Heat flow

Planets can be understood as thermal engines that are in some ways similar to steam engines. When the planets were formed, they were heated by the impacts of the planetesimals that collapsed upon them. Heat was also produced in the rock by the decay of radioactive uranium, thorium and potassium isotopes. Over the course of aeons, the planet cooled down – a process described as thermal evolution. This cooling takes place deep within the planet due to what is known as convective heat transport, or heat transfer through movement. Simply put, hot rock rises towards the surface, while cold rock sinks towards the interior. This is a familiar process from observing a cup of coffee or knowing how central heating works. The movement of hot or sometimes even molten rock causes mountain formation and volcanoes, which can be seen as the planet’s way of performing mechanical work, like a thermal engine. This mechanical work mainly consists of processes of formation, for example, mountain building. The stresses generated as a result are released through events such as earthquakes, or – to put it more accurately – marsquakes. It is possible to calculate the thermal stresses and thus predict quake zones.

The convective cooling of the rock mantle may even drive convection in the core and thus power a dynamo mechanism that generates a planet-wide magnetic field of the kind that is familiar on Earth. This is no longer the case on Mars, but it may have occurred in the distant past, around four billion years ago. This is supported by old, magnetised crustal rock. It is possible to characterise the heat transfer from the interior of the planet by measuring the heat flow at the surface. This was the objective of the Heat Flow and Physical Properties Packages (HP³) created by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR). In addition, the power of this planetary thermal engine can be characterised using the waste heat produced. The higher the heat flow, the more active the planet. The heat flow also helps to determine the chemical composition of the rocky sphere of the planet, as the rate of heat production depends on the concentration of radioactive elements within the interior of Mars, in particular within its crust. Made of basaltic rock, the crust was formed by volcanic activity, which enriched radioactive elements in the magma and thus in the basaltic crustal rock.

The heat flow is determined by multiplying the geothermal gradient – the increase in temperature per metre of depth – with the thermal conductivity. The geothermal gradient is determined by measuring the temperature at several depths in a borehole. The DLR HP³ heat flow probe accomplished this by drilling a small penetrometer, known as a ‘mole’, into the ground, pulling a ribbon cable equipped with 14 temperature sensors behind it. The temperature was measured every hour for several months. A long measuring period was necessary because the geothermal gradient is disturbed by various heat waves penetrating from the surface. These include the daily wave generated by the effect of the diurnal cycle on the surface temperature, although this penetrates only a few centimetres deep. The annual wave caused by seasonal variations in temperature is more significant, and can penetrate up to three metres in depth, depending on the level of thermal conductivity below the ground. HP³ measured the thermal conductivity of the soil as it penetrated it. During this process, the mole’s outer skin was heated at a constant power whilst the temperature of the outer skin was measured simultaneously. The rise in this temperature is inversely proportional to the thermal conductivity. If the thermal conductivity is low, the temperature rise is high, and vice versa.