain,
snow, sleet, or hail. It is the process of returning water from the atmosphere back to
the Earth’s surface.
«« Once returned to the surface, liquid water may runoff the surface into streams
and reservoirs (e.g. lakes, oceans). Water may then infiltrate the subsurface and
be incorporated into the groundwater system. It may be consumed and stored in
organisms, or trapped in glaciers.
«« Therefore, the atmosphere is an efficient medium to move water around the globe.
Fig. 5 The hydrological cycle
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Learning Tasks
1. Study the illustration of the greenhouse effect (Fig. 3).
a. How does the greenhouse affect Earth subsystems?
b. How do greenhouse gases affect solar radiation?
c. What human activities increase the amount of greenhouse gases? How do these
activities affect the energy budget?
2. Study the illustration of the hydrological cycle (Fig. 5).
a. What is the main difference between evaporation and evapotranspiration?
b. What is the importance of the atmosphere in the hydrological cycle?
Examples
Questions:
1. What are the layers of the atmosphere?
2. According to the homogenous accretion theory, how did the Earth form?
3. Why is the Earth’s energy balance important in maintaining the average temperature of
the Earth?
Answers:
1. The atmosphere has different layers – troposphere, stratosphere, mesosphere,
thermosphere, and exosphere.
2. According to homogeneous accretion hypothesis, the formation of Earth began after the
condensation of fine particles of the primitive nebula. When these particles accreted,
they formed a homogeneous primordial Earth. Its primary components were iron,
magnesium, nickel, silicates, and some radioactive elements such as uranium and
thorium. When the temperature of early Earth increased, molten iron and nickel sank
towards the center because of their high density. Eventually, they accumulated as Earth’s
core. As Earth cooled, a thin layer of solid rock formed the crust including the continents
and ocean basins.
3. The Earth’s energy balance describes the relationship between the solar energy that
enters Earth’s system and the energy that is radiated back to space. Keeping the Earth’s
energy balance ensures that the average temperature on Earth remains stable.
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Wrap Up
Water is a medium that
enabled the reaction of
the molecules on early
Earth.
Earth receives the right
amount of solar energy
that enables existence
and maintenance of life.
The atmosphere, with
its carbon dioxide,
oxygen, and water vapor
enable the existence of
life forms on Earth. It
also protects organisms
against harmful radiation
from the sun and deadly
impact of space debris.
Why Life on
Earth is Possible
KEY POINTS
•• Some of the unique characteristics of
why the Earth is habitable include the
presence and abundance of water, the
energy from the sun, and the presence
of gases in the atmosphere.
•• The Earth is considered as a dynamic
planet. This means that Earth has
continuously changed ever since its
formation 4.6 billion years ago.
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TIPS
•• Humans have a crucial role in
maintaining life on Earth. Their actions
largely impact the sustainability of
Earth for future generations. You
must take your part in ensuring the
sustainability of Earth by saving water
and energy and reducing your carbon
footprint (amount of greenhouse
emissions caused by an individual).
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Bibliography
NaotatsuShikazono. 2012. Introduction to Earth and Planetary System Science: New View of
Earth, Planets and Humans, Germany:Springer Science & Business Media.
RenuAnand. 2016.The Story of Planet Earth,New Delhi: The Energy and Resources Institute
(TERI).
Ronald Martin. 2012. Earth's Evolving Systems: The History of Planet Earth, Massachusetts: Jones
& Bartlett Publishers.
Michael Pidwirny. 2016. Chapter 4: Solar Radiation and Earth: Single chapter from the eBook
Understanding Physical Geography, Our Planet Earth Publishing.
Rubin, Kenneth. 2016. Geochemistry Lecture 33 Accessed March 17, 2017. https://www.soest.
hawaii.edu/krubin/GG325/lect33.pdf.
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