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The surface heat flow combined with rock thermal conductivity defines the geothermal gradient—how temperature increases with depth. Planetary geotherms constrain crustal composition, mantle temperature, and convection rates; steeper gradients on young or tectonically active planets reflect higher internal temperatures and vigor of mantle convection.
From your study of planetary thermal modeling and heat flow measurement, you know that planets are hot inside and that this internal heat drives geological activity. The geothermal gradient — the rate at which temperature increases with depth — is the most direct expression of how efficiently a planet moves its internal heat outward. On Earth, the average geothermal gradient in continental crust is about 25–30°C per kilometer of depth. This means that just 30 km down, temperatures reach 750–900°C — hot enough to partially melt some rock types. But this number varies enormously depending on tectonic setting, and those variations tell you about the processes operating below.
Surface heat flow is measured in milliwatts per square meter (mW/m²) and is the product of the geothermal gradient and the rock's thermal conductivity — how easily heat passes through it. Earth's average surface heat flow is about 87 mW/m², but it ranges from roughly 40 mW/m² in old, stable continental cratons to over 200 mW/m² at mid-ocean ridges. The heat itself comes from two sources: primordial heat left over from planetary accretion and differentiation, and radiogenic heat produced by the decay of uranium, thorium, and potassium in rocks. In Earth's continental crust, radiogenic heat production is concentrated in the upper crust (which is enriched in these elements), meaning a significant fraction of the surface heat flow originates within the crust itself rather than flowing up from the mantle.
The geotherm — the full temperature-depth profile through a planet — is constructed by integrating the thermal gradient downward, accounting for changes in heat production and thermal conductivity with depth. A geotherm is not just a temperature curve; it is a predictive tool. By comparing a planet's geotherm to the melting curves (solidus) of its constituent rocks, you can determine where melting occurs, where the lithosphere transitions to the convecting asthenosphere, and how viscous the mantle is at any given depth. A steep geotherm that intersects the solidus at shallow depth indicates active volcanism and thin lithosphere — the situation at mid-ocean ridges and hotspots. A gentle geotherm that stays well below the solidus implies a thick, rigid, cold lithosphere — the situation beneath ancient continental shields.
Comparing geotherms across planets reveals their thermal evolution. Mars, being smaller than Earth, has cooled more efficiently and likely has a relatively gentle modern geotherm, consistent with its lack of current plate tectonics and infrequent volcanism. The Moon, smaller still, cooled so effectively that its interior is largely solid and its surface heat flow is very low. Venus, similar in size to Earth, presents a puzzle: it may have a steep geotherm but lacks plate tectonics, suggesting its heat escapes through episodic global resurfacing events rather than steady-state convection. Understanding crustal heat flow and geotherms is therefore foundational for inferring the internal state, tectonic mode, and geological vitality of any rocky world.