DLR Mission Update
In his logbook, Instrument Lead Tilman Spohn who is back in Berlin since April and communicating with JPL via the web, gives us the latest updates regarding the InSight mission and our HP3 instrument – the ‘Mole’ – which will hammer into the Martian surface.
Logbook entry 27 August 2019
I hope you had – or are still having – a good summer!
Back in early July, I reported that we had safely removed the Support Structure Assembly (the SSA) to expose the mole. You may recall that the SSA’s function is to house the mole and the tethers before deployment and that it was sitting above the mole when the latter penetrated into the surface. The SSA was thus blocking the view of the mole. By removing it, we enabled us to view the mole up close, to eventually interact with it, and to work its immediate surroundings with the robotic arm. What we saw first showed us that our estimate of the length of the mole in the sand of 30-35 cm was pretty good (Figure 1).
We see now that about 5 cm are sticking out of the ground. This also confirmed our suspicion that the mole had stopped penetrating as it left the guiding springs in the SSA that had provided the necessary friction. It would have been extremely bad luck if a sufficiently large stone would be found just at that depth of 35 cm. What further confirmed the theory of lacking friction from the Martian sand was the size of the pit that the mole had cut out. We had indications before that the mole might have dug a hole or a pit but we had not expected it to be that large.
As you can see from Figure 1, the diameter of the “mole hole” (you may recall that we had nick-named our commanding office at JPL the “mole hole in December) is almost twice the mole diameter or about 6 cm. Thus, the mole must have precessed (like a spinning top) while it was digging. Moreover, the twist in the tether shows that the mole must have rotated clockwise about its long axis by about 135°. The image also showed that the feet of the SSA had left clear footmarks that had remained stable, indicative of at least some regolith cohesion as is the pit itself. The multiple footmarks are proof of the SSA having been lifted and bouncing with the mole during hammering.
Another image taken later under better lighting conditions (Figure 2) revealed the bottom of the pit being about 2.5-3 mole diameters or 7-8 cm deep. That image also suggested that there was a layer of quite some cohesion with clumps and concretions and maybe caverns, possibly overlying cohesionless sand. The interpretation is that we exposed a layer of duricrust about 5-10 cm thick (on Mars, the term duricrust is used to indicate a mechanically strong layer of regolith, somewhat different than in terrestrial geology). It is thought by geologists to consist of cemented sand.
In July one believed that the duricrust around the pit might be easily crushable. Thus, it was decided to go ahead with the plan of loading the surface with the scoop to increase pressure and thus friction on the mole hull, but, the pit would have to be collapsed first. Three rounds of pushing on the surface with the scoop followed until mid of August of two pushes each. The first push was done with the flat blade (65 sqcm), just as shown in Figure 3.
Then 4 pushes with the sharp tip (Figure 4a left,and 4b, right) and a final one again with the blade. The pushes had a force of about 50 N, equivalent to about 10 kPa of vertical stress with the blade and 300 kPa with the tip.
Figure 5 shows the result of gardening with the tip (mid of August 2019), and Figure 6 the result of the final push with the flat blade. As Figure 6 shows, none of these could fully collapse the pit, although a partial collapse can be seen on the right-hand side of the pit. This probably indicates some inhomogeneity in the duricrust rheological properties. Moreover, it is seen that the pit got partially filled to about half of the initial depth.
I conclude that the duricrust has a compressive strength of at least a few 100 kPa and is overlain by an about 1 cm thick layer of loose dust (which is the material that got mostly moved and compressed by the pushes). It is interesting to note that an independent estimate that I made on the basis of our tiltmeter recordings of the initial penetration of the mole resulted in a similar order of magnitude resistance (300 kPa) of the top layer to penetration. These recordings had suggested that the mole first lifted the SSA while at the same time penetrating slowly about 7 cm until it had hammered through the duricrust and the SSA resettled on the ground.
The mission is pausing now until 10 September because Mars is entering what is known as solar conjunction. This means that Mars is becoming invisible from the Earth because it is moving behind the sun (as seen from Earth) and communication with a spacecraft on Mars becomes impossible.
The project has prepared the lander for this hiatus and some in the team use the time for some well deserved vacation. For others, this is a break that will allow some thinking ahead of what to do next. I am leaning towards moving away from further trying to collapse the pit as it proved to be very time consuming. Rather, I am thinking towards pinning the mole with the scoop such that the pinning and the pressing of the mole against the wall of the pit would increase friction. This will be more risky than the previous strategy, but with the unexpectedly stiff duricrust, it may be worth a try.
That’s it for now. Stay tuned until we come back from conjunction with a report on what the project finally decided to do.