Currents in the cometary atmosphere

If the structure of the magnetic field and electric current in the cometary type I tail can be represented by an electric current circuit, disruption of the cross-tail current system may lead to a current discharging through the cometary ionosphere, and the dissipation of the magnetic energy stored in the tail. From the point of view of energy budget, a tail-aligned magnetic field on the order of 10γ will be sufficient to produce a strong ionization effect of the cometary atmosphere.     

Ion chemistry and N-containing molecules in Titan's upper atmosphere

High-energy photons, electrons, and ions initiate ion-neutral chemistry in Titan's upper atmosphere by ionizing the major neutral species (nitrogen and methane). The Ion and Neutral Mass Spectrometer (INMS) onboard the Cassini spacecraft performed the first composition measurements of Titan's ionosphere. INMS revealed that Titan has the most compositionally complex ionosphere in the Solar System, with roughly 50 ions at or above the detection threshold. Modeling of the ionospheric composition constrains the density of minor neutral constituents, most of which cannot be measured with any other technique. The species identified with this approach include the most complex molecules identified so far on Titan. This confirms the long-thought idea that a very rich chemistry is actually taking place in this atmosphere. However, it appears that much of the interesting chemistry occurs in the upper atmosphere rather than at lower altitudes. The species observed by INMS are probably the first intermediates in the formation of even larger molecules. As a consequence, they affect the composition of the bulk atmosphere, the composition and optical properties of the aerosols and the flux of condensable material to the surface. In this paper, we discuss the production and loss reactions for the ions and how this affects the neutral densities. We compare our results to neutral densities measured in the stratosphere by other instruments, to production yields obtained in laboratory experiments simulating Titan's chemistry and to predictions of photochemical models. We suggest neutral formation mechanisms and highlight needs for new experimental and theoretical data.     

Possible nature of cometary atmosphere particles

The polarimetric and spectrophotometric data of observations, the results of laboratory simulations, and theoretical calculations are considered as evidence in favor of the presence of large irregular particles in cometary atmospheres. The attempt is made to define more precisely the particle parameters. In particular, observations of some comets at small phase angles can be interpreted by light scattering on large icy grains. The results of laboratory experiments with ice at low temperatures and pressures are adduced; this can be explain the formation of a large icy grain cloud near the cometary nucleus. Changes of these particles under the effects of solar radiation are considered.     

The photochemistry and dynamics of a dusty cometary atmosphere

A self-consistent solution of the dynamical and thermal structure of an H₂O-dominated, two-phase, dusty-gas cometary atmosphere has been obtained by solving the simultaneous set of differential equations representing conservation of number density, momentum and energy together with the transfer of solar radiation in the streams responsible for the major photolytic processes and the heating of the nucleus. The validity of the model is restricted to the collision-dominated region where all the gas species are assumed to attain a common velocity and common temperature. Two models are considered for the transfer of solar radiation through the circum-nuclear dust halo. In the first only the direct extinction by the dust is considered. In the second, the finding of some recent models, that the diffuse radiation field due to multiple scattering by the dust halo more or less compensates for radiation removed by direct absorption when the optical depth is near unity, is approximated by neglecting the attenuation of the radiation by the dust altogether.As has been shown earlier, the presence of dust results in a transonic solution, and it is obtained by a two-step iterative procedure which makes use of the asymptotic behaviour of the radiation fields sufficiently far from the nucleus and a regularity condition at the sonic point.The calculations were performed for a medium sized comet (R _n =2.5 km) having a dust to gas production rate ratio of unity, at a heliocentric distance of 1 AU. The dust grains were assumed to be of the same radius (1), of low density (1g cm^–3) and be strongly absorbing (having the optical properties of magnetite).The main effect of the dust on the cometary atmosphere is dynamic. While the dust-gas coupling persists to about 20R _n , the strong throat effect of the dust friction on the gas causes the latter to go supersonic quite rapidly. Consequently the sub-sonic region around the nucleus is very thin, varying between 45 and 85m in the two models considered. On the other hand, while this highly absorbing dust has a temperature substantially above that of the gas in the inner coma, heat exchange between them does not significantly change the temperature profile of the gas. This is because of the predominance of the expansion cooling, and even more importantly, the IR-cooling by H₂O, in the inner coma. Consequently, the gas temperature goes through a strong inversion, as in the dust-free case, achieving a temperature as low as about 6K within about 50km of the nucleus, before increasing to about 700K atr=10⁴km, due to the high efficiency of photolytic heating over the cooling process in the outer coma. The Mach number achieves a maximum value of about 10 at the distance of the temperature minimum, thereafter steadily decreasing to a value of about 2.5 atr10⁴km.It is shown that while the dust attenuation has a strong effect on the production rate of H₂O, it also has an interesting effect on the electron density profile. It increases the electron density in the inner coma over the unattenuated case, while at the same time, decreasing it in the outer coma. In conclusion, the limitations of the present model and the necessity to extend it using a multi-fluid approach are discussed.     

Sulfur chemistry in the middle atmosphere of Venus

Venus Express measurements of the vertical profiles of SO and SO₂ in the middle atmosphere of Venus provide an opportunity to revisit the sulfur chemistry above the middle cloud tops (～58 km). A one dimensional photochemistry-diffusion model is used to simulate the behavior of the whole chemical system including oxygen-, hydrogen-, chlorine-, sulfur-, and nitrogen-bearing species. A sulfur source is required to explain the SO₂ inversion layer above 80 km. The evaporation of the aerosols composed of sulfuric acid (model A) or polysulfur (model B) above 90 km could provide the sulfur source. Measurements of SO₃ and SO (a¹Δ → X³Σ^-) emission at 1.7 μm may be the key to distinguish between the two models.     

A multi-fluid model of an H2O-dominated dusty cometary atmosphere

A self-consistent multi-fluid solution of the dynamical and thermal structure of an H₂O-dominated, two-phase dusty-gas cometary atmosphere has been obtained by solving the simultaneous set of differential equations representing conservation of number density, momentum and energy, together with the transfer of solar radiation in streams responsible for the major photolytic processes and the heating of the nucleus. The validity of this model, as in the earlier single-fluid ones, is restricted to the collision-dominated region where all the heavy species (ions and neutrals) are assumed to achieve a common temperature and velocity. However, recognizing that the photo-produced hydrogen is rather inefficient in exchanging energy with the heavier species we treat the hydrogen separately: it is assumed to be composed of a thermalized component (the second fluid) and a pre-thermal component.The present model, which is transonic due to the presence of the dust in the inner coma, causes the heavy species to expand subsonically from the nucleus and to smoothly traverse the sonic point within about 45 m of the nucleus, although the dust-gas coupling persists to about 50 km. While the temperature of the heavy species goes through a strong inversion within about 100 km from the nucleus, due to the effects of IR cooling and expansion, it increases to about 300–400 K in the outermost part of the collision-dominated coma due to UV photolytic heating. These temperatures are smaller by a factor of 2–3 from the predictions of the earlier single-fluid models, which assumed instant thermalization of the photo-produced hydrogen.While the velocities of the heavy species and the thermal hydrogen increase to, respectively, 1.1 km s^–1 and 1.6 km s^–1 in the outer (collisional) coma, the velocity of the pre-thermal component reaches about 15 km s^–1. This latter value is consistent with Ly- observations of a number of comets, which implies a fast (20 km s^–1) hydrogen component in the outer coma. The boundary of the exosphere, where the non-thermal hydrogen dominates, is predicted to be around 1.5×10⁴ km from the nucleus. The calculations are for a comet of radius 2.5 km with a dust/gas ratio of 1, at a heliocentric distance of 1 AU.     

Phase functions of polarization and brightness and the nature of cometary atmosphere particles

The dependence of brightness and polarization of cometary on the phase angle is studied. The similarity between the phase curves of comets, minor planets, and the zodiacal cloud is pointed out. The dependence found correspond to dielectric particles with dimensions greater than 1 μm.     

Negative ion chemistry in Titan's upper atmosphere

The Electron Spectrometer (ELS), one of the sensors making up the Cassini Plasma Spectrometer (CAPS) revealed the existence of numerous negative ions in Titan's upper atmosphere. The observations at closest approach (∼1000 km) show evidence for negatively charged ions up to ∼10,000 amu/q, as well as two distinct peaks at 22±4 and 44±8 amu/q, and maybe a third one at 82±14 amu/q. We present the first ionospheric model of Titan including negative ion chemistry. We find that dissociative electron attachment to neutral molecules (mostly HCN) initiates the formation of negative ions. The negative charge is then transferred to more acidic molecules such as HC₃N, HC₅N or C₄H₂. Loss occurs through associative detachment with radicals (H and CH₃). We attribute the three low mass peaks observed by ELS to CN⁻, C₃N⁻/C₄H⁻ and C₅N⁻. These species are the first intermediates in the formation of the even larger negative ions observed by ELS, which are most likely the precursors to the aerosols observed at lower altitudes.     

Nitrile gas chemistry in Titan’s atmosphere

This work presents the first study of the gaseous products resulting from the partial dissociation of methane and nitrogen in the PAMPRE experimental setup simulating Titan’s atmospheric chemistry.Using cryogenic trapping, the gaseous products generated from the chemical reactions occurring in the reactor have been trapped. Analyses of these products by gas chromatography coupled to mass spectrometry have allowed the detection and identification of more than 30 reaction products. Most of them are identified as nitrile species, accompanied by aliphatic hydrocarbons and a few aromatics compounds. The observed species are in agreement with the data from the recent Cassini-Huygens mission as well as from other laboratory setups capable of dissociating nitrogen and methane. This work emphasizes the probable importance of nitrogen-bearing compounds in the chemistry taking place in Titan’s atmosphere.Furthermore, a quantification of mono-nitriles with saturated alkyl chains has been performed relatively to hydrogen cyanide and shows a power law dependence in their concentration. This dependence is consistent with the Cassini-INMS data and Titan’s photochemical models.An empirical relationship has been extracted from our experimental data: [C_xH_2x−1N] = 100x⁻⁵, where x is the number of carbon atoms in the nitrile molecule. This relationship can be directly used in order to foretell the concentration of heavier nitriles induced by chemistry in Titan’s atmosphere.     

Light scattering by an inhomogeneous spherically symmetric cometary atmosphere: Solar flux impinging on the nucleus head

The generalized Eddington approximation, obtained by the three-stream division of the radiation field, is used to compute the intensity falling on the nucleus head of a spherically-symmetric comet illuminated by parallel solar radiation.     

Volatiles in the cometary nuclei

The argument for the similarity of the composition of cometary volatiles to that of interstellar molecules has been strengthened by the analysis of CO⁺ and CO ₂ ⁺ emission of the comet West. The strong 6300 Å emission of oxygen atoms can be interpreted in terms of photodissociation of OH by the solar Lyman-alpha radiation, and not as being due to photo-dissociation of CO₂ of speculatively large amount.     

Direct statistical simulation of the near-surface layers of a cometary atmosphere. II: A nonspherical nucleus

The study presents the results of numerical simulations of mass-transfer processes in the near-surface layer of the cometary nucleus and in the inner part of the cometary atmosphere, which is formed under the action of solar radiation. The gas-kinetic model of the inner part of the cometary atmosphere surrounding a spherical nucleus (Skorov et al., 2004) is extended to the case of a nonspherical nucleus with axial symmetry. After high-resolution images of comets 19P/Borrelly and Wild 2 have been obtained by Deep Space 1 and Stardust spacecraft, such an extension seems to be vital and important. The nucleus and the inner part of the coma are closely related to each other because of the permanent exchange of energy and mass; therefore, they are modeled consistently. As in the first part of our study, the boundary conditions at the inner boundary of the simulation domain, which are necessary for gas-kinetic simulations, were determined from the self-consistent model of heat and mass transfer in a porous cometary nucleus that was developed earlier by the authors. The model took into account the volumetric character of the radiation absorption in a porous sublimating medium, the kinetic regime of the transport of sublimation products in the pores, and the backward gas fluxes from the coma due to intermolecular collisions. We considered different models of the nucleus structure that determined the effective gas production. Using the direct simulation Monte Carlo method, we computed the two-dimensional gas flow from a heterogeneous nonspherical cometary nucleus. The simulations were performed using the SMILE software. The parallel computer implementation of the software made it possible to calculate the spatial structure of the gas flow for the entire circumnucleus zone.     

Inorganic chemistry of O2 in a dense primitive atmosphere

A simple steady-state photochemical model is developed in order to determine typical molecular oxygen concentrations for a comprehensive range of primitive abiotic atmospheres. Carbon dioxide is assumed to be the dominant constituent in these atmospheres since CO2 photodissociation may potentially result in the enhancement of the O2 partial pressure. The respective effects of the H2O content, temperature, eddy diffusion coefficient and UV flux on the results are investigated. It is shown that for any pressure at the surface, the partial pressure of molecular oxygen does not exceed 10 mbar. The peculiar case of a runaway greenhouse which has possibly taken place on Venus is qualitatively envisaged. Although O2 is basically absent in the present Venus atmosphere, a transient presence in a primitive stage cannot be ruled out. Possible mechanisms for O2 removal in such an atmosphere are reviewed. At the present stage, we think that the detection of large O2 amounts would be at least a good clue for the presence of life on an extrasolar planet.     

A review of selected issues concerning the chemistry in Venus’ middle atmosphere

Even after decades of study using advanced observing instruments and sophisticated numerical models, a number of significant questions remain unanswered concerning the composition and chemistry of Venus’ atmosphere. The primary chemical cycles and the interactions among sulfur and chlorine radicals in Venus’ middle atmosphere are reviewed to assess the current status of our knowledge, identify unresolved questions, and assess how the Venus Express mission may contribute to their resolution.     

The organic chemistry and biology of the atmosphere of the planet Jupiter

In order to understand the chemical processes which may be taking place in the Jovian atmosphere, we have conducted a number of simulation experiments in the laboratory. These reactions appear to be significant for our understanding of chemical evolution and the nature and origin of organic matter in the universe. Mixtures of methane and ammonia in varying proportions have been exposed to electric discharges and the products analyzed. We have found that, as the methane and ammonia disappear, hydrogen cyanide and acetylene are to be built up. The analysis of the volatiles has also provided us with a wide range of aminonitriles. It is conceivable that some of these nitriles, on hydrolysis, will give rise to amino acids. On cyclization, some of them would provide the pathways for the origin of pyrimidines. A characteristic result of these reactions has also been the appearance of a red polymer which may have a bearing on the color in the red spots of Jupiter. Spectral analysis in the laboratory may provide some clues in our search for organic material in the Jovian atmosphere by orbiting spacecraft, or ground-based observations.     

Asteroids in cometary orbits

Orbital integrations are presented for a total of 14 asteroids with perihelia inside 1.7 AU and with aphelion distances in excess of 4 AU, 10 of which were discovered in 1979–1984. The integrations were normally extended over approximately ±1000 years in a three-body model (Sun-Jupiter-asteroid). The effects of uncertainties of starting orbits are not treated in this work, and as far as the real asteroids are concerned, the results should be regarded mostly as preliminary indications. A wide variety of orbital evolutions is found, and some of them evidently belong to the cometary, chaotic type. Three such cases are identified with certainty (1983 SA, 1983 XF, and 1984 BC) and two or three more with various degrees of likelihood. An asteroidal motion is found for the well-observed object 1979 VA. A stable libration around the $\text{2}\text{1}$ resonance is found for 1981 FD, which obviously adds to the Griqua group. A long-lasting libration around the $\text{5}\text{3}$ resonance performed by 1982 YA is probably unstable. Temporary librations are also found for 1983 SA ($\text{4}\text{3}$ resonance) and 1983 XF ($\text{2}\text{1}$ resonance), but these objects appear to transit into irregular motions with close approaches to Jupiter (less than 0.01 AU for 1983 XF). A very rapid large-amplitude ω libration around 90° is found in the future motion of 1983 VA. If this will indeed occur for the real asteroid, the object will oscillate with a period of only 750 years between a main-belt orbit of very high inclination and a low-inclination Apollo-type orbit.     

A cometary aurora

It is proposed that the cometary analog of a terrestrial aurora was responsible for the enhanced fluxes of suprathermal (keV) electrons and associated plasma waves observed in the cometosheath of Comet Halley during its VEGA 2 encounter. The non-detection of such suprathermal electron fluxes during the GIOTTO encounter is ascribed to the much quieter solar wind conditions at that time.     

Impact of electron chemistry on the structure and composition of Io's atmosphere

Two-dimensional model calculations (altitude and solar zenith angle) are performed to investigate the impact of electron chemistry on the composition and structure of Io's atmosphere. The calculations are based upon the model of Wong and Smyth (2000, Icarus 146, 60-74) for Io's SO₂ sublimation atmosphere with the addition of new electron chemistry, where the interactions of the electrons and neutrals are treated in a simple fashion. The model calculations are presented for Io's atmosphere at western elongation (dusk ansa) for both a low-density case (subsolar temperature of 113 K) and a high-density case (subsolar temperature of 120 K). The impact of electron-neutral chemistry on the composition and structure of Io's atmosphere is confined primarily to an interaction layer. The penetration depth of the interaction layer is limited to high altitudes in the thicker dayside atmosphere but reaches the surface in the thinner dayside and/or nightside atmosphere at larger solar zenith angles. Within most of the thicker dayside atmosphere, the column density of SO₂ is not significantly altered by electrons, but in the interaction layer all number densities are significantly altered: SO₂ is reduced, O, SO, S, and O₂ are greatly enhanced, and O, SO, and S become comparable to SO₂ at high altitudes. For the thinner nightside atmosphere, the species number densities are dramatically altered: SO₂ is drastically reduced to the least abundant species of the SO₂ family, SO and O₂ are significantly reduced at all altitudes, and O and S are dramatically enhanced and become the dominant species at all altitudes except near the surface. The interaction layer also defines the location of the emission layer for neutrals excited by electron impact and hence determines the fraction of the total neutral column density that is visible in remote observation. Electron chemistry may also impact the ratio of the equatorial to polar SO₂ column density deduced from Lyman-α images and the north-south alternating and System III longitude-dependent asymmetry observed in polar O and S emissions.