Rocky Planet Geology and Interiors

The layered insides of rocky planets, from metallic cores to silicate mantles and crusts, and the geophysics that reveals them.

Definition

Rocky planet geology and interiors is the study of the differentiated internal structure, composition, dynamics, and magnetic-field generation of terrestrial planets and large rocky bodies.

Scope

This topic covers the internal structure, composition, and dynamics of terrestrial planets and large rocky moons: how they differentiate into core, mantle, and crust; how interior heat is generated and transported by conduction and convection; the rheology and mineralogy of the mantle; and the generation of magnetic fields by core dynamos. It includes the geophysical methods, seismology, gravity, magnetometry, and heat-flow measurement, used to probe interiors remotely and in situ.

Core questions

• How do rocky planets separate into core, mantle, and crust, and what sets the size of the core?
• How is heat generated and transported through a planet's interior over time?
• What conditions allow a planet to generate a global magnetic field by dynamo action?
• How do geophysical observations constrain the interior of a planet we cannot drill into?

Key theories

Core dynamo theory

Convective motion of electrically conducting liquid metal in a planet's core, driven by cooling and compositional buoyancy, can sustain a self-generating magnetic field through magnetohydrodynamic dynamo action.

Differentiation and core formation

Early heating melts a rocky planet enough for dense iron-rich metal to sink and form a core while lighter silicates rise to make the mantle and crust, fixing the planet's layered structure.

Mantle convection

Although solid, the mantle creeps and convects over geological time, transporting heat to the surface and driving tectonics, volcanism, and the long-term cooling of the planet.

Mechanisms

Accretional and radiogenic heat melt the early planet, allowing iron-rich metal to sink and form a core. As the planet cools, the mantle convects and the core may freeze an inner solid component, releasing buoyancy that drives the dynamo. Seismic waves, gravity variations, and magnetic measurements encode the resulting density, temperature, and conductivity structure.

Clinical relevance

Interior structure governs a planet's magnetic field, volcanic and tectonic activity, and outgassing, all of which feed back on atmospheric retention and surface habitability.

History

Seismology revealed Earth's core and mantle structure across the 20th century, and Lehmann's discovery of the inner core in 1936 was a landmark. Spacecraft magnetometry and gravity mapping, plus the InSight mission's seismic measurements of Mars, extended interior studies to other planets, while dynamo theory matured to explain why some bodies have magnetic fields and others do not.

Debates

Composition and light elements of planetary cores

Which light elements, such as sulfur, oxygen, or silicon, are mixed with iron in planetary cores, and how this affects freezing and dynamo behavior, remains an open question.

Key figures

• David J. Stevenson
• Donald Turcotte
• Gerald Schubert
• Inge Lehmann

Seminal works

• stevenson1981
• turcotteschubert2014
• stevenson2003

Frequently asked questions

Why does Earth have a magnetic field but Mars does not?

Earth's liquid metal core still convects vigorously enough to run a dynamo, whereas Mars's smaller core cooled and its global dynamo shut down billions of years ago, leaving only patches of ancient magnetized crust.

How do scientists study a planet's interior without digging in?

They use geophysics: seismic waves, the planet's gravity field, magnetic measurements, and heat flow, all of which depend on what lies beneath the surface.