Infrared moon - A review

Abstract

T brightness temperature of the lunar surface has been determined using telescopes on Earth, on balloons, and on aircraft. Observations revealed more than a thousand anomalous regions (hot spots) during an eclipse. It was also discovered that certain of the mare and portions of mare regions show an anomalous thermal behavior. In 1966 and 1968 Surveyors I, III, V, VI, and VII performed in situ thermal measurements. With the two Apollo landings in 1969, we have passed from a remote sensing to a physical sampling era. Apollo 11 and 12 provided samples of the lunar surface from which certain of its thermophysical properties will be determined. In the coming Apollo missions, it can be expected that a landing will be made on or at least near one of the so-called hot spots, and on one of the enhanced mare locations. The purpose of this paper is to describe the infrared measurements that have been made of the lunar surface. The earliest measurement was made one hundred years ago. The first significant work was done between 1924 and 1928 by Pettit and Nicholson of the Mount Wilson Observatory; their work has been reviewed in detail by others. They reported measurements of brightness temperatures under various conditions: distribution of temperature along the lunar diameter at full moon, the subsolar point temperatures as a function of phase, the antisolar point (nighttime) temperature and the transient temperature during a total lunar eclipse for a point near the disk center and at the limb. In the theoretical work that followed, Wesselink and Jaeger and Harper » assumed that the thermophysical properties of the lunar surface were constant with depth and temperature. Even with a two-layer model, it was not possible to match simultaneously the lunation and eclipse measurements with one set of thermal parameters. These early theoretical studies, however, did suggest that the uppermost layer of the moon was of a porous or dustlike nature. Further measurements of eclipse cooling were made by Sinton and Strong in 1953, and in 1958 and 1959 Sinton constructed isothermal contour maps over the lunar surface at nine different phases. The lunar infrared measurements up to 1960, including a description of certain theoretical models, have been discussed by Sinton. During the lunar eclipse of March 13, 1960, Shorthill, Borough, and Conley discovered that several rayed craters cooled less rapidly than their environs, and in particular, that Tycho was about 40° K warmer than its environs an hour into the umbral phase. This surprising observation in the infrared, revealing differential properties over localized regions, provided an impetus to the author as well as several others to make additional measurements. Subsequently, extensive measurements were made during eclipses and during the lunar daytime. Measurements during the lunar night are less extensive. Some results of these measurements will be discussed as well as -a new thermophysical model developed by Winter and Saari which fits both the lunation and eclipse data.