4 February 2021 | Space

InSight logbook – the Mars 'Mole' in the autumn of its mission

You can find more graphics explaining the instruments of the InSight mission on flickr
Credit:
DLR (CC-BY 3.0)

Since February 2019, the scientific director of DLR's HP³ instrument, Tilman Spohn, has been providing us with the latest news about the InSight mission in the DLR blog and regularly explains the current situation of the heat probe HP³, which we affectionately refer to as the Mars 'Mole'.

Logbook entry 4 February 2021

Last October, the HP³ team planned to fill the pit around the 'Mole' and conduct the 'Free Mole Test'. This was to test whether the Mole could burrow into the soil on its own, without support from the InSight lander's robotic arm.

By the end of December, the hole had actually been backfilled in three stages, with sand being pushed into the pit with the scoop during each stage. The three scraping movements performed by the scoop took place in the direction of the lander and were laid out in three parallel paths (see Figure 1). Between each stage, the material was compacted by pushing on the freshly piled sand with the scoop. After the first two sand scrapings but before the third, the heating circuits and temperature sensors built into the Mole were used to measure the thermal conductivity of the Martian soil. At this point, the probe was well covered with sand and the end was approximately three centimetres below the surface, so the measured average thermal conductivity could cover approximately the top 40 centimetres of soil.

The results are currently being prepared for publication, but it can already be reported here that the measured value of about 0.04 watts per metre-kelvin fits well with the value that had previously been derived for the uppermost five to 10 centimetres of the soil using data from the radiometer. This indicates that the soil should be quite homogeneous down to a depth of 40 centimetres, at least in terms of heat transport capacity. The value of 0.04 watts per metre-kelvin is comparatively small, lower by a factor of between three and five than that found in terrestrial sands and lower by a factor of 30 to 50 than that of basalt, of which Martian sands are composed.

The thermal conductivity of porous materials depends on the intrinsic conductivity of the particles – but especially on the porosity of the soil. The low value of conductivity suggests a high porosity of approximately 60 percent and thus a density of only 1.2 grams per cubic centimetre, significantly lower than the material density of basalt (about three grams per cubic centimetre). The strength of the soil, estimated from the earlier tests with the scoop, suggested salt-encrusted sand. However, the thermal conductivity is actually rather too low for this. The apparent contradiction between strength and thermal conductivity still needs to be understood. The high porosity of the soil and its strength would explain how the hole around the probe was originally formed. The impact movements of the Mole in March 2019 would have crumbled the material framework and the actual sand would have found itself at the bottom of the pit.

At this point, it should be said that the configuration of the probe reached at the time of the measurement has high scientific potential. Fully buried, it can serve as a thermal probe that, in addition to measuring thermal conductivity, allows measurement of temperature in the soil and thus the thermal interaction with the atmosphere and solar radiation. This interaction includes gas exchange between the ground and the atmosphere, which is an important element of the physics of the atmosphere. In this capacity, the probe complements the measurements of our radiometer and the temperature and pressure measurements of the Temperature and Winds for InSight (TWINS) sensor package used by the atmospheric scientists on the InSight team.

Nevertheless, until the very end, we pursued the goal of digging deeper into the subsurface with the probe to enable the original goal of measuring planetary heat flux. However, we were careful that further activities with the Mole did not jeopardise its usefulness as a thermal probe.

Figure 1 (left) shows traces of the two parallel scoop movements in the direction of the arrow, from top to bottom, used to push sand into the pit around the Mole on 17 October 2020. To the right of the tether is the imprint of the scoop after compression of the piled sand. On 14 November 2020, a third strip was pulled down the centre, which was then used to push most of the pile of sand seen in the figure to the left, in front of the arrowhead, into the remaining depression. The sand was then compacted again before the scoop was placed to the right of the cable on 19 December (Figure 1 right). The Mole sits roughly vertically in the soil with an angle of 30 degrees to the vertical. Its tip points roughly to the right.
Credit:
NASA/JPL-Caltech

Finally the 'Free Mole Test' before minimum solar insolation

The planning for the 'Free Mole Test' and the further work on Mars unfortunately had to be done under the conditions of decreasing energy supply for the lander with Mars’ increasing distance from the sun on its orbit and solstice approaching, which means that the solar radiation is continuously decreasing.

Since, for personnel reasons and because of the mission's limited budget, it was only possible to command every 14 days and, in addition, Thanksgiving and Christmas were just around the corner, it was necessary to hurry in the autumn if we wanted to have enough time to hammer to the target depth of five metres after a successful test. We wanted to proceed in three interval steps of 1.5 metres and repeatedly measure the thermal conductivity of the soil in between. Originally, it was even planned to measure the thermal conductivity about every 50 centimetres. Unfortunately, there was not sufficient time for this.

The schedule in the autumn called for the final 'Free Mole Test' to take place on 19 December. For this purpose, after further compression of the sand, the scoop was placed over the Mole, to prevent a possible escape of the probe upwards, although nobody expected the Mole to hammer backwards again. It had done this twice during the last few years, but at that time it was only partially in the ground and not as well packed in sand as it was this time. Still, no risk was to be taken in this regard – the Mole sticking out of the ground again would have lost its functionality as a thermal probe.

During the final check before 19 December, it was found that the scoop was positioned a little too far away from the cable to safely exclude an 'escape' by the Mole. With a heavy heart, the test was postponed to 9 January and the placement of the scoop was corrected beforehand (see Figure 1 right). The finally placed scoop then exerted a pressure of approximately seven kilopascals on the surface. This vertical pressure alone was thought to be sufficient to compensate for the expected recoil of the probe. With a commanded 500 strokes – more strokes than in previous tests – a clear result was thought to be achievable. There was a consensus among the team that the Mole had now been placed in a position where a successful test could be expected. However, given the lander's diminishing energy supply and with time running out, it was clear to all that failure would mean an end to the effort – at least for the foreseeable future.

Figure 2 shows the course of the 'Free Mole Test'. The movements of the Mole can only be seen indirectly. However, it can clearly be seen that the cable does not go any deeper. The multiple movements of sand particles and agglomerates are interesting, suggesting that the energy of the hammer blows was transferred mainly into elastic and kinetic energy of the sand particles and less into plastic deformation at the tip of the probe (burrowing into the soil).
Credit:
NASA/JPL-Caltech

By late afternoon on 9 January, the first images were on the ground, and Troy Hudson of the Jet Propulsion Laboratory (JPL) compiled them into a first animated GIF (Figure 2). From these images, it was already apparent that the probe had essentially hopped in place, with no clear resulting forward movement of the cable. However, the Mole did bring sand upwards, as can clearly be seen in the images, and some tracks on the cable could be interpreted as a result of a slight backward movement. Given the preparations that had been made, this result was a great disappointment with unfortunate consequences for the experiment. The attempts to get the Mole into the ground and the first measurement of planetary heat flux on Mars, indeed on an Earth-like planet, had failed.

The question arises as to why the measures taken, the filling of the cavities and the pressing of the scoop into the ground, were not sufficient to compensate for the recoil of the hammer mechanism due to friction and pressure. Or was it not due to a lack of friction, but to the resistance of the soil?

It is difficult to find simple answers to these questions, but it could be that the backfilling of the soil was insufficient and that the transfer of force from the scoop through the soil did not succeed to the extent that rough calculations had suggested. In addition, it was noticeable that the last observed penetration rates (also supported by the scoop) were significantly lower, by a factor of between three and five, than those measured in the laboratory. And this is at a depth where the soil pressure is comparatively low!

This may be due to the fact that two years ago – before the problem was known – prolonged hammering (approximately 9000 strokes) on the spot had consolidated the soil. However, estimates had indicated that the Mole should have penetrated or largely penetrated this consolidated layer – if it had been created – by then. The consolidated layer would be approximately a few diameters of the Mole thick and since the beginning of the effort we had, after all, brought the Mole almost 10 centimetres deeper into the ground. On the other hand, it cannot be excluded that the Mole is pushing a small stone in front of it. In any case, the movements of the ground visible in the video – sliding on the slopes and the material flowing out of the ground – suggest that the mechanical energy of the hammer mechanism hardly went into plastic deformation of the ground at the tip, but possibly to a large extent into elastic energy and into kinetic energy of sand particles and agglomerates on the surface and in the ground.

Figure 3 shows the Mole pit with the arm and scoop retracted. The image was acquired on Sol 775 (31 Jan). It is good to see that the Mole is covered with sand, an important condition for its functioning as a thermal probe.
Credit:
NASA/JPL-Caltech

The Mole remains close to the surface – as a thermal probe

Over the coming weeks and months, the team will once again go through the diverse records of the past two years. They will also try to understand what happened through model calculations and, if possible, derive values for soil mechanical parameters. The probe and the temperature sensors on the tether together with the radiometer – which is still working perfectly – will be of great use as a thermal probe. Depending on how the available solar energy on Mars develops in the coming weeks, the sensors will continue to provide useful data, even if the measurement of the heat flux has had to be abandoned for the time being.

Perhaps comforting is a recent New York Times article by Randall Munroe, which notes that understanding the motion of granular materials like sand is one of the great unsolved problems in physics.

It is appropriate at this point to say a few words of thanks and to highlight the cooperation in the team. First of all, it must be noted that the hardware worked without any problems until the very end. This is remarkable because the load was well above what the probe was originally designed for. A good part of this endurance is due to the reworking of the design that Jörg Knollenberg and his DLR colleagues from Bremen and Berlin, staff from Astro- und Feinwerktechnik in Berlin and from Astronika in Warsaw did between 2016 and 2018. The space-qualified motor from Maxxon that powers the hammer has also performed superbly.

Moreover, it must be emphasized that Cinzia Fantinati, Louise Thomas, Daniel May and Sven Jansen, under the direction of Christian Krause, were tirelessly available for commanding the instrument, which turned out to be far more time-consuming than originally planned. Matthias Grott, Nils Müller and Jörg Knollenberg have evaluated the radiometer and temperature data and will continue to do so.

All the attempts to get the probe into the ground would not have been possible without the intensive, trusting and friendly cooperation with colleagues from the Jet Propulsion Laboratory (JPL) in Pasadena. JPL and DLR had together formed an Anomaly Response Team (ART), coordinated by Troy Hudson of JPL. Weekly conference calls were used to plan all the steps and evaluate the results. Khaled Ali and his team managed to control the lander's robotic arm – which was not designed for such tasks – with millimetre precision. On behalf of the JPL Mission Operations Team, we would like to thank Elizabeth Barrett and Patrick Guske, as well as Matt Golombek, Justin Maki, Eloise Marteau, Robert Deen and Jim Garvin for their scientific support. In particular, thanks are in order for the integration of the SEIS seismometer recordings by Ken Hurst and Cedric Schmelzbach (ETH Zurich). Bruce Banerdt, as Principal Investigator of the mission, and his deputy Sue Smrekar as well as Mission Managers Tom Hoffman and Charles 'Chuck' Scott 'stuck their necks out' for the 'Mole Recovery' operation. NASA and DLR and the entire InSight Science Team are thanked for their unwavering support as this endeavour cost resources that could have been used elsewhere.

With this entry my logbook also comes to an end. For more than one Martian year I have reported about our efforts to get the Mars Mole to burrow into the Martian soil and to reach the target depth of three to five metres.

I was not privileged to conclude the report with a big success story, but it would also not be appropriate to speak of our joint efforts as a failure. It was clear from the outset that HP³ was the part of the InSight mission with by far the greatest risk. Getting a robotic instrument to burrow into the Martian soil and aiming for a target depth of a few metres was something no one had dared to do before. DLR and NASA/JPL can take credit for having attempted this, and I personally consider it the greatest space adventure of my time at DLR.

Next up, ESA's ExoMars 2022 rover Rosalind Franklin will attempt to drill to a depth of two metres on Mars using conventional drilling technology. I am sure that DLR will continue to develop the technology of the Mole, with which it is possible to reach greater depths in principle without using a drill pipe – that is, with less hardware mass and volume. In the design process, greater attention will be paid to the difficult interaction between the casing and the ground. Whether this will be attempted first on a Mars mission, a lunar mission or a mission to another body remains to be seen.

For now, the HP³ team will continue focusing on learning lessons from the Mole's ups and downs and will then evaluate the data that the instrument package will provide as a thermal ground probe and radiometer.

DLR blog posts about the Mars 'Mole' can be found here.

The original logbook of Principal Investigator Tilman Spohn, including previous contributions can be found here.

About InSight

5 May 2018 saw the launch of NASA's InSight mission, in which a lander will carry out geophysical measurements directly on the surface of Mars to explore the planet's inner structure and thermal balance. DLR has contributed to this mission in the form of the Heat Flow and Physical Properties Package (HP³) instrument. On 26 November 2018, InSight touched down north of the equator, on the Elysium Planitia plain.

For the first time since the astronaut mission Apollo 17 in 1972, heat flow measurements will be carried out on another celestial body using a drilling mechanism. The main aim of the experiment is to be able to determine the thermal state of the interior of Mars using heat flow measurements taken beneath the surface. Models of Mars’ formation, chemical composition and inner structure can be checked and refined on the basis of this data. The measurements from Mars can also be used to draw conclusions about Earth’s early development.

The depth achieved by the HP³ Mole can be tracked in the virtual control room!

More images of the mission can be found here.