Lightning activity on Jupiter

Photographic observations of the nightside of Jupiter by the Voyager 1 spacecraft show the presence of extensive lightning activity. Detection of whistlers by the plasma wave analyzer confirms the optical observations and implies that many flashes were not recorded by the Voyager camera because the intensity of the flashes was below the threshold sensitivity of the camera. Measurements of the optical energy radiated per flash indicate that the observed flashes had energies similar to that for terrestrial superbolts. The best estimate of the lightning energy dissipation rate of 0.4 × 10^?3 W/m² was derived from a consideration of the optical and radiofrequency measurements. The ratio of the energy dissipated by lightning compared to the convective energy flux is estimated to be between 0.27 × 10^?4 and 0.5 × 10^?4. The terrestrial value is 1 × 10^?4.     

Red clouds in reducing atmospheres

A dark reddish-brown high-molecular-weight polymer is produced by long wavelength ultraviolet irradiation of abundant gases in reducing planetary atmospheres. The polymer i examined by paper chromatography, mass spectrometry, and infrared, visible, and ultraviolet spectroscopy. High carbon-number straight-chain alkanes with NH₂ and, probably, OH and CO groups are identified, along with the previously reported amino acids. There are chemical similarities between this polymer and organic compounds recovered from carbonaceous chondrites and precambrian sediments. The visible and near-ultraviolet transmission spectrum of the polymer shows its absorption optical depth to be redder than λ^?2 and perhaps similar in coloration to the clouds of Jupiter, Saturn, and Titan. The near-ultraviolet absorption coefficient is ～10³ cm^?1, and typical grain sizes ～30 μm. The nitrile content is small, and the polymer should be semitransparent in the 5 μm atmospheric window. Such polymers may be a common constituent of clouds in the outer solar system and on the early Earth.     

Measurement of stratospheric aerosol on Saturn using an eclipse of Titan

An eclipse of Titan by Saturn was observed on December 20, 1979, to measure the aerosol content in the atmosphere of Saturn. The measurements were made with the 74-in. telescope of the Helwan Observatory, Egypt, in the bandpass 6300–7300 Å and extend to ～5 magnitudes of eclipse darkening. The faint portion of the lightcurve unambiguously requires the presence of aerosol in the lower stratosphere of Saturn. The aerosol extends to at least 20 km above the tropopause and has a one-way stratospheric vertical optical depth of 0.4_?0.02^+0.04 at 6700 Å. The results apply to the sunset terminator at a cronographic latitude of 23°S.     

Predictions of the electrical conductivity and charging of the aerosols in Titan's atmosphere

The electrical conductivity and electrical charge on the aerosols in atmosphere of Titan are computed for altitudes from 0 to 400 km. Ionization due to both galactic cosmic rays and electron precipitation from the Saturnian magnetosphere is considered. This ionization results in free electrons and the primary ions N₂⁺ and N⁺ which are then rapidly converted into secondary ions such as H₂CN⁺ and NH₄⁺ which in turn form ion clusters such as H₂CN⁺(HCN)_n and NH₄⁺(NH₃)_m. In contrast to the atmospheres of Venus and Earth, we find no species in the Titan atmosphere that lead to the formation of appreciable concentrations of negative ions. Consequently, the predicted conductivity is quite different in that a substantial concentration of electrons exists all the way to the surface of Titan. The ubiquitous aerosols observed in the Titan atmosphere also play an important role in determining the charge distribution in the atmosphere. At altitudes above 100 km and for aerosol concentrations above approximately 10/cc, the recombination of electrons and positive ions is controlled by the recombination on the surface of the aerosols rather than by the gas-kinetic recombination rate. For small aerosol concentrations, the ratio of the number of charges per particle to the radius of the particle is approximately 30, for radii in microns. This value is similar to that obtained by previous investigators for terrestrial noctilucent clouds. Because the aerosol particles are highly charged, coagulation is inhibited, particle sizes are smaller, and their settling rates are reduced. As a consequence, the optical depth of the atmosphere is much higher than it would be if the particles were uncharged.     

Models of the protosatellite disk of Saturn: Conditions for Titan’s formation

Models of the protosatellite accretion disk of Saturn are developed that satisfy cosmochemical constraints on the volatile abundances in the atmospheres of Saturn and Titan with due regard for the data obtained with the Cassini orbiter and the Huygens probe, which landed on Titan in January 2005. All basic sources of heating of the disk and protosatellite bodies are taken into account in the models, namely, dissipation of turbulence in the disk, accretion of gaseous and solid material onto the disk from the feeding zone of Saturn in the solar nebula, and heating by the radiation of young Saturn and thermal radiation of the surrounding region of the solar nebula. Two-dimensional (axisymmetric) temperature, pressure, and density distributions are calculated for the protosatellite disk. The distributions satisfy the cosmochemical constraints on the disk temperature, according to which the temperature at the stage of the satellite formation ranged from 60–65 K to 90–100 K at pressures from 10^?7 to ?10^?4 bar in the zone of Titan’s formation (according to estimates, r = 20–35R _Sat). Variations of the basic input parameters (the accretion rate onto the protosatellite disk of Saturn from the feeding zone of the planet ?; the parameter α characterizing turbulent viscosity of the disk; and the mass concentration ratio in the solid/gas system) satisfying the aforementioned temperature constraint are found. The spectrum of models satisfying the cosmochemical constraints covers a considerable range of consistent parameters. A model with a rather small flux of ? = 10^?8 M _Sat/ yr and a tenfold depletion of Saturn’s disk in gas due to gas scattering from the solar nebula is at one side of this range. A model with a much higher flux of ? = 10^?6 M _Sat/yr and a hundredfold decrease in opacity of the disk matter owing to decreased concentration of dust particles and/or their agglomeration into large aggregates and sweeping up by planetesimals is at the other side of the range.     

Influence of methane concentration on the optical indices of Titan’s aerosols analogues

This work deals with the optical constant characterization of Titan aerosol analogues or “tholins” produced with the PAMPRE experimental setup and deposited as thin films onto a silicon substrate. Tholins were produced in different N₂–CH₄ gaseous mixtures to study the effect of the initial methane concentration on their optical constants. The real (n) and imaginary (k) parts of the complex refractive index were determined using the spectroscopic ellipsometry technique in the 370–1000 nm wavelength range. We found that optical constants depend strongly on the methane concentrations of the gas phase in which tholins are produced: imaginary optical index (k) decreases with initial CH₄ concentration from 2.3 × 10^?2 down to 2.7 × 10^?3 at 1000 nm wavelength, while the real optical index (n) increases from 1.48 up to 1.58 at 1000 nm wavelength. The larger absorption in the visible range of tholins produced at lower methane percentage is explained by an increase of the secondary and primary amines signature in the mid-IR absorption. Comparison with results of other tholins and data from Titan observations are presented. We found an agreement between our values obtained with 10% methane concentration, and Imanaka et al. (Imanaka, H., Khare, B.N., Elsila, J.E., Bakes, E.L.O., McKay, C.P., Cruikshank, D.P., Sugita, S., Matsui, T., Zare, R.N. [2004]. Icarus, 168, 344–366) values, in spite of the difference in the analytical method. This confirms a reliability of the optical properties of tholins prepared with various setups but with similar plasma conditions. Our comparison with Titan’s observations also raises a possible inconsistency between the mid-IR aerosol signature by VIMS and CIRS Cassini instruments and the visible Huygens-DISR derived data. The mid-IR VIMS and CIRS signatures are in agreement with an aerosol dominated by an aliphatic carbon content, whereas the important visible absorption derived from the DISR measurement seems to be incompatible with such an important aliphatic content, but more compatible with an amine-rich aerosol.     

Production and condensation of organic gases in the atmosphere of Titan

The rates and altitudes for the dissociation of atmospheric constituents of Titan are calculated for solar UV, solar wind protons, interplanetary electrons, Saturn magnetospheric particles, and cosmic rays. The resulting integrated synthesis rates of organic products range from 10²–10³ g cm^?2 over 4.5 × 10⁹ years for high-energy particle sources to 1.3 × 10⁴ g cm^?2 for UV at $\text{λ < 1550}\text{A}\text{?}$, and to 5.0 × 10⁵ g cm^?2 if $\text{λ > 1550}\text{A}\text{?}$ (acting primarily on C₂H₂, C₂H₄, and C₄H₂) is included. The production rate curves show no localized maxima corresponding to observed altitudes of Titan's hazes and clouds. For simple to moderately complex organic gases in the Titanian atmosphere, condensation occurs below the top of the main cloud deck at 2825 km. Such condensates comprise the principal cloud mass, with molecules of greater complexity condensing at higher altitudes. The scattering optical depths of the condensates of molecules produced in the Titanian mesosphere are as great as ～ 10²/(particulate radius, μm) if column densities of condensed and gas phases are comparable. Visible condensation hazes of more complex organic compounds may occur at altitudes up to ～ 3060 km provided only that the abundance of organic products declines with molecular mass no faster than laboratory experiments indicate. Typical organics condensing at 2900 km have molecular masses = 100–150 Da. At current rates of production the integrated depth of precipitated organic liquids, ices, and tholins produced over 4.5 × 10⁹ years ranges from a minimum ～ 100 m to kilometers if UV at $\text{λ > 1550}\text{A}\text{?}$ is important. The organic nitrogen content of this layer is expected to be ～ 10^?1?10^?3 by mass.     

The greenhouse of Titan

Both non-gray radiative equilibrium and gray convective equilibrium calculations for Titan indicate that the discrepancy between the equilibrium temperature of an atmosphereless Titan and the observed infrared temperatures can be explained by a massive molecular hydrogen greenhouse effect. The convective calculations indicate a probable minimum optical depth of 14, corresponding to many tens of km-atm of H₂, and total pressures of ～0.1 bar. The tropopause is several hundred km above the Titanian surface and at a temperature of about 90°K. Methane condensation is likely at this level. Such an atmosphere is unstable against atmospheric blow-off unless typical mesosphere scale heights are < 25km, an unlikely situation. Blow-off can also be circumvented by exospheric temperatures near the freezing point of hydrogen. It is considered more plausible that the present atmosphere is in equilibrium between outgassing and blow-off of the one hand and accretion from protons trapped in a hypothetical Saturnian magnetic field on the other; or exhibits uncompensated blow-off of outgassing products. To maintain the present blow-off rate without compensation for all of geological time requires an outgassing equivalent to the volatilization of a few km of subsurface ices. Photo-dissociation of these volatilized ices produces the observed high abundance of H₂ as well as large quantities of complex organic chromophores which may explain the reddish coloration of the Titanian cloud deck. An extensive circum-Titanian hydrogen corona is postulated. Surface temperatures as high as 200°K are not excluded. Because of its high temperatures and pressures and the probable large abundance of organic compounds, Titan is a prime target for spacecraft exploration in the outer solar system.     

An upper limit to the abundance of lightning-produced amino acids in the Jovian water clouds

The effect of excess hydrogen on the synthesis of amino acids, by high-temperature shock waves in a hydrogen/methane/ammonia/water vapor mixture, was studied experimentally. The energy efficiency results, together with the best estimate of the lightning energy dissipation rate on Jupiter, from the Voyager data, were used to calculate an upper limit to the rate of amino acid production by lightning in the Jovian water clouds. Using reasonable values for the eddy diffusion coefficients within and below the water clouds, the column abundance of lightning-produced amino acids in the clouds was estimated to be 6.2 × 10^?6 cm-am. Hence, concentration of amino acids in water droplets would be 8 × 10^?8 mole liter^?1.     

Spectra of simulated lightning on Venus,Jupiter, and Titan

Laser-induced plasmas in various gas mixtures were used to simulate lightning in other planetary atmospheres. This method of simulation has the advantage of producing short-duration, high-temperature plasmas free from electrode contamination. The laser-induced plasma discharges in air are shown to accurately simulate terrestrial lightning and can be expected to simulate lightning spectra in other planetary atmospheres. Spectra from 240 to 880 nm are presented for simulated lightning in the atmospheres of Venus, Earth, Jupiter, and Titan. The spectra of lightning on the other giant planets are expected to be similar to that of Jupiter because the atmospheres of these planets are composed mainly of hydrogen and helium. The spectra of Venus and Titan show substantial amounts of radiation due to the presence of carbon atoms and ions and show CN Violet radiation. Although small amounts of CH₄ and NH₃ are present in the Jovian atmosphere, only emission from hydrogen and helium is observed. Most differences in the spectra can be understood in terms of the elemental ratios of the gas mixtures. Consequently, observations of the spectra of lightning on other planets should provide in situ estimates of the atmospheric and aerosol composition in the cloud layers in which lightning is occuring. In particular, the detection of inert gases such as helium should be possible and the relative abundance of these gases compared to major constituents might be determined.     

The upper ionosphere of Titan

Photoionization of the upper atmosphere of Titan by sunlight is expected to produce a substantial ionospheric layer. We have solved one-dimensional forms of the mass, momentum, and energy conservation equations for ions and electrons and have obtained electron number densities of about 10³ cm^?3, using various model atmospheres. The significant ions in a CH₄H₂ atmosphere are H⁺, H₃⁺, CH₅⁺, CH₅⁺, CH₃⁺, and C₂H₅⁺. Electron temperatures may be as high as 1000°K, depending on the abundance of hydrogen in the high atmosphere. Interaction of the solar wind with the ionosphere is also discussed.     

Estimated impact shock production of N2 and organic compounds on early Titan

Bombardment of Titan by Uranus-Neptune planetesimals and/or fragments of a disrupted Hyperion progenitor supplied more than enough energy to drive vigorous atmospheric shock chemistry. Chemical equilibrium modeling of the shock products in simulated atmospheres indicates that impact energy has produced large amounts of N₂ and organic compounds over Titan's history. The mole fraction of organic compounds in the shocked gas mixture (T = 1200−2500°K, P = 10⁻¹−10³bar) reaches a maximum of approximately 3% in a current Titan mixture and 12% in a primordial CH₄, NH₃-rich mixture. Atmospheric water mixing ratio controls the organic yield in shock reactions, but its limiting effect may have been reduced by cold-trapping of water in a cooling atmosphere. Kinetic inhibition of graphite formation in the shocked gas enhanced the yield of radicals and organic. The resulting mixture of carbonaceous soot and condensed hydrocarbons subsequently settled onto the surface; the depth of the generated layer was on the order of hundreds of meters. Impact shock energy was capable of converting massive amounts of NH₃ to N₂ early in Titan history—over twice the present atmospheric and 1.5 times the total ocean-atmospheric inventory of N₂. Shock conversion of NH₃ into N₂ bypasses the difficulties of other schemes of N₂ production and may have been of singular importance in Titan's atmospheric evolution.     

The response of selected terrestrial organisms to the Martian environment: A modeling study

An energy balance model has been developed to investigate how the Martian atmospheric environment could influence a community of photosynthetic microorganisms with properties similar to those of a cyanophyte (blue-green algal mat) and a lichen. Surface moisture and soil nutrients are assumed to be available. The model was developed to approximate equatorial equinox conditions and includes parameters for solar and thermal radiation, convective and conductive energy transport, and evaporative cooling. Calculations include the diurnal variation of organism temperature and transpiration and photosynthetic rates. The influences of different wind speeds and organism size and resistivity are also studied. The temperature of organisms in mats less than a few millimeters thick will not differ from the ground temperature by more than 10°K. Water loss is actually retarded at higher wind speeds, since the organism temperature is lowered, thus reducing the saturation vapor pressure. Typical photosynthetic rates lead to the production of 10^?6 to 10^?7 mole O₂ cm^?2 day^?1.     

The surface energy balance at the Huygens landing site and the moist surface conditions on Titan

The Huygens Probe provided a wealth of data concerning the atmosphere of Titan. It also provided tantalizing evidence of a small amount of surface liquid. We have developed a detailed surface energy balance for the Probe landing site. We find that the daily averaged non-radiative fluxes at the surface are 0.7 W m^?2, much larger than the global average value predicted by McKay et al. (1991) of 0.037 W m^?2. Considering the moist surface, the methane and ethane detected by the Probe from the surface is consistent with a ternary liquid of ethane, methane, and nitrogen present on the surface with mole fractions of methane, ethane, and nitrogen of 0.44, 0.34, and 0.22, respectively, and a total mass load of ～0.05 kg m^?2. If this liquid is included in the surface energy balance, only a small fraction of the non-radiative energy is due to latent heat release (～10^?3 W m^?2). If the amount of atmospheric ethane is less than 0.6×10^?5, the surface liquid is most likely evaporating over timescales of 5 Titan days, and the moist surface is probably a remnant of a recent precipitation event. If the surface liquid mass loading is increased to 0.5 kg m^?2, then the liquid lifetime increases to ～56 Titan days. Our modeling results indicate a dew cycle is unlikely, given that even when the diurnal variation of liquid is in equilibrium, the diurnal mass variation is only 3% of the total liquid. If we assume a high atmospheric mixing ratio of ethane (>0.6×10^?5), the precipitation of liquid is large (38 cm/Titan year for an ethane mixing ratio of 2×10^?5). Such a flux is many orders of magnitude in excess of the photochemical production rate of ethane.     

The role of organic haze in Titan's atmospheric chemistry: I. Laboratory investigation on heterogeneous reaction of atomic hydrogen with Titan tholin

In Titan's atmosphere consisting of N₂ and CH₄, large amounts of atomic hydrogen are produced by photochemical reactions during the formation of complex organics. This atomic hydrogen may undergo heterogeneous reactions with organic aerosol in the stratosphere and mesosphere of Titan. In order to investigate both the mechanisms and kinetics of the heterogeneous reactions, atomic deuterium is irradiated onto Titan tholin formed from N₂ and CH₄ gas mixtures at various surface-temperatures of the tholin ranging from 160 to 310 K. The combined analyses of the gas species and the exposed tholin indicate that the interaction mechanisms of atomic deuterium with the tholin are composed of three reactions; (a) abstraction of hydrogen from tholin resulting in gaseous HD formation (HD recombination), (b) addition of D atom into tholin (hydrogenation), and (c) removal of carbon and/or nitrogen (chemical erosion). The reaction probabilities of HD recombination and hydrogenation are obtained as ηabst=1.9(±0.6)×¹⁰⁻³×exp(−300/T) and ηhydro=2.08(±0.64)×exp(−1000/T), respectively. The chemical erosion process is very inefficient under the conditions of temperature range of Titan's stratosphere and mesosphere. Under Titan conditions, the rates of hydrogenation > HD recombination ? chemical erosion. Our measured HD recombination rate is about 10 times (with an uncertainty of a factor of 3-5) the prediction of previous theoretical model. These results imply that organic aerosol can remove atomic hydrogen efficiently from Titan's atmosphere through the heterogeneous reactions and that the presence of aerosol may affect the subsequent organic chemistry.     

A Saturnian gas ring and the recycling of Titan's atmosphere

Atoms which escape Titan's atmosphere are unlikely to possess escape velocity from Saturn, and can orbit the planet until lost by ionization or collision with Titan. It is predicted that a toroidal ring of between ～1 and ～10³ atoms or molecules cm^?3 exists around Saturn at a distance of about 10 times the radius of the visible rings. This torus may be detectable from Earth-orbit and detection of nondetection of it may provide some information about the presence or absence of a Saturnian magnetic field, and the exospheric temperature and atmospheric escape rate of Titan. It is estimated that, if Titan has a large exosphere, ～97% or more of the escaping atoms can be recaptured by Titan, thereby decreasing the effective net atmospheric loss rate by up to two orders of magnitude. With such a reduction in atmospheric loss rates, it becomes more plausible to suggest that satellites previously thought too small to retain an atmosphere may have one. It is suggested that Saturn be examined by Lyman-α and other observations to search for the gaseous torus of Titan. If successful, these could then be extended to other satellites.The effect of a hypothetical Saturnian magnetosphere on the atmosphere of Titan is investigated. It is shown that, if Saturn has a magnetic field comparable to Jupiter's (～10 G at the planetary surface), the magnetospheric plasma can supply Titan with hydrogen at a rate comparable to the loss rates in some of the models of Trafton (1972) and Sagan (1973). A major part of the Saturnian ionospheric escape flux (～ 10²⁷ photoelectrons sec^?1) could perhaps be captured by Titan. At the upper limit, this rate of hydrogen input to the satellite could total ～0.1 atm pressure over the lifetime of the solar system, an amount comparable to estimates of the present atmospheric pressure of Titan.     

Turbulent viscosity and Jupiter's tidal Q

A recent estimate of tidal dissipation by turbulent viscosity in Jupiter's convective interior predicts that the current value of the planet's tidal Q ～ 5 × 10⁶. We point out a fundamental error in this calculation, and show that turbulent dissipation alone implies that at present Q ～ 5 × 10¹³. Our reduced estimat for the rate of tidal dissipation shows conclusively that tidal torques have produced only negligible modifications of the orbits of the Galilean satellites over the age of the solar system.     

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.