ASTR 120 ยท Lecture 04
Seismology transformed Earth science; planetary missions extend similar inference to the Moon, Mars, and icy worlds.
Definition: The total mass of a world divided by the volume set by its mean radius.
Why it matters and connects: It constrains broad rock, metal, ice, gas, and porosity fractions before a detailed interior model is attempted.
Boundary or common confusion: Different mixtures and compression states can share the same mean density.
Definition: Separation of materials into layers according to density and chemical affinity while part of a body is mobile or molten.
Why it matters and connects: It explains metal-rich cores, silicate mantles, crusts, and the release or storage of heat-producing elements.
Boundary or common confusion: A layered body need not be fully molten today, and later impacts or convection can modify the original layering.
Definition: A measure of how mass is distributed relative to a rotation axis.
Why it matters and connects: For a known mass and radius, the normalized moment of inertia helps distinguish centrally concentrated from nearly uniform interiors.
Boundary or common confusion: It is usually inferred through rotational response or gravity measurements and is model-dependent when data are sparse.
Definition: The spatial pattern of gravitational acceleration produced by a body's total mass and its internal mass distribution.
Why it matters and connects: Tracking spacecraft Doppler shifts and trajectories reveals departures from spherical symmetry, buried structures, and interior concentration.
Boundary or common confusion: Gravity maps do not directly identify composition; several density distributions can reproduce similar fields.
Definition: A process in which motion of electrically conducting fluid sustains a planetary magnetic field.
Why it matters and connects: A field can indicate a conducting layer, energy to drive motion, and sufficient rotation or organization of flow.
Boundary or common confusion: The absence of a present global field does not prove that a body never had a dynamo.
Definition: Thermal energy crossing a unit area per unit time from a body's interior toward its surface.
Why it matters and connects: Heat flow constrains radioactive decay, leftover formation energy, differentiation, tidal dissipation, and the ability to sustain geology.
Boundary or common confusion: Surface heat flow may vary spatially and does not identify the heat source by itself.
Density, gravity, magnetic fields, and heat flow reveal interiors we cannot sample directly.
Density: average composition, not a layer-by-layer map.
Shape and gravity: mass distribution and possible differentiated interiors.
Magnetism: clues to conductive liquids, rotation, and present internal motion.
When a young body is warm enough to partially melt, dense metal can sink and lighter silicate or ice can rise. Accretion energy, radioactive decay, and giant impacts all help determine whether a world separates into layers.
Interpret the diagram as a model constrained by mass, radius, density, field measurements, and heat flow. The layers are an inference, not something directly photographed.
Using the planetary-property table, Saturn's mean density is about 0.69 g/cm3, below liquid water. A low mean density requires abundant hydrogen and helium, even though pressure deep inside produces unfamiliar material states.
The same average density can come from different mixtures of rock, metal, ice, porosity, or compression. Interior claims become stronger when density is paired with moment of inertia, gravity field, magnetic field, and seismic or tidal response.
A moon has density 1.9 g/cm3 and a weak induced magnetic signal. Which interior model is better supported: dry rock plus porosity, rock mixed with ice, or a salty subsurface ocean? What measurement would most reduce the ambiguity?
A magnetic field can be intrinsic, induced, or absent for different reasons. Distinguish a dynamo in a moving conductive layer from an induced response to a parent planet's field before turning magnetism into an interior claim.