Chemical sources of haze formation in Titan's atmosphere

A prominent feature of Titan's atmosphere is a thick haze region that acts as the end product of hydrocarbon and nitrile chemistry. Using a one-dimensional photochemical model, an investigation into the chemical mechanisms responsible for the formation of this haze region is conducted. The model derives profiles for Titan's atmospheric constituents that are consistent with observations. Included is an updated benzene profile that matches more closely with—recent ISO observations (Icarus 161 (2003) 383), replacing the profile given in the benzene study of Wilson et al. (J. Geophys. Res. 108 (2003) 5014). Using these profiles, pathways from polyynes, aromatics, and nitriles are considered, as well as possible copolymerization among the pathways. The model demonstrates that the growth of polycyclic aromatic hydrocarbons throughout the lower stratosphere plays an important role in furnishing the main haze layer, with nitriles playing a secondary role. The peak chemical production of haze layer ranges from 140 to 300 km peaking at an altitude of 220 km, with a production rate of 3.2×10⁻¹⁴ gcm⁻² s⁻¹. Possible mechanisms for polymerization and copolymerization and suggestions for further kinetic study are discussed, along with the implications for the distribution of haze in Titan's atmosphere.     

Bolides in the present and past martian atmosphere and effects on cratering processes

Abstract— We investigate the action of the martian atmosphere on entering meteoroids for present and past atmospheres with various surface pressures to predict the smallest observable craters, and to understand the implications for the size distributions of craters on Mars and meteoroids in space. We assume different strengths appropriate to icy, stone, and iron bodies and test the results against available data on terrestrial bolides. Deceleration, ablation, and fragmentation effects are included. We find that the smallest icy, stone, and iron meteoroids to hit the martian ground at crater forming speeds of ≥500 m/s have diameters of about 2 m, 0.03‐0.9 m (depending on strength), and 0.01 m, respectively, in the current atmosphere. For hypothetical denser past atmospheres, the cutoff diameters rise. At a surface pressure of 100 mb, the cutoff diameters are about 24 m, 5–12 m, and 0.14 m for the 3 classes. The weaker stony bodies in the size range of about 1–30 m may explode at altitudes of about 10–20 km above the ground. These figures imply that under the present atmosphere, the smallest craters made by these objects would be as follows: by ice bodies, craters of diameter (D) ?8 m, by stones about 0.5–6 m, and by irons, about 0.3 m. A strong depletion of craters should, thus, occur at diameters below about 0.3 m to 5 m. Predicted fragmentation and ablation effects on weak meteoroids in the present atmosphere may also produce a milder depletion below D ?500 m, relative to the lunar population. But, this effect may be difficult to detect in present data because of additional losses of small craters due to sedimentation, dunes, and other obliteration effects. Craters in strewn fields, caused by meteoroid fragmentation, will be near or below present‐day resolution limits, but examples have been found. These phenomena have significant consequences. Under the present atmosphere, the smallest (decimeter‐scale) craters in sands and soils could be quickly obliterated but might still be preserved on rock surfaces, as noted by Hörz et al. (1999). Ancient crater populations, if preserved, could yield diagnostic signatures of earlier atmospheric conditions. Surfaces formed under past denser atmospheres (few hundred mbar), if preserved by burial and later exposed by exhumation, could show: a) striking depletions of small craters (few meter sizes up to as much as 200 m), relative to modern surfaces; b) more clustered craters due to atmospheric breakup; and c) different distributions of meteorite types, with 4 m to 200 m craters formed primarily by irons instead of by stones as on present‐day Mars. Megaregolith gardening of the early crust would be significant but coarser than the gardening of the ancient lunar uplands.     

Chemical structure of the deep atmosphere of Jupiter

We report here the equilibrium abundances calculated for a system of over 500 compounds of 27 selected elements along a nominal Jupiter adiabat. Several species predicted to be of negligible abundance in the visible upper troposphere if chemical equilibrium is exactly attained are found to be potential tracers of rapid vertical motions. Vertical mixing of certain species, especially CO, PH₃, AsH₃, GeS, and GeH₄, may provide detectable quantities of these species near the visible cloudtops due to quenching and incomplete equilibration of the rapidly rising, rapidly cooling gas. Observational prospects for detecting such tracers of deep circulation are discussed in the light of the spectroscopic detection of CO in the 5-μm window on Jupiter and the confirmation of PH₃ on both Jupiter and Saturn.     

Infrared observations of the surface and atmosphere of Titan

Infrared photometry of Titan, Saturn, and Saturn's Rings at 3.5, 4.9, 17.8, and 18.4 μm is reported. Comparison of the albedo of Titan in the 4.9 μm “window” with the albedo of the rings and with laboratory spectra suggests that frost, possibly water ice, could be a major constituent. If thick clouds are present they must be very dark at 4.9 μm. The 17.8 and 18.4 μm data are not consistent with a clear, dense molecular hydrogen atmosphere.     

Multiple scattering in the atmosphere with a rough surface

In the present paper, the intensity of radiation emergent from the atmosphere bounded by a rough surface is discussed with the aid of the superposition method derived by Mukai (1973). The merit of this method is to express the laws of diffuse reflection and transmission for the planetary problem with a rough surface in terms of a scattering and a transmission function for the standard problem.Here the bottom surface is assumed to reflect light in accordance with the slope distribution given by Cox and Munk (1954a, b). The results are discussed in terms of the optical properties and roughness of the bottom surface.     

Chemical kinetic model for the lower atmosphere of Venus

A self-consistent chemical kinetic model of the Venus atmosphere at 0-47 km has been calculated for the first time. The model involves 82 reactions of 26 species. Chemical processes in the atmosphere below the clouds are initiated by photochemical products from the middle atmosphere (H₂SO₄, CO, S_x), thermochemistry in the lowest 10 km, and photolysis of S₃. The sulfur bonds in OCS and S_x are weaker than the bonds of other elements in the basic atmospheric species on Venus; therefore the chemistry is mostly sulfur-driven. Sulfur chemistry activates some H and Cl atoms and radicals, though their effect on the chemical composition is weak. The lack of kinetic data for many reactions presents a problem that has been solved using some similar reactions and thermodynamic calculations of inverse processes. Column rates of some reactions in the lower atmosphere exceed the highest rates in the middle atmosphere by two orders of magnitude. However, many reactions are balanced by the inverse processes, and their net rates are comparable to those in the middle atmosphere. The calculated profile of CO is in excellent agreement with the Pioneer Venus and Venera 12 gas chromatographic measurements and slightly above the values from the nightside spectroscopy at 2.3 μm. The OCS profile also agrees with the nightside spectroscopy which is the only source of data for this species. The abundance and vertical profile of gaseous H₂SO₄ are similar to those observed by the Mariner 10 and Magellan radio occultations and ground-based microwave telescopes. While the calculated mean S₃ abundance agrees with the Venera 11-14 observations, a steep decrease in S₃ from the surface to 20 km is not expected from the observations. The ClSO₂ and SO₂Cl₂ mixing ratios are ∼10⁻¹¹ in the lowest scale height. The existing concept of the atmospheric sulfur cycles is incompatible with the observations of the OCS profile. A scheme suggested in the current work involves the basic photochemical cycle, that transforms CO₂ and SO₂ into SO₃, CO, and S_x, and a minor photochemical cycle which forms CO and S_x from OCS. The net effect of thermochemistry in the lowest 10 km is formation of OCS from CO and S_x. Chemistry at 30-40 km removes the downward flux of SO₃ and the upward flux of OCS and increases the downward fluxes of CO and S_x. The geological cycle of sulfur remains unchanged.     

Chemical composition of Venus atmosphere and clouds: Some unsolved problems

In spite of many spacecrafts that visited Venus, chemical composition of the Venus atmosphere and clouds present many challenging problems in observation and theory. The following problems are briefly discussed below: (1) molecular oxygen above the clouds, (2) lightning, (3) the blue absorption in the clouds, (4) mode 3 particle controversy and the Vega X-ray fluorescent observations, (5) search for new chlorine and sulfur species, and (6) vertical and spatial variations of water vapor and CO.     

Chemical response of the middle atmosphere to solar variations

The possible variation of the trace species concentration in the middle atmosphere related to long term solar irradiance variability is estimated by means of a one-dimensional numerical model.Proceedings of the 14th ESLAB Symposium on Physics of Solar Variations, 16–19 September 1980, Scheveningen, The Netherlands.Aspirant au Fonds National de la Recherche Scientifique.     

Report to cross sections related to plasma-planetary atmosphere interaction processes

As a recent trend, the continuous increase of new technologies for space observations of new missions to Mars, Venus, and Titan, has stimulated vigorous experimental and theoretical studies on the collision process induced by interactions between plasma and planetary atmosphere. In order to facilitate the comprehension of these processes, this brief paper chose a collection of cross section data not always easily accessible. With the purpose of making a useful collection of such data we have collected both experimental and theoretical estimate for most of the expected collisions processes.     

Comment on element fractionation in the solar atmosphere driven by ionization-diffusion processes

It is emphasized that one-dimensional, steady-state models of a photoionization layer in which incoming photon fluxes ionize neutral particle fluxes cannot give rise to enhancements or depletions of one element over another. The so-called fractionation mechanism proposed by Marsch, von Steiger, and Bochler (1995) and Peter (1996, 1998) is shown to be a fallacy arising from applying an incorrect boundary condition at an arbitrary point.     

Exchange of water vapor between the atmosphere and surface of Mars

A model for exchange of water from the atmosphere to condensing CO₂ caps is developed. The rate of water condensation in the caps is assumed to be proportional to the meridional heat flux. It follows that the amount of water condensed in the caps varies inversely with the amount of CO₂ condensed. The seasonal phase of the release of water from the caps is not consistent with observed variations in the abundance of atmospheric water. Seasonal variations of atmospheric water abundance are most consistent with vapor exchange between the atmosphere and permafrost in the subtropics. Although water condensation in semipermanent caps is normally very slow, it may take place at a much faster rate at unusually high atmospheric temperatures, such as those produced by absorption of solar radiation by airborne dust.     

Theoretical interpretation of photometric properties of the Martian surface and atmosphere

Earth-based UBV photometry, high-quality photographs from the Lowell Observatory collection, and Mariner 9 data have been combined with a new radiative transfer theory to derive physical parameters for the Martian surface and atmosphere, both before and during the 1971 dust storm. We find that the dust particles of the storm had a single-scattering albedo of 0.84 ± 0.02 and an asymmetry factor of 0.35 ± 0.10 in green (V) light. The geometric albedo of Mars was 0.15 and the phase integral 1.83, which yield 0.27 for the Bond albedo. The mean optical thickness of the “clear” atmosphere averaged over the whole planet was 0.15 ± 0.05 and was not detectably dependent on wavelength. Geometric albedos for the surface are 0.25 (light areas) and 0.17 (dark areas) in V, 0.095 in B (both areas), and 0.060 in U (both areas). The soil particles are moderately backward scattering with an asymmetry factor of ?0.20, indicating them to be rather opaque. The mean surface roughness, on a scale larger than that of individual dust particles and therefore large compared with the wavelength, is 0.57. This represents the depth/radius ratio of an average hole and it is only one-half as large as values typical for the Moon and asteroids.     

Cascade processes in the surface layers of pulsars

The influence of the Landau-Pomeranchuk effect on the development of a shower generated by ultrarelativistic particles bombarding the surface of a pulsar is discussed. Because of this effect, the path length of the shower increases while low-energy photon generation is strongly suppressed. In view of this, the mechanism of pair production suggested by Cheng, Ruderman, and Jones for the pulsar magnetosphere, may be essential only for pulsars whose magnetic field intensity at the surface lies in a relatively narrow range of aroundB 10¹² G.     

Titan's atmosphere and surface: Parallels and differences with the primitive Earth

In spite of a marked resemblance with our planet, Titan should not be hastily considered as another Earth but rather as a useful tool in the study of chemical and physical processes in the primitive Earth.     

Mass transfer in the satellite system of Neptune: implications for Triton's crater asymmetry

In this paper, we shall analyse a promising way to explain the huge crater asymmetry observed on Triton, the largest of Neptune's satellites. Triton shows, as well as many other satellites in the Solar System, a non-symmetric crater distribution on its surface. This fact is principally due to the synchronous rotation of these satellites, as shown by many theoretical works (see Shoemaker and Wolfe, Satellites of Jupiter, University of Arizona Press, Tucson, 1992, p. 277; Horedt and Neukum, Icarus 60 (1984) 710; Zahnle et al., Icarus 136 (1998) 202; Zahnle et al., Icarus 153 (2001) 111). However, on Triton the asymmetry is much more pronounced than on other satellites, and it exceeds what the models, in which the source of the craters are bodies in heliocentric orbits, can account for. For this reason, many authors (Croft et al., Icarus 99 (1992) 94; Schenk and Sobieszczyk, American Astronomical Society, DPS Meeting, Vol. 31, 1999; Zahnle et al., Icarus 153 (2001) 111) proposed that the origin for Triton's asymmetry has to be found in a swarm of bodies having planetocentric orbits, instead of heliocentric ones. Here, we analyse from a dynamical point of view the possibility that such swarm of fragments was generated by a collision between an inner satellite and a third object (a process we call ‘mass transfer’). Moreover, we discuss the possibility that the observed crater distribution on Triton comes from two populations: heliocentric bodies responsible for a few big craters, plus planetocentric bodies responsible for the big asymmetry.Finally, we discuss some implications for ground observations.     

Chemical response of the middle atmosphere to changes in the ultraviolet solar flux

Time variations in the solar flux between 1000 and 4000 Å induce changes in the concentrations of minor constituents in the upper stratosphere and mesosphere. The response of mesospheric ozone to variations in the Lyman α line over the course of several solar rotations may be of measurable magnitude. Large Lyman α fluxes lead to small O₃ densities above 65 km due to the enhanced dissociation of H₂O and resultant destruction of odd oxygen by odd hydrogen. An increase in continuum and Lyman α fluxes causes a slight enhancement in both the odd oxygen and hydrogen concentrations in the upper stratosphere.     

Mass spectrometric analysis in planetary science: Investigation of the surface and the atmosphere

Knowing the chemical, elemental, and isotopic composition of planetary objects allows the study of their origin and evolution within the context of our Solar System. Landed probes are critical to such an investigation. Instruments on a landed platform can answer a different set of scientific questions than can instruments in orbit or on Earth. Composition studies for elemental, isotopic, and chemical analysis are best performed with dedicated mass spectrometer systems. Mass spectrometers have been part of the early lunar missions, and have been successfully employed to investigate the atmospheres of Mars, Venus, Jupiter, Saturn, Titan, and in comet missions. Improved mass spectrometer systems are foreseen for many planetary missions currently in planning or implementation.