ENHANCED LUNAR THERMAL RADIATION DURING A LUNAR ECLIPSE*

Transcription

1 ENHANCED LUNAR THERMAL RADIATION DURING A LUNAR ECLIPSE* R. W. Shorthill, H. C. Borough, and J. M. Conley Boeing Airplane Co. Aero-Space Division, Seattle, Washington In an attempt to detect variations from the previously de- scribed cooling characteristics of the lunar surface, bolometric measurements of infrared lunar radiation were made at the New- tonian focus of the 72-inch reflector of the Dominion Astrophysi- cal Observatory during the total lunar eclipse of March 12-13, Measurements by Pettit and Nicholson 1 and by Pettit 2 have revealed the rapid cooling of the lunar surface during eclipses (from. r 370 K to 180 K), and the effect has been interpreted by Epstein 3 and others 4 5 in terms of a dust layer. The purpose of these observations was to find regions of anoma- lous thermal characteristics, in particular areas that cool less rapidly than the general lunar surface material. Regions of in- terest in this regard are the flanks of steep mountains, bright and rayed areas, features reported to have shown activity, and others. The radiation measurements were made with a thermistor bolometer detector with a KRS-5 window that limited the spec- tral bandpass to p. The dimensions of the sensitive sur- face of the thermistor were 0.3 X 0.3 mm, subtending a field of 7" at the f/5 Newtonian focus. This corresponded to a linear dimension of about 12 km at the surface of the moon. A filter wheel with microscope cover glass, germanium disc, and open po- sitions was used. The cover glass provided a long-wavelength cut- off at about 5 p, and the germanium passband extended from 1.8 to about 15 p. The rms noise level of the device corresponded to a radiation flux of 3 X 10-9 watts for a 10 cps bandwidth, approximately that due to a lunar temperature change of 20 K at the background temperature of 180 K. It was desirable to observe many scattered features ; however, because of the limited * Presented at the Eugene meeting of the Astronomical Society of the Pacific, June I

2 482 SHORTHILL, BOROUGH, AND CONLEY Fig. 1. Scanning cycle: a. Aristarchus, b. end of scan off east limb, c. end of scan near the sea of Nectar, d. center of disk, end of 14.5-minute scan cycle, e. Copernicus. illumination during some eclipses, a restricted scan program was planned using the bright crater Aristarchus as the starting point (Fig. 1). Then, with the telescope clamped in declination, the right ascension slow setting motion (10'/min) was used to drive off the east limb. The slow motion was then reversed for 1.5 minutes, carrying the detector image to a point near the sea of Nectar, where the drive was again reversed and the detector again driven off the east limb. This procedure was repeated through a total of eight reversals, the elapsed time being 14.5 minutes. The moon s motion in declination during this time was 2f5, about five detector diameters per sweep cycle. The detector was then driven to the apparent central point of the disk, passing through Copernicus en route. The circumstances of the eclipse are tabulated below. The times listed are PST. Moon entered penumbra March h 34 m Beginning of totality End of totality Moon left penumbra

3 LUNAR RADIATION DURING ECLIPSE 483 The maximum zenith angle of the moon during the observations reported here corresponded to sec z = The fractional change in sec z during the observations was The scanning procedure was executed twice during the first penumbral phase with the filter wheel rotating to give three nearly continuous traces. However, clouds began to obscure the moon during the second scan and the data from these early scans have not been used for temperature calculations. The sky cleared again at 24:00 PST, and the scan was repeated with the filter wheel stationary on the germanium disc. The eclipse was then total, and the lunar temperature had dropped to below 200 K. The seeing was good and the sky background produced a constant deflection. The germanium filter was used to eliminate the effect of reflected light, but differential measurements of the thermal radiation could have been made even with an open filter, since the reflected radiation, which produced 10% of the deflection from the full moon, was now reduced to negligible proportions. All deflections during totality were, then, essentially due to thermal radiation. The craters Aristarchus and Copernicus produced deflections greater than those of the lunar background, but, because the recorder was not closely attended at the time, these were not fully appreciated until the records were more carefully examined after the eclipse. Within the limits of accuracy of the instrument no other deviations from the measurements of the Mount Wilson investigators were found. It was then decided to search elsewhere, and the crater Tycho was chosen because of its good definition. En route to Tycho we scanned the crater Alphonsus but no enhanced infrared emission was detected, indicating that the temperature of the part of Alphonsus scanned was within 20 K of the general lunar background. Upon reaching Tycho, we noted a deflection greater than that of its immediate background by a factor of two. The crater was scanned repeatedly from 00:38 to 01:03 PST. Figure 2 is a portion of a typical recording of the Tycho data with the same data also shown in smoothed form. For these observations the scan rate was reduced to 30" per minute, the slow tracking rate of the 72-inch telescope. On each trace a sharp rise occurred near the S rim, gradually decreasing toward the N rim. The

4 484 SHORTHILL, BOROUGH, AND CONLEY ~ a RIM -p WATTS MOON y ( MOON SKY-^ ' SKY TIME (PST) Fig. 2. Original and smoothed data from a typical scan of the crater Tycho. equivalent diameter of the crater is shown in Figure 2, but the collimation uncertainty was about one-third of the crater diameter, so the sharp rise cannot be definitely ascribed to the rim of the crater. The maximum signal in the vicinity of Tycho was three times that of the lunar background, corresponding to a temperature rise of about 50 K. Because of the low level of illumination it was not possible to follow exactly the same path from one scan to another and the radiation level fluctuated, but it was invariably two or more times that of the surrounding area. The only transit across Eratosthenes produced negative results, as did a large number of craters in the vicinity of Tycho and elsewhere. After these scans of Tycho, the standard scan cycle beginning at Aristarchus was repeated, and again the only exceptional features found were Aristarchus and Copernicus. Following these measurements the sky became hazy (first noted at 01:30 PST) and the observations ceased. The three lunar features found to exhibit enhanced radiation are all rayed craters, although a representative group of other features were scanned, including mountains, seas, and a large number of other craters. The most immediate interpretation of these observations is that the rayed craters are covered by a thinner dust layer, but other interpretations are open, such as vulcanism, radioactivity, and emissivity variations. The data are

5 LUNAR RADIATION DURING ECLIPSE 485 not in disagreement with the assumption that these rayed craters are among the younger of the lunar features. Further thermal radiation measurements, preferably of a differential nature, may add substantially to our knowledge of the evolution of the moon. Thermal measurements during eclipses are of greatest value as a result of the exaggerated manner in which a slight anomaly manifests itself, but it may be possible to obtain comparable measurements at the terminator during a lunation. Detailed maps of the transient temperature excesses may furnish further information on the lunar surface structure, and temperature studies of the faults, craters, mountains, and seas may be of value in the determination of their relative (or absolute) ages. Low-noise instruments capable of improving these measurements by several orders of magnitude are available. The writers wish to express their gratitude to the staff mem- bers of the Dominion Astrophysical Observatory for their assist- ance in the planning and execution of these observations E. Pettit and S. B. Nicholson, Ap. J., 71,102, E. Pettit, Ap. J., 91, 408, P. S. Epstein, Phys. Rev., 33,269, A. J. Wesselink, B.A.N., 10,351,1948 (No. 390). 5 J. H. Piddington and H. C. Minnett, Aust. J. Sei. Res. Ser. A, 2, 63,