Although InSight’s Mole is Completely Buried, it Might be Stuck Again

You’ve gotta hand it to NASA, and to the German Aerospace Center (DLR.) They’ve been struggling for over a year to get the InSight Lander’s Mole working. There’ve been setbacks, then progress, then more setbacks, as they try to get the Mole deep enough to do its job.

Now the Mole is finally buried completely in the Martian surface, but it might still be stuck.

The Mole is the nickname for the Heat Flow and Physical Properties Package (HP3) instrument on the InSight Lander. InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) is all about Mars’ interior, and the Mole is designed to measure the heat coming from the interior to the surface.

But to do that, it has to penetrate deep into the regolith. It’s designed to penetrate to a maximum depth of 5 meters (16.4 ft), but can still gather useful scientific data at a depth of only 3 meters (10 ft). But as of now, it’s nowhere near 3 meters deep.

In this image, the Heat Flow and Physical Properties Probe is shown inserted into Mars. Image: NASA

The problem is the nature of the soil. Rather than loose material, the soil is a cement-like substance they’re calling duracrust. And that’s bad news for the Mole.

The Mole slowly hammers its way into the soil. And to do that, it needs the friction provided by loose soil, as the soil falls into the hole with the hammering motion. Without that friction, the hammering motion has nothing to brace itself with, and the Mole just pops out of the hole.

The Mole was designed and built by the DLR, or German Aerospace Center, as their contribution to the InSight mission. Both the DLR and NASA have been trying to solve the problem, and their most recent rescue attempt involved the instrument placement arm. They used the scoop on the end of the arm to press down on the Mole as it hammered. The arm provided the friction that the Martian soil couldn’t.

As it stands now, that intervention worked. Sort of. The Mole is buried now, which is good. But now that it’s buried, the arm may no longer be able to help.

The movement of sand grains in the scoop, seen here, suggested that the mole had began bumping up against the bottom of the scoop while hammering on June 20, 2020. Credit: NASA/DLR/JPL-Caltech

Operators aren’t certain yet if the Mole is stuck. In the above images, grains of dirt are bouncing around in the scoop. That could indicate that the Mole is bouncing against the scoop, making no further progress.

Tilman Spohn is the DLR instrument lead for the Mole. Spohn keeps a blog detailing the Mole’s struggles. In a recent post, Spohn said that “The ‘Mole’ started bouncing in place after making some progress without direct support from the scoop on 13 June (Sol 550).”

Spohn points out that since the Mole is now buried, there are no images of what’s actually happening under the surface. The team can only make judgements based on what they can see. “Rather, we judge this from the motion of the tether, or more precisely, from the apparent motion of features on the tether with respect to the background,” Spohn wrote in his blog post. “The images clearly show that the tether moved back and forth and eventually stopped altogether, suggesting that the Mole did not dig further down on its own.”

Even though the Mole is now buried, and out of physical contact with the scoop on the instrument arm, it looked like the scoop might still be helping. By pressing down on the surface above the Mole, it might still provide some of the necessary friction for the Mole to continue deeper.

But those hopes were dashed. Spohn wrote: “On Sunday 21 June, when we looked at the images that had been sent to Earth after the hammering session on Saturday (Sol 557, 150 hammer strokes), we had to conclude that having the Mole two to three centimetres deeper in and below the surface was not providing the necessary friction, even when helped with pushing on the regolith.”

One of two Mars InSight’s 7-foot (2.2 meter) wide solar panels was imaged by the lander’s Instrument Deployment Camera, which is fixed to the elbow of its robotic arm. It’s dusty season on Mars now, and soon the team will need the instrument arm and its camera to check for dust accumulation on the panels. Credits: NASA/JPL-Caltech

Taking a closer look at the previous gif animation, it’s pretty clear that the movement of the tether indicates that progress has stalled.

Writes Spohn: “The tether moved back and forth and then to the left, reversing much of its forward progress from Sol 550.” And the moving dust particles mentioned previously were also a bad sign, according to Spohn. “The moving dust particles imply that the Mole had backed up again and was tapping on the flat side of the scoop from below,” he writes.

But Spohn and the rest of the InSight team aren’t giving up yet. Their next move is to move the scoop out of the way and get images of the Mole’s situation inside the hole. “We will be interested to see how deep in the Mole really is (it should be a centimetre or so below the surface), whether the morphology of the pit has changed and whether the sand that we had seen in the pit is still there or whether the pit has been drained by the hammering action.”

Depending on what they see, they may push more sand into the hole, and again use the scoop to put pressure on the top of the hole. But filling the pit with sand will take a long time.

An illustration of the Mole’s situation in its Martian hole. On the left is before trying to fill the pit with sand, and on the right is after that maneuver. The idea is that if they fill the pit, more of the scoop’s pressure will be directed to help the Mole’s hammering motion. Image Credit: DLR.

The Mole has taken a lot of time and attention, and Spohn says though the team is concerned, they are determined. “First of all, let me say that the team continues to be determined, although we appreciate that the task is not likely to become easier.”

But there are other issues that need to be addressed. It’s been one year since operators used the camera to check the status of InSight’s solar panels. It’s the dusty time of year on Mars, and dust load on the panels affects power availability. The team needs to measure the dust and determine how much power can be budgeted for the lander’s different operations. The camera is on the end of the arm, so the Mole will be left alone for a while.

The InSight Lander’s seismometer underneath its protective wind and thermal shield. Image Credit: NASA/JPL-Caltech

InSight also contains a seismometer to listen to Marsquakes, another way of learning about the interior of Mars. But Mars is subject to meteor strikes, and there has to be a way to differentiate meteor impacts from Marsquakes. The InSight team needs to use the instrument arm camera to watch the sky for meteors. They did this previously in the mission, and it allows them to cross-check seismic readings with meteor activity.

It’s been a remarkable journey and story for InSight’s Mole. For those of us who have been following closely, we can’t help but hope for a successful conclusion.

The over-arching goal of the InSight mission is to learn about Mars’ interior. That knowledge might help us understand other rocky bodies in the Solar System. Including our own Earth. But for that, we need the Mole.

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Evan Gough

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