Molecular fluxes in the lunar atmosphere

The properties of a neutral lunar atmosphere are investigated theoretically. A non-uniformity is shown to result from the temperature variations and non-uniform gas source distribution on the surface of the Moon. An integral equation governing the distribution of molecular fluxes, in the steady state, is formulated. This equation is solved by computer and analytical methods. Solutions are obtained and discussed for mass numbers ranging from hydrogen to the heavy gases. Characteristic relaxation times for approach to the steady state are estimated and found generally to be a small fraction of the synodic period. It is concluded that in all cases a marked anisotropy of molecular fluxes can be expected. By measuring these fluxes conclusions can be drawn about the distribution of gas sources, the physical properties of the surface and the composition of the lunar atmosphere.     

The lunar atmosphere

In contrast to earth, the atmosphere of the moon is exceedingly tenuous and appears to consist mainly of noble gases. The solar wind impinges on the lunar surface, supplying detectable amounts of helium, neon and ³⁶Ar. Influxes of solar wind protons and carbon and nitrogen ions are significant, but atmospheric gases containing these elements have not been positively identified. Radiogenic ⁴⁰Ar and ²²²Rn produced within the moon have been detected. The present rate of effusion of argon from the moon accounts for about 0.4% of the total production of ⁴⁰Ar due to decay of ⁴⁰K if the average abundance of potassium in the moon is 1000 ppm. Lack of weathering processes in the regolith suggests that most of the atmospheric ⁴⁰Ar originates deep in the lunar interior, perhaps in a partially molten core. If so, other gases may be vented along with the argon.     

Formation of the lunar atmosphere

Measurements of⁴⁰Ar and helium made by the Apollo 17 lunar surface mass-spectrometer are used in the synthesis of atmospheric supply and loss mechanisms. The argon data indicate that about 8% of the⁴⁰Ar produced in the Moon due to decay of⁴⁰K is released to the atmosphere and subsequently lost. Variability of the atmospheric abundance of argon requires that the source be localized, probably in an unfractionated, partially molten core. If so, the radiogenic helium released with the argon amounts to 10% of the atmospheric helium supply. The total rate of helium escape from the Moon accounts for only 60% of the solar windα particle influx. This seems to require a nonthermal escape mechanism for trapped solar-wind gases, probably involving weathering of exposed soil grain surfaces by solar wind protons.     

The 1999 Quadrantids and the lunar Na atmosphere

Enhancements of the Na emission and temperature from the lunar atmosphere were reported during the Leonid meteor showers of 1995, 1997 and 1998. Here we report a search for similar enhancement during the 1999 Quadrantids, which have the highest mass flux of any of the major streams. No enhancements were detected. We suggest that different chemical–physical properties of the Leonid and Quadrantid streams may be responsible for the difference.     

Solar-wind interactions: Nature and composition of lunar atmosphere

The solar wind interacts directly with the lunar surface material resulting in an essentially complete absorption of the corpuscles producing no upstream bowshock but a cavity downstream from the Moon. The main source of most neutral species of the atmosphere, except probably⁴⁰Ar, is the solar-wind interaction products. The other sources which appear to be minor contributors to the atmosphere are the interaction products of cosmic rays, planetary degassing, effects of meteorite impacts and radioactive decays. Most of the hydrogen atoms derived from the solar-wind protons contribute to the atmosphere as hydrogen molecules rather than atoms. Only on the basis of the solar-wind protons, alpha particles and ions of oxygen and carbon, the atmospheric species concentration (cm^–3) near the lunar surface at 300K are as follows: H₂ 3.3 to 9.9 × 10³; He 2.4 to 4.7 × 10³; H 3.7; OH 0.25; H₂O 0.24; and O₂, O, CO, CO₂ and CH₄ in concentrations smaller than H₂. Whatever the source, the OH and H₂O concentrations in the atmosphere are about the same. The calculated concentrations are in good agreement with the observations by the Apollo 17 lunar surface mass spectrometer and the Apollo 17 orbital UV spectrometer. At the time of sample collection from the Moon, the hydrogen content in the trapped gas layer of the lunar surface material was partly as hydrogen atoms and partly as hydrogen molecules, but at the time of sample analysis hydrogen was mostly in molecular form. The H₂O content at the time of sample analysis was only a few parts per million by weight.Paper presented at the Conference on Interactions of the Interplanetary Plasma with the Modern and Ancient Moon, sponsored by the Lunar Science Institute, Houston, Texas and held at the Lake Geneva Campus of George Williams College, Wisconsin, between September 30 and October 4, 1974.     

A noble gas data collection of lunar meteorites

We collected the published noble gas data of altogether 35 lunar meteorites. This compilation includes the stable isotopes of He, Ne, Ar, Kr, and Xe. We also give a summary of cosmogenic, trapped, and radiogenic noble gas components of lunar meteorites for which data are available in the literature.     

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.     

A gas dynamic model of the rotating solar atmosphere

A model is presented of a solar atmosphere which is heated by the periodic passage of shock waves. The outer atmosphere rotates and is assumed not to affect the strength of the shock waves. This constant shock strength hypothesis is used as the basis of the model of the outer solar atmosphere. From the model it is concluded that the chromospheric temperature rise and flow Mach number are slightly affected by the rotation of the atmosphere.     

Shock jump relations for a dusty gas atmosphere

This paper presents simplified forms of jump relations for one dimensional shock waves propagating in a dusty gas. The dusty gas is assumed to be a mixture of a perfect gas and spherically small solid particles, in which solid particles are continuously distributed. The simplified jump relations for the pressure, the temperature, the density, the velocity of the mixture and the speed of sound have been derived in terms of the upstream Mach number. The expressions for the adiabatic compressibility of the mixture and the change-in-entropy across the shock front have also been derived in terms of the upstream Mach number. Further, the handy forms of shock jump relations have been obtained in terms of the initial volume fraction of small solid particles and the ratio of specific heats of the mixture, simultaneously for the two cases viz., (i) when the shock is weak and, (ii) when it is strong. The simplified shock jump relations reduce to the Rankine-Hugoniot conditions for shock waves in an ideal gas when the mass fraction (concentration) of solid particles in the mixture becomes zero. Finally, the effects due to the mass fraction of solid particles in the mixture, and the ratio of the density of solid particles to the initial density of the gas are studied on the pressure, the temperature, the density, the velocity of the mixture, the speed of sound, the adiabatic compressibility of the mixture and the change-in-entropy across the shock front. The results provided a clear picture of whether and how the presence of dust particles affects the flow field behind the shock front. The aim of this paper is to contribute to the understanding of how the shock waves behave in the gas-solid particle two-phase flows.     

Molecular gas in young planetary nebulae

We review our recent results concerning the molecular gas content of young planetary nebulae NGC 2346, M 2–9, and NGC 6720 (the Ring Nebula in Lyra).Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

Rare gas diffusion studies in individual lunar soil particles and in artificially implanted glasses

The linear heating technique was applied to study the release of solar wind implanted He and Ne in single glass spherules and minerals of lunar soils. In addition, the diffusion of rare gases artificially implanted into simulated lunar glass was investigated. Activation energies derived for lunar glasses are much higher than for virginal glasses of similar chemical composition. Volume diffusion of the lunar surface cannot explain the high retentivity for the trapped gases. It is expermentally shown that various types of radiation damage are of paramount importance to the understanding of the secondary alterations of elemental abundances after solar wind implantation on the lunar surface.Paper dedicated to Prof. Harold C. Urey on the occasion of his 80th birthday on 29 April 1973.     

The sunlit lunar atmosphere: A comprehensive study by CHACE on the Moon Impact Probe of Chandrayaan-1

The altitudinal/latitudinal profile of the lunar atmospheric composition on the sunlit side was unraveled for the first time by the Chandra’s Altitudinal Composition Explorer (CHACE) on the Moon Impact Probe, a standalone micro-satellite that impacted at the lunar south pole, as a part of the first Indian mission to Moon, Chandrayaan-1. Systematic measurements were carried out during the descent phase of the impactor with an altitude resolution of ∼250 m and a latitudinal resolution of ∼0.1°. The overall pressure on the dayside and the neutral composition in the mass range 1-100 amu have been measured by identifying 44 and 18 amu as the dominant constituents. Significant amounts of heavier (>50 amu) species also have been detected, the details of which are presented and discussed.     

Photometry of the lunar surface during lunar eclipses

The photometric observations of the lunar surface during lunar eclipses were carried out on four nights between 1972 to 1978, using the 91 cm reflector of the Dodaira Station of the Tokyo Astronomical Observatory. The photometry was performed in B-, V-, and R-colours, and arranged in accordance with the angular distance from the centre of the Earth's shadow. The results do not show any large systematic differences between the four nights, showing no support for Danjon's proposition.     

Molecular gas associated with the IRAS-vela shell

We present a survey of molecular gas in theJ = 1 → 0 transition of¹²CO towards the IRAS Vela Shell. The shell, previously identified from IRAS maps, is a ring-like structure seen in the region of the Gum Nebula. We confirm the presence of molecular gas associated with some of the infrared point sources seen along the shell. We have studied the morphology and kinematics of the gas and conclude that the shell is expanding at the rate of ~ 13 km s^-1 from a common center. We go on to include in this study the Southern Dark Clouds seen in the region. The distribution and motion of these objects firmly identify them as being part of the shell of molecular gas. Estimates of the mass of gas involved in this expansion reveal that the shell is a massive object comparable to a GMC. From the expansion and various other signatures like the presence of bright-rimmed clouds with head-tail morphology, clumpy distribution of the gas etc., we conjecture that the molecular gas we have detected is the remnant of a GMC in the process of being disrupted and swept outwards through the influence of a central OB association, itself born of the parent cloud.     

Europa's atmosphere, gas tori, and magnetospheric implications

A two-dimensional kinetic model calculation for the water group species (H₂O, H₂, O₂, OH, O, H) in Europa's atmosphere is undertaken to determine its basic compositional structure, gas escape rates, and velocity distribution information to initialize neutral cloud model calculations for the most important gas tori. The dominant atmospheric species is O₂ at low altitudes and H₂ at higher altitudes with average day-night column densities of 4.5×¹⁰¹⁴ and 7.7×¹⁰¹³ cm⁻², respectively. H₂ forms the most important gas torus with an escape rate of ∼2×¹⁰²⁷ s⁻¹ followed by O with an escape rate of ∼5×¹⁰²⁶ s⁻¹, created primarily as exothermic O products from O₂ dissociation by magnetospheric electrons. The circumplanetary distributions of H₂ and O are highly peaked about the satellite location and asymmetrically distributed near Europa's orbit about Jupiter, have substantial forward clouds extending radially inward to Io's orbit, and have spatially integrated cloud populations of 4.2×¹⁰³³ molecules for H₂ and 4.0×¹⁰³² atoms for O that are larger than their corresponding populations in Europa's local atmosphere by a factor of ∼200 and ∼1000, respectively. The cloud population for H₂ is a factor of ∼3 times larger than that for the combined cloud population of Io's O and S neutral clouds and provides the dominant neutral population beyond the so-called ramp region at 7.4-7.8 RJ in the plasma torus. The calculated brightness of Europa's O cloud on the sky plane is very dim at the sub-Rayleigh level. The H₂ and O tori provide a new source of europagenic molecular and atomic pickup ions for the thermal plasma and introduce a neutral barrier in which new plasma sinks are created for the cooler iogenic plasma as it is transported radially outward and in which new sinks are created to alter the population and pitch angle distribution of the energetic plasma as it is transported radially inward. The europagenic instantaneous pickup ion rates are peaked at Europa's orbit, dominate the iogenic pickup ion rates beyond the ramp region, and introduce new secondary plasma source peaks in the solution of the plasma transport problem. The H₂ torus is identified as the unknown Europa gas torus that creates both the observed loss of energetic H⁺ ions at Europa's orbit and the corresponding measured ENA production rate for H.     

Studying methane and other trace species in the Mars atmosphere using a SOIR instrument

Solar Occultation in the InfraRed (SOIR) is one of three spectrometers of the SPICAV/SOIR instrument suite (Bertaux et al., 2007b) on board the Venus Express orbiter (VEX). VEX has been in orbit around Venus since April 2006 and to date SOIR has carried out over 674 measurements. Pre-launch and in-orbit performance analyses allow us to predict what SOIR would be capable of at Mars. SOIR spectra through the Martian atmosphere have been simulated with ASIMUT, a line-by-line (LBL) radiative transfer code also used for the retrieval of vertical profiles of atmospheric constituents of Venus (Vandaele et al., 2008, Bertaux et al., 2007a). The code takes into account the temperature and pressure vertical profiles as well as those of the atmospheric species, but also the instrument function and the overlapping of the diffraction orders of the echelle grating. We will show these spectra and the detection limits of species that could be studied using a SOIR spectrometer making solar occultation or nadir measurements in Mars orbit.     

Self-similar flows of a self-gravitating gas with increasing energy in uniform atmosphere

Self-similar flows of a gas, moving under the gravitational force of attraction behind a spherical shock wave, which are driven out by a propelling contact surface and propagating into a uniform atmosphere at rest are investigated. The energy of the expanding wave has been assumed to be time-dependent, obeying a power law. In the last section the self-similar homothermal flows of self-gravitating gas has been also discussed. A comparative study has been made between the nature of flow variables for adiabatic and homothermal flows.