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 is 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. If you were to measure the heat flow at various sites, you would be able to roughly represent the process through which the crust is developed (see diagram above). As InSight is only flying one station to Mars, only one value can be measured, so the hope is that this will be representative of the planet. Model calculations performed in the run-up to the mission have shown that this is likely to be the case.
The surface heat flow on Mars is probably much smaller than that on Earth, which gives off an average of 80 milliwatts of heat per square metre. This is partly because Mars is smaller than Earth and therefore has less stored heat, and may also contain slightly less radioactive material per kilogram of rock. We are also familiar with the effect whereby smaller bodies cool more quickly because their surface area is larger in relation to their volume. The surface heat flow for Mars is expected to be around 20 milliwatts per square metre, or a quarter of the value for Earth. This value corresponds approximately to the ratio of the volumes and masses for both bodies. HP3 will attempt to measure the heat flow with a maximum error margin of ±2 milliwatts per square metre.
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 will accomplish this by drilling a small penetrometer, known as a ‘mole’, into the ground, pulling a ribbon cable equipped with 14 temperature sensors behind it. Once the probe has reached its target depth, the temperature will be measured at all of the measuring points every 15 minutes for several months. A long measuring period is 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 will measure the thermal conductivity of the soil roughly every half metre as it descends to the target depth. The temperature of the mole’s outer shell, which will be subject to constant heat, will also be measured at this time. The increase in this temperature behaves inversely to thermal conductivity; if thermal conductivity is low, the temperature increase is high, and vice versa.