An in situ lunar heat flow measurement is planned using the Japanese Lunar-A penetrators. The temperature gradient of the regolith is expected to be obtained within 12% error

Fujimura, A.

Hagermann, A.

Hayakawa, M.

Mizutani, M.

Tanaka, S.

Yoshida, S.

English

Open Research Online (OU)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsIn situ lunar heat flow experiment using the LUNAR-ApenetratorConference or Workshop ItemHow to cite:Tanaka, S.; Yoshida, S.; Hagermann, A.; Hayakawa, M.; Fujimura, A. and Mizutani, M. (2001). In situ lunarheat flow experiment using the LUNAR-A penetrator. In: 32nd Lunar and Planetary Science Conference, 12-16 Mar2001, Houston, Texas, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1495.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukIN SITU LUNAR HEAT FLOW EXPERIMENT USING THE LUNAR-A PENETRATOR . S. Tanaka, S. Yo-shida, A.Hagermann, M. Hayakawa, A. Fujimura, and H. Mizutani;Institute of Space and Astronautical Sci-ence,Yoshinodai, Sagamihara Kanagawa, 229-8510, JapanIntroduction: An in situ lunar heat flow measure-ment is planned using the Japanese LUNAR-A pene-trators which will be deployed on the near and far sideof the lunar surface in the year 2003. The penetrator isa cone shaped probe which is 0.84m in length and0.16m in maximum diameter[1]. The lunar heat flow isof vital importance in lunar science, because itprovides a basic data for inferring the thermal state ofthe lunar interior and also a strong constraint of bulkabundance of radioactive elements in the moon. Re-cently, the thorium abundance of the whole lunar sur-face area has been reported from the gamma ray remotesensing observation[2]. The most feasible candidates oflanding sites of the two penetrators are near the Apollo12 landing site and almost at the center of the Men-dellev crater. The thorium abundance of A12 site has aconcentration  2.5ppm larger concentration comparedwith the A15 site(3.6ppm); on the other hand, theMendellev site has an abundance 1.5ppm smaller com-pared with the A17 site(1.9ppm).  The relationshipbetween the thorium abundance and the heat flow dataof such a wide range of thorium abundance will give usbetter estimation of the averaged heat flow value. Sensors for the measurement of heat flow by theLUNAR-A penetrator: The penetrators contain 17temperature sensors (Fig. 1). Seven of these are abso-lute temperature sensors made of platinum resistancewires and eleven are type-K thermo-couple sensors forthe measurement of relative temperature. Five of theeleven thermo-couple sensors are attached to smallsheet-shaped heaters which are used to heat the regolithto measure its thermal conductivity/diffusivity. Thetemperature change of the regolith upon supplying aheating power of 50mW for 120sec will provide thedata of the thermal conductivity of the regolith[3,7].The relationship between the thermal properties andthe temperature change has been calibrated well andthe accuracy of determination of the thermal conduc-tivity with this method is found to be about 10 %. Allthe relative temperature sensors are mounted on or nearthe surface of the body for the purpose of the heat flowmeasurement. On the contrary, four of the five absolutetemperature sensors are mounted inside the penetratorfor the purpose of house-keeping the electronics orbatteries.The resolution of temperature is an important specifi-cation for the temperature gradient measurement. Con-sidering both the results of numerical simulations andtechnical feasibility, we designed the hardware to ob-tain 1/100 degree resolution for the relative tempera-ture measurement. With this resolution, we will be ableto determine the temperature gradient within about10% error.Temperature field around the penetrator: Sincethe thermal properties of the penetrator and that of theregolith are significantly different from each other, thetemperature field in the regolith around the penetratoris expected to be disturbed significantly from the origi-nal one by placing the penetrator itself in the regolith.For example, the thermal conductivity of CFRP (Car-bon Fiber Reinforced Plastic), which is a main compo-nent of the penetrator’s body, is about 4.2 W/m/K,whereas that of the lunar regolith is about 0.01W/m/K[4]. Therefore the penetrator will become amajor heat path for the lunar heat flow, changing thetemperature gradient  to a value lower than the originaltemperature gradient. Figure 2 shows a temperaturecontour map around the penetrator based on the as-sumption of suitable thermal properties of each com-ponent of the penetrator and the 1deg/m temperaturegradient of the regolith. The result shows that the tem-perature difference gradient measured on the surface ofthe penetrator body is reduced to 1/10 compared to theoriginal temperature gradient in the regolith.Precise thermal model of the penetrator: It is es-sential to quantitatively correct the thermal disturbancecaused by the penetrator to infer the original tempera-ture field. In order to correct for the thermal distur-bance, we need to construct a precise thermal model ofthe penetrator by using measured thermal conductivity,specific heat, and density of every component of thepenetrator. The thermal conductivity, specific heat, anddensity of every component of the penetrator weremeasured by a stationary method within 10 % error [5].In order to refine the thermal model of the final model,a fully integrated penetrator was suspended in a tem-perature control unit  1.5m in height and 0.5m in di-ameter. The unit was placed in a large space chamber4m in height and 4m in diameter[6]. The space cham-ber was evacuated to below 10-5 Pa, and was cooled toa temperature of 77K. We modified the initial thermalmodel to fit the temperature data obtained duringcooling. When adjusting the thermal properties of ourmathematical model to fit this temperature data, weallowed for a 10% change for those materials that hadbeen measured on component-level and for a 20%change for  components not measured experimentally.With this constraint, we found good agreement be-tween the obtained data and the expected temperatureprofile. On the basis of the experimental data, we cal-culated the temperature disturbance of the penetratorplaced in the regolith by changing thermal properties ofcomponents within the range of their uncertainties. Itshould be noted that the effect varies with the observa-tion timing after the penetration because the differenceof initial temperatures between the penetrator and theregolith (20 degrees assumed in this case). For earlierobservation, the errors are induced by both thermalconductivity(K) and heat capacity because the pene-trator is still in the transient cooling phase. After 30days, only the effect of K becomes dominant since thepenetrator has nearly reached its  steady state. Amongthe error contribution factors, the axial conductivity ofCFRP body gives about as high as 4% an effect ,and a12% error is expected on estimating the original tem-perature gradient in total when we observe the tem-perature of the penetrator 10 days after their penetra-tion.Lunar and Planetary Science XXXII (2001) 1495.pdfHEAT FLOW EXPERIMENT USING THE LUNAR-A PENETRATOR: S.Tanaka et al. References:[1]Mizutani et al.(2000) ESA SP462107 [2]Lawrence et al.(2000) JGR 105 20307[3]Horai et al. (1990) Proc. Lunar Planet Symp 277[4]Langseth et al.,(1976) LPSC 3143  [5]Yoshida(1999) Ph.D  thesis, University of Tokyo [6]Tanaka etal. (2000) ESA SP462 187 [7]Hagermann et al.(2001)LPSC(This issue)Figure 2. Steady state temperature distribution around the penetratorsurrounded by lunar regolith.  The interval of temperature contour is0.02degree.  The temperature gradient of the lunar regolith is assumed tobe 1 deg/m. The ogive shape penetrator has a length of 800mm and 15cmin diameter, and weight of 13.5kg.Figure 1. Schematic cross section of the penetrator and position of thetemperature sensors and thermal conductivity sensors in LUNAR-Apenetrator. The figure also schematically shows the schematic mechani-cal components; P-BAT: battery block, P-GYM: seismometer mountedon a rotational mechanism, P-MSU and P-DPU: electronics circuitblocks, UHYB/P: hybrid circuit for Tx and Rx, and U-ANT/P: antenna.Lunar and Planetary Science XXXII (2001) 1495.pdf

In situ lunar heat flow experiment using the LUNAR-A penetrator

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In situ lunar heat flow experiment using the LUNAR-A penetrator

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