Restored methane band images of Uranus and Neptune

Charge-coupled device images of Uranus and Neptune taken in the 8900-Å absorption band of methane are presented. The images have been digitally processed by means of nonlinear deconvolution techniques to partially remove the effects of atmospheric seeing. The restored Uranus images show strong limb brightening consistent with previous observations and theoretical models of the planet's atmosphere. The computer-processed images of Neptune show discreted cloud features similar to those reported previously by B. A. Smith, H. J. Reitsema and S. M. Larson (1979 Bull. Amer. Astron. Soc.11, 570). A time series of the restored Neptune images shows a continuous variation which may be due to the planet's rotation.     

Direct imaging of planetary systems around nearby stars

We show that by utilizing a space-borne telescope it may be possible to directly image planetary systems around the nearest stars. Direct imaging, while limited to the nearest stars, would provide a great deal of information on the planet, over and above the planet's orbital elements—estimates of the planet's size and rotation rate, and the presence or absence of an atmosphere are all possible if light from the planet can be separated from light from the star. It is shown that a Jupiter-like planet would be detectable around several of the nearest stars.     

The brightness temperature of Saturn at decimeter wavelengths

Observations of the planet Saturn at wavelengths of 49.5 and 94.3 em are reported. The equivalent disk brightness temperatures were found to be 400 ± 65°K and 540 ± 110°K, respectively. It is suggested that the enhanced portion of the spectrum of the disk brightness temperature favours the idea that the observed long wavelength radiation comes from the planet's atmosphere.However, the possibility of a magnetic field associated with Saturn is not rejected by the observations. Part of the excess temperature could be attributed to weak synchrotron emission coming from a region outside the ring system.     

The Jovian ammonia abundance from interferometric observations of limb darkening at 3.4 mm

The interferometer visibility of Jupiter, observed at a wavelength of 3.4 mm, is used to determine the global limb darkening of the planet's brightness. From a single-parameter fit to the visibility curve, we find an ammonia-to-molecular hydrogen mixing ratio of 6.4[+5.1, ?1.9] × 10^?5, which corresponds to 35[+28, ?10]% of the solar nitrogen abundance if all of the nitrogen is in the form of ammonia. The fitting procedure uses a simple model atmosphere for the Jovian atmosphere which is based on other observations of the planet. The dependence of the result on the various model parameters is studied.     

The rotation period of Neptune's upper atmosphere

Significant variations in the near-infrared brightness of Neptune during July and August 1980 were observed. These observations show a well-defined, large-amplitude variation in Neptune's J-K color, with a period of 17.73 ± 0.1 hr and are interpreted as diurnal variations resulting from the 17.73-hr rotation period of the upper atmosphere of Neptune in the presence of inhomogeneous weather. These results qualitatively corroborate those of D. P. Cruikshank (1978, Astrophys. J.220, L57-L59) in an earlier study using similar techniques. In addition, variations were observed in the 5-μm spectral region which are in phase with the variations seen at shorter wavelengths. A new 5-μm measurement of Uranus is also reported.     

Ammonia photolysis on Jupiter

Ammonia photolysis under simulated Jovian conditions indicates that the photochemical reaction would rapidly convert all the ammonia of Jupiter to nitrogen even in a large excess of hydrogen. It is suggested that ammonia is observed because the planet's atmosphere is deep and hot and/or because electrical discharge phenomena are important.     

The internal structures and the relative rotation rates of Uranus and Neptune

The difference between the interior structures of Uranus and Neptune is presented, based on models which fit the observed mass, radius, and gravitational moments for the assumed rotation periods of these planets. If Uranus and Neptune are assumed to be as similar in internal structure as they are in mass and radius, the rotation period for Neptune must be shorter than that for Uranus. It is suggested that the true rotation period is given by Neptune's oblateness, while the photometric period corresponds to the motion of Rossby waves in the upper atmosphere.     

Comparative thermal evolution of Uranus and Neptune

We extend a Jovian convective-cooling model to Uranus and Neptune. The model assumes that efficient interior convection prevails, so that escape of interior heat is governed by the radiative properties of the atmosphere. A comparison of the thermal evolution of Uranus and Neptune indicates that the larger amount of solar radiation absorbed in Uranus' atmosphere tends to differentially suppress the escape of interior heat. The model is shown to be consistent with recent infrared observations of the thermal balance of Uranus and Neptune, and with the presumed age of these planets.     

Radiation pressure and dust particle dynamics

The dynamics of small dust grains orbiting a planet are investigated when solar radiation pressure forces are added to the planet's gravitational central field. In the first part a set of differential equations is derived in a reference frame linked to the solar motion. The complete solution of these equations is given for particles lying in the planet's orbital plane, and we show that the orbital eccentricity may undergo considerable variation. At the same time the pericenter longitude librates or circulates according to initial conditions. With this result we establish a criterion for any orbiting particle (because of its highly eccentric orbit) to collide with its planet's atmosphere. The case of inclined orbit is studied through a numerical integration and allows us to draw conclusions related to the stability of the orbital plane. All solutions are periodic, with the period being independent of the initial conditions. This last point permits us to investigate the different time scales involved in that problem. Finally, the Poynting-Robertson drag is included, along with the radial radiation pressure forces, and the secular trend is considered. A coupling effect between the two components is ascertained, yielding a systematic behavior in the eccentricity and thus in the pericenter distance. Our solutions generalize the results of S. J. Peale (1966, J. Geophys. Res.71, 911–933) and J. A. Burns, P. Lamy, and S. Soter (1979, Icarus40, 1–48) by allowing eccentricities to be large (of order 1) and inclinations to be nonzero and by considering Poynting-Robertson drag.     

Saturn's electrostatic discharges: Could lightning be the cause?

It is proposed that Saturn's electrostatic discharges (SED) might be generated in the planet's equatorial atmosphere, perhaps as lightning from a storm system. The 10^h10^m periodicity of the signal envelope duplicates that of Saturn's equatorial jet. The rings shield the atmosphere from solar EUV photons, and thereby substantially reduce the local ionospheric cutoff frequency to allow low-frequency SED to leak out. Many of the unusual properties of SED could be explained in terms of changes in the storm system, the relative spacecraft position in the beaming pattern of the source, local refraction of the signal by the highly disturbed ionosphere, and the influence of the ring particles on the highest frequency component of SED. A comparison of SED with planetary lightning on other planets shows that the two are similar in general character and some time behavior; the power output of SED may be higher than most planetary lightnings but that is unclear because of uncertainties in the measurements and variations in the signal's spectrum. Our simple discussion suggests that lightning could be a viable source for SED and that exotic ring mechanisms are not necessarily required.     

When and where were the satellites of uranus formed?

Uranus exhibits an unusually large obliquity compared to other planets of the solar system; its equator is inclined by 98° to the plane of its orbit. However its five satellites are remarkably regular, with eccentricities and inclinations very nearly zero, but of course with orbit planes that are tilted by ～98° to the plane of the ecliptic. This circumstance is used here to relate the formation of satellites to planet formation. Six different cases are discussed, of which two can be ruled out and two others are highly improbable. In the analysis, use is made of the fact that satellites in near-equatorial orbits could not follow a rapid (“non-adiabatic”) change of the planet's obliquity. We assume, also, that the observed obliquity is the result of the last stages of planet accumulation. We can therefore exclude contemporaneous formation of planet and satellites, and conclude instead that the satellites were formed or acquired after the planet's axis had been tilted. A plausible scenario involves the tidal capture of a body having 5% to 10% of the planet's mass—sufficient to account for the tilt—followed by its accretion. However, tidal forces break up the body into chunks, slow the accretion, and allow ～1% of the chunks to form the satellites through interaction with a temporary dense atmosphere. The same reasoning may apply also for Saturn and Jupiter. It should be noted that the synchronous orbit it well within the Roche limit for all three planets.     

Impact craters on Venus

Calculations are made to determine the sizes of stone and iron meteoroids which could penetrate the atmosphere of Venus and cause hypervelocity impact craters on the planet's surface. Using scaling relationships based on kinetic energy, impact crater size is related to meteoriod size. Finally, it is determined that the smallest impact craters that might exist on Venus are on the order of 150 to 300 meters in diameter.     

Observations of Ganymede,Callisto, Ceres,Uranus, and Neptune at 3.33 mm wavelength

We have measured the 3.33 mm wavelength disk brightness temperatures of Ganymede (136 ± 21°K), Callisto (95 ± 17°K), Ceres (137 ± 25°K), Uranus (125 ± 9°K), and Neptune (126 ± 9°K). Our observations of Ganymede are consistent with the radiation from a blackbody in solar equilibrium, whereas Callisto's microwave spectrum indicates a surface similar to that of the Moon. The disk temperature for Ceres agrees with that expected from a rapidly rotating blackbody. The millimeter temperatures of Uranus and Neptune greatly exceed solar equilibrium values, implying atmospheres with large temperature gradients.     

The Martian hydrologic cycle: Effects of CO2 mass flux on global water distribution

The Martian CO₂ cycle, which includes the seasonal condensation and subsequent sublimation of up to 30% of the planet's atmosphere, produces meridional winds due to the consequent mass flux of CO₂. These winds currently display strong seasonal and hemispheric asymmetries due to the large asymmetries in the distribution of insolation on Mars. It is proposed that asymmetric meridional advection of water vapor on the planet due to these CO₂ condensation winds is capable of explaining the observed dessication of Mars' south polar region at the current time. A simple model for water vapor transport is used to verify this hypothesis and to speculate on the effects of changes in orbital parameters on the seasonal water cycle.     

Voyager U.V. spectrometer observations of He 584 Å dayglow at Jupiter

The intensity of Jupiter's He 584 Å airglow has been measured by the Voyager U.V. spectrometers. The disc-averaged brightness is about 4 Rs and limb darkening is present. The intensity probably varies with longitude, the variation being out of phase with the H Lyman-α intensity bulge. Modelling of resonance scattering of the solar He 584 Å line by Jupiter's atmosphere has shown that the hydrogen and helium emissions can be explained about equally well by at least two self-consistent scenarios involving the structure (temperature and eddy diffusion coefficient) and excitation of the atmosphere. All our evidence points to a dramatic change of conditions in the Jovian atmosphere in the time between Pioneer and Voyager encounters.     

Large seasonal variations in Triton's atmosphere

Triton's seasons differ materially from those of Pluto owing to four important differences in the governing physics: First, the obliquity of Triton is significantly less than Pluto's obliquity. Second, Triton's inclined orbit precesses rapidly about Neptune so that a complicated seasonal variation in the latitude of the Sun occurs for Triton. Third, Neptune's orbit is much more circular than Pluto's orbit so that the sunlight intercepted by Triton's disk does not vary seasonally. Finally, Triton's atmosphere cannot be saturated at the lower latitudes so that the mass of the atmosphere is controlled by the temperature of the high-latitude ices or liquids (polar caps), as for CO₂ on Mars. The consequences of Triton's entire surface being covered with volatile substances have been examined. It is found that the circularity of Neptune's orbit then implies that Triton would have hardly any seasonal variation at all in surface temperature or atmospheric bulk, in spite of the complicated precessional effects of Triton's orbit. The only seasonal effect would be the migration of surface ices and liquids. This scenario is ruled out because it implies a column CH₄ abundance much higher than that observed and because it quickly depletes the lower latitudes of volatiles. It is concluded that Triton's most volatile surface substances are probably relegated to latitudes higher than 35° and probably form polar caps. The temperature of the polar caps should be nearly equal, even during midwinter/midsummer when the insolation of the summer pole is greatest. If the summer pole completely sublimates during one of the “major” summers, Triton's atmosphere may begin to freeze out over the winter caps. It is therefore expected that Triton's atmosphere undergoes large and complex seasonal variations. Triton is currently approaching a “maximum southern summer”, and over the remainder of this century, a dramatic increase in CH₄ abundance above the current upper limit of 1 m-Am may be witnessed.     

Photochemical aerosols in Titan's atmosphere

The optical properties of polymers, produced photolytically from ethylene, which was detected in Titan's atmosphere and from acetylene or hydrogen cyanide which may be present there, were studied experimentally. It is shown that an aerosol consisting of polyethylene provides an excellent fit to the variation of Titan's albedo with wavelength, while polymers of acetylene or hydrogen cyanide do not. This fit seems to remove the requirement of nitrogen-bearing polymers, which was proposed earlier to account for Titan's red coloration. Therefore, Titan's coloration does not necessarily imply the presence of nitrogen in its atmosphere. It is also proposed that above the layer of larger aerosol particles, whose scattering determines the phase function, there are smaller particles of the same material, which act as an absorbing haze to darken and slightly redden the underlying aerosol. This high-altitude haze also causes the observed strong limb-darkening.     

Models for planetary eclipses of the uranian satellites

A model for computing the brightness of a satellite in the shadow of a planet is described, which takes into account the Sun–planet–satellite–sensor geometry, the satellite bi-directional reflectance function, and the refraction of sunlight in the planetary atmosphere. Synthetic light curves for eclipse ingress or egress of the five large satellites of Uranus are generated. The model luminosities can be fitted to photometric observations in order to calculate a precise distance between the centers of the satellite and the planet. Alternately, when the satellite ephemeris is accurately known the atmospheric state of the planet can be studied.     

A technique to deduce atmospheric temperature and constituent profiles from a planet's limb radiance profile

The concept is described of deducing the temperature and constituent profile of a planetary atmosphere from orbiter measurements of the planet's ir limb radiance profile. Expressions are derived for the weighting functions associated with the limb radiance profile for a Goody random band model. Analysis of the weighting functions for the Martian atmosphere indicates that a limb radiance profile in the 15 μm CO₂ band can be used to determine the Martian atmospheric temperature profile from 20 to 60 km. Simulation of the Martian limb radiance profile in the rotational water vapor band indicates that Martian water vapor mixing ratios can be inferred from limb radiance observations in a water vapor band.     

Cosmic dust,planets and related problems

The study of micrometeorites which reach the Earth and of cosmic dust in general in inter-planetary space and in a planet's atmosphere may contribute significantly to the resolution of important astrophysical and geophysical problems, such as the origin and evolution of our solar system and the universe, and the problem of medium range weather forecasting (for about one month), in addition to the practical problem of NASA's “Man in Space” Programme. In this paper, the questions related to the fall of matter of cosmic origin on Earth, a possible mechanism for the capture of micrometeoric particles by the Earth and other planets of the solar system, indicated by the author, will be dealt with.