Questions: Planetary Magnetic Field Evolution and Decay

Question 1

Two planets are identical in size and rotation rate, but one has a core with significant dissolved light elements (sulfur, oxygen) while the other has a nearly pure iron core. Which is more likely to sustain a magnetic field longer, and why?

• A. The pure iron core planet — light elements disrupt orderly convective flow
• B. The light-element core planet — expelled light elements and latent heat from inner core crystallization provide compositional convection, extending dynamo life
• C. Both sustain identical fields — core composition has no effect on dynamo duration
• D. The pure iron core planet — light elements depress the freezing point, causing the core to solidify faster

Answer: (B)

When a liquid iron core begins freezing from the center outward, it releases latent heat and expels dissolved light elements into the remaining liquid. Both effects drive compositional convection that supplements — and ultimately outlasts — purely thermal convection. A core enriched with light elements therefore sustains a dynamo far longer. Option D reverses the logic: depressing the freezing point *slows* solidification, giving the dynamo more time, not less.

Question 2

Mars has ancient crustal magnetic anomalies detectable from orbit, but no present-day global magnetic field. What is the most likely explanation within the framework of planetary magnetic field evolution?

• A. Mars rotates too slowly today to organize convective flow into dynamo-sustaining patterns
• B. Mars's small size caused rapid core cooling, stalling convection and shutting down the dynamo roughly 4 billion years ago
• C. The solar wind stripped Mars's magnetic field directly from the planet's surface
• D. Mars's core is entirely solid, preventing any fluid motion

Answer: (B)

Small bodies have a higher surface-area-to-volume ratio and lose heat faster. Mars's core cooled rapidly, weakening and eventually stopping convection within the first billion years. The crustal anomalies are fossil magnetism — rock that preserved the field direction when it solidified during the active dynamo era. The solar wind cannot directly strip a magnetic field from solid rock; it strips the atmosphere of an unmagnetized planet. Mars's rotation rate is not the limiting factor here.

Question 3

Earth's inner core crystallization provides an additional energy source that helps sustain the geodynamo beyond what purely thermal convection could maintain.

• T. True
• F. False

Answer: True

As the liquid iron core freezes outward from the center, two things happen: latent heat is released (additional thermal energy) and light elements (sulfur, oxygen, silicon) are expelled into the remaining liquid. The density contrast between the light-element-enriched liquid and the overlying denser liquid drives compositional convection. This is a second energy mechanism operating even when the thermal gradient alone would be insufficient to sustain vigorous convection — which is why Earth's dynamo has persisted for at least 3.5 billion years.

Question 4

A planet's magnetic field will inevitably strengthen as its interior cools, because a steeper temperature gradient drives more vigorous core convection.

• T. True
• F. False

Answer: False

This reverses the actual relationship. Early in a planet's history, a large temperature difference between the hot core and cooler mantle drives vigorous convection — and a strong dynamo. As the planet radiates heat to space and the core cools, this gradient flattens. Convection weakens and eventually stalls. The dynamo then dies. Planetary cooling is the mechanism of magnetic field *decay*, not strengthening. Mercury's weak present-day field persists despite small size precisely because special conditions (a large inner core and sulfur enrichment) have kept a thin liquid shell active.

Question 5

Why does inner core crystallization help sustain Earth's magnetic field, and what are the two distinct physical mechanisms it provides?

Model answer: Inner core crystallization releases latent heat (thermal mechanism) and expels light elements such as sulfur, oxygen, and silicon into the overlying liquid (compositional mechanism). The latent heat adds thermal energy that helps drive convection. The expelled light elements are less dense than the surrounding liquid, creating a buoyancy contrast that drives compositional convection independently of the thermal gradient. Together, these two mechanisms continue powering the dynamo even as purely thermal convection weakens with cooling.

The key insight is that crystallization is not just 'freezing' — it actively injects energy and creates density contrasts that sustain fluid motion. Earth's dynamo is essentially being powered by its own ongoing solidification. This is why the geodynamo has outlasted what simple thermal cooling models would predict, and why core composition (what light elements are present) matters as much as core size for predicting a planet's magnetic lifetime.