For two years, the Mars 'Mole' has endeavoured to dig ever deeper into the Martian soil. Now its journey has come to an end. Until recently, scientists and engineers at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) and the Jet Propulsion Laboratory (JPL) had been working to get the mole to dig to a target depth of five metres. The DLR Mole, which travelled to Mars on board the NASA InSight lander, was designed to measure temperature and heat flow beneath the surface of Mars. In recent months, the InSight team at JPL in Pasadena, California, used InSight's robotic arm to get the Mole back beneath the surface and backfill the cavities beside it with Martian soil. It was thought that the additional lateral friction would allow the Mole to advance further. Unfortunately, despite careful preparations, another 500 hammer blows conducted on Saturday 9 January did not result in any visible progress.
The Mole is part of the Heat Flow and Physical Properties Package (HP3) experiment. It is essentially a 40-centimetre-long penetrometer – like a self-hammering nail – with a diameter of 2.7 centimetres. It was designed to pull a ribbon cable fitted with temperature sensors behind it. These sensors were intended to measure heat flowing from the interior of the planet to the surface. To do this, the Mole needed to burrow to a depth of at least three metres. Almost two years ago, on 28 February 2019, having been set down on the Martian soil, the Mole was activated for the first time within its housing, and began to hammer its way underground.
Almost two years of running through every possible option
"We did absolutely everything that we could, but Mars and our plucky Mole were simply not a good fit," says Tilman Spohn of the DLR Institute of Planetary Research. Spohn is the Principal Investigator for the HP3 experiment, which was developed by DLR and various partners. He is now also the Executive Director of the International Space Science Institute in Bern, Switzerland. “Fortunately, we have gained a lot of experience that can help future Mars missions get beneath the surface," says Spohn. Interest in exploring the planet's subsurface remains high. Due to harmful aspects of solar radiation, very little of which is filtered out by the thin Martian atmosphere, the surface of the planet is rather inhospitable for potential lifeforms. However, suspecting that microbial life may be viable in the subsurface, researchers believe that life could once have been present on Mars and may even exist there today.
To find out how the robotic arm of the InSight lander could help the Mole dig, JPL – which is conducting the InSight mission on behalf of NASA – carried out numerous experiments in its 'Indoor Mars Yard'. Among other findings, the researchers established how the scoop on the robotic arm could apply pressure to and alongside the Mole. "The Mole was developed based on soil analysis by NASA rovers such as Spirit and Opportunity," says JPL scientist Troy Hudson. "But after the landing, we found that the soil in this area is completely different to anything that we have seen before." The DLR Test Laboratory in Bremen also conducted research into the reasons why the Mole had come to a standstill.
Ongoing measurements by parts of the experiment
"Given the many years of planning, development and construction that have gone into our heat-flow experiment, we are naturally disappointed that not all of HP3's components are working as we had expected," says Professor Heike Rauer, Director of the DLR Institute of Planetary Research in Berlin-Adlershof. "However, HP3 will at least be able to provide us with temperature measurements from the top layer of the Martian surface. While this is not all that we had been hoping for, it will nevertheless help us to gain new knowledge about Mars. This planet remains a difficult neighbour to explore. We will continue trying to unlock the mysteries of Mars, so as to find out whether life ever actually existed there. The next experiments are already being developed."
HP3 is one of several scientific experiments on board the InSight lander, which recently had its mission extended by two years to December 2022. Another experiment is the French seismometer SEIS, which continues to record Marsquakes and local tremors caused by atmospheric phenomena. Furthermore, a radio wave experiment is collecting data about the position and orientation of the planet's axis of rotation to provide insights into whether the core of Mars is liquid or solid. At the same time, weather sensors provide detailed meteorological data. HP3 will now continue as a partial experiment.