Paul Morgan, senior geothermal geologist at the Colorado Geological Survey, was recently interviewed by Motherboard, a Vice Media website focused on tech and science news, about the Heat Flow and Physical Properties Package (HP3) that was aboard the Mars InSight launch May 5. Upon landing on the Martian surface in late November, HP3's job will be to take the planet's temperature, revealing just how much heat is still flowing out of the interior of Mars.
From the article:
According to HP3 instrument co-investigator Paul Morgan, a senior geothermal geologist at the Colorado School of Mines, five meters is the target depth because it is the estimated distance that the mole should be able to descend before hitting tougher bedrock.
"The regolith is the broken-up layer, and as you get deeper, it tends to compact and get harder to penetrate," Morgan told me in a phone call. "There's an outcrop not too far from the landing site where we think we can see the base of the regolith, so five meters is an estimate of what we should be able to penetrate."