Earth's Burning Secret
Why hasn’t our planet cooled after 4.5 billion years?
Begin Investigation
Begin Investigation
Learning Objectives
Hover over each objective to explore the clues that explain why Earth still burns within
Model
Explain
Analyze
Apply
Mission Briefing
Click here to read again.
GEOLOGICAL SURVEY HEADQUARTERS CASE ASSIGNMENT: URGENT
Dear Scientist, Yesterday, our deep-Earth monitoring stations detected something unusual. Heat readings from Earth’s core showed levels of activity that should not be possible after 4.5 billion years. The board of directors is demanding answers. How can a planet this old still be generating heat? Where is this energy coming from, and will it last? We have compiled all available data, but we need someone with your investigative skills to make sense of it. The evidence is scattered across several fields of geological research. Each mission will take you deeper into the mystery of Earth’s internal heat. Time is critical. The International Geological Conference is in three days, and the organizers are expecting a full report. We're counting on you. Your investigation begins now. Director Sarah Chen Earth Sciences Division
Accept Mission
Select Your Mission
Explore inside the vault for clues about the secret source of Earth’s internal heat.
Explore B
Explore A
Engage
Explain
Review
Case File: #001
Why Isn't Earth's Interior Cold?
Engage
The Birth of a Hot Planet
Nebula to Solar System
Approximately 4.6 billion years ago, a giant cloud of gas and dust called a solar nebula began to collapse under its own gravity.
The Birth of a Hot Planet
Nebula to Solar System
As the cloud collapsed, it began to spin faster and flatten into a disk.
The Birth of a Hot Planet
Nebula to Solar System
Most of the material was drawn toward the center, where pressure and temperature increased, eventually forming the Sun.
The Birth of a Hot Planet
Nebula to Solar System
Surrounding the young Sun was a rotating disk of leftover gas and dust.
The Birth of a Hot Planet
Nebula to Solar System
Within this disk, tiny dust particles began to collide and stick together through electrostatic forces, forming planetesimals—small, solid objects that were the building blocks of planets.
The Birth of a Hot Planet
Nebula to Solar System
Over time, planetesimals continued to collide and merge through a process called accretion, forming larger bodies called protoplanets.
The Birth of a Hot Planet
In the region near the Sun, temperatures were extremely high, allowing only materials made of metal and silicate minerals to remain solid and form planets.
The Birth of a Hot Planet
This is where the inner planets, Mercury, Venus, Earth, and Mars, formed.
The Birth of a Hot Planet
Farther from the Sun, where temperatures were cooler, ice and gas condensed to form the gas giants Jupiter, Saturn, Uranus, and Neptune.
Forged in Heat
Earth formed from repeated collisions of planetesimals. Each impact released energy, adding heat to the growing planet.
Forged in Heat
The energy from impacts, along with pressure from gravity, caused much of Earth’s surface to melt, creating a global ocean of magma.
Forged in Heat
As the molten planet cooled, dense metals like iron sank to form the core, while lighter rocks rose to form the mantle and crust.
Forged in Heat
Even after the surface cooled and solidified, Earth’s interior remained intensely hot. This lasting heat came from residual energy left over from formation and radioactive decay deep within the planet.
The Cooling Mystery
Long ago, both Mars and Earth burned hot inside. But something changed...
Why did one world go cold while the other kept its fire? Clues are hidden in each world... Click each planet to uncover a clue.
Make Your Case
Work with your team to develop three possible scientific hypotheses for why Earth's interior remains hot. Consider these questions: 1. What processes could generate heat inside a planet? 2. What could prevent heat from escaping quickly? 3. How long might these processes last? Record your team's three hypotheses in your Student Journal. Then begin exploring the evidence to discover what keeps Earth's interior hot.
Select Your Mission
Now it’s time to test your hypotheses and begin your next mission to uncover the evidence.
Explore B
Explore A
Engage
Explain
Review
Case File: #001
Modeling Radioactive Decay
Explore A
What Lies Beneath
Let’s start by examining what science reveals about Earth’s internal heat. Click the magnifying glass to begin exploring what lies beneath Earth’s surface.
Click the magnifying glass to see how evidence from Earth’s rocks led to a surprising discovery about heat inside the planet.
Clues in the Rocks
Clues in the Rocks
This is an atom — the smallest building block of matter. Some atoms, like this one, hold the secret to Earth’s internal heat. Click the arrow to see what happens when this atom changes.
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
Atomic Identity
p+
=Proton
=Neutron
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
Potassium-39 19 Protons, 20 Neutrons
Potassium-40 19 Protons, 21 Neutrons
Each element is defined by the number of protons in its nucleus. For example, an atom with 19 protons is always Potassium.
Atomic Identity
p+
=Proton
=Neutron
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
Potassium-39 19 Protons, 20 Neutrons
Potassium-40 19 Protons, 21 Neutrons
However, the same element can have different numbers of neutrons. For example, Potassium-39 has 20 neutrons while Potassium-40 has 21 neutrons.
Atomic Identity
p+
=Proton
=Neutron
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
Potassium-39 19 Protons, 20 Neutrons
Potassium-40 19 Protons, 21 Neutrons
These different forms of the same element are called isotopes. Both have 19 protons, but one has 20 neutrons and the other has 21.
Atomic Identity
p+
=Proton
=Neutron
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
Radioactive
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
Potassium-39 19 Protons, 20 Neutrons
Potassium-40 19 Protons, 21 Neutrons
When an atom has too many or too few neutrons compared to its protons, it becomes unstable and radioactive.
Atomic Identity
p+
=Proton
=Neutron
Beta Particle
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
e-
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
Unstable
Stable
Energy (heat)
Potassium-40 19 Protons, 21 Neutrons
Calcium-40 20 Protons, 20 Neutrons
Radioactive atoms emit particles and energy as they change to become stable, a process known as radioactive decay.
Atomic Identity
p+
=Proton
=Neutron
Beta Particle
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
e-
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
Unstable
Stable
Energy (heat)
Potassium-40 19 Protons, 21 Neutrons
Calcium-40 20 Protons, 20 Neutrons
In Potassium-40, one neutron changes into a proton and releases a beta particle. This increases the number of protons from 19 to 20, changing the atom into Calcium-40.
Atomic Identity
Parent
Daughter
Beta Particle
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
e-
Stable
Unstable
Energy (heat)
Potassium-40 19 Protons, 21 Neutrons
Calcium-40 20 Protons, 20 Neutrons
During radioactive decay, the original unstable atom, referred to as the parent isotope, transforms into a more stable atom called the daughter isotope.
Modeling Radioactive Decay
Materials
Procedure
Graphing
Scientists cannot directly observe atoms decaying inside Earth, but they can model this process. In this hands-on simulation, you will work with your team to model radioactive decay using coins to represent atoms.
Select Your Mission
The investigation continues... Use your decay model data to uncover the pattern scientists call half-life.
Explore B
Engage
Explore A
Explain
Review
Case File: #001
Half-Life
Explore B
What is Half-Life?
20 Parent 0 Daughter Time=0 Ma
The data collected in your simulation illustrates a relationship between time and the decay of radioactive isotopes.
What is Half-Life?
10 Parent 10 Daughter Time=100 Ma
20 Parent 0 Daughter Time=0 Ma
100
(Ma)
During each interval, approximately half of the parent isotopes undergo radioactive decay, producing daughter isotopes.
What is Half-Life?
10 Parent 10 Daughter Time=100 Ma
5 Parent 15 Daughter Time=200 Ma
20 Parent 0 Daughter Time=0 Ma
100
(Ma)
100
(Ma)
This measurable rate of decay represents the half-life of the isotope.
What is Half-Life?
10 Parent 10 Daughter Time=100 Ma
5 Parent 15 Daughter Time=200 Ma
20 Parent 0 Daughter Time=0 Ma
Half-Life
Half-Life
100
(Ma)
100
(Ma)
Half-life is the time it takes for half of a quantity of a substance to decay.
Team Analysis
Work with your team to analyze the results of your radioactive decay model from Explore A and B. Use your data table, graphs, and half-life model to explain the patterns you observed in how the number of parent isotopes and total energy changed over time. Answer the Analysis Questions in your Student Journal.
Select Your Mission
The investigation continues... You discovered that radioactive decay releases heat. Now let's explore the other sources that keep Earth’s interior hot.
Explore B
Engage
Explore A
Elaborate
Explain
Case File: #001
Explain
Earth's Heat Sources
Mystery of Earth’s Heat
p+
p+
p+
p+
p+
p+
Radioactive
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
Radioactive
Residual
You learned that radioactive decay releases energy as heat. But that process is only part of the story.
Mystery of Earth’s Heat
p+
p+
p+
p+
p+
p+
Radioactive
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
Radioactive
Residual
Scientists know that Earth’s interior holds radioactive elements that release energy as they decay, making radioactive decay a major source of internal heat.
Mystery of Earth’s Heat
p+
p+
p+
p+
p+
p+
Radioactive
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
Radioactive
Residual
But when scientists measure Earth’s total heat flow, the values are much higher than what radioactive decay alone can explain. Something else must be contributing to the heat inside Earth.
Mystery of Earth’s Heat
p+
p+
p+
p+
p+
p+
Radioactive
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
Radioactive
Residual
Click each heat source to uncover what has kept Earth’s interior hot for billions of years.
Mystery of Earth’s Heat
Radioactive
Residual
You learned that radioactive decay releases energy as heat. But that process is only part of the story. What does radioactive decay have to do with the heat deep inside Earth? Click the heat sources to uncover what has kept Earth’s interior hot for billions of years.
Mystery of Earth’s Heat
Radioactive
Residual
You learned that radioactive decay releases energy as heat. But that process is only part of the story. What does radioactive decay have to do with the heat deep inside Earth? Click the heat sources to uncover what has kept Earth’s interior hot for billions of years.
Power of Decaying Atoms
We know that radioactive decay is a key source of heat within Earth’s interior. The elements shown in the table—uranium, thorium, and potassium—each release energy as their unstable atoms transform into more stable forms over time.
Power of Decaying Atoms
Because these isotopes have half-lives measured in billions of years, they continue to produce heat long after Earth’s formation.
Earth's Heat Over Time
Scientists can estimate how much heat these elements have generated throughout Earth’s history by modeling their decay rates over time.
Back to Heat Sources
Earth's Heat Over Time
What patterns do you notice in the heat flow over time?
Back to Heat Sources
A Violent Beginning
Earth's formation 4.5 billion years ago generated enormous quantities of heat through two primary processes. First, accretional heating occurred as countless asteroids, comets, and planetesimals collided to form the growing Earth. Each impact converted the kinetic energy of the incoming object into thermal energy, as when a hammer strikes metal, warming both the object and the hammer.
A Violent Beginning
Earth's formation 4.5 billion years ago generated enormous quantities of heat through two primary processes.
A Violent Beginning
First, accretional heating occurred as countless asteroids, comets, and planetesimals collided to form the growing Earth.
A Violent Beginning
Each impact converted the kinetic energy of the incoming object into thermal energy (heat), as when a hammer strikes metal, warming both the object and the hammer.
A Violent Beginning
Less Dense Rock Rises
More Dense Metal Sinks
Iron-Nickel Core
Second, planetary differentiation released additional heat as Earth's materials separated by density.
A Violent Beginning
Less Dense Rock Rises
More Dense Metal Sinks
Iron-Nickel Core
During Earth's first 100 million years, dense iron and nickel sank toward the center while lighter silicate minerals rose toward the surface.
A Violent Beginning
Less Dense Rock Rises
More Dense Metal Sinks
Iron-Nickel Core
This massive reorganization converted gravitational potential energy into thermal energy through friction and compression, similar to how a sliding object generates heat through friction.
Sinking Metal
Second, planetary differentiation released additional heat as Earth's materials separated by density. During Earth's first 100 million years, dense iron and nickel sank toward the center while lighter silicate minerals rose toward the surface. This massive reorganization converted gravitational potential energy into thermal energy through friction and compression, similar to how a sliding object generates heat through friction.
Less Dense Rock Rises
More Dense Metal Sinks
Iron-Nickel Core
Trapped Heat
This ancient heat remains trapped within Earth because rock acts as an excellent insulator. Heat moves extremely slowly through thousands of kilometers of solid rock, requiring millions of years to escape to the surface. Scientists estimate that Earth still retains approximately half of its original heat from formation.
Select Your Mission
The investigation continues... You discovered how radioactive decay produces heat inside Earth. Now is the time to explore how that heat has changed over time and what it reveals about Earth’s thermal evolution.
Explore B
Engage
Explore A
Elaborate
Explain
Elaborate
Case File: #001
Shifting Balance of Earth’s Heat
Earth’s Changing Heat
Over Earth’s history, the balance between internal heat generation and surface heat loss has shifted. Early in Earth’s history, total heat flow was nearly twice what it is today. As short-lived isotopes decayed away and primordial heat slowly escaped through the crust, the planet’s overall heat flow declined.
Earth’s Changing Heat
Even after billions of years, Earth’s interior is far from quiet. Heat rises from deep within the planet, but somewhere between the intensely hot core and the cold surface, the process becomes uncertain. How does this heat move through thousands of kilometers of solid rock to reach the surface?
Team Analysis
Work with your team to analyze how Earth’s internal heat has changed over time. Use the heat flow graph, your notes on isotope decay, and the Trapped Heat data to explain how the balance between heat generation and heat loss has shifted throughout Earth’s history. Discuss what evidence suggests that Earth’s interior is still active today. Answer the Analysis Questions in your Student Journal.
Select Your Mission
Engage
Explore 1
Explore 2
Review
Explain
Select Your Mission
Engage
Explore 1
Explore 2
Explain
Review
Level 4
Level 2
Level 1
Level 3
Level 3
Earth's Age
Earth formed about 4.5 billion years ago. If you counted one number every second without stopping, it would take you more than 142 years to reach that number.
Materials:- 250 coins or colored discs
- Covered container
- Radioactive Decay Graph Analysis Sheet
Mars grew cold and silent, its surface frozen in time.
Earth stayed alive—its core still burning after 4.5 billion years.
Using the Radioactive Decay Graph Analysis Sheet, plot your data from Table 1.
For Figure 1: Time versus Parent Isotopes Remaining
- Plot Time (x-axis) versus Parent Isotopes Remaining (y-axis).
- Connect the points with a smooth curve.
For Figure 2: Time versus Total Energy Released
- Plot Time (x-axis) versus Total Energy Released (y-axis).
- Connect the points with a smooth curve.
Earth stayed alive—its core still burning after 4.5 billion years.
Procedure:
1. Begin with 250 coins (Earth's initial radioactive isotopes) 2. Place all tokens in the container and shake thoroughly 3. Empty contents onto work surface 4. Sort: HEADS (parent isotopes) vs TAILS (daughter isotopes). 5. Record data in Table 1 in your Student Journal. 6. Remove daughter isotopes, return parent isotopes to the container 7. Repeat until fewer than 10 parent isotopes remain
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Transcript
Earth's Burning Secret
Why hasn’t our planet cooled after 4.5 billion years?
Begin Investigation
Begin Investigation
Learning Objectives
Hover over each objective to explore the clues that explain why Earth still burns within
Model
Explain
Analyze
Apply
Mission Briefing
Click here to read again.
GEOLOGICAL SURVEY HEADQUARTERS CASE ASSIGNMENT: URGENT
Dear Scientist, Yesterday, our deep-Earth monitoring stations detected something unusual. Heat readings from Earth’s core showed levels of activity that should not be possible after 4.5 billion years. The board of directors is demanding answers. How can a planet this old still be generating heat? Where is this energy coming from, and will it last? We have compiled all available data, but we need someone with your investigative skills to make sense of it. The evidence is scattered across several fields of geological research. Each mission will take you deeper into the mystery of Earth’s internal heat. Time is critical. The International Geological Conference is in three days, and the organizers are expecting a full report. We're counting on you. Your investigation begins now. Director Sarah Chen Earth Sciences Division
Accept Mission
Select Your Mission
Explore inside the vault for clues about the secret source of Earth’s internal heat.
Explore B
Explore A
Engage
Explain
Review
Case File: #001
Why Isn't Earth's Interior Cold?
Engage
The Birth of a Hot Planet
Nebula to Solar System
Approximately 4.6 billion years ago, a giant cloud of gas and dust called a solar nebula began to collapse under its own gravity.
The Birth of a Hot Planet
Nebula to Solar System
As the cloud collapsed, it began to spin faster and flatten into a disk.
The Birth of a Hot Planet
Nebula to Solar System
Most of the material was drawn toward the center, where pressure and temperature increased, eventually forming the Sun.
The Birth of a Hot Planet
Nebula to Solar System
Surrounding the young Sun was a rotating disk of leftover gas and dust.
The Birth of a Hot Planet
Nebula to Solar System
Within this disk, tiny dust particles began to collide and stick together through electrostatic forces, forming planetesimals—small, solid objects that were the building blocks of planets.
The Birth of a Hot Planet
Nebula to Solar System
Over time, planetesimals continued to collide and merge through a process called accretion, forming larger bodies called protoplanets.
The Birth of a Hot Planet
In the region near the Sun, temperatures were extremely high, allowing only materials made of metal and silicate minerals to remain solid and form planets.
The Birth of a Hot Planet
This is where the inner planets, Mercury, Venus, Earth, and Mars, formed.
The Birth of a Hot Planet
Farther from the Sun, where temperatures were cooler, ice and gas condensed to form the gas giants Jupiter, Saturn, Uranus, and Neptune.
Forged in Heat
Earth formed from repeated collisions of planetesimals. Each impact released energy, adding heat to the growing planet.
Forged in Heat
The energy from impacts, along with pressure from gravity, caused much of Earth’s surface to melt, creating a global ocean of magma.
Forged in Heat
As the molten planet cooled, dense metals like iron sank to form the core, while lighter rocks rose to form the mantle and crust.
Forged in Heat
Even after the surface cooled and solidified, Earth’s interior remained intensely hot. This lasting heat came from residual energy left over from formation and radioactive decay deep within the planet.
The Cooling Mystery
Long ago, both Mars and Earth burned hot inside. But something changed...
Why did one world go cold while the other kept its fire? Clues are hidden in each world... Click each planet to uncover a clue.
Make Your Case
Work with your team to develop three possible scientific hypotheses for why Earth's interior remains hot. Consider these questions: 1. What processes could generate heat inside a planet? 2. What could prevent heat from escaping quickly? 3. How long might these processes last? Record your team's three hypotheses in your Student Journal. Then begin exploring the evidence to discover what keeps Earth's interior hot.
Select Your Mission
Now it’s time to test your hypotheses and begin your next mission to uncover the evidence.
Explore B
Explore A
Engage
Explain
Review
Case File: #001
Modeling Radioactive Decay
Explore A
What Lies Beneath
Let’s start by examining what science reveals about Earth’s internal heat. Click the magnifying glass to begin exploring what lies beneath Earth’s surface.
Click the magnifying glass to see how evidence from Earth’s rocks led to a surprising discovery about heat inside the planet.
Clues in the Rocks
Clues in the Rocks
This is an atom — the smallest building block of matter. Some atoms, like this one, hold the secret to Earth’s internal heat. Click the arrow to see what happens when this atom changes.
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
Atomic Identity
p+
=Proton
=Neutron
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
Potassium-39 19 Protons, 20 Neutrons
Potassium-40 19 Protons, 21 Neutrons
Each element is defined by the number of protons in its nucleus. For example, an atom with 19 protons is always Potassium.
Atomic Identity
p+
=Proton
=Neutron
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
Potassium-39 19 Protons, 20 Neutrons
Potassium-40 19 Protons, 21 Neutrons
However, the same element can have different numbers of neutrons. For example, Potassium-39 has 20 neutrons while Potassium-40 has 21 neutrons.
Atomic Identity
p+
=Proton
=Neutron
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
Potassium-39 19 Protons, 20 Neutrons
Potassium-40 19 Protons, 21 Neutrons
These different forms of the same element are called isotopes. Both have 19 protons, but one has 20 neutrons and the other has 21.
Atomic Identity
p+
=Proton
=Neutron
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
Radioactive
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
Potassium-39 19 Protons, 20 Neutrons
Potassium-40 19 Protons, 21 Neutrons
When an atom has too many or too few neutrons compared to its protons, it becomes unstable and radioactive.
Atomic Identity
p+
=Proton
=Neutron
Beta Particle
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
e-
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
p+
Unstable
Stable
Energy (heat)
Potassium-40 19 Protons, 21 Neutrons
Calcium-40 20 Protons, 20 Neutrons
Radioactive atoms emit particles and energy as they change to become stable, a process known as radioactive decay.
Atomic Identity
p+
=Proton
=Neutron
Beta Particle
p+
p+
p+
p+
p+
p+
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e-
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Unstable
Stable
Energy (heat)
Potassium-40 19 Protons, 21 Neutrons
Calcium-40 20 Protons, 20 Neutrons
In Potassium-40, one neutron changes into a proton and releases a beta particle. This increases the number of protons from 19 to 20, changing the atom into Calcium-40.
Atomic Identity
Parent
Daughter
Beta Particle
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e-
Stable
Unstable
Energy (heat)
Potassium-40 19 Protons, 21 Neutrons
Calcium-40 20 Protons, 20 Neutrons
During radioactive decay, the original unstable atom, referred to as the parent isotope, transforms into a more stable atom called the daughter isotope.
Modeling Radioactive Decay
Materials
Procedure
Graphing
Scientists cannot directly observe atoms decaying inside Earth, but they can model this process. In this hands-on simulation, you will work with your team to model radioactive decay using coins to represent atoms.
Select Your Mission
The investigation continues... Use your decay model data to uncover the pattern scientists call half-life.
Explore B
Engage
Explore A
Explain
Review
Case File: #001
Half-Life
Explore B
What is Half-Life?
20 Parent 0 Daughter Time=0 Ma
The data collected in your simulation illustrates a relationship between time and the decay of radioactive isotopes.
What is Half-Life?
10 Parent 10 Daughter Time=100 Ma
20 Parent 0 Daughter Time=0 Ma
100
(Ma)
During each interval, approximately half of the parent isotopes undergo radioactive decay, producing daughter isotopes.
What is Half-Life?
10 Parent 10 Daughter Time=100 Ma
5 Parent 15 Daughter Time=200 Ma
20 Parent 0 Daughter Time=0 Ma
100
(Ma)
100
(Ma)
This measurable rate of decay represents the half-life of the isotope.
What is Half-Life?
10 Parent 10 Daughter Time=100 Ma
5 Parent 15 Daughter Time=200 Ma
20 Parent 0 Daughter Time=0 Ma
Half-Life
Half-Life
100
(Ma)
100
(Ma)
Half-life is the time it takes for half of a quantity of a substance to decay.
Team Analysis
Work with your team to analyze the results of your radioactive decay model from Explore A and B. Use your data table, graphs, and half-life model to explain the patterns you observed in how the number of parent isotopes and total energy changed over time. Answer the Analysis Questions in your Student Journal.
Select Your Mission
The investigation continues... You discovered that radioactive decay releases heat. Now let's explore the other sources that keep Earth’s interior hot.
Explore B
Engage
Explore A
Elaborate
Explain
Case File: #001
Explain
Earth's Heat Sources
Mystery of Earth’s Heat
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Radioactive
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Radioactive
Residual
You learned that radioactive decay releases energy as heat. But that process is only part of the story.
Mystery of Earth’s Heat
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Radioactive
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Radioactive
Residual
Scientists know that Earth’s interior holds radioactive elements that release energy as they decay, making radioactive decay a major source of internal heat.
Mystery of Earth’s Heat
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Radioactive
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Radioactive
Residual
But when scientists measure Earth’s total heat flow, the values are much higher than what radioactive decay alone can explain. Something else must be contributing to the heat inside Earth.
Mystery of Earth’s Heat
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Radioactive
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Radioactive
Residual
Click each heat source to uncover what has kept Earth’s interior hot for billions of years.
Mystery of Earth’s Heat
Radioactive
Residual
You learned that radioactive decay releases energy as heat. But that process is only part of the story. What does radioactive decay have to do with the heat deep inside Earth? Click the heat sources to uncover what has kept Earth’s interior hot for billions of years.
Mystery of Earth’s Heat
Radioactive
Residual
You learned that radioactive decay releases energy as heat. But that process is only part of the story. What does radioactive decay have to do with the heat deep inside Earth? Click the heat sources to uncover what has kept Earth’s interior hot for billions of years.
Power of Decaying Atoms
We know that radioactive decay is a key source of heat within Earth’s interior. The elements shown in the table—uranium, thorium, and potassium—each release energy as their unstable atoms transform into more stable forms over time.
Power of Decaying Atoms
Because these isotopes have half-lives measured in billions of years, they continue to produce heat long after Earth’s formation.
Earth's Heat Over Time
Scientists can estimate how much heat these elements have generated throughout Earth’s history by modeling their decay rates over time.
Back to Heat Sources
Earth's Heat Over Time
What patterns do you notice in the heat flow over time?
Back to Heat Sources
A Violent Beginning
Earth's formation 4.5 billion years ago generated enormous quantities of heat through two primary processes. First, accretional heating occurred as countless asteroids, comets, and planetesimals collided to form the growing Earth. Each impact converted the kinetic energy of the incoming object into thermal energy, as when a hammer strikes metal, warming both the object and the hammer.
A Violent Beginning
Earth's formation 4.5 billion years ago generated enormous quantities of heat through two primary processes.
A Violent Beginning
First, accretional heating occurred as countless asteroids, comets, and planetesimals collided to form the growing Earth.
A Violent Beginning
Each impact converted the kinetic energy of the incoming object into thermal energy (heat), as when a hammer strikes metal, warming both the object and the hammer.
A Violent Beginning
Less Dense Rock Rises
More Dense Metal Sinks
Iron-Nickel Core
Second, planetary differentiation released additional heat as Earth's materials separated by density.
A Violent Beginning
Less Dense Rock Rises
More Dense Metal Sinks
Iron-Nickel Core
During Earth's first 100 million years, dense iron and nickel sank toward the center while lighter silicate minerals rose toward the surface.
A Violent Beginning
Less Dense Rock Rises
More Dense Metal Sinks
Iron-Nickel Core
This massive reorganization converted gravitational potential energy into thermal energy through friction and compression, similar to how a sliding object generates heat through friction.
Sinking Metal
Second, planetary differentiation released additional heat as Earth's materials separated by density. During Earth's first 100 million years, dense iron and nickel sank toward the center while lighter silicate minerals rose toward the surface. This massive reorganization converted gravitational potential energy into thermal energy through friction and compression, similar to how a sliding object generates heat through friction.
Less Dense Rock Rises
More Dense Metal Sinks
Iron-Nickel Core
Trapped Heat
This ancient heat remains trapped within Earth because rock acts as an excellent insulator. Heat moves extremely slowly through thousands of kilometers of solid rock, requiring millions of years to escape to the surface. Scientists estimate that Earth still retains approximately half of its original heat from formation.
Select Your Mission
The investigation continues... You discovered how radioactive decay produces heat inside Earth. Now is the time to explore how that heat has changed over time and what it reveals about Earth’s thermal evolution.
Explore B
Engage
Explore A
Elaborate
Explain
Elaborate
Case File: #001
Shifting Balance of Earth’s Heat
Earth’s Changing Heat
Over Earth’s history, the balance between internal heat generation and surface heat loss has shifted. Early in Earth’s history, total heat flow was nearly twice what it is today. As short-lived isotopes decayed away and primordial heat slowly escaped through the crust, the planet’s overall heat flow declined.
Earth’s Changing Heat
Even after billions of years, Earth’s interior is far from quiet. Heat rises from deep within the planet, but somewhere between the intensely hot core and the cold surface, the process becomes uncertain. How does this heat move through thousands of kilometers of solid rock to reach the surface?
Team Analysis
Work with your team to analyze how Earth’s internal heat has changed over time. Use the heat flow graph, your notes on isotope decay, and the Trapped Heat data to explain how the balance between heat generation and heat loss has shifted throughout Earth’s history. Discuss what evidence suggests that Earth’s interior is still active today. Answer the Analysis Questions in your Student Journal.
Select Your Mission
Engage
Explore 1
Explore 2
Review
Explain
Select Your Mission
Engage
Explore 1
Explore 2
Explain
Review
Level 4
Level 2
Level 1
Level 3
Level 3
Earth's Age
Earth formed about 4.5 billion years ago. If you counted one number every second without stopping, it would take you more than 142 years to reach that number.
Materials:- 250 coins or colored discs
- Covered container
- Radioactive Decay Graph Analysis Sheet
Mars grew cold and silent, its surface frozen in time.
Earth stayed alive—its core still burning after 4.5 billion years.
Using the Radioactive Decay Graph Analysis Sheet, plot your data from Table 1.
For Figure 1: Time versus Parent Isotopes Remaining
For Figure 2: Time versus Total Energy Released
Earth stayed alive—its core still burning after 4.5 billion years.
Procedure:
1. Begin with 250 coins (Earth's initial radioactive isotopes) 2. Place all tokens in the container and shake thoroughly 3. Empty contents onto work surface 4. Sort: HEADS (parent isotopes) vs TAILS (daughter isotopes). 5. Record data in Table 1 in your Student Journal. 6. Remove daughter isotopes, return parent isotopes to the container 7. Repeat until fewer than 10 parent isotopes remain