Lesson 1 of 6 · Differentiation, heat & interior evidence
Inside a world: sorting and moving material
9 minute lesson · 3 checks along the way
Your goal for this lessonExplain how internal layering can develop and distinguish density, thermal state, and the evidence needed to infer an interior.
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A layered world needs a physical history
A rocky world can contain a metal-rich core, a rocky , and an outer , but those labels describe the result of a history. Planetary differentiation is the separation of initially mixed material into regions with different compositions. When conditions permit movement, relatively dense metal can collect inward while other material remains farther out. Gravity cannot simply sort a completely rigid mixture as though every grain could pass freely through its neighbors.
Accretional impacts and radioactive decay supply heat; rearranging mass can release gravitational energy too. Whether extensive separation happens depends on when a body assembles, its ingredients, melting behavior, and cooling. can move some components while leaving others solid. therefore need not produce three perfectly uniform, sharply bounded shells, and a present-day layer diagram should not be mistaken for a literal photograph of the early process.
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Composition and motion are different descriptions
Crust and mantle are compositional terms. describes a relatively rigid mechanical outer layer, which can include crust and upper mantle. Below it, hot rock can deform slowly while remaining mostly solid. does not require a planet-wide ocean of liquid magma. Conduction transfers heat through material without its bulk motion; convection carries heat with moving material. Both can operate within one world at different depths.
Size matters because heat is stored throughout a volume and escapes across a surface. Other things equal, a larger spherical body has less surface area per unit volume. This favors heat retention, but it is not a complete cooling calculation. Radioactive inventory, insulating layers, material properties, impacts, and can change the outcome. A small body in a strongly heated environment need not be inactive merely because it is small.
Work through a decision
Separate a geometric result from a cooling prediction
Consider two hypothetical spheres of identical material, with radii R and 2R. Surface area scales with radius squared, so the larger has four times the area. Volume scales with radius cubed, so it has eight times the volume. Its is therefore 4/8, or half the smaller body’s ratio. This comparison isolates geometry without assigning either object a measured temperature or age.
If both initially have the same thermal energy per volume and the same heat loss per area, the larger stores eight times the energy but initially loses it four times as fast. Stored energy divided by that initial loss rate doubles. That is a simple characteristic timescale under stated conditions, not proof that a real planet cools in exactly twice the time; the flux changes as its interior and surface evolve.
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Use Vesta to test a familiar picture
Dawn mapped Vesta’s surface and tracked its gravitational influence on the spacecraft. Earlier interpretations commonly pictured a substantial iron core. A NASA report in 2025 described a new analysis of rotation and gravity that favored a much smaller core or none. The authors considered incomplete differentiation or formation from another body’s debris. This revised inference does not erase evidence that some Vesta-related material experienced melting.
The archive image reveals an irregular, cratered exterior; it cannot show where dense material lies underground. A better interior investigation combines gravity, rotation, shape, rock chemistry, and their uncertainties. Ask which observations a proposed interior explains and which alternatives remain possible. The lesson is a method: distinguish the process of separation from the particular internal structure that available data support for an individual world.
Dawn’s clear-filter view of Vesta from July 24, 2011 records surface shape and texture. It is an exterior observation, not an image of the inferred internal layers.
Look closely
Choose one visible crater and the irregular limb. Which extra measurement could test whether mass is concentrated toward Vesta’s center?
See what to notice
Precise tracking of spacecraft motion and Vesta’s rotation, combined with its shape, can constrain mass distribution. Surface brightness and crater outlines alone cannot reveal a buried core.
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