Von R. Eshleman

The atmosphere of Titan: An analysis of the Voyager 1 radio occultation measurements

Abstract Two coherently related radio signals transmitted from Voyager 1 at wavelengths of 13 cm (S-band) and 3.6 cm (X-band) were used to probe the equatorial atmosphere of Titan. The measurements were conducted during the occultation of the spacecraft by the satellite on November 12, 1980. An analysis of the differential dispersive frequency measurements did not reveal any ionization layers in the upper atmosphere of Titan. The resolution was approximately 3 × 10 3 and 5 × 10 3 electrons/cm 3 near the evening and morning terminators, respectively. Abrupt signal changes observed at ingress and egress indicated a surface radius of 2575.0 ± 0.5 km, leading to a mean density of 1.881 ± 0.002 g cm −3 for the satellite. The nondispersive data were used to derive profiles in height of the gas refractivity and microwave absorption in Titans troposphere and stratosphere. No absorption was detected; the resolution was about 0.01 dB/km at the 13-cm wavelength. The gas refractivity data, which extend from the surface to about 200 km altitude, were interpreted in two different ways. In the first, it is assumed that N 2 makes up essentially all of the atmosphere, but with very small amounts of CH 4 and other hydrocarbons also present. This approach yielded a temperature and pressure at the surface of 94.0 ± 0.7°K and 1496 ± 20 mbar, respectively. The tropopause, which was detected near 42 km altitude, had a temperature of 71.4 ± 0.5°K and a pressure of about 130 mbar. Above the tropopause, the temperature increased with height, reaching 170 ± 15°K near the 200-km level. The maximum temperature lapse rate observed near the surface (1.38 ± 0.10°K/km) corresponds to the adiabatic value expected for a dry N 2 atmosphere—indicating that methane saturation did not occur in tbis region. Above the 3.5-km altitude level the lapse rate dropped abruptly to 0.9 ± 0.1°K/km and then decreased slowly with increasing altitude, crossing zero at the tropopause. For the N 2 atmospheric model, the lapse rate transition at the 3.5-km level appears to mark the boundary between a convective region near the surface having the dry adiabatic lapse rate, and a higher stable region in radiative equilibrium. In the second interpretation of the refractivity data, it is assumed, instead, that the 3.5 km altitude level corresponds to the bottom of a CH 4 cloud layer, and that N 2 and CH 4 are perfectly mixed below this level. These assumptions lead to an atmospheric model which below the clouds contains about 10% CH 4 by number density. The temperature near the surface is about 95°K. Arguments concerning the temperature lapse rates computed from the radio measurements appear to favor models in which methane forms at most a limited haze layer high in the troposphere.

Occultation Experiment: Results of the First Direct Measurement of Mars's Atmosphere and Ionosphere

Changes in the frequency, phase, and amplitude of the Mariner IV radio signal, caused by passage through the atmosphere and ionosphere of Mars, were observed immediately before and after occultation by the planet. Preliminary analysis of these effects has yielded estimates of the refractivity and density of the atmosphere near the surface, the scale height in the atmosphere, and the electron density profile of the Martian ionosphere. The atmospheric density, temperature, and scale height are lower than previously predicted, as are the maximum density, temperature, scale height, and altitude of the ionosphere.

The atmosphere of Mars analyzed by integral inversion of the Mariner IV occultation data.

Abstract The aim of this paper is to present results of a detailed analysis of the Mariner IV radio occultation data, using the data as directly as possible with a minimum number of assumptions and interpretations. Integral inversion of the basic measurements is used to obtain height profiles of refractivity over the immersion and emersion points on Mars. For the upper atmosphere this yields directly the ionospheric profile, for which a profile representing plasma temperature divided by mean molecular mass is derived assuming either diffusive or photoionization equilibrium conditions. Molecular number density, temperature, and pressure profiles for the lower atmosphere are derived for two constituent models assuming only mixed, non-polar molecules in hydrostatic equilibrium. It is believed that rather high precision has been obtained for conditions at the immersion point, but that the emersion results are less precise due to a change in the mode of operation of the radio system.

Particle size distributions in Saturn's rings from Voyager 1 radio occultation

Abstract Observations of microwave opacity τ[λ] and near forward scatter from Saturns rings at wavelengths λ of 3.6 and 13 cm from the Voyager 1 ring occultation experiment contain information regarding ring particle sizes in the range of about a = 0.01 to 15 m radius. The opacity measurements τ[3.6] and τ[13] are sufficient to constrain the scale factor n(a0) and index q of a power law incremental size distribution n(a) = n(a0)[a0/a]q, assuming known minimum and maximum sizes and a many-particle-thick model. The families of such distributions are highly convergent in the centimeter-size range. Forward scatter at 3.6 cm can be used to solve for a general distribution over the radius range 1 ⪷ a ⪷ 15 m by integral inversion and inverse scattering methods, again assuming a many-particle-thick slab-type radiative transfer model. Distributions n(a) valid over 0.01 ⪷ a ⪷ 15 m are obtained by combining the results from the two types of measurements above. Mass distributions may be computed directly from n(a). Such distributions, partly measured and partly synthesized, have been obtained for four features in the ring system centered at 1.35, 1.51, 2.01, and 2.12 Saturn radii (Rs). The size and mass distributions both cut off sharply at a ≅ 4–5 m; the mass distribution peaks over the narrow size range 3 ⪷ a ⪷ 4 m for all four locations. No single power law distribution is consistent with the data over the entire interval 0.01 ⪷ a ⪷ 5 m , although a power law-type model is consistent with the data over a limited size range of 0.01 ⪷ a ⪷ 1 m , where the indices q = 3.4 and 3.3 are obtained from the slab model for the features located at 1.51 and 2.01 Rs. The fractional contribution of the suprameter particles to the microwave opacity in each feature appears to be about 1 3 , 1 3 , 2 3 , and 1 , respectively, with the fraction at 2.12 Rs being the least certain. The cumulative surface mass per unit area obtained for the classical slab model is approximately 11, 16, 41, and 132 g/cm2 for the four features, respectively, if the particles are solid H2O ice. Both the fractional opacity and the mass density estimates represent upper bounds implied by the assumption of a uniformly mixed set of particles in a many-particle-thick vertical profile; lower estimates would result if the rings were assumed to be nearly a monolayer or if the vertical distribution of particles were size dependent.

Voyager 2 radio science observations of the Uranian system Atmosphere, rings, and satellites

Voyager 2 radio occultation measurements of the Uranian atmosphere were obtained between 2 and 7 degrees south latitude. Initial atmospheric temperature profiles extend from pressures of 10 to 900 millibars over a height range of about 100 kilometers. Comparison of radio and infrared results yields mole fractions near the tropopause of 0.85 and 0.15 � 0.05 for molecular hydrogen and helium, respectively, if no other components are present; for this composition the tropopause is at about 52 kelvins and 110 millibars. Distinctive features in the signal intensity measurements for pressures above 900 millibars strongly favor model atmospheres that include a cloud deck of methane ice. Modeling of the intensity measurements for the cloud region and below indicates that the cloud base is near 1,300 millibars and 81 kelvins and yields an initial methane mole fraction of about 0.02 for the deep atmosphere. Scintillations in signal intensity indicate small-scale stucture throughout the stratosphere and upper troposphere. As judged from data obtained during occultation ingress, the ionosphere consists of a multilayer structure that includes two distinct layers at 2,000 and 3,500 kilometers above the 100-millibar level and an extended topside that may reach altitudes of 10,000 kilometers or more. Occultation measurements of the nine previously known rings at wavelengths of 3.6 and 13 centimeters show characteristic values of optical depth between about 0.8 and 8; the maxim value occurs in the outer region of the ∈ ring, near its periapsis. Forward-scattered signals from this ring have properties that differ from those of any of Saturns rings, and they are inconsistent with a discrete scattering object or local (three-dimensional) assemblies of orbiting objects. These signals suggest a new kdnd of planetary ring feature characterized by highly ordered cylindrical substructures of radial scale on the order of meters and azimuthal scale of kilometers or more. From radio data alone the mass of the Uranian system is GMsys = 5,794,547– 60 cubic kilometers per square second; from a combination of radio and optical navigation data the mass of Uranus alone is GMu = 5,793,939� 60 cubic kilometers per square second. From all available Voyager data, induding imaging radii, the mean uncompressed density of the five major satellites is 1.40� 0.07 grams per cubic centimeter; this value is consistent with a solar mix of material and apparently rules out a cometary origin of the satellites.

Radio science with Voyager 1 at Jupiter - Preliminary profiles of the atmosphere and ionosphere

A preliminarv profile of the atmosphere of Jupiter in the South Equatorial Belt shows (i) the tropopause occurring at a pressure level of 100 millibars and temperature of about 113K, (ii) a higher warm inversion layer at about the 35-millibar level, and (iii) a lower-altitude constant lapse rate matching the adiabatic value of about 2 K per kilometer, with the temperatutre reaching 150 K at the 600-millibar level. Preliminary afternoon and predawn ionospheric profiles at 12� south latitude and near the equator, respectively, have topside plasma scale heights of 590 kilometers changing to 960 kilometers above an altitucde of 3500 kilometers for the dayside, and about 960 kilomneters at all measured heights above the peak for the nightside. The higher value of scale height corresponds to a plasma temperature of 1100 K under the assumption of a plasma of protons and electrons in ambipolar diffusive equilibrium. The peak electron concentration in the upper ionosphere is approximately 2 x 105 per cubic centimeter for the dayside and about a factor of 10 less for the nightside. These peaks occur at altitudes of 1600 and 2300 kilometers, respectively. Continuing analyses are expected to extend and refine these results, and to be used to investigate other regions and phenomena.

Radio science with voyager 2 at saturn: atmosphere and ionosphere and the masses of mimas, tethys, and iapetus.

Voyager 2 radio occultation measurements of Saturns atmosphere probed to the 1.2-bar pressure level, where the temperature was 143 � 6 K and the lapse rate apparently equaled the dry adiabatic value of 0.85 K per kilometer. The tropopause at both mid-latitude occultation locations (36.5�N and 31�S) was at a pressure level of about 70 millibars and a temperature of approximately 82 K. The stratospheric structures were very similar with the temperature rising to about 140 K at the 1-millibar pressure level. The peak electron concentrations sensed were 1.7 x 104 and 0.64 x 104 per cubic centimeter in the predawn (31�S) and late afternoon (36.5�N) locations. The topside plasma scale heights were about 1000 kilometers for the late afternoon profile, and 260 kilometers for the lower portions and 1100 kilometers for the upper portions of the topside predawn ionosphere. Radio measurements of the masses of Tethys and Iapetus yield (7.55 � 0.90) x 1020 and (18.8 � 1.2) x 1020 kilograms respectively; the Tethys-Mimas resonance theory then provides a derived mass for Afimas of (0.455 � 0.054) x 1020 kilograms. These values for Tethys and Mimas represent major increases from previously accepted ground-based values, and appear to reverse a suggested trend of increasing satellite density with orbital radius in the Saturnian system. Current results suggest the opposite trend, in which the intermediate-sized satellites of Saturn may represent several classes of objects that differ with respect to the relative amounts of water, ammonia, and methane ices incorporated at different temperatures during formation. The anomalously low density of lapetus might then be explained as resulting from a large hydrocarbon content, and its unusually dark surface markings as another manifestation of this same material.

The microwave opacity of Saturn's rings at wavelengths of 3.6 and 13 cm from Voyager 1 radio occultation

Abstract Radio occultation observations of Saturns rings with Voyager 1 provided independent measurements of complex (amplitude and phase) microwave extinction and near-forward scattering cross section of the rings at wavelengths (λ) of 3.6 and 13 cm. The ring opening was 5.9°. The normal microwave opacities, τ[3.6] and τ[13], provide a measure of the total cross-sectional area of particles larger than about 1 and 4 cm radius, respectively. Ring C exhibits gently undulating (∼ 1000 km) structure of normal opacity τ[3.6] ≲ 0.25 except for several narrow imbedded ringlets of less than about 100 km width and τ[3.6] ∼ 0.5 to 1.0. The normalized differential opacity Δτ/τ[3.6], where Δτ = τ[3.6] − τ[13], is about 0.3 over most of ring C, indicating a substantial fraction of centimeter-size particles. Some narrow imbedded ringlets show marked increases in Δτ/τ[3.6] near their edges, implying an enhancement in the relative population of centimeter-size and smaller particles at those locations. In the Cassini division, several sharply defined gaps separate regions of opacity τ ∼ 0.08 and τ ∼ 0.25; the opacity in the Cassini Division appears to be nearly independent of λ. The boundary features at the outer edges of ring C and the Cassini Division are remarkably similar in width and opacity profile, suggesting a similar dynamical control. Ring A appears to be nearly homogeneous over much of its width with 0.6 τ[3.6] ⪞ 1.2 . The differential opacity for the inner one-fourth of ring B is Δτ/τ[3.6] ∼ 0.15. There are no gaps in ring B exceeding about 2 km in width. Ring F was observed at 3.6 cm as a single ringlet of radial width ⪝ 2 km , but was not detected in 13 cm data.

Radio science with voyager at jupiter: initial voyager 2 results and a voyager 1 measure of the io torus.

Voyager 2 radio signals were observed essentially continuously during a grazing occultation of the spacecraft by the southern limb of Jupiter. Intensity data show a classic atmospheric occultation profile and the effects of turbulence and ionospheric focusing and defocusing. No reliable profile of the neutral atmosphere has yet been obtained, primarily because of a combination of large trajectory uncertainties and error multiplication effects associated with the grazing geometry of the Voyager 2 occultation. Analysis of the dispersive ionospheric refraction data yields preliminary profiles for the topside ionosphere at 66.7�S (entry in the evening) and 50.1�S (exit in the morning) that are reversed with respect to corresponding Voyager 1 profiles in terms of plasma concentration at a fixed altitude. Plasma scale heights and temperatures of 880 kilometers, 1200 K and 1040 kilometers, 1600 K were obtained for morning and evening conditions, respectively. Preliminary reduction of the pre-encounter occultation of Voyager 1 by the Io torus yields an average plasma density of about 1000 electrons per cubic centimeter.

Radio Communication by Scattering from Meteoric Ionization

By a consideration of the amplitude and duration of echoes forward-scattered from individual meteor ionization trails, and of the probability of detecting randomly oriented trails over an oblique radio propagation path, an estimate of the contribution of meteoric ionization to extended range hf and vhf radio transmission has been obtained. It has been concluded that meteoric ionization alone would give a virtually continuous signal for a transmission path of about 1,000 km at frequencies near 15 mc. For the very high frequencies, scattering from meteor trails has been found to be at least an important contributing factor to the propagation of a signal over an oblique path. A precise evaluation of the role of this process must await a better determination of the number of trails as a function of their ionization density.