Measurements of lunar radio brightness distribution and certain properties of its surface layer

Abstract

The construction of high resolution telescopes, including the 22 m radio telescope RT 22, developed at the P. N. Lebedev Physical Institute, made it possible to investigate the distribution of lunar radio brightness in the millimetre and centimetre wavelength ranges. Such research provides information which makes it possible to deduce certain properties of the surface layer which is responsible for radiation emission. During the first stage of the studies, which were carried out at the wavelengths of 4 mm, 8 mm, 2 cm and 3·2 cm, we observed the law of the decrease of the surface temperature from the equator to the poles, which turned out to be close to √cos ψ . The average effective dielectric constant was esti­mated as ∊ = 1·5. The measured phase dependence of the brightness temperature in the centre of the lunar disk, measured at the indicated wavelengths, is in good agreement with the results of calculations in terms of the one-layer model. With values of the material dielectric constant and density taken equal to those of terrestrial rock materials, the obtained measurements lead to a value of effective density of the surface layer of approximately ρ = 0·6 g/cm 3 . Using γ = ( kpc ) –½ = 400–600 (which follows from the phase dependence curves of the brightness temperature for the centre of lunar disk) and c = 0·2 cal./g, we obtain k ≃ 2 x 10 –5 cal cm –1 deg –1 s –1 . The penetration depth of the thermal wave under these conditions is close to 10 cm. A method of relative measurements has been developed for the detection of brightness temperature distinctions (‘radio contrasts’) of ‘sea’ and ‘continental’ regions on the Moon, using the new known averaged data characteristics of lunar radio emission. It was shown that the radio contrasts may depend mainly upon the differences among night surface temperatures connected with different values of the δ parameter, as well as upon the difference between the values of variable component of the brightness temperature, connected with different values of the δ parameter, which expresses the ratio of the depths of penetration of electromagnetic and thermal waves. The comparative measurements which were carried out in the 4 and 8 mm wavelength ranges showed relatively small contrasts between the brightness temperatures of ‘sea’ and ‘continental’ regions. The average excess temperature of the ‘sea’ region during a period of lunation was 1·5 ± 0·5 %. We came to the conclusion, which is in agreement with the results of measurements in 8 to 13 μ m interval, that the difference in night surface temperatures in the regions compared is about 8 degK. The difference in the values of the parameter γ corresponding to this difference is not more than 25 %. This indicates a high degree of homo­geneity of the material in the compared regions. The results of analogous measurements, carried out by B. Ya. Losovsky in the 16 mm wavelength range confirms this conclusion.