Nature of the surface of the moon [1]

Abstract

THE question of the nature and temperature of the lunar surface has recently acquired a new interest since microwave radar observations of the temperature have become possible. Twenty years ago Pettit, working in the infra-red, measured the fall of surface temperature during a lunar eclipse, and from this information Epstein1 deduced that most of the surface is covered with a material the thermal properties of which are comparable with those of pumice. If K, ρ and c are the thermal conductivity, density and specific heat of the lunar material (assumed to be constant), t0 is the duration of penumbra, and A is the insolation before the eclipse, he gave the formula for the fall of temperature up to time t after the beginning of penumbra, and from the observations deduced (Kρc)- Combining double low line 120, the units being C.G.S. and°C. The source of this formula was not stated, but it is, in fact, that for the surface temperature of a semi-infinite solid from which heat is extracted at the rate At/t0 per unit time per unit area. This is equivalent to assuming that the surface loses heat during the eclipse at a rate proportional to the fourth power of its initial temperature, instead of to the fourth power of its actual temperature, and since the temperature-range involved is from 370°K. to about 200°K., formula (1) will give values which are far too large. To get an accurate result, the non-linear equations must be studied numerically: this has been done recently by Wesselink2, who finds a value (Kρc)- Combining double low line 920, and shows that this value is consistent with dust at low pressures (he does not point out the incorrectness of Epstein's calculation, and, indeed, states that their results agree, which by a coincidence they do, though, in fact, they are in different units). © 1950 Nature Publishing Group.