Question

Choose the best answer to each of the following. Explain your reasoning with one or more complete sentences. Earth has retained a lot of internal heat primarily because of its (a) distance from the Sun; (b) large iron core; (c) relatively large size.

Short answer

Earth retains internal heat mainly due to its relatively large size.

Step-by-step solution

Understanding Earth's Heat Retention

First, we need to consider the factors that might contribute to Earth retaining its internal heat. Earth's internal heat comes from various sources, including radioactive decay, heat from when the planet was formed, and friction from tectonic movements.

Analyzing the Options

Let's analyze each option: (a) Distance from the Sun affects solar energy reaching Earth but not the planet's internal heat. (b) An iron core contributes to magnetic fields, not significantly to internal heat retention. (c) A relatively large size could mean retention of heat because a larger body cools more slowly.

Evaluating the Most Relevant Option

Option (c) is the most relevant because larger planets like Earth can retain heat longer due to their volume-to-surface-area ratio. This means that the heat inside takes longer to escape, keeping the planet's core warm over extended periods of time, compared to smaller celestial bodies.

Key concepts

Earth's Internal Heat

The heat within Earth is not just a simple leftover from its fiery formation. Earth's internal heat comprises various components that play a role in its temperature and behavior.

• Primordial Heat: This is heat retained from when Earth originally formed around 4.5 billion years ago. As it coalesced from a disk of dust and gas, immense gravitational energy was converted into heat.
• Radioactive Decay: The decay of radioactive isotopes like uranium, thorium, and potassium in Earth's interior also produces a significant amount of heat.
• Tectonic Activity: Movements within tectonic plates generate friction, which produces additional heat.

Earth manages to retain this internal heat due to its thick, insulating mantle, which hampers the escape of heat, allowing the core to stay relatively warm over eons.

Radioactive Decay

Radioactive decay is crucial to the story of Earth's thermal dynamics. It is a process through which unstable atomic nuclei lose energy.
Unstable isotopes such as uranium-238, thorium-232, and potassium-40 break down naturally, releasing particles and energy. These decay processes occur at predictable rates and are a lasting, continuous source of heat.
The significance of radioactive decay includes:

• Long-Term Heat Source: Radioactive decay provides a steady, long-term heat source, crucial for sustaining geological activity within Earth.
• Driving Tectonic Processes: By contributing to Earth's heat, radioactivity powers mantle convection, driving plate tectonics that shape Earth's surface.

In essence, radioactive decay acts as a geothermal energy source within Earth, continually generating heat that influences tectonic and volcanic activities.

Volume-to-Surface-Area Ratio

When considering heat retention in planets, the volume-to-surface-area ratio is an essential concept. This ratio influences how quickly a body can cool.
A larger planet like Earth has a high volume relative to its surface area, meaning:

• Slow Cooling: Heat generated within the planet takes longer to dissipate through its surface, keeping the interior warmer for extended periods.
• Comparative Resistance to Heat Loss: Compared to smaller planets or celestial bodies, larger ones lose heat more slowly, maintaining higher internal temperatures.

This principle helps explain why Earth still retains much of its internal heat, even billions of years after its formation. Essentially, its size contributes significantly to how its internal heat persists over time.