A SUSPECTED PARTIAL OBSCURATION OF THE FLOOR OF ALPHONSUS. Dinsmore Alter Griffith Observatory, Los Angeles, California

Transcription

1 A SUSPECTED PARTIAL OBSCURATION OF THE FLOOR OF ALPHONSUS Dinsmore Alter Griffith Observatory, Los Angeles, California Visual observations of the moon have been reported from time to time in which an area is hazy enough that well-known markings cannot be observed, despite the fact that adjacent areas show all known features quite sharply. Some of these observations, at least, have been made reliable and skillful observers. In general the observers have interpreted them as indicating a temporary local atmosphere. Usually they have been reported under a very low sun. Beginning in April 1954, systematic lunar photographic observations have been undertaken at the Cassegrain focus of the Mount Wilson 60-inch telescope in an attempt to secure evidence concerning the existence or nonexistence of such obscurations. Of course, evidence of nonexistence would be merely statistical. If, within a crater of the moon, there should exist a thin atmosphere of sufficient density to produce some obscuration of the surface, two facts necessarily would apply to observations : ( 1 ) The effect should be much more noticeable under a low sun than a high one. This is true because any scattering of light varies little with the altitude of the sun. The illumination of the crater floor, however, is a trigonometric function and decreases very rapidly with the approach of the sun to the horizon. The same amount of scattering, therefore, near sunrise or sunset would produce a greater effect on the visibility of markings on the floor. (2) If plates are exposed almost simultaneously in blue-violet and in infrared light, the scattering effect should be much stronger on the blue-violet plate than on the infrared plate. Table I gives data for a series of plates of the region of the craters Arzachel and Alphonsus taken under favorable seeing conditions. Blue and infrared exposures were alternated, and the plates of a pair were exposed in quick succession. These eight plates are reproduced in the present paper. No filter was used with the II-O plates, while a Pyrex CS7-69, CG 2600 was used with the I-N plates. The Pyrex filter 158

2 OBSCURATION OF ALPHONSUS 159 TABLE I Plate Pacific Standard Time Type of Plate I II III IV V VI VII VIII Oct. 26,1956 3h 59m 50s Kodak Spectroscopic II-O I-N II-O I-N II-O I-N II-O I-N has its 10 percent cut-off at À 7200 at the short end. Its maximum transmission is at À 7650 and it transmits 35 percent even at À 10,000. The II-O plates all were exposed 0? 25, and the I-N plates 1? 20. All were developed 5 m in D 19 at approximately 70. Basic seeing was 4 on the Mount Wilson Observatory scale. The earth s atmosphere complicates the problem very much. The effect of the unsteadiness of our air is far greater for the short waves than for the long waves, especially at times of average or of less than average seeing. When our air is unusually steady, the difference between plates exposed to blue-violet and to infrared lessens, so that if they could be exposed under the rare conditions when seeing is 6 or 7, one plate would be almost as good as the other. This is illustrated quite well by Plates VII and VIII. The basic seeing at the time was marked 4 +, but at the instant when Plate VII was exposed, it must have been much better. The detail in the crater Arzachel {upper) is excellent enough on Plate VII that one must take a second look when comparing it to Plate VIII before one notices any inferiority. The blue-violet Plate VII is almost as good as Plate II, the poorest infrared plate of the series. Another instant of seeing which must have been better than 4 was that at which the infrared Plate IV was exposed. This plate perhaps shows more detail on the floors of Arzachel and of Ptolemaeus than does any other photograph that has been made. Unfortunately it was impossible to reproduce more than a small section of Ptolemaeus here and still retain the desired large-scale reproduction. In the early morning of October 26, seeing conditions became unusually good. The basic rat-

3 PLATE I Blue-Violet Light

4 PLATE II Infrared Light

5 PLATE III Blue-Violet Light

6 PLATE IV V i Infrared Light

7 PLATE V Blue-Violet Light

8 PLATE VI Infrared Light

9 PLATE VII Blue-Violet Light

10 PLATE VIII Infrared Light

11 160 DINSMORE ALTER in g was 4, and from that rating the seeing fluctuated upward for short intervals of time. The unsteadiness of the air follows various patterns, and that morning it consisted of very short waves in exceedingly rapid oscillation. In each pair of plates the comparison should be made between the superiority of the infrared over the blue in the western sections of Arzachel {upper) and of Alphonsus {lower). In each of these craters the infrared reveals a rill in the western side of the floor. (South is up and west is to the left.) In Arzachel this rill shows plainly in all eight photographs. In the crater Alphonsus the result is different. There each blue photograph shows much less detail than can be seen in its infrared mate. This holds even for the pair VII and VIII. The dark spots seen on the floor of Alphonsus are the famous black spots. Each of them has one or more tiny craters within it. For some reason the blue-violet photographs lose more detail in the west side of the floor of Alphonsus than they do in the floor of Arzachel. This is not true of the infrared ones. Unfortunately there is only a short time during each month when such comparisons can be made. When the rising sun is low, the shadows of the western walls extend beyond the rills. When the sun gets higher, it reaches the bottoms of the shallow rills and they are difficult to observe on any plates. Only a low setting sún leaves their area in sunlight, but presents them with black floors. There is a temptation to interpret these results immediately as being due to a thin atmosphere, either temporary or permanent, over the floor of Alphonsus. The theoretical difficulties inherent in such a hypothesis are, however, strong enough to forbid a wholehearted acceptance of it. Theoretically there must be a thin atmosphere on the day side of the moon. The heavy gases which would not escape from the moon are those which constitute approximately 1/10,000 of the earth s atmosphere. These gases, of course, must freeze at night. Under the laws of diffusion, they would not remain markedly in deep pockets. The pressure gradient on the moon is much less than it is on the earth. A thin atmosphere that has remained from ancient leakage should not exhibit a differential effect between adjacent craters. A current slow

12 OBSCURATION OF ALPHONSUS 161 leakage of gas from some of the small craters on the moon would explain the effect that has been observed. Such a leakage, however, would not be expected on the lunar surface and, therefore, its existence cannot be accepted as proved until all other possible explanations of the photographic effect have been examined. Color effects have been suggested for such observations as this, and proper choice of different colors for the floors could be made to produce these results. It is not believed, however, that anyone who has made any long series of lunar visual observations would be willing to accept the reality of an explanation by color difference. This is doubly true because of the fact that the principal detail that has been obscured in each is a rill and colorless. Another explanation that cannot be ruled out is that the difference is due to the dissimilarity of the floors of the two craters. The floor of the western part of Arzachel is rougher than is the corresponding part of the floor of Alphonsus. Therefore, it will exhibit more of strong contrast because of the additional amount of shadow. This objection is answered partially by the fact that the comparison is made between rills in the two floors. It appears probable that the rills are equally dark and that, therefore, the differential effect would not exist for them unless there is a true obscuration. Indeed, a lack of other contrasting detail in Alphonsus might even cause the rill to be seen more easily. Nevertheless, this explanation is a possible one and some additional form of evidence is needed before it can be rejected completely. All that can be stated definitely at present is that the series of photographs demands a more intense search of this and of other areas when near the terminator. I wish to express my thanks to Dr. Ira S. Bowen, Director of the Mount Wilson and Palomar Observatories, for permission to use the Mount Wilson 60-inch reflector for lunar observations during the past two and a half years.