The structure of the atmosphere of Uranus

Models of the interior of Uranus (Podolak, 1976) suggest that the abundances of such substances as CH₄ are greatly enhanced with respect to solar abundances of heavy elements. Such enhancement leads to a new type of model atmosphere for Uranus, which agrees with observation if the internal energy flux is small (?10%) compared with the absorbed solar energy. An important feature of the models is the presence of a cloud of CH₄ droplets whose top is at a temperature of ?90°K and a pressure of ?4atm. Above the cloud, the atmosphere is stable because of the rapid decrease of the thermal flux with depth. Being saturated, most of the observable gaseous CH₄ is near the cloud; the CH₄ abundance above the cloud, of the order of 5 km-am, is a very sensitive function of the cloud-top temperature.     

Axel dust on Saturn and Titan

The scattering and absorption properties of Axel dust were investigated by means of Mie theory. We find that a flat distribution of particle radii between 0 and 0.1 μm, and an imaginary part of the index of refraction which varies as λ^?2.5 produce a good fit to the variation of Titan's geometric albedo with wavelength (λ) provided that τ_ext, the extinction optical depth of Titan's atmosphere at 5000 Å, is about 10. The real part of the complex index is taken to be 2.0. The model assumes that the mixing ratio of Axel dust to gas is uniform above the surface of Titan. The same set of physical properties for Axel dust also produces a good fit to Saturn's albedo if τ_ext = 0.7 at 5000 Å. To match the increase in albedo shortward of 3500 Å, a clear layer (containing about 7 km-am H₂) is required above the Axel dust. Such a layer is also required to explain the limb brightening in the ultraviolet. These models can be used to analyze the observed equivalent widths of the visible methane bands. The analysis yields an abundance of the order of 1000 m-am CH₄ in Titan's atmosphere. The derived CH₄/H₂ mixing ratio for Saturn is about 3.5 × 10^?3 or an enhancement of about 5 over the solar ratio.     

Interpretation of spatial and temporal variations of hydrogen quadropole absorptions in the Jovian atmosphere observed during the 1972 apparition

During the 1972 apparition of Jupiter, we carried out a patrol of the (3,0) S(1) and (4,0) S(1) quadrupole lines of molecular hydrogen in the equatorial region and in bands bounded by ±15 and ±49° zenographic latitude from the McDonald and Table Mountain Observatories. At the center of the Jovian disk, we found evidence of temporal variability of both lines over the duration of our observing period. We employ a technique which takes into account all radiative transfer processes in an inhomogeneous model of Jupiter's atmosphere, and use it to derive the effective level of formation of the spectral lines and the relative abundance of hydrogen. In this way, we are able to correlate measured changes in the equivalent widths of the hydrogen lines with variations in cloud structure. The effective pressure level at which the (4,0) S(1) line is formed varies in the range 2 ± 0.5 to 1.3 ± 0.2 atm, while for the (3,0) S(1) line, the pressure varies between 1.6 ± 0.5 and 1 ± 0.4 atm. If these variations are interpreted in terms of changes in elevation of the top of a dense lower cloud deck, the elevation apparently varied with an amplitude of 25 km during the observational period.Spatial variations in the strengths of both lines were also found. Both lines are weaker at the east limb than at the center of the disk (15–19%) while the variations toward the west limb are less pronounced (5%). Similar center-to-limb variations were found in the latitude bands bounded by ±15 and ±49°, although the lines were stronger in the northern component at the time of the observations.     

Venus mesosphere and thermosphere temperature structure: II. Day-night variations

A two-dimensional nonlinear hydrodynamic model has been developed for studying the global scale winds, temperature, and compositional structure of the mesosphere and thermosphere of Venus. The model is driven by absorption of solar radiation. Ultraviolet radiation produces both heating and photodissociation. Infrared solar heating and thermal cooling are also included with an accurate NLTE treatment. The most crucial uncertainty in determining the solar drive is the efficiency by which $\text{λ < 1080}\text{A}\text{?}$ solar radiation is converted to heat. This question was analyzed in Part I, where it was concluded that essentially all hot atom and O(¹D) energy may be transferred to vibrational-rotational energy of CO₂ molecules. If this is so, the minimum possible euv heating occurs and is determined by the quenching of the resulting excess rotational energy. The hydrodynamic model is integrated with this minimum heating and neglecting any small-scale vertical eddy mixing. The results are compared with predictions of another model with the same physics except that it assumes that 30% of $\text{λ < 1080}\text{A}\text{?}$ radiation goes into heat and that the heating from longer-wavelength radiation includes the O(¹D) energy. For the low-efficiency model, exospheric temperatures are ?300°K on the dayside and drop to < 180°K at the antisolar point. For the higher-efficiency model, the day-to-night temperature variation is from ?600°K to ?250°K. Both versions of the model predict a wind of several hundred meters per second blowing across the terminator and abruptly weakening to small values on the nightside with the mass flow consequently going into a strong tongue of downward motion on the nightside of the terminator. The presence of this circulation could be tested observationally by seeing if its signature can be found in temperature measurements. Both versions of the model indicate that a self-consistent large-scale circulation would maintain oxygen concentrations with ?5% mixing ratios near the dayside F-1 ionospheric peak but ?40% at the antisolar point at the same pressure level.     

Sources and sinks for atmospheric H2: A current analysis with projections for the influence of anthropogenic activity

Oxidation of CH₄ provides the major source for atmospheric H₂ which is removed mainly by reaction with OH. Biological activity at the Earth's surface appears to represent at most a minor sink for H₂. Anthropogenic activity is a significant source for both H₂ and CO in the present atmosphere and may be expected to exert a growing influence in the future. Models are presented which suggest a rise in the mixing ratio of H₂ from its present value of 5.6 × 10^?7 to about 1.8 × 10^?6 by the year 2100. The mixing ratio of CO should grow from 9.7 × 10^?8 to 2.3 × 10^?7 over the same time period and there should be a rise in CH₄ by about a factor of 1.5 associated with anthropogenically induced reductions in tropospheric OH.     

An upper limit on the aperture separation of ion drift meters

An upper limit has been calculated on the effective aperture separation or detector thickness of ion drift meters of two fundamental types. The limit applies to meters which compare currents collected by detectors with different view directions at the same retarding potential and to meters which measure the entire thermal ion distribution function. For both types, a single scanned detector may be employed instead of multiple detectors, in which case the limit applies to twice the radius of curvature of the path followed by the detector aperture during a scan, (i.e. the diameter of a spinning payload). The limit was found to be important in two cases. First, in the F region on spacecraft with stringent electrostatic cleanliness requirements, the 10% error limit was found to be 40 cm. Second, in the E region, the limit was found to be 1 cm.Originally submitted to the journalSpace Science Instrumentation.Deceased.     

The structure of a type-II supernova remnant G 160.5+2.8 (HB-9) observed at λ=12m

A two-dimensional brightness distribution of the old supernova remnant G 160.5+2.8 (HB-9) has been measured at a frequency of 25 MHz with the T-Shaped UTR-2 radio telescope, with the resolution 29×27. The source has a distinctly pronounced, almost spherical envelope. At the same time, individual features located near the internal boundary of the source are observable.Certain considerations are put forward to validate a suggestion that the brightest feature in the decameter range (maximum at=4^h55^m.3 and =46°.37) might be of an extragalactic origin.From structure considerations, the velocity of the pulsar PSR 0459+47 projected on HB-9 is estimated. The estimate does not contradict the hypothesis of Damasheket al. (1978) concerning a possible genetic relation between HB-9 and PSR 0459+47.