W. Liller

Orbits of nine Uranian rings

Observations of a stellar occultation by Uranus and its nine rings are presented and used to examine the structures and kinematics of the rings. The observations of the occultation of the K giant star KM 12 were obtained in the K band with the 4-m CTIO telescope at a signal-to-noise ratio higher than any previously obtained. Ring occultation profiles reveal the alpha ring to possibly have a double structure and less abrupt boundaries than the gamma ring, which exhibits diffraction fringes, while the eta ring is a broad ring with an unresolved narrow component at its inner edge. The present timing data, as well as previous occultation timings, are fit to a kinematic model in which all nine rings are treated as coplanar eclipses of zero inclination, precessing due to the zonal harmonics of the Uranian gravitational potential to obtain solutions for the ring orbits. Analysis of the residuals from the fitted orbits reveals that the proposed model is a good representation of ring kinematics. The reference system defined by the orbit solutions has also been used to obtain a value of 0.022 + or - 0.003 for the ellipticity of Uranus and a Uranian rotation period of 15.5 h.

The occultation of beta Scorpii by Jupiter. I - The structure of the Jovian upper atmosphere

The light curves of beta Scorpii AB and C were recorded during occultation by Jupiter with a time resolution of 0.01 sec. The spikes on the curves, correlated with flashes observed through the eyepiece, are shown to be due to density fluctuations in Jupiters atmosphere. Using the delays in spike arrival times, the ratio of the refractivities at 3934 and 6201 A is found to be 0.9713 + or - 0.0015. Temperature profiles for three different assumed atmospheric compositions are generated from the light curves. Assuming the terrestrial value for the eddy diffusion coefficient, the turbopause is calculated to occur at the 10 to the 13th power per cu cm level, below which a positive temperature gradient of about 1 K/km is indicated.

The thermal structure and energy balance of the Uranian upper atmosphere

Abstract Two occultation observations of the upper atmosphere of Uranus are reported: (i) the 15–16 August 1980 occultation of KM 12 from Cerro Tololo Inter-American Observatory, and (ii) the 26 April 1981 occultation of KME 13 from the Anglo-Australian Telescope and the 1-m telescope of the Australian National University. Mean atmospheric temperatures of 154 ± 15°K for the 1980 event and 132 ± 15°K for the 1981 event are derived from the lightcurves. A comparison of all available Uranus occultation data since March 1977 suggests that the mean atmospheric temperature of Uranus has changed significantly, with a typical variation of 15°K year−1. We investigate plausible energy sources that might account for such large temperature variations. We conclude that molecular and eddy diffusion, and atmospheric dynamics, are potentially as important as radiation to the upper atmospheric heat balance. There is evidence that no significant radiative cooling had occurred at two suboccultation points that had been in darkness for more than 0.5 terrestrial year, suggesting upper limits to the mixing ratios of CH4 and C2H2 of 6 × 10−5 and 5 × 10−7, respectively. The consistently close agreement of immersion and emersion mean temperatures for each occultation, in spite of apparently large secular changes in the atmospheric mean temperature, suggests that effective meridional transport occurs on Uranus. A continuing program of occultation observations in future years should reveal whether this pattern is significant.

Observations of Icarus: 1968

Abstract Icarus is a nearly spherical object that rotates once every 2.25 (±0.05) hr. From the degree of polarization of the reflected light as a function of phase angle, we estimate that the reflectivity of the surface (in the visual) does not exceed 0.20. Combining this estimate with the inferred absolute magnitude g v = 17.05, we obtain a lower limit on the radius of 750 m.

The rings of Uranus - Occultation profiles from three observatories

Abstract Occultation profiles for the nine confirmed Uranian rings obtained from Las Campanas, the European Southern Observatory, and Cerro Tololo on 15–16 August 1980 are compared. The α ring shows a “double-dip” structure; the η ring shows a broad and narrow component (similar to Saturns F ring); and the e ring shows six features that appear in the data from all three observatories. Diffraction fringes appear at the edges of several of the occultation profiles.

Occultation of beta Scorpii by Jupiter. V - The emersion of beta Scorpii C

Use of multicolor photometry to remove planetary limb contamination from occultation light curves is demonstrated. This color restoration procedure is shown to be more advantageous than limb scanning techniques since it allows continuous observation of the occultation. Sharp spikes appearing in the light curve of beta Sco C are compared with those observed during the occultation of beta Sco AB, and a model is developed for estimating the effects of all factors affecting the observed shape of a spike. The widths of the narrowest spikes are shown to correspond closely to the angular diameters of the occulted stars, suggesting that occultation spikes can be used to determine stellar diameters.

The occultation of Beta Scorpii by Jupiter. III - Simultaneous high time-resolution records at three wavelengths

Abstract Simultaneous high time-resolution records at three wavelengths of the May 13, 1971 Jupiter occultation of Beta Scorpii are presented. These observations are unique and contain important information about the structure and composition of the Jovian atmosphere. An equally favorable Jupiter occultation should not occur again for several centuries.

Galilean satellites: Observations of mutual occultations and eclipses in 1973

Abstract During the fall of 1973 we observed seven Galilean satellite mutual events: two occultations and two eclipses of Europa and three eclipses of Io. The observations were carried out simultaneously at three wavelengths (0.35, 0.50, and 0.91 μm) with a time resolution of 0.1 sec. Our principal aim was to obtain color information about albedo distributions and limb darkening on the satellites, but it is now clear from our analysis that before such information can be fully extracted from the observed light curves, values of the various geometric parameters (such as satellite radii and impact parameters) must be established accurately using all of the mutual-event data obtained during 1973–1974. At this stage, our observations do not yield any conclusive information about Ios limb darkening. For Europa, however, our best data indicate that this satellite is limb-darkened at both 0.50 and 0.91 μm. A variety of computer-generated model light curves are presented to clarify the question of how sensitive observed light curves are to the brightness distribution on the surface of the occulted or eclipsed satellite.