InSight’s Mole May Not Deliver on Heat Flow, But Reveal Mysteries of Martian Soil Instead

One of the instruments on NASA’s Insight Mars lander, called a mole, is behaving unexpectedly.  Scientists are still trying to understand why, but even if it ultimately cannot deliver on providing measurements of the planet’s heat flow, all is not lost.  In an interview, Principal Investigator Bruce Banerdt pointed out that new data about the properties of Martian soil could be an unexpected bonus that will inform future exploration by robots and humans.

NASA’s Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) spacecraft landed on Mars almost exactly a year ago.  It carries two scientific instruments, one built by the French space agency CNES and the other by Germany’s space agency DLR, plus radio equipment that also is being used for science investigations.

DLR’s Heat Flow and Physical Properties Package, or HP3, is designed to take Mars’s temperature to determine how much heat is flowing out of the planet’s interior. That will tell scientists whether Earth and Mars formed from the same “stuff” and how active Mars is today.

To collect the data, a probe affectionately called the “mole” needs to be inserted deep into the soil — as much as 5 meters (16 feet).  It uses a self-hammering device that relies on friction with the Martian soil to burrow downwards instead of bouncing in place due to recoil from the hammer.

When mission controllers first tried it, however, the mole got stuck after getting 35 centimeters (13.8 inches) into the soil.  Theorizing the soil was not providing enough friction, they used a scoop at the end of InSight’s robotic arm to pin the mole against the side of its hole and get that necessary friction.  In this NASA video, the black scoop is in the center, with the cylindrical mole and its trailing tether of wires to the left.

Initially it worked and the mole descended another 2 centimeters (0.8 inch), but at that point mission controllers worried that the scoop might damage the tethers connected to the top of the mole according to a JPL spokesman.  They resumed hammering without pinning and inexplicably the mole started backing out of the hole instead of going deeper.

No one knows why.  DLR and NASA scientists and engineers are assessing the situation before deciding on what to try next.  They do not want to make anything worse.

Determining Mars’s heat flow is very important scientifically, but typically would be obtained using a drill, which is quite expensive.  InSight is a cost-capped Discovery-class mission and this probe method was chosen because it was affordable even though it meant a higher risk of failure.

In an interview on Friday, JPL’s Banerdt told SpacePolicyOnline.com that when he proposed the project to NASA as part of the Discovery selection process, he listed 10 Level 1 science requirements it would achieve.  Because of the cost cap and the possibility that some experiments might have to be descoped or even eliminated to stay under it, those were separated into six “threshold” requirements that must be met to claim overall mission success and four “baseline” requirements that were highly desired but not as critical.

The HP3 data is one of the baseline, not threshold, Level 1 requirements.  The threshold requirements will be met by the Seismic Experiment for Interior Structure (SEIS) that is listening for Marsquakes or the radio science Rotation and Interior Structure Experiment (RISE).

The CNES-built SEIS experiment had its own troubles before launch, causing a two-year delay in InSight’s launch.  It appears to be working well now.

Banerdt said the design of  the HP3 mole was based on scientific understanding of the Martian soil from previous NASA landers and rovers dating back to the 1970s.  The soil at the sites explored by Viking, Spirit, Opportunity and Curiosity had differences, but were sufficiently consistent to inform the design of HP3.

Or so they thought.

What is causing the mole to behave so unexpectedly remains to be determined, but Banerdt sounded intrigued.  Scientists discovered a thin layer of duricrust, a type of cemented soil, in the area explored by Spirit (which is part of the reason it got stuck), but the duricrust is much thicker at InSight’s location.  That may suggest a different formation process on that part of Mars.

“We’re getting some really fascinating data” about the mechanical properties of the soil and near subsurface that are likely to yield many scientific papers, he enthused.

As NASA develops its plans to send humans to Mars as early as the 2030s, understanding soil characteristics will be critical for everything from habitation to resource extraction and utilization. “We have limited experience with digging on Mars,” so whatever happens with HP3’s efforts to measure heat flow, we “definitely will get some good science.”

As for measuring the temperature of Mars, if the problem cannot be resolved, it may require the more expensive drilling approach. That would have to be prioritized against other Mars science questions in the next planetary science Decadal Survey.

But it also would inform what needs to be done to establish self-sufficient habitation on Mars.  Heat flow is a “key measurement for understanding the equivalent of geothermal power” or the depth of possible aquifers where temperatures are conducive to liquid water.  “Doing it on a Discovery budget” was a known risk, Banerdt emphasized, but HP3 will result in important scientific knowledge even if that goal is not met.

Correction: the original version of story said that using the scoop to pin the mole to the side of its hole did not work.  However, according to a JPL spokesman, it did work initially, but its use was discontinued to avoid damaging the tethers.