Robert E. Murphy

Temperatures of Titan and the Galilean satellites at 20 microns.

Determination of the effective temperatures of the satellites by means of broad-band observations made at Mauna Kea in 1971. The temperatures of the Galilean satellites are in fair agreement with observations at shorter wavelengths and with theory. The 20-micron flux of Titan is nearly an order of magnitude lower than expected, suggesting either low surface emissivity or high atmospheric opacity between 18 and 25 micron. Hydrogen could produce the required opacity, but the abundance needed is large.

The two faces of Iapetus

Abstract Combined photometry and radiometry of Iapetus can be used to investigate the nature of its surface and, in particular, the distribution of albedo that is responsible for the large variations in its visible and infrared brightness as it rotates. We present new 20-μm radiometric observations made in 1971–1973 and discuss these together with the photometric studies by Widorn (in 1949), Mills (in 1971), Noland et al. (in 1972–1973), and Franklin and Cook (in 1972–1974). The linear phase coefficient varies as the satellite rotates from 0.028 to 0.068 mag deg −1 . When corrected for this effect, the photometric variations suggest an albedo distribution characterized by a dark area covering most of the leading hemisphere and a bright trailing hemisphere and bright south polar cap. A combined analysis of the photometry and radiometry yields a radius of 800 to 850 km and mean geometric albedos for the light and dark faces of about 0.35 and 0.07, respectively. The average phase integral of the bright hemisphere is between 1.0 and 1.5. We offer no explanation for the unique photometric properties of this satellite.

Temperatures of Saturn's rings.

The 20-micron brightness temperatures of the rings were determined using the 224-cm telescope of the Mauna Kea Observatory, and the standard University of Hawaii radiometer with a 17- to 25-micron filter. The observations were made on the nights of Aug. 20 and 21, and Sept. 26 and 27, 1972. The brightness temperatures of the A, B, and C rings are, respectively, 89 plus or minus 3 K, 94 plus or minus 2 K, and 89 plus or minus 4 K. A possible explanation of the relatively high temperature of the C ring is that Saturn has radiation belts and the inner ring is heated by particle bombardment.

The post-eclipse brightening of Io

Abstract We review the photometric work on eclipse reappearances of Io. New observations of eclipse reappearances of Io confirm the post-eclipse brightness anomaly reported by Binder and Cruikshank (1964) but testify to its intermittent nature. A post-eclipse anomaly of approximately 0.07 mag was observed on two occasions in 1972, while observations of Europa and Ganymede showed no brightness anomaly greater than 0.01 mag. The atmospheric condensation model for the anomaly on Io is reviewed in terms of the quantity of frost required to produce the effect and the corresponding amount of gas liberated to the atmosphere upon sublimation. The observational data and the results from a stellar occultation are in general accord with the theoretical predictions of the stability of heavy gases on Io, while both observational and theoretical criteria are satisfied by a tenuous atmosphere of a heavy gas such as methane or ammonia having a surface pressure ∼10 −7 bar.

Thermal inertia of Ganymede from 20-micron eclipse radiometry

Ganymede thermal inertia data from simultaneous visual photometry and IR radiometry observations during 17 March 1971 eclipse

Spatial variations in the Jovian 20-micrometer flux

Abstract The 17–28 μm brightness temperature of the center of the disk of Jupiter is 136 ± 4 K. Model calculations yield an effective temperature of 142 ± 4 K at the center of the disk for a helium to hydrogen ratio He/H 2 of 0. This corresponds to an effective temperature of the entire disk of 136 ± 5 K. The NEB, SEB, and STeB are shown to emit an excess flux at 20 μm when compared to the neighboring zones. The hot belts were grey in color at the time of the observations and were the source of excess 5-μm flux as well (Keay et al. 1973). The relationships between 5-μm and 20-μm flux excesses and the cloud structures are discussed.