The Wavelength Dependence of the Albedos of Uranus

Home , Geometric albedo

THE ASTROFHYSICAL JOURNAL, 184:1007-1016, 1973 September. IS O 1979. Tat Amnfkiw Aiuonoiniul Society. AM righu retcrved. Printed in U.S.A.

THE WAVELENGTH DEPENDENCE OF THE ALBEDOS OF URANUS AND NEPTUNE FROM 0.3 TO 1.1 MICRON WlLLEM WAMSTEKER* Spice Sciences Laboratory, NASA, Marshall Space Flight Center, Alabama Received 1973 February 7 ABSTRACT Narrow-band photoelectric photometry was made of Uranus and Neptune over a wavelength interval from 0.3 to 1.1 /*. The wavelength dependence of the geometric albedo was determined for these planets. Evidence ii given that the comparison star used resembles the Sun very closely in its energy distribution. It is shown that, apart from methane, another opacity source seems to be necessary in the atmospheres of these planets to explain the observed wavelength dependence of the geometric albedo for the two. planets simultaneously. Radiative transfer calculations were made to determine if the previously, suggested pressure-induced dipole absorptions of Ha result in a sell-consistent explanation. This seems to be the case. The Ha abundance in this case is limited for both planets between 350 km amagat s Af(Ha). < .800 km amagat. This agrees with a previous determination of the Ha abundance for. Uranus. The relative CH, abundance is determined from the observations to •be Ata(CH4)/Aty(CH«) > 1; the actual value depends on the saturation in the absorptions. This does not agree with previous determinations. Subject headings: atmospheres, planetary — Neptune — photometry — Uranus I. INTRODUCTION In the course of a narrow-band photometric study of tie reflecting properties of the brighter planets and satellites, observations were obtained of Uranus (,•,) and Neptune (^).'The'observations will be described and some comments made on the calibration of the standard star used for accurate compensation of the solar energy distribution in the albedo spectra of the planets. Spectral absorptions observed in Uranus and Nep- tune have been identified with GH« (Wildt 1932; Dunham 1933) and Ila (Hcr/.berg 1952). Herzberg also concluded that the presence of large amounts of helium must be considered very likely. The low temperature expected to dominate in these atmospheres makes the presence of other gases rather unlikely. Various models have been proposed for the Uranus atmosphere. The reflecting properties of a cloudless semi-infinite atmosphere have been explored in some detail by Belton, McElroy, and Price (Il)71). hereafter referred to as BMP. Sinton (1972) concluded that the observed limb brightening at A = 0.887 /i for Uranus requires the presence of a thin haze high in the atmosphere in addition to Raylcigh scattering. A similar conclusion was reached by Hinder and McCarthy (1972), based upon their observations of the infrared albedo of Uranus. •From the limb darkening observed for Uranus in a band 0.40> < A < 0.60 /i, with the Stratoscope II balloon telescope, Daniclson, Tomasko, and Savage (1972) concluded that a cloud deck should be present on Uranus. However, their data do not agree.with a high haze, but seem to require the cloud level below a finite Rayleigh scattering atmosphere (r = 0.5). We will attempt to sec if the present observations allow a decision between these various propositions. Also, the CH, abundance for Uranus relative to that of Neptune will be determined.

II. OBSERVATIONS AND CALIBRATION the observations of Uranus were made around the oppositions of 1971 and 1972; the observations of Neptune were made around ths 1972 opposition. The photometric * NASrNRC Postdo.it i Resident Research Associate. 1007

83 1008 WILLEM WAM3TEKER TABLE 1 BROAD-BAND MAGNITUDES FOR 35 LEONTS

V B- V U- B V- R R- I

5.980. ... +0.646 +0.206 +0.570 + 0.294 system, which is described elsewhere (Wamslek:r 1973, hereafter called Paper D, gives a spectral resolution A/AA — 30 over a wavelength interval extending from C.30 to. I.I /x. This is obtained by narrow-band interfeience filters The observalions were made with the 1.5-m and the 1.0-m telescopes a the Catalina Observatory of the University of Arizona. The standard atmospheric extinction coefficients, derived for the calibration of the photometric system in 1970, vere checked. A redetermination of these coefficients was not necessary. " The procedure d"?cribed in Paper I resulted in a reliable calibration of the photometric system. It was, however, considered useful tc obtain an independent check on this calibration. Therefore, broad-band observations in the Arizona UBVRI system (Johnson, 1965) were obtained by Lee of the prim;- y solar standard of the narrow- band system, 35 Leo (HR 4030). Table I lists the broad-band magnitude and colors measured for this star. The standard deviation for the UBVcolors is ~ 1 percent and for the RI colors ~2 percent. Since the broad-band system is calibrated (Johnson 1965), it is possible to obtain from the observed colors a brqadtband absolute energy distribution." Figure 1 shows the solar energy distribution as determined by Labs and Neckel (1968), the energy distribution of 35 Leo (Paper I), and the broad-band data for 35 Leo. Since these three data sets are completely independent, the good agreement indicates that the calibration of the narrow-band system is reliable"arid that the energy distribution of 35 Leo is, at the resolution of the narrow-band system, very similar to that of the Sun. Thus, 35 Leo is very well suited for investigations concerning the nature of the albedo spectra of solar system bodies

III. THE GEOMETRIC ALBEDO To derive the geometric albedo from the observed magnitude of a planet at zero phase and unit distance, m(l, 0), a solar magnitude Vm = —26.74 was used (Johnson 1965): The radii used to derive the geometric albedo were for Uranus'/?-,' = 25,900 km (Danielson et al. 1972) and for Neptune Ry == 24,600 km (Bixby and Van Flanderen 1969). . . To compare the narivW-band measurements with the observations of Harris (1952) and Appleby and Irvine (1971), the narrow-band observations; were transformed into m(l, 0) at the appropriate wavelengths of the other two investigations. Table 2 lists

TABLE 2* PHOTOELECTRIC MAGNITUDES or UKANUS AND NEPTUNE

URANUS m(l, 0) NEPTUNE m(l,0) COLOR Harris A + I LPL Harris A LPL

U -6.35 -6.33 -6.30 -6.25 -6.26 -6.16 B -6.63 -6.61 -6.57 -6.46 -6.45 -6.41 V -7.19 -7.12 -7.06 -6.87 -6.90 -6.85 R -7.04 -6.86 -6.54 -6.60 . /..... -6.24 -6.19 -5.74 -6.04

• Harris, Harris (1952); A + I, Appleby and Irvine (1971); A, Appleby (1973); LPL, this paper. 2.2

2.0

1.8

1 1.6 SJ *E o 1.4

1.2

CONST . C 1.0 CO X

0.9 1.0 1.1

FIG. I.—Flux distribution of 35 Leo compared .with the Sun. Dotted line, flux distribution of the Sun from Labs and Neckel (1968); solid line, flux distribution or 35 Leo derived in Paper I; heavy dots, broad-band magnitudes of 35 Leo in the UBVRI system (Johnson 1965). The constant in the ordinaie allows Tor the apparent brightness difference between 35 Leo and the Sun. ' - • 00 T

FIG. 2.—The geometrical albedo-wavclength-dependence of Uranus (y»j) and Neptune (Py), a. derived from the observed ratio ,FA(plane0/f;i(35 Leo). The error bars indicate the average deviation. The shortest wavelength is extremely uncertain. Shown as dotted lines are the narrow-band geometrical albedos from Appleby and Irvine (1V71) for Uranus and from Appleby (1973) for Neptune. Note the different P = 0 levels for Ihe two planets. ALBEDOS OF URANUS AND NEPTUNE 1011 the data for both Uranus and Neptune. Although Uranus seems to show a systematic decrease in brightness since 1952, the reality of this is doubtful. The determination of m(l,0) has, from all Uranus data combined, an accuracy of ±0.02 mag in one observation. Since the spectrum of these planets for A > 0.50 p is rather strongly disturbed by the absorptions of methane (see fig, 2), a comparison at these wavelengths is rather sensitive to the effective wavelengths of the photometric system. Therefore, one must conclude, since the deferences in the U and B band do not exceed the above derived mean error in one observation, that these photoelectric observations do not indicate any variation in the brightness of Uranus. In the narrow-band system of Appleby and Irvine (197;) the difference between the Boydcn data and the present observations gives rise to a standard deviation o(LPL - Boyden) = 0.01 mag. This comparison does not include the //-band in Appleby and Irvine's data, since this filler is very narrow and is located on a strong CH, absorption. For Neptune, the comparison between the data of Harris (1952), Appleby (1973), and the present data also docs not indicate any variability. In figure 2 is plotted the geometric albedo as a function of wavelength for both Uranus and Neptune (N.B. the different P «• 0 levels). At some wavelengths, error bars are given to indicate the accuracy of observations. These bars give Zn ICoba - ^«v.iw)l/«- The measurements at A = 0.30 p are extremely uncertain and included only for completeness. Also shown are the narrow-band geometric albedos for Uranus and Neptune as determined by, respectively, Appleby and Irvine (1971) and Appleby (1973).

IV. DISCUSSION

The wavelength dependence of the geometric albedo is very similar for both planets. For 0.30/x < A < 0.50 /i, the spectrum is rather flat and the values of the geometric albedo are rather high in this interval. The calculations by Wallace (1972) for scmi- infinile Rayleigh and Raman scattering indicate that in this region of the spectrum Rayleigh scattering is possibly the process determining the optical depth penetrated by the reflected light. For longer wavelengths (A > 0.5 /i), the geometric albedo de- creases considerably and the spectrum begins to show strong depressions caused by the CH4 absorption bands. Two reasonable explanations for the general decrease toward longer wavelengths are possible. First, the presence of the strong CHt absorptions in the spectrum leads to the suggestion lhat overlapping of'methane bands of increasing strength and spectral density will also depress considerably the continuum outside the band centers. In their discussion of the Uranus atmosphere, BMP derived from" the 5(0) and the 5(1) lines of the 4-0 vibration-rotation band of Hz, a molecular hydrogen abundance of ~ 500 km-amagat in a vertical column on Uranus. This led to the second suggestion—namely, that apart from ihc-CH4 absorptions, the short- 1 wavelength wing, of the pressure-induced dipole absorption of H=(y0 = 4160 cm" ) might also contribute considerably to the atmospheric opacity for Uranus. BMP followed McTagg'art and Hunt. (1969), who found the best agreement with the observed line shape when the absorption in the shorl-wavclcngth wing varied as (y —. v,)-.*•*', extending toward shorter wavelengths as far as v = 3i'O for the pressure- 1 induced dipole absorption which *'iey studied. For the v0 = 4160cm" band, this would extend until A = 0.83 /i. In the next section, it will be argued that the methane absorptions alone cannot explain the observed properties of Uranus and Neptune. After that we will attempt to see if the presence of the pressure-induced dipole absorption of H3, as additional opacity source, allows a consistent explanation of the observed wavelength dependence of the geometric albedos of both planets..

87 WILLEM WAMSTEKER Vol. 184

0.4-

0.7 0.8 0.9 1.0 1.1

Flo. 3.—The ratio spectrum of the geometric albedos of Uranus and Neptune. Note the steady increase in the ratio between 0.4 n< A < 0.7 M and the depressions of increasing depth for A > 0.7 p caused by the strong absorptions of methsne on these planets.

a) CH4 Absorptions Alone To permit a better comparison of the geometric albedo spectra of Uranus and Neptune, figure 3 shows the ratio spectrum (PJPy) versus wavelength. Apart from' the filters more or less coinciding with the strong CH4 absorptions, this ratio is nearly always larger than unity. C>nly at.A » 0.81,-.0.86, 0.89, 0.99, 1.03, and l.IO/i is the ratio P^Py less than unity. The filters at these wavelengths cover the.bulk of the stronger methane absorptions. For wavelengths adjacent to the aforementioned, the ratio Pi,lP>v > 1. Kuiper (1950) published spectra of Uranus and Neptune. A visual inspection of these spectra (Plate 111, spectra e and/, Kuiper 1950) seems to confirm qualitatively the observed behavior of the ratio spectrum shown in figure 3 in the overlapping wavelength region. Although the actual values of the ratio spectrum for longer wavelengths, where the geometric albedos are small, arc sensitive to the accuracy of the albedos themselves, there can hardly be any doubt that in the stronger CH4 absorptions the geometric-albedo ratio is considerably less than unity, while outside the absorptions the ratio P/JPy > 1. For X < 0.54 ji, the value of the ratio approaches unity, toward shorter wavelengths, in agreement with the possible domin- ance of Raylcigh scattering at shorter wavelengths. Under the temperature conditions prevailing in the atmospheres of these planets, the curve of growth for methane will hot have a significant Doppler part if the pressure exceeds 0.1 atmosphere (Fink, private communication). Such a low pressure in the. regions penetrated by the radiation, reflected from the planets, in the wavelength, interval under consideration, is highly improbable, if only in view of the H3 abundance derived by BMP. The absence of a significant Doppler part will prevent the curves of growth for CHt for Uranus and Neptune from crossing, and will require that, if one

88 No. 3, 1973 ALBEDOS OF URANUS AND NEPTUNE 1013 CH« absorption is stronger on one planet than on the other, all CH, absorptions for that planet have to be stronger. Then the fact that the strongest CH4 bands give rise to a value of the ratio (Pi/Py) less than unity implies necessarily that the ratio should be less than unity for all wavelengths where the CH« bands are relatively strong, if CH4 were the only opacity source in the atmospheres. Since at A s 0.7 p the geometric albedo of the planets is already more than a factor 2 lower than the value at A a: 0.5/i, the CH4 absorption must be considered to be relatively strong. However, outside the CH4 band at A as 0.72 n the value of the ratio is still larger than1.2. This controversy can, of course, be removed by an adjustment of the radii ofthe planet". However, in view of the accuracy with which these are known— ±400 km—this would require aa unacceptably large change (AJ? > 2500 km); it would also make the ratio in the ultraviolet much less than unity, while Raylcigh (and Raman) scattering should make the planets look quite similar. It thus seems that CH4 absorption alone cannot explain the geometric albedo spectra of Uranus and Neptune at the same time. It is, therefore, reasonable to consider the presence of another opacity source in the atmospheres of these planets to explain the general decrease ofthe geometric albedo at longer wavelengths.

b) The Pressure-induced Dipole Absorption

The short-wavelength wing of the pressure-induced dipole absorption of Ha supplies a possible opacity source needed for the explanation of the wavelength dependence ofthe geometric albedo shown by Uranus and Neptune, as suggested by BMP. To see if this will allow a consistent explanation of the observations, a number of radiative transfer calculations were made for conditions anproximating the situation for these planets. The computations were made with the CDC 6400 computer of the University of Arizona. The program, which was made available to the author by Tomasko, is described in more detail elsewhere (Torr.asko 1973). A very short descrip- tion will be given here. The calculation procedure consists ofthe layer-doubling method described by Hansen (1969). Polarization was not included. Since pressure-induced absorptions give rise to an increase ofthe opacity with depth, this is not sufficient for • the present purpose. Therefore, the Chandrasekhar scattering and transmission functions Sm(r: p,no) and TW{T\ y., y^), calculated by the doubling method, are used for. a layer addition calculation. The addition of a single layer of optical thickness AT allows a variation of the single scattering albedo -m at each AT step. The step size can be chosen to allow a close, approximation of the function W(T) for an inhomogeneous atmosphere. The calculations were made for four wavelengths, A = 0.50, 0.60, 0.80, and 0.90 p. Step sizes for A — 0.50 and 0.60 /x were AT = 0.2, 0.4, and 0.8. The step sizes for A = 0.80 and 0.90 fi were AT = 0.0125, 0.2, and 0.4, because of the very strong decrease of •OT(T) at small values of the optical depth. The calculations included the presence of a Lambertian cloud surface with an albedo varying from. 1.0 to 0.0 in steps of A/4. = 0.1. The addition of layers was continued until the calculated geometric albedo—assuming a Rayleigh phase function—became independent ofthe albedo of the cloud surface. This represents the extreme case of a semi-infinite atmosphere. The atmospheres were assumed to be isothermal at 100° K, in which case BMP's formulae (17) and (19) supply the nr(r) relation. Only Rayleigh scattering and pressure-induced absorption were included; Raman scattering was neglected. The values for the pressure- induced monochromatic absorption coefficient of BMP were used, and the previously mentioned variation with wavelength was assumed. Some pf the results are shown in figure 4. Since the radiative transfer neglects polarization, all calculated geometric albedos in figure 4 are 5 percent higher than the actually calculated values; this should give values which are not too far from the real values (Tomasko, private communication). The lines in this figure represent a smoothed

89 1014 WILLEM WAMSTEKER Vol. 184 0.6

0.5

0.3

0.2 URANUS r(X"05}»0.e ; Ad >0.7 r(X'0,5)« 1.6 J Ac -0.7 0.1 T(X»O.5)«3.2 ; A^."0.8 T(X-0.3)*6.4

0.4

0.3

0.2 NEPTUNE TUPO.S)-O.8 r(X«OJ5)»l.6 0.1 f(X-O5)-2.e r(X>O9>-6.0

0.5 0.6 0.7 0,8 0.9.

Fici. A.—Wavelength dependence of the "continuum level" of'the geometric albedo of Uranus and Neptune (nonmcthane continuum, sec text) drawn through the peaks in fig. 2. Also shown nrc the albedos calculated (increased by 57.) for various optical depths at A.= 0.5 /i [T(A = 0.5)] and "cloud" layers in the atmosphere with a Lambert albedo A«; The calculations were for Raylcigh scattering and pressure-induced dipolc absorption of H3 following a power-law cxtrapoialion, assuming a Raylcigh phase function. The different symbols refer to the situations indicated and are plotted for A a o.5l 0.0, 0.8, and 0.9 p. . continuum drawn oyer the observed pK curves of figure 2. The different symbols illustrate the geometric albedo calculated for a cloud cover with a reasonable albedo at various depths in the atmospheres. The selected sets were required to have a geometric albedo of A = 0.50 p reasonably close to the observed values at this wavelength. The single dots in figure 4 show the results for the semi-infinite case. It is clear that this will result in geometric albedos for both Uranus and Neptune which are too

90 No.,3, 1973 ALBEDOS OF URANUS AND NEPTUNE 1015 low at all wavelengths, especially since the low resolution of the spectrophotometry will cause the continuum to be drawn lower than the actual non-CH4 continuum. The squared dots in figure 4 give the results obtained for a situation as suggested by Danielson et al. (1972^; i.e., a cloud layer with an albedo of 0.7 at an optical depth T(A - 0.5 /i) intermediate between 0.5 and 1.0. This gives rise to geometric albedos which are somewhat unroalistically high at longer wavelengths, even if an increase in the resolution would raise tiiC observed continuum drastically. The circled dots in figure 4 are the best fit obtainable from the calculational grid used. In view of the spectrophotometry of Uranus by Younkin (1970), which had considerably higher spectral resolution, the crosses in figure 4 should represent the true continuum for Uranus and Neptune most closely, in the author's opinion. The fact that the radiative transfer calculations do not predict limb brightening at A = 0.90 n for r(A = 0.5 /<) = 1.6 and AQl = 0.7, while the conditions represented by T(A = 0.5 /*) = 3.2 and Arl = 0.8 do give some limb brightening at A = 0.9, is not a very serious objection. Limb brightening was observed by Sinton (1972) for Uranus in a very strong CM, band. Thus, the calculations, which concern only the nonmcthanc continuum, arc not valid at tlvat specific wavelength. This situation can, however, be considered qualitatively by a decrease of the presumed cloud albedo if the CM,, is concentrated deep in the atmosphere. Then, for T(A = 0.5/t) = 1.6, limb brightening is predicted for /frl <, 0.4. Considering the situation represented by the crosses and the circled dots ir. figure 4 as the two extreme possibilities, one can derive an upper and a lower limit to the molecular hydrogen abundance for Uranus and Neptune. It should be kept in mind that the true value will be closer to the lower limit than to the upper limit, due to the low resolution of the present observational data. For the two planets, the following values were obtained:

Uranus: 350 km-amagat <, A^(Ha) < 800 km-amagat;

Neptune: 350 km-amagal < AV(H2) < 700 km-amagat.

V. THE RELATIVE CH4 ABUNDANCE The arguments presented above indicate that methane is probably not the only opacity source in the atmospheres of Uranus and Neptune. It is then reasonable to, assume that the peaks in the geometric albedo spectra represent the non-CH, continuum at the spectral resolution of our observations. The behavior of the ratio spectrum confirms this assumption. Under these conditions, it is possible to derive from the observed geometric albedo spectra a measure for the equivalent-width ratio of the integrated methane bands (^/HV)CIU.This equivalent-width ratio, which is a measure of the relative CH4 abundance, is given by (W-IW^)CIU = (1 - P^J.P^YI nc (1 •- ^abn/PonnOv• T values of Pcmt were read o(T the drawn curves of figurc4, which are smoothed conlinua drawn through the observed peak values of the geometric albedo spectra of figure 2. For the temperature conditions under consideration, the integrated band strength is independent of temperature if the individual lines arc unsaturalcd. If saturation occurs, which is very probable for Uranus and Ncpuinc, a temperature dependence will be present. For these two planets, however, that will have no significant influence on the derived relative CH4 abundance. From the continua derived above and the minima in the geometric albedo spectra at A z 0.54 /<( 1), 0.62 /i( I). 0.72 /i'(3), 0.81 /i(2), 0;89/t(2)i 1.03/i(2), arid 1.10 /t(l), we obtained an equivalent-width ratio of (WVWy)cH. = 1.13 ± G percent (m.c.) for the integrated methane bands. (The number of litters considered to cover the CH4 absorption band is given in parentheses for each 'wavelength.) This gives rise to a relative CH4 abundance JV,-(CH4) = 1.I3JVV(CH4) if the lines are unsaturated. Since there is probably considerable satura- tiori'on both planets, the relative abundance will probably be Nt{CKJ » 1.3Aty(CHf).

91 1016 WILLEM WAMSTEKER Since this determination is essentially a null method, it is not very strongly afacted by the low spectral resolution of our geometric albedo spectra. This relative CH« abundance does not agree very well with Kuiper's (1952) determination of the methane abundance of Uranus and Neptune. From his values, one derives tfi(CH«) - 0.6tfv(CH4).

VI. CONCLUSION In the previous discussion we have shown that it docs not seem possible to obtain a consistent explanation of the geometric albedo spectra of Uranus and Neptune together, if methane were the only opacity source in the atmospheres of these plnncts. Radiative transfer calculations were made to see if the power-law extrapolation of the short wavelength wing of the pressure-induced dipote absorption of H3 will allow a consistent explanation. It is shown that with a large Rayleigh scattering optical depth, T(A = 0.5 fi) ft; 1.6, limited by a reflecting surface with a Lambert albedo A = 0.7, the observed geometric albedo spectra can be matched reasonably well.. Also, the derived H2 abundance is in agreement with that derived independently from the 4-0 5(0) and S(l) lines of H2. The behavior of the ratio spectrum can also be explained, which could not be done in terms of methane opacity alone. Although the.power-law extrapolation of the Ha absorption, including a rather deep cloud'layer, used in this discussion fiis the observations rather well, it is not necessarily a unique solution. The derived H2 abundance seems, however, to support this suggestion. The difference between the relative CH4 abundance derived in this paper and the one obtained from previous abundance determinations is not very weir understood and justifies an additional observational effort. The author is indebted to Dr. M. G. Toniasko for making his radiative transfer program available. A considerable part of this work was done while the author held a position at the Lunar and Planetary LaboraU-ry of the University of Arizona. The ideas expressed in this paper are not necessarily shared by the principal investigator for NASA grant NGL 03-002-002, under which grant the described research was supported. REFERENCES Appleby, J. F. 1973, A.J., 78, 110. Appleby, J. F., and Irvine, W. M. 1971. A.J., 76, 6i7. Bclton, M. J. S., McElroy. M. B.. and Price, M. J. 1971, Ap. J., 164, 191 (BMP). Binder, A. B., and McCarthy, D. W., Jr. 1972. Ap. J. {Utters), 171, LI. Bixby, J. E., and Van Flandcren, T. C. 1969, A.J., 74. 1220. Danielson, R. E., Tomasko, M. G., and Savage, B. O. 1972, Ap. J., 178, 887. Dunham. Th. 1933, Pub. A.S.P., 45, 42. Hanscn. J. 1969, Ap.J., 155, 565. Harris, D. L. 1952, in Planets and Satellite.*, ed. G. P. Kuiper and B. Middlchurst (Chicago: University or Chicago Press), p. 272. Hcrzbcrg, G. 1952, Ap. /.; H5, 337. Johnson, H. L. l965i.C6mm. Lunar, and Planetary Lab., 3, 67. Kuipcr, G. P. 1950, Rrpt. Progr. Phys., 13, 247. . 1952, in 77w Atmospheres of the Earth and Planets, ed. G. P. Kuiper (Chicago: University of Chicago Press), p. 306. .' Labs, O., and Ncckcl, H. 1968, Zs. f. Ap., 69, I. MacTaggarf, J. W., and Hun:, J. L. 1969, Canadian J. Phyi., 47, 65. Sinton, W. M. 1972, Ap. J. {Utters), 176, L131. Tomasko, M. G. 1973, Ap.J., in preparation, Wallace. L. 1972, Ap. J:, 176, 249. Waimteker, W. 1973, Comm. Lunar and Planetary Lab., 9, in press (Paper 1). Wildt, R. 1932, Veroff. Univ. Sterw. Coll., No. 22. . Younkin, R. L. 1970, thesis, Univenily of California at Loi Angeles.