NASA’s InSight Mars lander acquired this image using its robotic arm-mounted, Instrument Deployment Camera (IDC) on October 27, 2019, Sol 326.
Credit: NASA/JPL-Caltech



A troublesome update on the Heat Flow and Physical Properties Package (HP3) experiment deployed by NASA’s InSight lander on Mars.

Reports Tilman Spohn of the German Aerospace Center’s (DLR) Institute of Planetary Research in Berlin: “This time, the news are not so good, unfortunately!”

Spohn explains that the mole had backed-out of the Martian soil instead of going deeper as scientists had expected it to do.

InSight’s scoop has touched the Mole as shown in this image using the robotic arm-mounted, Instrument Deployment Camera (IDC). This image was acquired on October 3, 2019, Sol 302.
Credit: NASA/JPL-Caltech



Martian atmosphere

“How could that happen? After all, this mole does not have a reverse gear,” Spohn adds. “But, we have seen the phenomenon of backing-up in the lab at small atmosphere pressure such as on Mars.”

Spohn points out that the Martian atmosphere pressure is only 0.6% of the Earth’s. At Earth’s atmosphere pressure, if the mole starts to rebound and if the rebound is not compensated by friction on the wall, the mole rapidly opens a cavity below the tip of the mole.

Engineers in a Mars-like test area at NASA’s Jet Propulsion Laboratory try possible strategies to aid the Heat Flow and Physical Properties Package (HP3) on NASA’s InSight lander, using engineering models of the lander, robotic arm and instrument.
Credit: Tilman/NASA/JPL-Caltech.


“The difference in pressure in the expanding cavity and in the atmosphere creates a suction effect that helps damping the rebound. The atmosphere pressure on Mars, however, is so small that the suction effect does not play a role and cannot really help the mole,” Spohn explains. “This is one more reason why the friction on the mole hull…is so important! It is the major force to balance the recoil! In addition to the recoil you need some collapsing of the borehole at the tip in order to let the mole move up.”

Playing it safe

When mole researchers analyzed the images from the hammering before, there was an indication that the mole forward motion had slowed down towards the end of that session.

Credit: NASA/JPL-Caltech

“Therefore, we played  it safe by commanding a smaller than originally planned  number of hammer strokes and then a readjust of the pushing of the arm and scoop on the soil,” Spohn says. “And indeed, the backing out occurred before the readjustment of the push. But I, for one, would have never thought that the mole could back out as much during a few tens of hammer strokes. Maybe, if we were out of luck, it would not penetrate or possibly come up a bit.”

Spohn now speculates that the low gravity on Mars helped the rapid backward movement.

“Well, operating on Mars is not only time consuming. It is full of surprises! The interactions of low atmosphere pressure, low gravity, unknown regolith mechanically properties and mole dynamics are a challenge,” Spohn relates.

Credit: NASA/JPL

What to do next?

First, mole researchers want to be sure that the mole will not tip over. Then, there’s need to inspect the hole it is sitting in, the view of which is blocked by the scoop.

“We may then do another pinning trying to bring the mole back to where it was before the recent hammering,” Spohn continues, “and then start again to get it to dig below the surface. But give us some time to think!”

Bottom line: “Stay tuned,” Spohn concludes, “it is not at all over, but the mole is not making our lives easier these days!”

“After all, our goal is to eventually go much deeper and measure the heat flow! We will not give up so easily…if only we eventually learn more about the Martian soil and operations on Mars,” Spohn says. 

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