WASHINGTON — Engineers are continuing to work to free an instrument on NASA’s InSight Mars lander that remains stuck just below the surface.

The Heat Flow and Physical Properties Probe (HP3), one of the two main instruments on the spacecraft, features a probe, or “mole,” designed to hammer its way into the surface to a depth of about five meters. Once in place, it will measure how much heat is flowing out of the planet’s interior.

The instrument, placed on the surface weeks after the spacecraft’s landing last November, started the hammering process in late February, but project scientists stopped that work days later when it appeared the mole was stuck about 30 centimeters below the surface. Engineers have since been trying to determine why the mole is stuck and how to get it moving again.

The instrument team identified three potential causes for the mole becoming stuck. The mole may have hit a rock that blocks its progress. The tether trailing behind the mole could be stuck in the instrument’s support structure. Another possibility is that the mole’s hull doesn’t have enough friction with the surrounding regolith to keep it from rebounding when fires its hammer.

That third explanation now appears to be the most likely one. “We’ve already moved one rock away,” noted Pascale Ehrenfreund, chair of the executive board of the German space agency DLR, during a panel discussion about the mission May 14 at the Humans to Mars Summit here. DLR developed the instrument for the InSight mission.

She said it also appeared unlikely that the tether was tangled within the instrument. “It looks like that it’s actually the hull friction that is limited.”

Tilman Spohn, the principal investigator for the instrument, wrote in a May 6 blog post that data collected by InSight’s other main instrument, a seismometer, showed that the mole’s response to the hammering was between that of one bouncing freely and one progressing normally into the surface.

He wrote previously that a lack of friction could be caused by a “duricrust” of regolith at the surface that has a higher cohesion than layers beneath. “If the mole is sitting in the duricrust, its hull may very well have lost friction and upon time, the mole may have widened the hole in the duricust,” he wrote.

Ehrenfreund said the instrument performed some “diagnostic hammering” in the last few days, but that the results of that latest effort was still being analyzed.

She remained optimistic that the mole would be able to get to its desired depth. “All the tests have shown that HP3 is really healthy and operative. We just have to optimize to hammer further down,” she said.

InSight’s seismometer has been free of technical issues, but has encountered a different problem: a lack of seismic activity. “It turns out that Mars indeed is, so far, extremely quiet,” said Robert Fogel, InSight program scientist at NASA Headquarters, on the same panel.

The instrument has detected six seismic events to date, he said, including one “just the other day.” One of them is a likely Marsquake, but a weak one: an estimated magnitude of 2.5 about 150 kilometers from the landing site.

A notable aspect of that quake, he said, is that the train of waves lasted 12 minutes, far longer than any terrestrial earthquake but similar to quakes detected by seismometers on the moon placed there by the Apollo missions. That can be explained if the Martian crust is very shattered, scattering waves and elongating the wave train, he explained. It also suggests that the crust is very dry, since water would “heal” shattered minerals.

Fogel said that despite the limited number of quakes, they still hope to detect stronger events, with magnitudes greater than four. “It’s what we need in order for us to help determine exactly how Mars is stratified: core, mantle and crust,” he said.