Lateral inhomogeneities in the Venus atmosphere: Analysis of thermal infrared maps

The thermal infrared maps of Venus published by Murray, Wildey, and Westphal (1963) and Westphal, Wildey, and Murray (1965) have been analyzed systematically in order to separate the observed intensity into a limb-darkening component and a solar-associated component representing fixed patterns of intensity corotating with the earth and sun, respectively. Interesting new results are obtained for the solar-associated component. Regions near the subsolar point and the poles are not covered in the original maps or in the analysis.The solar-associated pattern of intensity is very nearly symmetric about the equator. In both northern and southern hemispheres, an intensity minimum seems to occur near the morning terminator at middle to high latitudes, slightly beyond the limit of the maps. An intensity maximum occurs on the equator slightly to the east of the antisolar point. Three broad ridges of relatively high intensity radiate away from this point, one pointing to the west along the equator, the others pointing to the northeast and southeast, respectively. The eastward tilt of the latter two ridges may indicate that horizontal exchange is important in maintaining the equatorial maximum of zonal momentum which is associated with the 4-day circulation of the Venus atmosphere.     

A comment on the insolation at Mercury

E. Van Hemelrijck and J. Vercheval [Icarus48, 167–179 (1981)] presented calculations of the insolation at Mercury and Venus which neglect the finite angular size of the Sun. To determine the temperature structure in the subsurface a more accurate calculation is needed, especially at longitudes ±90° on Mercury, where the Sun takes 18 days to rise or set. These calculations are presented here.     

Comment on absorbing regions in the atmosphere of Venus as measured by radio occultation

Two recent papers, one by A.J. Kliore, C. Elachi, I.R. Patel, and J.B. Cimeno, Icarus37, 51-2- 72, 1979, and one by B. Lipa and G.L. Tyler, Icarus39, 192–208, 1979, reach fundamentally different conclusions concerning microwave absorption in the atmosphere of Venus, even though they are based on the same Mariner 10 radio occultation data. The Lipa and Tyler results are in general agreement with earlier Mariner 5 measurements analyzed by G. Fjeldbo, A.J. Kliore, and V.R. Eshleman, Astron. J.76, 123–140, 1971. We find that in the Kliore et al. treatment: (1) the effects of measurements and analysis uncertainties in the derived values of absorption are underestimated; (2) an incorrect formula is used for computation of the refractive effects needed to determine the absorption; (3) detailed features of a derived profile of absorption would have been created in an optically thin region by known motions of the spacecraft antenna, if its axial direction were biased about 0.5° from the computed directions; and (4) this particular angular bias is consistent with other available information about an apparent residual difference between true and reconstructed antenna pointing directions. We conclude that: (1) there is no credible evidence for measurable microwave absorption in the atmosphere of Venus at heights greater than 55 km for any of the wavelengths that have been used in radio occultation experiments, even though Kliore et al. indicate that there are significant amounts up to at least 70 km for both Mariner 10 wavelengths (13 and 3.6 cm); (2) absorption in the region 35 to 50 km has been reasonably well determined from the two concordant Mariner 5 and 10 analyses, but only at one wavelength (13 cm); and (3) improved instrumentation and careful planning and analysis will be required for the radio occultation technique to realize its potential for the study of absorbing regions in the atmospheres of Venus and the major planets.     

Optical signature of Venus lightning as seen from space

The calculations of M. A. Williams, L. W. Thomason, and D. M. Hunten (Icarus52, 166–170, 1982) for the light transmitted to space by a Venus lightning flash by dropping the use of similarity relations have been improved. This revised model increases their escape fractions and image sizes by about a factor of 2; however, their conclusions remain valid.     

Angular momentum drain: A mechanism for despinning asteroids

It is proposed that a new mechanism—angular momentum drain—helps account for the relatively slow rotation rates of intermediate-sized asteroids. Impact ejecta on a spinning body preferentially escape in the direction of rotation. This material systematically drains away spin angular momentum, leading to the counterintuitive result that collisions can reduce the spin of midsized objects. For an asteroid of mass M spinning at frequency ω, a mass loss δM correspond to an average decrease in rotation rate $\text{δω ≈}\text{ωδM}\text{M}$. A. W. Harris' (1979), Icarus40, 145–153) theory for the collisional evolution of asteroidal spins is significantly altered by inlusion of this effect. While the modified theory is still somewhat artificial, comparison of its predictions with the data of S. F. Dermott, A. W. Harris, and C. D. Murray (1984, Icarus57, 14–34) suggests that angular momentum drain is essential for understanding the statistics of asteroidal rotations.     

Stability of Pluto's atmosphere

L. Trafton (1980, Icarus44, 53–61) has pointed out that a substantial methane atmosphere, observed on Pluto by U. Fink, B.A. Smith, D.C. Benner, J.R. Johnson, and H.J. Reitsema (1980, Icarus44, 62–71), appears to be unstable against blowoff. The difficulty is shown to disappear if the actual heat balance and thermal structure are considered, instead of the classic assumption that the upper atmosphere is isothermal. An energy-limited flux (referred to the surface area) of 3.9 × 10¹⁰ cm^?2 sec^?1 is found. The loss of methane ice over the age of the solar system is an acceptable 3 km.     

Venus mesosphere and thermosphere: II. Global circulation,temperature, and density variations

Recent Pioneer Venus observations have prompted a return to comprehensive hydrodynamical modeling of the thermosphere of Venus. Our approach has been to reexamine the circulation and structure of the thermosphere using the framework of the R. E. Dickinson an E. C. Ridley (1977, Icarus 30, 163–178), symmetric two-dimensional model. Sensitivity tests were conducted to see how large-scale winds, eddy diffusion and conduction, and strong 15-μm cooling affect day-night contrasts of densities and temperatures. The calculated densities and temperatures are compared to symmetric empirical model fields constructed from the Pioneer Venus data base. We find that the observed day-to-night variation of composition and temperatures can be derived largely by a wave-drag parameterization that gives a circulation system weaker than predicted prior to Pioneer Venus. The calculated mesospheric winds are consistent with Earth-based observations near 115 km. Our studies also suggest that eddy diffusion is only a minor contributor to the maintenance of observed day and nightside densities, and that eddy coefficients are smaller than values used by previous one-dimensional composition models. The mixing that occurs in the Venus thermosphere results from small-scale and large-scale motions. Strong CO₂ 15-μm cooling buffers solar perturbation such that the response by the general circulation to solar cycle variation is relatively weak.     

The angular momenta of solar system bodies: Implications for asteroid strengths

The angular momentum H is plotted versus mass M for the planets and for all asteroids with known rotation rates and shapes, primarily taken from D. C. McAdoo and J. A. Burns [Icarus18, 285–293 (1973)]. An asteroid's angular momentum is derived from its rotation rate as determined by the period of its lightcurve, its shape as indicated by the lightcurve amplitude, and where possible its size as given by polarimetry or radiometry. The asteroid is assumed to be rotating about its axis of maximum moment of inertia. As previously found by F. F. Fish [Icarus7, 251–256 (1967]) and W. K. Hartmann and S. M. Larson [Icarus7, 257–260 (1967)], H is approximately proportional to $\text{M}{}^{\mathrm{<mtext>5</mtext><mtext>3</mtext>}}$, which shows that the asteroids and most planets spin with nearly the same rate. The very smallest asteroids on the plot deviate from the above reaction, usually containing excess angular momentum. This suggests that collisions have transferred substantial angular momentum to the smallest asteroids, perhaps causing their internal stress states to be substantially modified by centrifugal effects.The forces produced by gravitation are then compared to centrifugal effects for a rotating, triaxial ellipsoid of density 3 g cm^?3. For all asteroids with known properties the gravitational attraction is shown to be larger than the centrifugal acceleration of a particle on the surface: thus the observed asteroid regoliths are gravitationally bound. Poisson's equation for the gravitational potential is investigated and it is shown by mathematical and physical arguments that any arbitrarily shaped ellipsoid with the attractive surface force boundary condition found above will have only attractive internal forces. Thus the internal stress states in asteroids are always compressive so that asteroids could be internally fractured without losing their integrity.     

Theoretical interpretation of the 0.7820 μm CO2 band and 0.8226 μm H2O line on Venus

We have analyzed the P6, P8, and P10 lines in the 0.7820 μm CO₂ band of Venus using a scattering model. Our new results compare favorably with previous results from the 1.05 μm CO₂ band. We considered nonabsorbing and absorbing clouds. We found that the anisotropic scattering mean free path for both models at the 0.2atm level is between 0.55 and 0.73km, a range close to the value of 1 km for terrestrial hazes. We used our scattering models to synthesize the 0.8226 μm H₂O line, assuming that the clouds are composed of sulfuric acid drops, and found our nonabsorbing cloud required a sulfuric acid concentration of 82% by weight, while our thicker absorbing cloud required a concentration of 89%. A comparison of the variation of optical depth with height for our cloud models with the variation reported by Prinn (1973, Science182, 1132–1134) showed that, within a factor of 2, the variation for Prinn's thinnest cloud agreed with ours. Whitehill and Hansen (1973, Icarus20, 146–152) have recently confirmed the work of Regas et al. (1973a, J. Quant. Spectry. Radiative Transfer13, 461–463) which showed that two cloud layers are not required to explain the CO₂ phase variation of Venus. Prinn's recent photochemical study of sulfuric acid clouds further supports a single, continuous cloud layer in the line formation region instead of two cloud layers with an extensive clear region between. The single layer model appears more likely because the maximum particle density in Prinn's cloud occurs in the clear region between the two layers in the models of Hunt (1972, J. Quant. Spectry. Radiative Transfer12, 405–419) and Carleton and Traub (1972, Bull. Amer. Astron. Soc.4, 362.).     

Net thermal radiation in the atmosphere of Venus

The four entry probes of the Pioneer Venus mission measured the radiative net flux in the atmosphere of Venus at latitudes of 60°N, 31°S, 27°S, and 4°N. The three higher latitude probes carried instruments (small probe net flux radiometers; SNFR) with external sensors. The measured SNFR net fluxes are too large below the clouds, but an error source and correction scheme have been found (H. E. Revercomb, L. A. Sromovsky, and V. E. Suomi, 1982, Icarus52, 279–300). The near-equatorial probe carried an infrared radiometer (LIR) which viewed the atmosphere through a window in the probe. The LIR measurements are reasonable in the clouds, but increase to physically unreasonable levels shortly below the clouds. The probable error source and a correction procedure are identified. Three main conclusions can be drawn from comparisons of the four corrected flux profiles with radiative transfer calculations: (1) thermal net fluxes for the sounder probe do not require a reduction in the Mode 3 number density as has been suggested by O. B. Toon, B. Ragent, D. Colburn, J. Blamont, and C. Cot (1984, Icarus57, 143–160), but the probe measurements as a whole are most consistent with a significantly reduced mode 3 contribution to the cloud opacity; (2) at all probe sites, the fluxes imply that the upper cloud contains a yet undetected source of IR opacity; and (3) beneath the clouds the fluxes at a given altitude increase with latitude, suggesting greater IR cooling below the clouds at high latitudes and water vapor mixing ratios of about 2–5 × 10^?5 near 60°, 2–5 × 10^?4 near 30°, and 5 × 10^?4 near the equator. The suggested latitudinal variation of IR cooling is consistent with descending motions at high latitudes, and it is speculated that it could provide an important additional drive for the general circulation.     

A perturbative treatment of motion near the 3/1 commensurability

A semianalytic perturbation theory for motion near the 3/1 commensurability in the planar elliptic restricted three-body problem is presented. The predictions of the theory are in good agreement with the features found on numerically generated surfaces of section; a global understanding of the phase space is achieved. The unusual features of the motion discovered by J. Wisdom (1982, Astron. J.87, 577–593; 1983a, Icarus56, 51–74) are explained. The principal cause of the large chaotic zone near the 3/1 commensurability is identified, and a new criterion for the existence of large-scale chaotic behavior is presented.     

Settling and growth of dust particles in a laminar phase of a low-mass solar nebula

It is considered that some vertical convection as well as possible turbulence in an early phase of solar nebula soon terminates owing to diminution of the temperature dependence of dust opacity due to rapid growth of dust particles. We reexamine settling and growth of dust particles in the subsequent laminar phase of the solar nebula in detail, treating a dust layer as a two-component fluid composed of the dust and the gas. We obtain analytic expressions for the settling path, the growing size, and the settling time. The settling process is divided into two phases, i.e., an early gas-dominant phase and a later dust-dominant phase. So far, only the former phase, where the particle path finally turns from vertical to radial, has been investigated. In the latter phase, dust particles drag the gas, rather than the gas does dust particles. Consequently, the particle path turns from radial to vertical. Dust particles grow most appreciably and rapidly in a radially sweeping phase. The final radii of dust particles at the onset of gravitational instability of the dust layer are 20, 5.9, and 0.60 cm in the Earth's, Jupiter's, and Neptune's zones, respectively. These values are much smaller than those obtained previously by S.J. Weidenschilling [1980, Icarus44, 172–189] and Y. Nakagawa et al. [1981, Icarus45, 517–528]. The total settling times are 1.9 × 10³, 4.6 × 10³, and 2.8 × 10⁴ years in the above-mentioned three zones, respectively. These are somewhat smaller than those obtained by the previous studies. Most of the settling time is spent in the early vertically settling phase.     

Variability of carbon monoxide in the mars atmosphere

In January of 1982 we measured a microwave spectrum of CO in the Martian atmosphere utilizing the rotational J = 1 → 2 transition of CO. We have analyzed data and reanalyzed the microwave spectra of R. K. Kakar, J. W. Waters, and W. J. Wilson, (Science196, 1090–1091, 1977, measured in 1975) and J. C. Good and F. P. Schloerb, (Icarus47, 166–172, 1981 measured in 1980) in order to constrain estimates of the temporal variability of CO abundance in the Martian atmosphere. Our values of CO column density from the data of Karar et al., Good and Schloerb, and our own are 1.7 ± 0.9 × 10²⁰, 3.0 ± 1.0 × 10²⁰, and 4.6 ± 2.0 × 10²⁰cm^?2, respectively. The most recent estimate of CO column density from the 1967 infrared spectra of J. Connes, P. Connes, and J.P. Maillard, (Atlas de Spectres Infarouges de Venus, Mars, Jupiter, et Saturne, Editions due Centre National de la Recherche Scientifique, Paris, 1969), is 2.0 ± 0.8 × 10²⁰ cm^?2 (L.D.G. Young and A.T. Young, Icarus30, 75–79, 1977). The large uncertainties given for the microwave measurements are due primarily to uncertainty in the difference between the continuum brightness temperature and atmospheric temperatures of Mars. We have accurately calculated the variation among the observations of the continuum (surface) brightness temperature of Mars, which is primaroly a function of the observed aspect of Mars. A more difficult problem to consider is variability of global atmospheric temperatures among the observations, particularly the effects of global dust storms and the ellipticity of the orbit of Mars. The large bars accompanying our estimates of CO column density from the three sets of microwave measurements are primarily caused by an assumed uncertainty of ±10°K in our atmospheric temperature model due to possible dust in the atmosphere. A qualitative consideration of seasonal variability of global atmospheric temperatures among the measurements suggests that there is not strong evidence for variability of the column abundance of CO on Mars, although variability of 0–100% over a time scale of several years is allowed by the data set. The implication for the variability of Mars O₂ is, crudely, a factor of two less. We found that the altitude distribution of CO in the atmosphere of Mars was not well constrained by any of the spectra, although our spectrum was marginally better fitted by an altitude increasing profile of CO mixing ratios.     

An analysis of bending waves in Saturn's rings using voyager radio occultation data

The oblique geometry of the Voyager 1 radio occulation of Saturn's rings resulted in a strong coupling between the local slope of the ring midplane and the associated radio opacity (optical depth). We apply a model of this relationship to those regions of the rings where bending waves have been observed in the radio data. Using the Shu et al. linear model for a bending wave (F.H. Shu, J.N. Cuzzi, and J.J. Lissauer, 1983,Icarus53, 185–206), we obtain height profiles for the Mimas 5:3 and 7:4 bending waves. The first oscillation of the Mimas 5:3 bending wave has an amplitude of about 800 m, in agreement with the prediction of the Shu et al. model. However, the rest of the wave may be explained only by either a greatly decreased amplitude in the region beyond the second cycle, or by a significant enhancement in radio optical depth in the region of the bending wave. The shape of the enhancement necessary is similar to that of the enhancement at photopolarimetry wavelengths (L.W. Esposito, M. O'Callaghan, and R.A. West, 1983,Icarus56, 439–452), but differs in the region of the first cycle. Our solution gives 131,901±6 km as the resonance location, and a surface mass density of 35±6g cm⁻². The error bars on the resonance location do not include the uncertainty in the radial scale of the radio occultation data, which is approximately 10 km (R.A. Simpson, G.L. Tyler, and J.B. Holberg, 1983,Astron. J.88, 1531–1536). The Mimas 7:4 bending wave conforms more closely to the linear model, and requires no reduction in amplitude or enhancement in optical depth. We find a surface mass density of 30.5±9 g cm⁻², and resonance location at 127,765±7km.     

Some remarks on the capture of Triton and the origin of Pluto

Harrington and Van Flandern (1979, Icarus39, 131–136) suggests that the irregular features of the Neptunian satellite system and Pluto's escape were caused by an encounter with a massive external body. They rule out the alternative mechanism based on the capture of Triton (which seems more plausible because it does not appeal to any unobserved object) on the basis of an incorrect deduction from McCord's (1966, Astron. J.71, 585–590) analysis on the tidal decay of Triton's orbit. As a matter of fact, many recent results show that satellite captures are possible, and in the case of Triton several arguments support this interpretation.     

Size distribution of particles in planetary rings

Harris (Icarus24, 190–192) has suggested that the maximum size of particles in a planetary ring is controlled by collisional fragmentation rather than by tidal stress. While this conclusion is probably true, estimated radius limits must be revised upward from Harris' values of a few kilometers by at least an order of magnitude. Accretion of particles within Roche's limit is also possible. These considerations affect theories concerning the evolution of Saturn's rings, of the Moon, and of possible former satellites of Mercury and Venus. In the case of Saturn's rings, comparison of various theoretical scenarios with available observational evidence suggests that the rings formed from the breakup of larger particles rather than from original condensation as small particles. This process implies a distribution of particle sizes in Saturn's rings possibly ranging up to ～100 km but with most cross-section in cm-scale particles.     

Color photometry of surface features on Ganymede and Callisto

Voyager imaging data demonstrate that the scattering properties (“phase curves”) of all major terrain types on Ganymede and callisto are not significantly wavelength dependent between 0.4 and 0.6 μm. Our data suggest that the phase curves may be slightly steeper at the shorter wavelengths, consistent with the trend of telescopic observations near opposition. However, the differences are small and entirely within the uncertainties of our analysis. Our result indicates that the phase integrals (0.8 for Ganymede and 0.6 for Callisto) derived by S. W. Squyres and J. Veverka [Icarus46, 137–155 (1981)] from the abundant Voyager clear filter observations are reliable measures of the radiometric phase integrals. The corresponding values of the Bond albedo turn out to be 0.35 for Ganymede and 0.11 for Callisto.     

Numerical Evaluation of the General Yarkovsky Effect: Effects on Eccentricity and Longitude of Periapse

J. N. Spitale and R. Greenberg (2001, Icarus149, 222-234) developed a nonlinearized, finite-difference solution to the heat equation that yields orbital rates of change due to the Yarkovsky effect for small, spherical, bare-rock asteroids and used it to investigate changes in semimajor axis caused by the Yarkovsky effect. Here, we present results for changes in eccentricity and longitude of periapse. These results may be useful as benchmarks for simplified analytical solutions. Moreover, we explore a range of parameters, some of which are inaccessible to most other approaches. Instantaneous rates can be quite fast: For a 1-m scale body rotating with a 5-h period, de/dt can be as fast as 0.1 per million years (da/dt rates for similar test bodies were reported in J. N. Spitale and R. Greenberg (2001, Icarus149, 222-234)). For more typical rotation periods, these rates would be considerably slower. Output from our calculation method could be used in simulations of asteroid population evolution such as that by W. F. Bottke, D. P. Rubincam, and J. A. Burns (2000, Icarus145, 301-331). On long time scales, impacts would randomize the spin axis before significant orbital evolution could occur. Nevertheless, occasional favorable rotation states might persist long enough for substantial eccentricity changes to accumulate (1) if the body is decoupled from the main belt (e.g., many near-Earth asteroids), (2) if the population of very small (mm-scale) main-belt impactors is less than expected, or (3) if our numerical results are scaled up to km-size bodies.     

Spatially resolved infrared observations of Saturn: I. Equatorial limb scans at 20 μm

Drift scans of the equator of Saturn have been obtained through narrow band filters at 17.8, 19.7, and 22.7 μm. Spatial resolution was ?17% of the equatorial diameter. These observations clearly differentiate otherwise tenable atmospheric models. A published model by A. Tokunaga and R.D. Cess [Icarus32, 321–327 (1977)] is shown to represent these new observations significantly better than other models from the literature.