Density measurements of lunar internal heat by penetrating thermal probe

Аuthors

Dudkin K. K.^*, Alifanov O. M.^**

Moscow Aviation Institute (National Research University), 4, Volokolamskoe shosse, Moscow, А-80, GSP-3, 125993, Russia

*e-mail: lord-konst@mail.ru
**e-mail: o.alifanov@yandex.ru

Abstract

The article considers the scheme of the buried thermal probe for determining the lunar heat flow density. The probe represents a rod, on which thermometers are fixed at certain intervals. The thermal probe is placed in a pre-drilled well. A certain time is needed for the probe-soil system reaches thermal equilibrium. After it measurements are starting. If the soil thermal conductivity is known, then it remains only to measure a series of temperature values at various depths, to obtain its differences and, according to Fourier's law, obtain the heat flow density. However, the question arises on what material the rod should be made of. If it is made of a material with high thermal conductivity, then, given the ultra-low thermal conductivity of the soil (λ = 0.001 – 0.03 W/m K), then the heat flow-over through the rod from the lower warmer part to the upper one would occur. And the upper part, in its turn, would transfer the heat to the adjacent sections of the soil. The soil in the upper layers around the rod will be locally heated, and this means that the thermometers will not show the real temperature drops corresponding to natural conditions, but the distorted ones. Thus, the rod material should be of the lowest possible thermal conductivity to minimize these distortions. Direct measurements on the lunar surface have already been carried out by NASA in the Apollo 15 and Apollo 17 missions according to this scheme. Following NASA, fiberglass is assumed in calculations as a rod material, due to its adequate strength for the set problems, and herewith a low thermal conductivity. Two calculations were performed to assess a temperature difference deviations while such thermal probe scheme application.

- For a homogeneous soil model, in which the soil thermal characteristics the are constant;

- For a multi-layer soil model, in which the soil thermal characteristics are changing with depth.

According to the results of numerical simulation for various soil models, a penetrating thermal probe gives errors in determining the heat flux from 13.8% to 18.5%. Summarizing, we can say that the penetrating thermal probe has good prospects for application in various scientific lunar exploration missions.

Keywords:

lunar soil, thermal flow density, thermal probe, Moon exploration

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