QUOTE (JRehling @ Feb 6 2021, 02:11 PM)
For the heat probe objective specifically, would the summer ice cap be an appealing target? The martian regolith holds arbitrarily many differences from Earth, but H2O ice is a known material, and the rock problem (even if moderate elsewhere) would be near zero on the ice cap, right?
There's a few points to this. The 'ultimate' goal of HP3 was to measure the planetary heat flow (well, to measure the crustal heat flow at one point, from which the global heat flow might be estimated... there's more that could be said about that..but let's skip that today..) You do that by measuring the local thermal conductivity (which MP3 can still do, at its shallow depth) and the temperature gradient.
But, there are additional gradients imposed by transient events, by the diurnal cycle, by the annual (seasonal) cycle, and by longer-term variations (global dust storms on some years not others.... perhaps even a Little Ice Age signal, if the LIA was caused by solar variations rather than e.g. volcanism on Earth). The 3-5m depth goal of HP3 was driven by the need to get under the annual heat wave to pick up the underlying gradient, and you need a big enough depth range that the measurement errors on temperature itself do not degrade the measurement (e.g. at a 10K/km gradient not untypical for Earth, a 10m borehole gives you a 0.1K temperature difference, so if your measurement error is 0.02K, you can be 20% off, but if you could drill a 100m hole, you'd be within 2%...) . The annual wave depth is predicated on an assumption of conductivity - specifically the e-folding penetration depth of a wave is (kappa * tau)^0.5 where kappa is the thermal diffusivity and tau the timescale. For regolith, kappa is maybe 1E-7 m2/s , for solid ice, maybe 1E-6 (depends on temperature). So in solid ice, the thermal wave penetrates deeper.
(I published a paper a while back showing that if the LIA were caused by a solar luminosity variation, it could affect the HP3 heat flow retrieval. You can see a LIA signal in Greenland ice cores, down to 50-100m IIRC, which would correspond to a few meters in regolith)
So yes, from a penetration mechanics point of view, maybe an ice sheet has the prospect of better homogeneity (no rocks - although surface textures on glaciers and ice sheets can in fact be rather forbidding). But from the geothermal heat flux measurement standpoint, you get a much poorer measurement for a given depth in solid ice than in regolith.
There is also the question of how to attain that depth. I don't think a self-hammering drill like HP3 could penetrate solid ice effectively. A kinetic penetrator (e.g. DS-2) similarly would only go in 0.5-1m into ice, far too shallow to do heat flow. So to do an ice cap drilling mission (and that would be cool, and I think there have been Discovery/Scout/DSMCE proposals) you'd need a conventional drill or a thermal drill, a much more complicated proposition.
In this context, HP3 was a relatively inexpensive thing to try.
Recall that heat flow has been done on the moon, albeit with astronauts drilling a hole and emplacing a probe with the temperature sensors. Yet even that effort yielded only 2 measurements out of 4 planned (Apollo 13 never made it to the moon; Apollo 16 a cable broke). Space is hard.