Apollo lunar heat flow experiment revisited: A critical reassessment of the in situ thermal conductivity determination

Abstract

[1] Lunar heat flow was determined in situ during the Apollo 15 and 17 missions, but some uncertainty is connected to the value of the regolith's thermal conductivity, which enters as a linear factor into the heat flow calculation. Different approaches to determine the conductivity yielded discordant results, which led to a downward correction of the obtained heat flow values by 30%-50% subsequent to the publication of the first results. We have reinvestigated likely causes for the observed discrepancies and find that neither poor coupling between the probe and regolith nor axial heat loss can explain the obtained results. Rather, regolith compaction and compression likely caused a local increase of the regolith's thermal conductivity by a factor of 2-3 in a region which extends at least 2-5 cm from the borehole wall. We conclude that the corrected lunar heat flow values, which are based on thermal diffusivity estimates sampling a large portion of undisturbed regolith, represent robust results. Future in situ measurements of regolith thermal conductivity using active heating methods should take care to both minimize regolith disturbance during probe emplacement and maximize heating time to obtain reliable results. We find that for the Apollo measurements, heating times should have exceeded at least 100 h, and ideally 200 h.Citation: Grott, M., J. Knollenberg, and C. Krause (2010), Apollo lunar heat flow experiment revisited: A critical reassessment of the in situ thermal conductivity determination,

Application of Finite Difference Method for determining lunar regolith diurnal temperature distribution

“…Since early 70's several models of lunar regolith thermal conductivity temperature dependence have been proposed [12,[17][18][19][20][21]23]. There is a general agreement between thermal conductivities calculated using all these models for dense regolith, i.e.…”

“…Unfortunately, there were no precise in situ temperature measurements taken down to 5cm, due to relatively large thermocouples span along used bore stems [20,23,28].…”

“…for depths greater several centimetre's. However, there are significant differences between thermal conductivities calculated for loose regolith, so called fluff, that exists down to two centimetres beneath Moon's surface [12,23].…”

“…It is to be pointed out, that the very process of boring into lunar regolith disturbed and compacted the material that surrounded later inserted bore stems [12,23]. Moreover, there were only four boreholes made for temperature measurements during the Apollo programme [24], thus it is very difficult, if possible, to provide accurate thermal model of lunar soil basing only on these data.…”

“…According to data provided by manned Apollo lunar missions, lunar soil has very low thermal conductivity, that changes significantly with temperature, bulk density and applied pressure [12,[17][18][19][20][21]23]. Specific heat of lunar regolith is also highly temperature dependent [22].…”