The HP3 probe used for studying the geothermal features of Martian surface, hammers upto 5m deep and inserts a tether lined with heat sensor probes.

This link shared earlier by @uhoh, is an excellent read, which outlines in detail about the instrument.

But one is tempted to wonder why the heat sensor probes are arranged in a particular fashion? i.e. the spacing between the sensors keep decreasing as the tether reaches deeper.

heat sensor probe arrangements in HP3 probe

Is there a specific advantage to this arrangement of heat sensor probes?

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    $\begingroup$ I can venture a guess. As illustrated in this answer for the Moon, there is more uncertainty in values closer to the surface due to local perturbations of the descending day/night heat flux due various reasons that I won't speculate on yet. Once you get to deeper levels there may be less uncertainty so the tiny changes in temperature per centimeter may more accurately reflect the net rising heat flux from the core. Just a guess. $\endgroup$ – uhoh Nov 27 '18 at 6:48
  • $\begingroup$ @uhoh in the link you shared earlier, there is a mention of this "The scientists behind HP3 have agreed on a target depth of five metres, because from three metres down most of the seasonal temperature variations are likely to cause only negligible interference with the measurement results.". $\endgroup$ – karthikeyan Nov 27 '18 at 6:50
  • $\begingroup$ @uhoh - so effectively it is like cramping sensors closer to the depth than spreading it out evenly involving undesirable accuracy in upper level which is of no interest! Did I get it right? $\endgroup$ – karthikeyan Nov 27 '18 at 6:52
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    $\begingroup$ The daily variation damps out in a much shorter depth than that, but I'd totally forgotten about the Seasons; Mars' inclination is so much larger than the Moon's. About the other stuff, I can imagine you might want more data in the areas with the most valuable data, and fewer in the other areas, but "is of no interest" does not apply. Every scrap of data is known to be valuable, or could turn out to be crucial for unexpected reasons. But this is just speculation, let's see what fact-based answers have to offer. $\endgroup$ – uhoh Nov 27 '18 at 7:00
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    $\begingroup$ Maybe it's designed to prevent sensor overlapping as the probe descends. If the sensors were spaced evenly and after every hammering phase the mole has moved, let's say 10 cm, certain points along the depth would be measured repeatedly. By varying the distances, you can make sure that every point along the mole's decent is measured. $\endgroup$ – Dragongeek Nov 27 '18 at 14:55

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