Journal of Research of the National Bureau of Standards Vol. 5 1, No, 2, August 1953 Research Paper 2434 Ultraviolet Spectral Radiant Energy Reflected From the Moon Ralph Stair and Russell Johnston R esults are given of some measurements on t he ultraviolet and short-wavelengtll visi,b le s ectral radiant energy reflected from t he surface of the full moon, made from October to ~ rnber 1952 at Washington D , C. Although t he reflected lunar spectrum co ntarn s a ll t h~eF raunh o re r' bands as found in ?irect sunlight .with approximately t he s ame r~ e l aL r ve intensities in t he visible spectrum, In tense ab so rp t r ~ n occur's for some. of the ult raviolet wavelengths. Selective absorption for wavelengths III the spe?tT1!-l regrons of 380 to ~9 0 millimicrons and less t han 360 millimicrons indicates t he pOSSIbility of a lunar re flectrn g surface s imi la r to t hat of powdered glassy silicates. 1. Introduction range incident on th e ~oon . l'!le co ll e~ti~m Iron Man has long speculated abo:ut the I1?-0on- lts and other dark substances may be expected also to orio'in its surface features, and Its path III space. darken the lunar surface. . This interest has stimulated the search for facts The study of the moon through m easurem ent of Its regarding the exact nature and origin of the surface effect upon r efl ected sunlight may ~e .appro~ ch ed panorama visible through the telescope. from several angles: througb changes m mtenslty or It is, however, generally agreed that th e lunar polarization [11 to 14,. inclusive] of the. re.flected urface features have been sculptured by cata- radiant enerO'y as a functIOn of the angle of lllCldence, trophic agents (either meteoric ~r volcanic, or both ) or through ellanges in the l:eflected spectrum. caused [1 to 10, inclusiveJ.l The resultmg surfaee featm'es by the lunar sur!aee, ThIS report d .e~l s WIth th e differ greatly from anything on the ear th , except fo~ ' a integrated ultraVIOlet ~p e~tI'al mtensltlCs re ~l ecte d certain resemblanee to the few known tel'l'estn~l for the fixed angular .m Cldences cOl'l'espol"!-dmg to meteor craters. Because gravity on the moon IS near full moon and WIth the moon neal' ItS most only about one-sixth that on the earth and th ere northern posit ion in the sky , . . . is an absence of an atmosphere, the lunar craters The observed relative spectral dlstl'lbutlOn of a re probably 25 to 50 times as large [l1J as would ultraviolet radiant energy reflected from the moon is result on the earth. verv similar in quali ty to that emitted by the s.un As the result of the absence of an atmosphere il:nd itseH. All the Fraunhofer lines appeal' and WJ th moisture and, h ence, of the usuil:l types of weathermg approximately the same relative intensities. A.ny and erosion the moon has retamed records of many differences in the two spectra result from solectlVe of its early catastrophic experience . D,uring its op tical a,bsorption by the lunar .surface. Except ~or history about 16 times as ma:ny m eteontes have a slight yellowing of the lun~r .lillage and for var~a­ collided with the earth , but thOl[' records have been tions in the ultraviolet and VISIble spectrum over ItS largely erased [1], unless we conclude. that t he surfaee features [20], no thi:ng has been r el?o.rted in encounters with the larger ones resulted m som e of the available literature no tmg any other chfIerences the geologic transitions indicated by abrupt changes between the two spectra . between certain layers of the earth 's strata. The surface of the moon does change, however. 2. Instruments and Procedure It is affected b y the sun's rays, by gravity, and by The apparatus employed in this iu-yestiga:tion t idal forces by the temperature change from about consists of a Carl L eiss double quartz-pnsm mnTor 250 0 F [1 5 'to 18, inclusive] during the luna:r .day to spectrometer, using ~n R CA 1~28 photomultiplier about - 150° F during its night, and by attl'ltIOr,t ~u e as a detector. The lIght b eam IS m ~ d~lla~e d at. 5] 0 to falling meteorites, estimated abt. over one dmillron cis, and the ou tput of the photomultrplter IS fed mto pel' day [2]. These eff ects com me . to pro uce a a tuned amplifier [21] and recorder. (see fig. 1). A pulverization of the surfac.e l ~yer, whICh acts as the siderostat was employed for reflectmg ~he beall?- of effi cien t insulating surface mdICated by the character li O'ht from the moon into the spectroradlOmeter m a of the temperature changes on the moon's surface ~anner similar to that previously used with SUll- ob erved during solar eGlipses [1 9]. light [22] . . , The blackening of the old surface areas, or "maria", N o condensmg lens or mu'l'Ol' was employed, so may b e due in part to exposure of the surface ;ma- that the resultant m easurem en t was that for the terials to high frequency (short wavelength) radIant integrated surface of t~ e. moon. The s pe ctr~l­ energy. Oxygen, ozone, and other components of energy-response .charactel'.lstIC of the complete ll"!-­ the earth 's atmosphere act as a blanket that prevents strument mcludmg the sIderostat and photomultl­ the ear th from receiving ultraviolet and other solar plier, wa~ determined by using a special tungsten­ radiant energy of wavelengths shorter than about filament-in-quartz lamp [21 , 23], together WIth a 295 mil· Laboratory experiments show that many numb ~r of optical filters to reduce the lamp energy crystalline and glassy substances 0:r: tJ:.e earth darken for the various parts of the spectrum (see fig. 2) to upon exposure to wavelengths wlthm the spectral values approximating that of the moonbeam at the

I F igures in brackets indicate the literature references at tbe end of tbis paper. speetrometer slit, The radiant energy from the 81 lamp was reflected into the spectrometer by the quite similar (sec fig. 5), although different spectro­ same siderostat mirrors, so that the spectral enel'g ~' radiometers were employed. calibration for the moonbeam reduced to a simple com­ Measurements were made during four nights near parison of the recorder indications in the two cases. the ends of October and November 1952, when the The high sensitivity of the detecting and l'ecording moon was near its full phase and also near its maxi­ equipment permitted the use of relatively nanow slit mum northern position, hence near its highest alti­ widths (spectral width approximately 1 m}.! at 310 m}.! tude at the latitude of Washington, D . C. The and 2 to 3 m}.! at longer wavelengths) . These values best data were obtained dlll'ing the night of Novem­ are comparable to those employed in previous worl.;: ber 30- December 1, when the moon was not onl.v with sunlight [24], so that the Fl'aunhofer structlll'e nearest its full phase but was also at the highest of the measured radiant energy in the two cases is altitude for any night during the series of measure­ ments. Also , the atmosphere was entirely free of clouds and showed least dust or haze scattering on A SIDEROSTAT ~ SPECTROMETER PHOTO- T UNED this night. Data on the lunar altitude and air mass ~ ------MULTIPLIER AMPLIFIER for the four nights were calculated in the usual MOON manner by means of the celestial triangle through I I the use of the pertinent data published in the SECTOR HIGH RECTIFIER American Ephemeris and Nautical Almanac for 1952 DISK VOLTAGE AND for the solar and lunar positions. The resul ting SUPPLY FI LTER data are charted in figure 3. I I 3 . Spectral Radiant Energy Reflected From RECORDER\ the Moon The spectral radiant energy reflected from the FICURE 1. Instrumental layout. moon depends upon the optical and other physical characteristics of the lunar sllrface. Changes in the § 20 solar radiant energy are admittedly small [25] for b 18 all wavelengths penetrating the terrestrial atmos­ .... phere. Variations at the moon as a function of time o,., 16 may be considered insignificant. In view of the E o 14 fact that previous observers have found a marked ..' variation in the light reflected from the moon as a ~ 12 L AMP x!.M function of the angles of incidence and reflection ~ E 10 [11, 15], a similar behavior might be expected for the 8 WW 6 X 10 ultraviolet rays. However, at present the amount 0:~ W of this effect is unknown. Furthermore, variations ~ 6 in ultraviolet intensities over the lunar surface are I- ~ 4 known to be appreciable [20]. Future studies for is WW 8 X 10 specific areas of the moon and at various angles of ;2 2 incidence and l'eAection should be interesting and ~0~0~~~~~~~~42~0~~4~60~~5~070 ~~5~4~0~~5~eo~ informative. WAV ELENGTH. MILLI MICRONS T errestrial atmospheric absorption further modi­ FICURE 2. Spectral energy distribution of the standard lamp fies the lunar reflected radiant energy. M ean spec­ through the filters used in the calibration of the instrument. tral values (for ascending and descending moon) for AIR MASS a lunar altitude of 65 degrees (air mass l.1O) are 90 r-~-.----':--.--"---r-.----,--.-"---r-T 1.00 given in figure 4. -When spectral radiant energy data are taken over a range of ascending (or descend­ 80 MOON, WASH INGTON , D.C •• 1952 ing) positions of the moon and plotted logarithmi­ 70 cally [24J as a function of airmass and extrapolated ." ... 1.1 0 to airmass equals 0, the intercepts represent the 0: 60 co logarithms of the spectral intensities outside the ... 1. 20 a terrcstrial atmosphere. The data illustrated in ·50 1. 30 ...a 1. 40 figure 5 were obtained in this manner. ::> ... 4 0 1. 50 In order to illustrate better the similarity between 0: the spectral radiant energy refl ected from the moon

WASHINGTON,O.C., 195Z. - SUN, CLIMAX , COLORAOO 180 SEPTEMBER 1"1

MOO N AIR MA SS · I.IO NO V 30 -0E C I ~ ·:f,lO MOO N, WA SH IN GTON, D.C. _ r NOVE MBER 19'2 ~.

~ 90 SC ALE RAT IO ' 1/300000

~ :i 60 :;; 30

o 0 310 :no 3' 0 370 390 4 10 430 "50 470 4 90 510 530 glo 330 350 370 390 410 430 450 4 70 490 5 10 530 WAVELENGTH , MILLIMICRONS WAVELENGTH, MILLIMIC RONS FLGUlm 5. A compmison between the spectml distribution of F I GlJ R ~~ 4. Spectral d-is tT-i bution of the radiant energy Te./lee ted the radiant eneTgy Fom the sun and the Te./lected en el'gy from from the moon. the moon. s uIt in greater scattering of the data plotted in IOr--r--'--'--'--.---.--r--r--.--.--.-~--.--. o _2.-JJ----6-~-- figure 6. 9 An inspection of the lunar relative to the . solar ~~6.J>-O- radiant energy curve (fi g. 5) discloses I!reater difl'er­ 8 0 / 9- ... ences between the two toward the shorter wave­ o 0 ' 7 ~6~o..?- ~ ... o lengths. A quantitative plot of this ratio (fig. 6) qo 0 0 gives the relative spectral reflectivity ot the lUl:ar 6 cf~~ s urface. Much of the scatter of the data, as 111- )cb dieated above , res ults from slight differences in the 5 8 dispersions of the two spectroradiometers, inasmuch 4 I as a close inspection of the individual plotted points 10 C\i closes that the higher values resul t from ratios be­ 3 I " t~\ - een peaks on the cur ves, whereas the lower values arc associated with the Fraunhofer absorption bands. 2 Three importan t characteristic of this reflectivi ty c urve are worthy of no te. First, th e curve decl'NtSeS :in ordinate value with wavelength , thus indicating gO~O~~23~4~O~~3~8~O~~4~2~O--~~46~O~~~5~O~O~~5~4~O~~5~80 t hat, the lunar s urface h as a lower ref-te ctivitv for WAVELENGTH. MILLIMICRONS t he shorter ·wavelengths. Second, the ):land at, 380 FI GlJRIi] 6. R elative spectral reflectivity of the moon. to 390 m,u indicates selective absorption of the lunar surface materials. Third, th e sharp cuLoff beginn.ing measurements of the albedo of Lhe moon are not at about 360 m/-! ma.v be considered indicati ve of some precise, most of them faU below about 12 percen t. special composition. H ence, the surface of the moon may be composed, If the thTee special characteristics of the lUJl ar at least in par t, of powdered glassy silicates. In­ r efl ectivity curve are considered in terms of possible eiden tally, tbe high percen tn,ge of 8i02 in the earth 's materials present, and ot.he1' Imown factors about crust might suggest the possibility of a terrain the moon , such as its albedo, polarization, and heat similar to that of the moon had not air, vvater, conductivity are kept in mind, it appears no t un­ erosion , etc., been present. likely that a yellowish glass-like composition could The observations on polarization at the s urface of be responsible for the observed phenomena. Certain the moon b y Wrigh t [12] indicate refi ection by a fine silica glasses [2 6] have an ultraviolet cu toff corre­ texture and " point to pumiceous substances high in spon.ding closely with the observed CUl'\Te. In a silica, to powders of transparent substances and to splintered or crushed form they would refi ect a quartz porphyries and possibly to trachytes and m easurable amount of radiant energy after trans­ granites as the materials w e see at the moon's sur­ miLtance through an appreciable thickness of ma­ face." Similarly, the relative spectral refi ectivity terial. A small iron content would result in selective curve for the moon obtained in the present investiga­ absorption at 380 to 39 0 m,u and would give the tion points to the possibility that the surface ma­ material a slightly yellowish color. A pulverized terials are, at least in part, composed of powdered glassy silicate lunar surface would ?e highly insulat­ glassy silicates. Further refinemen ts in the observa­ ing and would produce characten stlCs compatIble tions of this interesting satellite are needed , however, with Lemperature measuremen ts obtained during before definitive conclusions can be drawn. eclipse and with lunar phase changes [15 , 16, 19]. The low average albedo [7 , 27 to 30, inclusive], 4. Atmospheric transmittance and ozone' about 7.3 percen t, corresponds closely to the ex­ The atmospheric transmission curve depicted in pected refi ectivi ty from glassy 111aterial. Although figure 7 is plotted in the usual way in terms of the 83 ------_.

ultraviolet spectral intensities. Preliminary tests 9.S already made of weak fluorescent sources, reflections WASH INGTON, D. C. from dull surfaces, and of radiant energy from small 9 .6 NOVEMSER 1952 sky areas (even during rainfall or after sundown) w ..~ 9.4 indicate a wide range of possible application for the l- I- 9.2 equipment. 6. References ~ If) ~ 9.0 [1] Ralph E . Baldwin, The face of the moon (University of a: Chicago Press, Chicago, Ill., 1949) . I- 8.S [2] L . J . Spencer, Meteorites and craters on the moon, o (!)- Nature, 139,655 (1937) . 3 S.6 [3] R. A. McIntosh, Origin of lunar features, J. Roy. Astro­ !lorn. Soc. Can. 37, 24 (1943). S · ~1'"::-0---:3~2-:-0--:::33~0--:3::-4""0---::-35:!cO:--::C3~60,....-Jc..,3::-:8""0-'-4""0""'0""4~20""'"450~-=5~00,,--,-l600 [4] S. J. Hacker and J. Q. Stewart, Lunar ray craters, WAVELENGTH, MILLIM IC RONS Astrophys. J. 81, 37 (1935). FIG URE 7. Atmospheric transmittance (from moon data). [5] Allan O. Kelly, The geology of the moon, Popular Astron. 55,530 (1947) . logarithm of the observed transmittances of unit [6] A. C. Gifford, The mountains of the moon, New Zealand atmosphere (at Washington) for the different wave­ J. Sci. Techno!. 7, 129 (1924). [7] Henry Korris Russel, Raymond Smith Dugan, and John lengths as a function of the wavelengths. This, in Quincy Stewart, Astronomy (Ginn & Co., Chicago, turn, is expanded [31] according to the function Ill. , 1926). - (,u- l )2}. -~ of the Rayleigh law of molecular scatter­ [8] Robert H. Baker, Astronomy (D. Van Nostrand Co, New York, N. Y., 1950). ing, [9] Fred L . Whipple, Earth, moon and planets (Blakiston Co., Philadelphia, Pa., 1941). [10] W. S. Krogdahl, The astronomical universe (Macmillan Co., New York, N . Y ., 1952). in which }. is the 'wavelength of the radiant energy, [11] F. E. 'Wright, The surface features of the moon, Sci. and ,u is the index of refraction of the atmosphere. Monthly 40,101 (1935). Since, for the zenith position the atmospheric [12] F . E . vVright, Polarization of li ght reflected from rough depth, Fl, and the molecular density, N, arc constant, surfaces with special reference to light reflected by the moon Proc. Nat. Acad. Sci. 13,535 (1927). the resulting plot of the logarithm of the atmos­ [13] F . E. vi~right , The surface of the moon, Carnegie Inst., pheric transmittances becomes a straight line in Wash., Pub. Ko. 501, p. 59 (1938) . those spectral regions wherein the Rayleigh law of [14] B . Lyot, Etude des surfaces planetaires par la polariza­ pUl"e molecular scattering is applicable. In as much tion, Compt. rend. 177, 1015 (1923). [15] Edison P ettit, Lunar radiation as related to phase, as appreciable ozone absorption occurs only at Astrophys. J . 81, 17 (1935). wavelengths shorter than about 330 m,u, the data [16] Edison Pettit and Seth B. Nicholson, Lunar radiation and herein recorded are inadequate for use in ozone temperature, Astrophys. J. 71, 102 (1930) . [17] Donald H . Menzel, vVater-cell transmissions and plane­ determinations. Between 300 and 330 m,u (fig. 4), tary temperatures, Astrophys. J . 58, 65 (1923). the observed intensities were extremely low and the [18] W. vI'. Coblentz, Further tests of stellar radiometers and instrumental noise levels relatively high. With some measurements of planetary radiation, BS Sci. certain improvements in the equipment, it is hop cd Pap. 18,535 (1922) S460. to reach sensitivities adequate for use of the appa­ [19] Edison Pettit, Radiation measurements on the eclipsed moon, Contr. Mt. Wilson Obs. No. 627, 26, 165 (1940) . ratus in ozone determinations at night. As an [20] R. W. Wood, The moon in ultraviolet light, and spectro­ alternative,the use of a condensing lens or mirror selenography, Popular Astron. 18, 67 (1 910). may be advantageous in supplying sufficient radiant [21] Ralph Stair, Photoelectric spectroradiometry and its energy from the moon for this purpose. a,pplication to the measurement of fluorescent lamps, J . Research NBS 46, 437 (1951) RP2212. S. Summary and Conclusions [22] R. Stair and W. O. Smith, A tungsten-in-quartz lamp and This report presents the first observed ultraviolet its application in photoelectric radiometry, J . Research NBS 30, 449 (1943) RP 1543. photometric CUl"ve of moonlight from data obtained [23] Ralph Stair, Ultraviolet spectral distribution of radiant primarily on a single .night, although measurements energy from the sun, J . Research NBS 46, 353 (1951) were made on three additional nights when the moon RP2206. was near its full phase and at high altitudes. De­ [24] Ralph Stair, Ultraviolet radiant energy from the sun observed at 11 ,190 feet, J . Research NBS 49, 227 spite the fact that the measurements were made (1952) RP2357. through the dense blanket of atmosphere over a sea­ [25] Donald H . Menzel, Our sun (Blakiston Co. , Philadelphia, level station, interesting information was obtained Pa., 1949). having a bearing on the composition of the lunar [26] Ralph Stair, Spectral-transmissive properties and use of eye-protective glasses, NBS Ci rcular 471 (1948). surface. Further nighttime measurements at higher [27] Henry Norris Russell, On the albedo of the planets and altitude stations with improved equipment should t heir satellites, Astrophys. J . 43, 173 (1916). result in additional information not only on lunar [28] G. Rougier, Photometric comparison of moon and sun, reflectivity but also on ozone concentration. Photoelectric albedo of the moon, Compt. rend. 202, 463 (1936). The extremely high sensitivity of the equipment [29] Edison Pettit, The co-albedo of the moon, Contr. Mt. lends its usefulness in other fields of research, in Wilson Obs. No. 705, 28, 173 (1945). particulal' to stellar investigations. With telescopic [30] Handbuch d el' Astrophysik 2, part 1, Verlag von Julius magnification sufficient radiant energy from many Springer, Berlin, 1929) . [31] Edison Pettit, Spectral energy-curve of the sun in the stars, and also from small areas on the moon should ultraviolet, Astrophys. J. 91, 159 (1940) . be available to permit precise determinations of WASHINGTON, May 15, 1953. 84