The InSight mission logbook
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 16 October 2020
In my last logbook entry on 10 August, I reported that we had succeeded in pushing sand into the pit better than expected. However, we still wanted to continue to press the back end of the Mole with the slanted scoop to get it a little deeper into the ground. We then planned to test again whether the probe would move further into the ground without the help of the arm. In other words, we would perform what we refer to as a 'Free Mole Test'.
Unfortunately, these tests took place under conditions that had become more difficult. In particular, dust in the Martian atmosphere as a result of nearby dust storms and the settling of dust on the solar cells significantly reduced the available electrical power. This led to the HP3 radiometer no longer conducting measurements as we wanted. Furthermore, the increased demands on the operations team associated with managing the reduced power availability meant that the Mole and the scoop could only be commanded fortnightly from September onwards. A total of three hammering operations have been performed since then, twice with 100 strokes on 22 August (Sol 618) and 5 September (Sol 632) and finally once with 250 strokes on 19 September (Sol 645).
We found that during the first two rounds of hammering and during the first half of the third round of hammering, the scoop went further into the sand. Since the Mole was hidden under the scoop, the penetration of the probe itself could not be observed directly. During the hammering, the flat tether running to the probe moved considerably, but these could only be clearly identified as forward movements during the hammering on 22 August. Overall, we could estimate from the movements of the scoop that the Mole moved at most one centimetre further into the ground. It was interesting to observe that during the second half of the round of 250 hammer blows on 19 September, the scoop did not go any further, probably because it encountered duricrust. This was certainly a desired outcome, as it allowed a second Free Mole Test to be conducted. In fact, the probe continued to move according to the movements of the tether, but it could not be clearly determined that these movements brought the Mole deeper into the ground.
In view of the inconclusive movements of the probe and the considerable amount of time involved, the team, after extensive discussions, decided to leave this path and instead proceed with filling the pit. For this purpose, the scoop was lifted on 3 October, making the pit visible.
After some discussion about the next steps, we decided that two parallel scoop movements should be conducted on Saturday 17 October (Sol 659). Afterwards, a thermal conductivity measurement will be carried out, which should also give us indirect indications about the backfilling. Then, the filling will be pressed to compress the sand and press on the Mole. Depending on the result of the back filling, further actions to fill the pit will be planned before further hammering and another Free Mole Test will take place later on.
Logbook entry 10 August 2020
The Mole is ‘in’ and the ‘finishing touches’ are ‘in sight’ ... or rather the finishing pushes. But let’s go back to where we left off. Following the Free Mole Test conducted in June (see logbook entry from 7 July 2020), the ‘Mole’ team decided to lift InSight’s arm and scoop and take a look at the Mole in the pit. Some of us had expected – or feared – that the previous hammering actions would have drained the sand from the pit. The sand, so the thinking went, would have loosened and fallen into possible deeper cracks and cavities in the duricrust. After all, we are still puzzled about where all the material – about 300 cubic centimetres or 10 ounces – went when the pit formed back in March 2019.
'Undercover' Mole
Instead, we were pleasantly surprised to see that the Mole was largely covered with sand (see image above). Only the back cap and a few centimetres of the hull are sticking out. It seems that instead of being drained of sand, more sand accumulated in the pit, likely because of some duricrust having been crushed during the hammerings.
After reviewing the images from Sol 577, the team’s discussion quickly turned to strategies for the next move. Some were in favour of filling the pit, compacting the sand in the pit, and then pushing the scoop onto the surface to provide force, which would then be transmitted to the Mole by the sand. The force should be enough to offset the hammer mechanism's recoil of about seven newtons.
Others argued that we should first try to get the Mole a few centimetres deeper by pushing the back cap with the tip of the scoop. After intense discussion, the team decided to first do a push on the back cap, similar to the successful back cap pushes conducted in the past months. The only problem is that in the previous configuration - bottom down - the scoop no longer fits in the pit. You can lift the scoop and push with the blade, but this means a higher risk of slipping and either damaging the cable or not being able to prevent the Mole from 'hammering backwards'. I have already mentioned that the placement of the scoop is risky and must be done with millimetre accuracy. With the blade down, this is even more difficult than before.
At that point, both options lacked important information. Two things were uncertain: how effectively could we scrape sand into the pit and how could we safely push on the Mole without causing potentially irreparable damage? We decided to do a scrape test first to give us more time to see how the scoop might fit into the pit using CAD models.
I had estimated that the first scrape of 12 centimetres swath length would raise the bottom of the pit but leave the Mole sticking out of the sand. By the way, this was the condition for some to agree to the quite controversial ‘scratch test’. As one can see in the image from Sol 600 shown below, that estimate was not quite right. The scraping was a complete success! The scrape was much more effective than expected and the sand filled the pit almost completely. The Mole is now covered, but there is only a thin layer of sand on the back cap.
Most of the miscalculation was due to the fact that the shovel went much deeper into the ground than planned. As a result, almost twice as much material was brought in. In addition, the Mole is a bit deeper in the ground than initially deduced from the stereo images.
The scraping had also had the effect of partially levelling the differences in height at the edge of the pit - making it easier to place the scoop. With this knowledge and with the support of the project management we decided to bring the Mole a little deeper into the ground with the help of the scoop. To do this, however, we would not press with the blade, but with the shovel at an angle of 20 to 30 degrees with respect to the surface. This is first of all a somewhat simpler, more predictable and less time-consuming operation compared to a sequence of scraping movements; possibly combined with movements of the shovel to fill the pit. I think, at the latest after filling the pit, we should be able to counter the recoil with sufficient force and the Mole will hopefully 'dig' deeper into the Martian soil on its own. Keep your fingers crossed!
Promising thermal values
As a supporting indication, I note that a recent measurement of the thermal conductance from the Mole to the regolith shows increased values over earlier measurements. This suggests that both the thermal and mechanical contact have improved. So we're feeling optimistic!
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!
Follow us on Twitter to get the newest information and pictures of our #MarsMaulwurf.
More images of the mission can be found here.
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