Grott, Matthias

Hudson, T.L.

Knollenberg, Jörg

Scharringhausen, Marco

Smrekar, S.

Wippermann, Torben

Institute of Transport Research:Publications

THE INFLUENCE OF REGOLITH COMPACTOIN ON INSIGHT-HP3 THERMAL CONDUCTIITY MEASUREMENTS, M. Grott1, J. Knollenberg1, T. Wippermann2, T. Spohn1, M. Scharringhausen2, T.L. Hudson3, S.E. Smrekar3, 1German Aerospace Center (DLR), Berlin, Germany, 2German Aerospace Center (DLR), Bremen, Germany, 3Jet Propulsion Laboratory, California Institute of Technology, Oak Grove Drive, Pasadena, CA 91109, USA Introduction:  The InSight [1] Discovery-class mission to study the martian interior landed in the Elysium Planita region [2] of Mars on November 26th, 2018, and  will install the first geophysical station on the planet. A heat flow probe termed the Heat Flow and Physical Properties Package (HP3) [3] is one of the mission’s three geophysical instruments, and HP3 will emplace a suite of temperature sensors to a depth of up to 5 m using a self-hammering mechanism called the HP3-mole. During penetration, the regoltith’s thermal conductivity will be measured using the mole’s TEM-A sensors [3] as a modified line heat source [3,4]; measurements will be performed at depth intervals of 50 cm.  The principle of the mole relies on moving the reg-olith in front of the probe out of the mole path, thus reducing regolith porosity in the vicinity. Depending on  regolith relative density, the volume affected ex-tends to 2-3 mole diameters [5]. Such compaction can increase regolith thermal conductivity, and similar effects have been reported for the Apollo lunar heat flow experiments [6], resulting in a downward correc-tion of the reported values by 30-50% [7]. While the higher Martian atmosphere pressure largely mitigates the influence of compaction on thermal conductivity, a quantitative analysis of the effect has so far not been performed.  Experimental Setup: We have used a flight-equivalent HP3 mole model to perform compaction and thermal conductivity tests under representative envi-ronmental conditions. The general test setup is shown in the top panel of Fig.1 and schematics of the setup are given in the bottom panel. The test cylinder con-tains regolith simulant, mole, and a reference conductivi-ty measurement using a Transient Hot Strip (THS) [4]. The test cylinder was placed inside a vacuum chamber, and air pressure was held constant to within 0.5 mbar dur-ing each experimental run. Depending on the experi-ment, background pressure was between 6 and 10 mbar using ambient air. The experimental procedure was as follows: 1) The container was filled and the reference THS measure-ment strip was installed. Then, the mole was installed in its mounting fixure (compare top panel of Fig. 1) and the container was moved into the vacuum cham-ber. 2) A THS measurement was conducted to deter-mine the pre-compaction thermal conductivity of the regolith simulant. The mole then penetrated to a 55 cm tip depth and another THS measurement was per-formed to verify that hammering does not influence the THS reference. 3) The chamber was closed and pumped down. 4) Another THS measurement and a TEM-A measurement were performed after the soil had thermally equilibrated.      Figure 1. Top: Experimental setup showing the HP3 mole in the center prior to penetration. The THS refer-ence measurement strip is buried on the right hand side of the test cylinder. Bottom: Schematic setup of the experiment showing the THS reference strip on the left and the HP3 mole in the center of the test cylinder, fully buried. 1269.pdf50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)   Figure 2. Top: Grainsize distributions for the three different simulants tested. Bottom: Minimum, maxi-mum, and average density as a function of grain densi-ty for the three simulants tested.   Regolith simulants used in the tests were a broken up basalt mixed with 25% dust (Syar), quartz sand (Quartz), as well as a dunite powder (MSS-D). Grain-size distributions as well as the minimum (loose) and maximum (compacted) densities of the simulants are shown in Fig. 2. While the quartz sand has a very nar-row grain-size distribution and is not very compactale, the MSS-D is highly compactable. Syar is in-between the two. During soil preparation, care was taken to place the soil in a loosely compacted state, as the influ-ence of compaction is expected to be most pronounced in this setting [5]. Results: Results of the tests are summarized in Ta-ble 1, where conductivities determined using the THS reference as well as the HP3-mole TEM-A measure-ments are shown. For all tests the influence of compac-tion at the location of the THS reference strip was below 2-3%, within the uncertainty of the method. Some TEM-A measurements suffer from increased background temperature drift, resulting in increased systematic uncertainties for the associated conductivity values.  During the tests, some soils inside container settled by a few cm, indicating soil compaction. For MSS-D, settling was ~3 cm during hammering, and pre- and post-hammering TEM-A thermal conductivities were 0.055 and 0.06 W/mK, respectively. This indicates a 10% increase of thermal conductivity due to the ham-mering action. For Quartz sand, no soil settling was observed and TEM-A results are indistinguishable between pre- and post-hammering in a first measure-ment, while a repetition of the experiement indicates a 9% increase of conductivity. However, the soil was not fully thermally equilibrated during the second test run, such that the apparent increase is likely due to system-atic measurement errors. For Syar, soil settled by ~1.5 cm during hammering, and observed compaction amounts to a 7% increase in thermal conductivity. However, these results again  suffer from background temperature drift, which makes them less reliable.   Soil THS [W/mK] TEM-A [W/mK] Δk [%] MSS-D 0.055 0.060 9 WF-34 0.094 0.092 -2 WF-341 0.089 0.098 10 Syar1 0.142 0.167 18 Syar1,2 0.142 0.152 7  Table 1 Results using the THS reference method as compared to conductivity values determined using the HP3 TEM-A after penetration. 1Measurements suffer from background temperature drift. 2Same measure-ment as above, but evaluated at shorter times to reduce the influence of background drift.   Discussion: On Mars, we expect moderately com-pacted soil with a grain size distribution in-between that of Quartz and Syar, but without the dust fraction [2,8]. Therefore, some effect of compaction on thermal conductivity may be expected, but this should be less than the ~10% worst case conditions studied here. On Mars, results obtained using TEM-A will furthermore be compared to thermal inertia estimates using the HP3 radiometer [3] as well as estimates derived from the attenuation of the annual temperature wave. These should be useful to verify/correct the TEM-A meas-urements.  References: [1] Banerdt, W.B., Russell, C.T., Space Sci. Rev., 211, 1–4, 1–3 (2017). [2] Golombek, M., Kipp, D., Warner, N., et al., Space Sci. Rev., 211, 1–4, 5–95 (2017).  [3] Spohn, T., Grott, M., Smrekar, S.E., et al., Space Sci. Rev., 214:96 (2018). [4] Ham-merschmidt, U., Sabuga, W., Int. J. of Thermophysics, 21, 1, 217-248 (2000). [5] Marshall, J.P., Hudson, T.L., Andrade, J.E, Space Sci. Rev., 211, 1–4, 239–258  (2017). [6] Grott, M. Knollenberg, J., Krause, C., J. Geophys. Res., 115, E11005, (2010). [7] Langseth, M.G., Keihm, S.J., Peters, K., Lunar Science Confer-ence, 3, 3143-3171 (1976). [8] Morgan, P.,  Grott, M., Knapmeyer-Endrun, B., et al.,  Space Sci. Rev., 214:96, 2018.  1269.pdf50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)

The Influence of Regolith Compaction on InSight-HP3 Thermal Conductivity Measurements

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