HP³, an instrument developed by DLR, measured the thermal conductivity of the Martian regolith and the heat flow from the interior of the planet after been placed on the planet's surface.
Components of the HP³ (Heat Flow and Physical Properties Package) heat flow probe
Components of the HP³ heat flow probe. Top left: the radiometer (RAD), which is used to measure the radiation temperature (roughly equivalent to the ground temperature) of the surface. Right: the casing with the mole penetrometer, the temperature measuring cable (TEM-P) and the data cable (ET) connected to the lander. In addition, the casing contains an optical length meter for determining the length of the temperature measuring cable that has been pulled from the casing. The mole contains the TEM-A active thermal conductivity sensor and the STATIL tiltmeter. Bottom left: the electronic control unit, known as the back end electronics (BEE), which remains on the lander and is connected to the probe via the ET.
In determining the heat flow, the increase in temperature with depth, referred to as the geothermal gradient and the thermal conductivity are measured. The latter measures the rock's ability to transfer heat. Multiplying these two values together gives the heat flow – in other words the amount of heat that flows to the surface per unit of time and per unit of surface area – and is released from there into the atmosphere. Geothermal gradient measurements may be distorted by heating cycles, which cause regular temperature variations on the surface (diurnal cycle, changing seasons), and these influence the underground temperature at different depths.
In some places on Earth, the geothermal gradient has been measured at depths of up to several kilometres. People are aware of geothermal gradients from mines, whereby it becomes increasingly hot the deeper one goes, and from geothermal energy. Due to the extensive technological requirements and thus the very high cost of conducting such measurements in many different places, this was not an option on Mars. The scientists behind HP3 agreed on a target depth of five metres, because from three metres down most of the seasonal temperature variations are likely to cause only negligible interference with the measurement results. There then remain another two metres in which to measure the geothermal gradient.
The DLR HP3 heat flow probe shown in the diagram above measured the geothermal gradient by driving a small penetrometer, known as 'the Mole', into the ground. The Mole pulled a ribbon cable equipped with 14 temperature sensors behind it. During the flight to Mars, after the landing and before entering the ground, the Mole and the TEM-P temperature measuring cable were stowed in a casing known as the support structure (SS).
The Mars Mole penetrometer
The Mole penetrometer was developed at the DLR Institute of Space Systems. It draws upon earlier developments at DLR and in Russia. An earlier version of the probe was built at the former DLR Institute of Space Simulation in Cologne as a sample collector for the Beagle II lander – flown as part of the Mars Express mission – which crashed onto the Martian surface in 2003. The hammering mechanism for the HP3 mole was developed by Astronika in Warsaw, Poland.
Sensor on the temperature measuring cable
Sensor on the temperature measuring cable (centre). The temperature sensors are PT100 sensors with an accuracy of one millikelvin. Feed lines to other sensors and the mole can also be seen. The markings along the edge allow optical measurement of the length of the measurement ribbon cable that has been extended and, when used together with the inclinometer, allow the depth of the Mole to be determined.
HP3 measured the thermal conductivity of the soil roughly as it descended. To do this, the Mole’s outer shell was heated with constant electrical power and its temperature monitored. The increase in this temperature is inversely proportional to the thermal conductivity. If the thermal conductivity is low, the temperature increase is high, and vice versa. Similar technology is used in thermal needle sensors.
Radiometer (RAD)
The Radiometer (RAD) measured the thermal infrared radiation from the surface of Mars and gauged its radiation temperature, which corresponds approximately to the actual surface temperature. This is useful for interpreting temperature readings at various depths, as it can record the trend of the surface temperature. The RAD also measured the thermal inertia of the surface of Mars, which allowed conclusions to be drawn about the top few centimetres of the Martian soil. The RAD was mounted underneath the platform of the lander and monitored the area beside the lander where HP3 was installed.
Deployment and operation of HP3
During the flight to Mars, the instrument was mounted on the deck of the lander and remained there after the landing on 26 November 2018. The instrument was deployed in early January 2019. During the waiting period, the camera and the HP3 radiometer were used to explore the landing site and find the most suitable site for deploying the seismometer and the heat flow probe. This search is, however, limited by the range of the robotic arm (see diagram).
Unfortunately, the affectionately named Mars ‘Mole' probe of DLR’s HP3 did not fully deliver the expected measurements, because the heat flow probe could not successfully penetrate to the required depths beneath the Martian surface. Originally, the Mole was supposed to penetrate to a depth of five metres and drag a tether with temperature sensors behind it. The self-hammering probe, capable of penetrating through the familiar loose, sandy soil of other missions, was unable to find a foothold in the unexpectedly hard soil around InSight’s surroundings. The instrument was eventually able to bury its 40-centimetre probe just below the surface, collecting some valuable data on the mechanical and thermal properties of the Martian soil. The fact that the Mole was finally able to burrow in is thanks to a team effort by engineers from JPL and DLR. They used the InSight lander's robotic arm in a creative way to give the Mole additional support.
