John S. Neff

Absolute spectrophotometry of Titan, Uranus, and Neptune: 30,500–10,500 Å

Absolute measurements of the geometric albedo spectra of Titan, Uranus, and Neptune from 3500 to 10,500 A are reported. The measurements have spectral resolution of about 7 A and high signal-to-noise ratio owing to the superb efficiency of the spectrograph and Reticon detector used to acquire them. Agreement is excellent with albedo measurements of Uranus G. W. Lockwood, B. L. Lutz, D. T. Thompson, and A. Warnock (1983, Astrophys. J., 266, 402–414). The high precision and spectral resolution of the data make possible quantitative measurements of the effects of Raman scattering by H2 in the atmospheres of Uranus and Neptune.

Numerical simulation of the emission and motion of neutral and charged dust from P/Halley

We have developed a numerical model to predict the distributions of Neutral or charged dust particles in P/Halley. The spatial and temporal variations of the plasma parameters and magnetic field, factors which govern the charging of the dust and its subsequent motion, were included. In computing the electric charge of the dust particles electron collection, photoemission, and secondary electron emission were taken into account. We found that the trajectories of charged dust particles with radii less than about 0.1 μm (10−14 g) were significantly different than the neutral dust trajectories. By charging the dust and including temporal fluctuations, the maximum particle-nucleus distances along the Giotto, Vega 1, and Vega 2 trajectories were increased and made more consistent with the measured values. Most of the model and in situ mass spectra were in good agreement except for a deficiency of the lowest mass model particles, and an offset in the total counting rate for two outbound mass spectra. The best agreement was obtained with a combination of two Lorentzian particle mass density functions with densities of 0.03–2.0 g cm−3 and 0.06–4.0 g cm−3, and a ratio of 100: 1 for the number of particles with the lower and higher density functions, respectively. We also needed particle ejection velocities of about 1.7 times the theoretical decoupling velocities, a high secondary electron yield, and small particles which were more absorbing than large particles

Bolometric albedos of Titan, Uranus, and Neptune

Abstract The geometric albedos of Titan, Uranus, and Neptune have been measured from 2000 to 3175 A by J. Caldwell, T. Owen, A. R. Rivolo, V. Moore, G. E. Hunt, and P. S. Butterworth (1981, Astron. J. 86 , 298–305), from 3500 to 10,500 A by J.S. Neff, D. C. Humm, J. T. Bergstralh, A. L. Cochran, W. D. Cochran, E. S. Barker, and R. G. Tull (1984, Icarus 60 , 221–235), and from 6440 to 25,360 A by J. Apt, R. N. Singer, and R. N. Clark (private communication). The integrated solar flux in this spectral interval amounts to 97% of the solar constant. These data sets were combined and integrated to find the bolometric geometric albedo of each object. Preliminary determinations of the phase functions were used to compute the Bond albedos and effective temperatures. The effective temperatures are compared with bolometric temperatures determined from brightness temperatures in the 10-μm to 5-mm region of the spectrum. An improved value for the internal luminosity of Neptune is (3.9 ± 1.1) × 10 15 W and an upper limit to the internal luminosity of Uranus 15 W. Titan was found to have an effective temperature greater than the observed brightness temperatures in the thermal infrared indicating that the emissivity in this spectral region is less than unity.

Spatially resolved absolute spectrophotometry of Saturn - 3390 to 8080 A

Abstract Absolute spectrophotometry of four regions on the visible disk of Saturn (north and south polar regions, equatorial band, south “temperate” region) from 3390 to 8080 A is reported. Spectral resolution is 10 A in the interval 3390–6055 A, and 20 A; aperture size is 1.92 arcsec. The explicit purpose of our observations was to provide ground-based photometric calibration for the Pioneer Saturn Imaging Photopolarimeter (IPP). We also compare our data with earlier spectrophotometric measurements of Saturn ( R.L. Younkin and G. Munch, 1963 ,Mem. Soc. Roy. Sci. Liege 7, 123–136; W.M. Irvine and A.P. Lane, 1971 ,Icarus 16, 10–26; T.B. McCord, T.V. Johnson, and J.H. Elias, 1971 ,Astrophys. J. 165, 413–424) and with the M. Podolak and R.EE. Danielson (1977) Icarus 30, 479–492) parameterization of “Axel Dust.” The latter reproduces the broad features but not the details of the observed spectral reflectivity (I/F). We find that large depths of clear molecular hydrogen (>14 km-am in the temperate regions) are needed to match the observed upturn in reflectivity shortward of 3800 A.

AN INVESTIGATION OF THE ROTATIONAL PERIOD OF THE PLANET PLUTO

New photometry of Pluto was obtained to resolve the longstanding ambiguity in the synodic period of Pluto. Measurements obtained at Kitt Peak National Observatory on four consecutive nights in April 1973 show that the shorter period of 1d1819 is spurious and confirm the 6d3867 i 0.0003 period found by Hardie (1964). The times of mean light crossing on the rising branch yield an improved synodic period of 6d3874 i 0.0002 based on 20 years of observations. Key words: Pluto - photometry - rotational period

Absolute spectrophotometry of Neptune: 3390 to 7800 Å

Abstract Absolute spectrophotometry of Neptune from 3390 to 7800 A, with spectral resolution of 10 A in the interval 3390–6055 and 20 A in the interval 6055–7800 A, is reported. The results are compared with filter photometry ( Appleby, 1973 ; Wamsteker, 1973 ; Savage et al. , 1980 ) and with synthetic spectra computed on the basis of a parameterization proposed by Podolak and Danielson (1977) for aerosol scattering and absorption. A CH 4 /H 2 ratio of 1 × 10 −2 CH 4 −1 is derived for the convectively mixed part of Neptunes atmosphere, and constrains optical properties of hypothetical aerosol layers.

A MEASUREMENT OF THE RELATIVE REFLECTANCE OF PLUTO AT 0.86 MICRON.

We have examined Plutos near infrared reflectance by means of photoelectric photometry using a filter whose effective wavelength is 0.86 micron. The ratio of Plutos reflectance at 0.86 micron to that at 0.55 micron is 1.33 plus or minus 0.04. Our measurements, combined with the UBVR colors of Pluto as measured by Hardie, show that Plutos reflectance increases almost linearly with wavelength in the region between 0.36 micron and 0.86 micron. Plutos reflectance in the spectral region resembles the reflectance of the asteorid 5 Astrae as well as some stony-iron meteorites and iron meteorites having low nickel content.

Absolute spectrophotometry of Comet Kohoutek 1973f: I. Column densities of cyanogen

Abstract Absolute spectrophotometry of the coma of Comet Kohoutek 1973f is discussed for the nights of January 24 and 26, 1974. Specific intensities are measured for spectral features and a continuum band in the wavelength region λλ3460–6062A. The (0, 0) band of the Δν = 0 sequence of the violet system of the cyanogen molecule is analyzed and column densities of 1.7 × 10 15 m −2 and 3.4 × 10 14 m −2 are found for January 24 and 26, 1974, respectively. The analysis of the bands of C 2 will be reported in a second paper of this series.

A comparison of modeled and observed intensity profiles for C2, C3, CN, and the continuum for P/Halley

Abstract Intensity profiles were obtained for C2, C3, and CN emission and the continuum of P/Halley. The observations were made on 14 December 1985 and 6 and 8 January 1986, when the comet was between 0.90 and 1.28 AU from the Sun. Model intensity profiles were compared with the observations, and used to constrain the dust and gas parameters. Most of these parameters were consistent with expected values; however, the lifetimes of C3 and its parent were much smaller than expected on each night, and the dust ejection velocity on 14 December was only 60% of the expected value. The day:night production rate ratio was found to be approximately 1:1 for the gas, and ranged from 4:3-1:0 for the dust.

Absolute spectrophotomery of comets 1973XII and 1975IX: II. Profiles of the Swan bands

Abstract Absolute spectrophotometric measurements of the Swan bands of two comets have been compared with computed synthetic spectra using modern Franck-Condon and Honl-London factors, and varying rotational, vibrational, and electronic excitation temperatures. Rotational and vibrational temperatures were obtained for the comets. Although the electronic excitation temperature and the molecular column density cannot be separated, a relationship is found between these two quantities. A review is made of recent determinations of column densities for CN in comets.