Andrew James Friedson

DISEQUILIBRIUM CARBON, OXYGEN, AND NITROGEN CHEMISTRY IN THE ATMOSPHERES OF HD 189733b AND HD 209458b

We have developed a one-dimensional photochemical and thermochemical kinetics and diffusion model to study the effects of disequilibrium chemistry on the atmospheric composition of “hot-Jupiter” exoplanets. Here we investigate the coupled chemistry of neutral carbon, hydrogen, oxygen, and nitrogen species on HD 189733b and HD 209458b and we compare the model results with existing transit and eclipse observations. We find that the vertical profiles of molecular constituents are significantly affected by transport-induced quenching and photochemistry, particularly on the cooler HD 189733b; however, the warmer stratospheric temperatures on HD 209458b help maintain thermochemical equilibrium and reduce the effects of disequilibrium chemistry. For both planets, the methane and ammonia mole fractions are found to be enhanced over their equilibrium values at pressures of a few bar to less than an mbar due to transport-induced quenching, but CH4 and NH3 are photochemically removed at higher altitudes. Disequilibrium chemistry also enhances atomic species, unsaturated hydrocarbons (particularly C2H2), some nitriles (particularly HCN), and radicals like OH, CH3, and NH2. In contrast, CO, H2O, N2, and CO2 more closely follow their equilibrium profiles, except at pressures 1 μbar, where CO, H2O, and N2 are photochemically destroyed and CO2 is produced before its eventual high-altitude destruction. The enhanced abundances of CH4, NH3, and HCN are expected to affect the spectral signatures and thermal profiles of HD 189733b and other relatively cool, transiting exoplanets. We examine the sensitivity of our results to the assumed temperature structure and eddy diffusion coefficients and discuss further observational consequences of these models.

Impact debris particles in Jupiter's stratosphere

The aftermath of the impacts of periodic comet Shoemaker-Levy 9 on Jupiter was studied with the Wide Field Planetary Camera 2 on the Hubble Space Telescope. The impact debris particles may owe their dark brown color to organic material rich in sulfur and nitrogen. The total volume of aerosol 1 day after the last impact is equal to the volume of a sphere of radius 0.5 kilometer. In the optically thick core regions, the particle mean radius is between 0.15 and 0.3 micrometer, and the aerosol is spread over many scale heights, from approximately 1 millibar to 200 millibars of pressure or more. Particle coagulation can account for the evolution of particle radius and total optical depth during the month following the impacts.

Viscosity of rock-ice mixtures and applications to the evolution of icy satellites

Theory and experiments are used to establish lower and upper bounds on the ratio of actual viscosity to pure ice viscosity for a suspension of rock particles in a water ice matrix. For typical conditions encountered in icy satellites, this ratio is of order ten or possibly larger, depending on unknown factors such as the particle size distribution. It is shown that even this modest increase in viscosity may be enough to have caused a failure of solid state convective self-regulation early in the evolution of a homogeneous, rock-water ice satellite, provided the satellite is large enough and sufficiently silicate rich. The criteria for this failure are satisfied by Ganymede and are marginal for Callisto, if the silicates are hydrated. Failure of self-regulation means that the viscosity is too high for the interior to remain completely solid and eliminate the heat production of long-lived radioisotopes by solid state convection. Partial melting of the ice then occurs. It is further shown that satellites of this size may then undergo runaway differentiation into a rock core and almost pure ice mantle, because the gravitational energy release is sufficient to melt nearly all the ice and the Rayleigh-Taylor instability time scale is short. (Although the high pressure phases of ice melt, the resulting water quickly refreezes at a higher level.) It is conjectured that these results explain the striking surface dissimilarity of Ganymede and Callisto, if these satellites accreted cold and undifferentiated. Ganymede may have gone supercritical (melted and differentiated) because of a failure of self-regulation, whereas Callisto remained undifferentiated to the present day. Like all proposed explanations for the Ganymede-Callisto dichotomy, this conjecture cannot be quantified with confidence because of inadequate or incomplete observations, theory, and experimental data.

Thermal maps of jupiter: spatial organization and time dependence of stratospheric temperatures, 1980 to 1990.

The spatial organization and time dependence of Jupiters stratospheric temperatures have been measured by observing thermal emission from the 7.8-micrometer CH4 band. These temperatures, observed through the greater part of a Jovian year, exhibit the influence of seasonal radiative forcing. Distinct bands of high temperature are located at the poles and mid-latitudes, while the equator alternates between warm and cold with a period of approximately 4 years. Substantial longitudinal variability is often observed within the warm mid-latitude bands, and occasionally elsewhere on the planet. This variability includes small, localized structures, as well as large-scale waves with wavelengths longer than ∼30,000 kilometers. The amplitudes of the waves vary on a time scale of ∼1 month; structures on a smaller scale may have lifetimes of only days. Waves observed in 1985, 1987, and 1988 propagated with group velocities less than �30 meters per second.

Jovian large-scale stratospheric circulation

We infer Jupiters stratospheric mean meridional residual circulation by balancing annual-average radiative net heating/cooling with adiabatic cooling/heating associated with vertical motions. Voyager 1 IRIS observations were used to define the thermal structure in the pressure range 0.3 to 270 mbar. Aerosol radiative heating plays an important role. Several independent observations indicate aerosols to be concentrated at high latitudes in the region near and just below the 1-mbar altitude. Voyager 2 photopolarimeter ultraviolet data also indicate a distinct north/south asymmetry in radiative heating which is not balanced by radiative cooling. A prominent feature of these models is a two-cell structure centered near the 10-mbar level, with subsidence occurring at the equator, and upwelling at high latitudes. This large-scale circulation did not appear in previous work (Conrath et al. 1990, Icarus 83, 255–281) in which aerosol heating was neglected. At the base of the stratosphere, where there is no aerosol heating in our model, we confirm the meridional circulation pattern proposed by Gierasch et al. (1986, Icarus 67, 456–483), with upwelling over zones and subsidence in belt regions. The magnitude of the upwelling/subsidence is a factor of 2 more vigorous than that proposed by Gierasch et al. Our model also estimates the seasonally averaged Eliassen-Palm flux divergence. This quantity is small at low latitudes and reaches extreme values, both positive and negative, in the latitude ranges 80 S to 40 S and 30 N to 80 N. At these latitudes it is most strongly negative near the 230-mbar level and most strongly positive near the 5-mbar level. Gravity waves propagating from the deeper atmosphere seem to be the most likely mechanism responsible for this pattern of eddy forcing.

Evidence for methane escape and strong seasonal and dynamical perturbations of Neptune's atmospheric temperatures

Aims. We studied the distribution of mid-infrared thermal emission from Neptune to determine the spatial variability of temperatures and the distribution of trace constituents, allowing us to determine the relative strengths of radiation and dynamics in its atmosphere. Methods. Mid-infrared images of the planet were taken at the Very Large Telescope on 1–2 September 2006. Results. These images reveal strong inhomogeneities in thermal emission. 17.6 and 18.7-µm images exhibit strong seasonally elevated south polar temperatures near Neptune’s tropopause. These high temperatures allow tropospheric methane, elsewhere cold-trapped at depth, to escape into the stratosphere. Poleward of 70 ◦ S, 8.6- and 12.3-µm emission from stratospheric methane and ethane is enhanced, and a distinct, warm stratospheric feature near 65–70 ◦ S latitude is rotating with the neutral atmosphere. This feature may