Bolometric Bond Albedo Research Articles

Voyager photometry of Triton: Haze and surface photometric properties

We have analyzed Voyager whole‐disk observations of Triton at violet (0.41 μm), blue (0.48 μm),and green (0.56 μm) filter wavelengths, using a model which combines an improved version of Hapke's photometric equation with a thin atmospheric haze layer in the appropriate spherical geometry. The model accurately describes the phase curves over a wide range of phase angles (10° ≤ α ≤ 159°) and agrees well with disk‐resolved brightness scans along the photometric equator and mirror meridian. An upturn in the phase curves seen at the highest phase angles observed (Smith et al., 1989) can be explained by including scattering in a thin atmospheric haze layer with optical depths ranging from 0.066 in the violet to 0.036 for the green filter data. The haze is forward scattering with g ∼ +0.6, requiring particle sizes of ≳0.2 μm. The haze single‐particle scattering albedo is near unity at all three wavelengths, suggesting the haze particles are relatively neutral ice condensates. The geometric albedo, phase integral, and spherical albedo of Triton in each filter corresponding to our best fit Hapke and atmospheric parameters are derived. The bolometric Bond albedo of Triton calculated from our results is 0.85 ± 0.05. If the 16‐μbar N2 atmosphere detected by the Voyager radio occultation experiment (Tyler et al., 1989) is in vapor equilibrium with the surface (therefore implying a surface temperature of 37.7 K), then our Bond albedo implies a surface emissivity of 0.46 ± 0.16.

The albedo, effective temperature, and energy balance of Neptune, as determined from Voyager data

Data from the Voyager infrared spectrometer and radiometer (IRIS) investigation are used in determining the albedo, effective temperature, and energy balance of Neptune. From broadband radiometric observations made at phase angles of 14° and 134°, together with measurements at intermediate phase angles from the literature, an orbital mean value for the bolometric Bond albedo, Ā = 0.290 ± 0.067, is obtained; this yields an equilibrium temperature Teq = 46.6 ± 1.1 K. From thermal spectra obtained over latitudes from pole to pole an effective temperature = 59.3 ± 0.8 K is derived. This represents a substantial improvement over previously determined values. The energy balance of Neptune is therefore E = 2.61 ± 0.28, which is in agreement with previous results. The reduced uncertainty in this value is due to the improved determination of the effective temperature.

SPECTRAL GEOMETRIC ALBEDO AND BOLOMETRIC BOND ALBEDO OF NEPTUNE'S SATELLITE TRITON FROM VOYAGER OBSERVATIONS

We have calculated the spectral geometric albedo and bolometric Bond albedo of Triton by combining the published data from the Voyager spacecraft Photopolarimeter (PPS) experiment and the Voyager imaging science (ISS) experiment. We have used the PPS ultraviolet (0.25 μm) and infrared (0.75 μm) filter data and the ISS violet (0.41 μm) and green (0.56 μm) filter data to produce geometric albedos and phase integrals at each of the four wavelengths. The resulting spectral geometric albedo is not inconsistent with the presence of weak absorption feature in Triton's spectrum near 0.75 μm. The phase integrals were determined by Russell's approximation and by 2‐point Gaussian quadrature. The geometric albedo at the uv, v, g and ir filters is 0.59, 0.68, 0.81, and 0.75 respectively. The resulting bolometric Bond albedo (∼0.65) is· consistent with the 38 K, unit emissivity, daytime surface temperature for Triton reported as a best fit to the data of the Voyager Infrared Specrotrometer and Radiometer (IRIS). Our results are also in agreement with the 37.5 K temperature of nitrogen at an inferred basal pressure of 14 μbar as reported by the Voyager Ultraviolet Spectrometer (UVS) investigation.The bolometric Bond albedo that is determined from tile phase integral and spherical albedo reported originally by the ISS investigation yields a surface temperature which is consistent with the PPS, IRIS and UVS temperatures only for a surface with a substantially lower emissivity. There are differences between the PPS and the ÎSS phase curves which cause the differences in bolometric bond albedo reported in the two studies.

The albedo, effective temperature, and energy balance of Uranus, as determined from Voyager IRIS data

Data from the Voyager infrared spectrometer and radiometer (IRIS) investigation are used to determine the albedo, effective temperature, and energy balance of Uranus. From broadband radiometric observations made over a range of phase angles 15° < α < 155°, an orbital mean value for the bolometric Bond albedo, A = 0.300 ± 0.049 , is obtained, which yields an equilibrium temperature T eq = 58.2 ± 1.0°K. From thermal spectra obtained over latitudes from pole to pole, an effective temperature T eff = 59.1 ± 0.3°K is derived. This represents a substantial improvement over previously determined values. The energy balance of Uranus is therefore E = 1.06 ± 0.08; the one-standard-error upper limit of 1.14 is lower than previous results.

Voyager disk-integrated photometry of Io

Voyager full-disk images of Io, available at solar phase angle of α = 2−29° and 101−159°, allow comparisons of the satellite's near-opposition photometric behavior with Earth-based results and the determination of the phase curve out to very high phase angles. The near-opposition data were reduced iteratively for self-consistent phase and rotation curves in each Voyager filter; the resulting phase coefficients, geometric albedos, and rotational lightcurves are consistent with Earth-based findings, except for a previously noted tendency for Voyager to yield somewhat redder spectral information. The derived near-opposition phase coefficients, ranging between 0.016 and 0.024 mag/ deg, decrease with increasing wavelength, a trend weakly noted in some Earth-based observations. The full, α = 2−159° phase curves allow the first direct determination of the phase integral of Io at several wavelengths: q rises from ≈0.7 in the ultraviolet to ≈0.8 in the orange. Combination of the Voyager phase integrals with Earth-based albedo information leads to a best estimate of the bolometric Bond albedo of 0.50 ± 0.10, a value consistent with, but slightly below, previous estimates.

Voyager photometry of Europa

Ninety voyager images ranging in phase angle from 3 to 143° and covering the spectral range from 0.34 to 0.58 μm were analyzed to derive the photometric properties of Europa. At small phase angles the disk-integrated phase curve is remarkable in that it shows little or no evidence of an opposition effect (in agreement with earlier Earth-based observations by Millis and Thompson, Icarus 26, 408, 1975). The phase integral determined in the Voyager clear filter (centered near 0.47 μm) is 1.09 ± 0.11, in good agreement with previous estimates based on radiometry. The bolometric Bond albedo is 0.62 ± 0.14. The scattering properties of Europa in general, and of the two major terrain types (bright plains and darker mottled terrain) in particular, cannot be represented by a lunar-like photometric law. However, an equation which is a linear superposition of a lunar-like scattering law and a Lambert component provides an adequate simple representation of the scattering properties. The plains are photometrically more homogeneous than the darker mottled terrain. In the Voyager clear filter, the average normal reflectance is 0.71 for the plains on both the leading and trailing hemispheres; for the darker mottled terrain the values are 0.60 on the leading hemisphere, and 0.48 on the trailing one.

The thermal balance of venus in light of the Pioneer Venus Mission

Instruments flown on the Pioneer Venus orbiter and probes measured many of the properties of the atmosphere of Venus which control its thermal balance and support its high surface temperature. Estimates based on orbiter measurements place the effective radiating temperature of Venus at 228±5 K, corresponding to an emission of 153±13 W/m², and the bolometric Bond albedo at 0.80±0.02, corresponding to a solar energy absorption of 132±13 W/m². Uncertainties in these preliminary values are too large to interpret the flux difference as a true energy imbalance. A mode of submicron particles is suggested as an important source of thermal opacity near the cloud tops to explain the orbiter and probe thermal flux measurements. Comparison of the measured solar flux profile with thermal fluxes computed from the measured temperature structure and composition shows that the greenhouse mechanism explains essentially all of the 500 K difference between the surface and radiating temperatures of Venus. Precise comparison of the observed and computed value of this difference is hindered by uncertainties in the local variability of H2O and in the thermal opacity of CO2 and H2O at high temperature and pressure. The directly measured thermal flux profiles at the small probe sites are surprisingly large and variable in the lower atmosphere. Observed zonal and meridional circulation are qualitatively as required to produce the observed uniformity of temperature structure. However, the present lack of quantitative estimates of the horizontal and vertical dynamical heat transports implied by these measurements is a significant gap in the understanding of the thermal balance of the atmosphere of Venus.

The 16- to 38-micron spectrum of Callisto

The emission spectrum of Callisto was measured between 16 and 38 μm with a spectral resolution λ Δλ ≈ 330 , using the NASA Kuiper Airborne Observatory on the night of October 30–31, 1975 Within the errors, the observed spectrum is like that of a 155°K blackbody, in both shape and absolute intensity. The infrared emission and diameter of Callisto indicate a bolometric Bond albedo of 0.05±0.14, which is consistent with heating of the surface by absorbed sunlight.

The infrared photometric function of Mars and its bolometric albedo

The photometric properties of local areas on Mars are studied using Minnaert's rule of surface scattering to analyze Mariner 6 and 7 Infrared Spectrometer data. Several bright deserts, Hellas, and the south polar cap are found to obey Minnaert's function well. The coefficients B0(α, λ) and k(α, λ) are obtained at α = 39, 48, 56, 84° and λ = 1.85, 2.23, 3.50 μm. Observed bright regions all have similar values of k, except for Hellas and the south polar cap. The lower k of Hellas is apparently caused by microscopic effects rather than by large-scale roughness due to cratering. The higher k of the cap is similar to terrestrial snows in the visual at the same phase angle. Using existing Earth-based observations, at smaller α and λ, a bolometric Bond albedo of A∗ = 0.24 ± 0.05 is calculated.

Infrared radiometry of the rings of Saturn

Broad-band radiometry with a spatial resolution of 5 arc sec is presented of Saturn and its rings. The brightness temperature of the B ring is 96 ± 3°K at 20 μm and 91 ± 3°K at 11 μm. These values constrain the bolometric Bond albedo of the ring particles to be less than 0.6, thus requiring a phase integral of less than unity. From differences in the thermal emission of the ansae, I suggest that the leading side of the particles has higher albedo than the trailing side. A measured drop in temperature of the B ring following eclipse of 2.0 ± 0.5°K is consistent with radii for the ring particles of 2 cm or larger.

The albedo of Titan

The irradiance of Titan has been measured from 0.50 to 1.08μ in 30 Å band-passes spaced 0.01–0.02μ apart. Geometric albedos have been computed at the wavelenghts of measurement using a standard solar flux distribution after Labs and Neckel. The maximum value of p λ (0) is 0.37 at 0.68, 0.75, and 0.834μ, the minimum value, in the centers of the strongest methane absorption bands, is 0.10 at 0.887 and 1.012μ. The brightness of Titan at the time of the present measurements has been compared with that of previous modern photoelectric measurements. Within the apparent consistency of the different photoelectric systems, the brightness of Titan appears to undergo changes with time. A provisional curve of the geometric albedo from 0.30 to 4.0μ has been made by combining the present results with those of other authors, i.e., relative measurements of Titan from 0.30 to 0.50μ, and measurements of Jupiter and Saturn from 1.08 to 4.00μ. The latter are used to estimate the strengths of the methane absorption bands of Titan in that spectral range. The bolometric geometric albedo, p ∗(0) , is computed to be 0.21. A variety of current measurements of Titan indicate a substantial atmosphere, suggesting a value of the phase integral q = 1.30 ± 0.20 . The bolometric Bond albedo, A ∗ , is then 0.27 ± 0.04, giving an effective radiative temperature T e= 84 ± 2°K. The absorption band contours of Titan have been compared with those of Jupiter and Saturn at the same resolution. The bands of the planets are known to be due primarily to methane, and they show a very regular relationship, with those of Saturn being consistently deeper and wider. For Titan, the strengths of the bands are equal or less than those of Jupiter in the band centers, while the wings are stronger than those of Saturn. Previous photoelectric and photographic spectra have been examined for evidence of temporal variation of the methane path length in the atmosphere of Titan. Differences in measurement techniques prohibit detection of small differences. The only potential differences beyond experimental uncertainties are those of Kuiper (1944) and Harris (mid-fifties). Taking Kuiper's results at face value, Titan appears to have a shorter methane path length in 1972. Harris's results can be reconciled only by the doubtful hypothesis of an almost complete absence of methane at that time.

Spectrophotometry of Jupiter's 3400–10000 Å spectrum and a bolometric albedo for Jupiter

Photoelectric measurements of the monochromatic flux from Jupiter in the region from 3400 to 10 000 Å are reported. From these measurements and published data for the ultraviolet and infrared outside the region of measurement a bolometric geometrical albedo of 0.28 was obtained. With an adopted bolometric phase integral of 1.6 the bolometric Bond albedo is then found to be 0.45, corresponding to a temperature of equilibrium with absorbed sunlight of 105°K. If the difference between this temperature and the radiometric temperature of 128°K is due to heat from the interior the mantle is probably melted. Rough calculations suggest that even with convective cooling the initial contraction energy could explain the present radiometric temperature. Radioactivity, meteor bombardment, tidal friction, and magnetic field decay do not appear to be feasible alternative energy sources for the radiometric temperature. A simple interpretation of the radiometric temperature is that Jupiter is radiating only absorbed sunlight with a temperature of 145°K and emissivity of 0.27.