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The Wavelength Dependence of the Albedos of Uranus

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The Wavelength Dependence of the Albedos of Uranus 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 brighten- ing at A = 0.887 /i for Uranus requires the presence of a thin haze high in the atmo- sphere 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) con- cluded 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 photo- metric 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 C . 1.2 m CONST 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.
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