[6][7][8][9] Additional science goals included determining the thickness of Mars' crust, the composition of its mantle, and thermal characteristics of the interior, such as the temperature gradient and heat flux.

[13][14][15] The HP3 heat flow probe is made up of the following subsystems:[16] HP3 was conceived by Gromov V. V. et al. in 1997,[4][17] and first flown as the PLUTO instrument on the failed 2003 Beagle 2 Mars lander mission.

[4] HP3 evolved further and it was proposed in 2001 for a mission to Mercury,[18] in 2009 to the European Space Agency as part of the Humboldt payload on board the ExoMars lander,[19][18] in 2010 for a mission to the Moon,[20] and in 2011 it was proposed to NASA's Discovery Program as a payload for InSight Mars lander, known at that time as GEMS (Geophysical Monitoring Station).

The mole is described as a "self-hammering nail" and was designed to burrow below the Martian surface while trailing a tether with embedded heaters and temperature sensors.

The goal was to measure the thermal properties of Mars' interior, and thus reveal unique information about the planet's geologic history.

It contains a heater to determine thermal conductivity during descent, and it trails a tether equipped with precise heat sensors placed at 10 cm (3.9 in) intervals to measure the temperature profile of the subsurface.

[21] The total mass of the system is approximately 3 kg (6.6 lb) and it consumes a maximum of 2 watts while the mole is active.

[7] For displacement, the mole uses a motor and a gearbox (provided by Maxon) and a cammed roller that periodically loads a spring connected to a rod that functions as a hammer.

Meanwhile, a suppressor mass travels upwards and its kinetic energy is compensated by gravitational potential and compression of a brake spring and wire helix on the opposite side of the mole.

The HP3 mole was originally expected to take about 40 days to reach 5 m (16 ft) deep,[23] but ultimately achieved only a few centimeters after more than a year of effort.

[33] In February 2020, the team reevaluated the risks of pushing the back cap of the mole directly using the robotic scoop, and determined the procedure to be acceptable.

[35] In July 2020, it was revealed that the mole was bouncing in place, underneath the scoop, suggesting insufficient friction to continue digging.

A proposed solution was to fill the hole with sand in order to distribute pressure from the robotic scoop, thereby increasing friction.

On January 14, 2021, NASA announced that, as the final attempt to bury the "mole" had failed, the team had given up, with the heat probe portion of the mission declared to be over.

The seismometer (SEIS), radio experiment (RISE) and the weather instruments (TWINS) continued to operate until the end of December 2022.