Question

Choose the best answer to each of the following. Explain your reasoning with one or more complete sentences. Which of the following fundamental properties of Mars could explain why it once had a global magnetic field but later lost it? (a) its small size; (b) its larger distance than Earth from the Sun; (c) a rotation rate that is slightly slower than Earth's.

Short answer

The best answer is (a) its small size.

Step-by-step solution

Understand the Problem

The problem asks us to identify which property of Mars could explain the change in its magnetic field over time. We have three potential answers to evaluate: Mars' size, its distance from the Sun, and its rotation rate.

Consider the Impact of Size

Mars is smaller than Earth, which means it has less gravity to retain its internal heat. Over time, smaller planets like Mars cool more quickly than larger ones. Cooling affects the core, which plays a critical role in generating a magnetic field through the dynamo effect.

Evaluate the Distance from the Sun

The distance of Mars from the Sun primarily affects its surface temperature and solar irradiation but is less directly related to the workings of its internal magnetic field. The presence or absence of a magnetic field is not significantly influenced by its distance.

Assess the Rotation Rate

Mars rotates on its axis in about 24.6 hours, which is only slightly slower than Earth's 24 hours. While a faster rotation can enhance magnetic field generation, the slight difference in rotation speed is unlikely to be the primary cause of the loss of Mars' magnetic field.

Choose the Best Answer

Considering the factors, Mars' small size is the most plausible reason for the loss of its global magnetic field. As it cooled faster, the core solidified, disrupting the dynamo mechanism that sustains the magnetic field.

Key concepts

Planetary Size

The size of a planet plays a crucial role in the development and maintenance of its magnetic field. Mars is significantly smaller than Earth, which results in a weaker gravitational pull. This weak gravity impacts Mars' ability to retain internal heat, leading to a faster cooling rate compared to larger planets like Earth.

When a planet cools, its core begins to solidify. The core's liquid state is essential for sustaining the dynamo effect, the process responsible for generating a magnetic field. In Mars' case, its smaller size allowed it to lose internal heat quickly, causing its core to cool and partially solidify.

• The smaller the planet, the faster it loses heat.
• A solidified core can't sustain a magnetic field.
• Mars' small size contributed significantly to the loss of its magnetic field over time.

Core Cooling

Core cooling is an important process when discussing planetary magnetic fields. It directly affects the dynamo effect since this phenomenon depends on the movement of molten metals within a planet's core.

In the case of Mars, its core cooled and solidified relatively quickly because of its small size. As the liquid core transforms into a solid state, the dynamic motion necessary for generating a magnetic field ceases. Eventually, this led to Mars losing its once global magnetic field, unlike Earth, which maintains its field due to its larger core and slower cooling process.

• The speed of core cooling is crucial to magnetic field sustainability.
• Once a core solidifies, magnetic field generation halts.
• Core cooling is a natural consequence of planetary size and composition.

Dynamo Effect

The dynamo effect is fundamental to understanding how planets generate magnetic fields. It occurs when the movement of conductive materials, like molten iron, within a planet's outer core generates electricity and, consequently, a magnetic field.

Mars once had the conditions necessary for the dynamo effect, but this changed as its core cooled due to its small size. Without the fluid motion necessary to sustain the dynamo effect, Mars' magnetic field gradually diminished. In contrast, Earth's larger size allows it to maintain a fluid core, which continuously generates its magnetic field.

• The dynamo effect relies on fluid motion within the core.
• A solid core cannot support the dynamo effect.
• The loss of the dynamo effect resulted in Mars' magnetic field weakening over time.