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