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1.
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. 相似文献
2.
In this paper, we look at space weathering processes on the icy surface of Jupiter’s moon Europa. The heavy energetic ions of the jovian plasma (H+, O+, S+, C+) can erode the surface of Europa via ion sputtering (IS), ejecting up to 1000 H2O molecules per ion. UV photons impinging the Europa’s surface can also result in neutral atom release via photon-stimulated desorption (PSD) and chemical change (photolysis). In this work, we study the efficiency of the IS and PSD processes for ejecting water molecules, simulating the resulting neutral H2O density. We also estimate the contribution to the total neutral atom release by the Ion Backscattering (IBS) process. Moreover, we estimate the possibility of detecting the sputtered high energy atoms, in order to distinguish the action of the IS process from other surface release mechanisms. Our main results are: (1) The most significant sputtered-particle flux and the largest contribution to the neutral H2O density come from the incident S+ ions; (2) the H2O density produced via PSD is lower than that due to sputtering by ∼1.5 orders of magnitude; (3) in the energy range below 1 keV, the IBS can be considered negligible for the production of neutrals, whereas in the higher energy range it becomes the dominant neutral emission mechanism; (4) the total sputtering rate for Europa is 2.0 × 1027 H2O s−1; and (5) the fraction of escaping H2O via IS is 22% of the total sputtered population, while the escape fraction for H2O produced by PSD is 30% of the total PSD population. Since the PSD exosphere is lower than the IS one, the major agent for Europa’s surface total net erosion is IS on both the non-illuminated and illuminated side. Lastly, the exospheric neutral density, estimated from the Galileo electron density measurements appears to be higher than that calculated for H2O alone; this favors the scenario of the presence of O2 produced by radiolysis and photolysis. 相似文献
3.
Observations of Saturn's distant moon Phoebe were made at far-ultraviolet (FUV) (1100-1900 Å) and extreme-ultraviolet (EUV) (600-1100 Å) wavelengths by the Cassini Ultraviolet Imaging Spectrograph (UVIS) during the Cassini spacecraft flyby on June 11, 2004. These are the first UV spectra of Phoebe and the first detection of water ice on a Solar System surface using FUV wavelengths. The characteristics of water ice in the FUV are presented, and Hapke models are used to interpret the spectra in terms of composition and grain size; the use of both areal and intimate mixing models is explored. Non-ice species used in these models include carbon, ice tholin, Triton tholin, poly-HCN and kerogen. Satisfactory disk-integrated fits are obtained for intimate mixtures of ∼10% H2O plus a non-ice species. Spatially resolved regions of higher (∼20%) and lower (∼5%) H2O ice concentrations are also detected. Phoebe does not display any evidence of volatile activity. Upper limits on atomic oxygen and carbon are 5×1011 and 2×1012 atoms/cm2, respectively, for solar photon scattering. The UVIS detection of water ice on Phoebe, and the ice amounts detected, are consistent with IR measurements and contribute to the evidence for a Phoebe origin in the outer Solar System rather than in the main asteroid belt. 相似文献
4.
C/2006 P1 McNaught is a dynamically new comet from the Oort cloud that passed very close to the Sun, driving overall volatile production rates up to about 1031 molecules s−1. Post-perihelion observations were obtained in a target-of-opportunity campaign using the CSHELL instrument at the NASA Infrared Telescope Facility atop Mauna Kea, Hawaii, on UT 2007 January 27 and 28. Eight parent volatiles (H2O, CH4, C2H2, C2H6, HCN, CO, NH3, H2CO) and two daughter fragments (OH and NH2) were detected, enabling the determination of a rotational temperature and production rate for H2O on UT January 27 and absolute and relative production rates for all the detected parent species on UT January 28. The chemical composition measured in the coma suggests that this close perihelion passage stripped off processed outer surface layers, likely exposing relatively fresh primordial material during these observations. The post-perihelion abundances we measure for CO and CH4 (relative to H2O) are slightly depleted while C2H2, NH2 and possibly NH3 are enhanced when compared to the overall comet population. Measured abundances for other detected molecular species were within the range typically observed in comets. 相似文献
5.
Darrell F. Strobel 《Icarus》2010,208(2):878-886
The third most abundant species in Titan’s atmosphere is molecular hydrogen with a tropospheric/lower stratospheric mole fraction of 0.001 derived from Voyager and Cassini infrared measurements. The globally averaged thermospheric H2 mole fraction profile from the Cassini Ion Neutral Mass Spectrometer (INMS) measurements implies a small positive gradient in the H2 mixing ratio from the tropopause region to the lower thermosphere (∼950-1000 km), which drives a downward H2 flux into Titan’s surface comparable to the H2 escape flux out of the atmosphere (∼2 × 1010 cm−2 s−1 referenced to the surface) and requires larger photochemical production rates of H2 than obtained by previous photochemical models. From detailed model calculations based on known photochemistry with eddy, molecular, and thermal diffusion, the tropospheric and thermospheric H2 mole fractions are incompatible by a factor of ∼2. The measurements imply that the downward H2 surface flux is in substantial excess of the speculative threshold value for methanogenic life consumption of H2 (McKay, C.P., Smith, H.D. [2005], Icarus 178, 274-276. doi:10.1016/j.icarus.2005.05.018), but without the extreme reduction in the surface H2 mixing ratio. 相似文献
6.
Karen Magee-Sauer Michael J. Mumma Neil Dello Russo Boncho P. Bonev G.L. Villanueva 《Icarus》2008,194(1):347-356
We used the NIRSPEC instrument on the Keck-2 telescope atop Mauna Kea, HI to observe Comet C/2001 A2 (LINEAR) in a Target of Opportunity campaign on UT 2001 July 9.5, 10.5 August 4.4, 10.5. We measured seven organic parent volatiles (C2H6, C2H2, HCN, CH4, CO, CH3OH, H2CO) simultaneously with H2O. We obtained absolute production rates and relative abundances for parent volatiles, and also measured rotational temperatures for several of these species. The chemical composition of C/2001 A2 differs substantially from any comet we have observed to date. The abundances we measure (relative to H2O) for C2H6, C2H2, HCN, and CH3OH are enriched by a factor of ∼2 to 3 in C/2001 A2 compared with most comets in our database. Other molecular species were detected within the typical range of measured abundances. C/2001 A2 presented a unique opportunity to study the chemistry of a fragmenting comet where pristine areas are exposed to the Sun. 相似文献
7.
Galina M. Chaban 《Icarus》2007,187(2):592-599
An absorption band at ∼4.26 μm wavelength attributed to the asymmetric stretching mode of CO in CO2 has been found on two satellites of Jupiter and several satellites of Saturn. The wavelength of pure CO2 ice determined in the laboratory is 4.2675 μm, indicating that the CO2 on the satellites occurs either trapped in a host material, or in a chemical or physical complex with other materials, resulting in a blue shift of the wavelength of the band. In frequency units, the shifts in the satellite spectra range from 3.7 to 11.3 cm−1. We have performed ab initio quantum chemical calculations of CO2 molecules chemically complexed with one, two, and more H2O molecules and molecules of CH3OH to explore the possibility that the blue shift of the band is caused by chemical complexing of CO2 with other volatile materials. Our computations of the harmonic and anharmonic vibrational frequencies using high levels of theory show a frequency shift to the blue by 5 cm−1 from pure CO2 to CO-H2O, and an additional 5 cm−1 from CO2-H2O to CO2-2H2O. Complexing with more than two H2O molecules does not increase the blue shift. Complexes of CO2 with one molecule of CH3OH and with one CH3OH plus one H2O molecule produce smaller shifts than the CO2-2H2O complex. Laboratory studies of CO2:H2O in a solid N2 matrix also show a blue shift of the asymmetric stretching mode. 相似文献
8.
Jupiter exhibits bright H+3 auroral arcs at 3-4 microns that cool the hot (>1000 K) ionosphere above the ∼10−7 bar level through the infrared bands of this trace constituent. Below the 10−7 bar level significant cooling proceeds through infrared active bands of CH4, C2H2, and C2H6. We report the discovery of 3-micron line emission from these hydrocarbon species in spectra of the jovian south polar region obtained on April 18 and 20, 2006 (UT) with CGS4 on the United Kingdom Infrared Telescope. Estimated cooling rates through these molecules are 7.5×10−3, 1.4×10−3, and , respectively, for a total nearly half that of H+3. We derive a temperature of 450 ± 50 K in the 10−7-10−5 bar region from the C2H2 lines. 相似文献
9.
We present results on the energy balance of the Deep Impact experiment based on analysis of 180 infrared spectra of the ejecta obtained by the Deep Impact spacecraft. We derive an output energy of 16.5 (+9.1/−4.1) GJ. With an input energy of 19.7 GJ, the error bars are large enough so that there may or may not be a balance between the kinetic energy of the impact and that of outflowing materials. Although possible, no other source of energy other than the impactor or the Sun is needed to explain the observations. Most of the energy (85%) goes into the hot plume in the first few seconds, which only represents a very small fraction (<0.01%) of the total ejected mass. The hot plume contains 190 (+263/−71) kg of H2O, 1.6 ± 0.5 kg of CO2, 8.2 (+11.3/3.1) kg of CO (assuming a CO/H2O ratio of 4.3%), 27.9 (+25.0/−8.9) kg of organic material and 255 ± 128 kg of dust, while the ejecta contains ∼107 kg of materials. About 12% of the energy goes into the ejecta (mostly water) and 3% to destroy the impactor. Volatiles species other than H2O (CO2, CO or organic molecules) contribute to <7% of the energy balance. In terms of physical processes, 68% of the energy is used to accelerate grains (kinetic energy), 16% to heat them, 6% to sublimate or melt them and 10% (upper limit) to break and compress dust and/or water ice aggregates into small micron size particles. For the hot plume, we derive a dust/H2O ratio of 1.3 (+1.9/−1.0), a CO2/H2O ratio of 0.008 (+0.009/−0.006), an organics/H2O ratio of 0.15 (+0.29/−0.11) and an organics/dust ratio of 0.11 (+0.30/−0.07). This composition refers to the impact site and is different from that of the bulk nucleus, consistent with the idea of layers of different composition in the nucleus sub-surface. Our results emphasize the importance of laboratory impact experiments to understand the physical processes involved at such a large scale. 相似文献
10.
A time-dependent one-dimensional model of Saturn's ionosphere has been developed as an intermediate step towards a fully coupled Saturn Thermosphere-Ionosphere Model (STIM). A global circulation model (GCM) of the thermosphere provides the latitude and local time dependent neutral atmosphere, from which a globally varying ionosphere is calculated. Four ion species are used (H+, H+2, H+3, and He+) with current cross-sections and reaction rates, and the SOLAR2000 model for the Sun's irradiance. Occultation data from the Voyager photopolarimeter system (PPS) are adapted to model the radial profile of the ultraviolet (UV) optical depth of the rings. Diurnal electron density peak values and heights are generated for all latitudes and two seasons under solar minimum and solar maximum conditions, both with and without shadowing from the rings. Saturn's lower ionosphere is shown to be in photochemical equilibrium, whereas diffusive processes are important in the topside. In agreement with previous 1-D models, the ionosphere is dominated by H+ and H+3, with a peak electron density of ∼104 electrons cm−3. At low- and mid-latitudes, H+ is the dominant ion, and the electron density exhibits a diurnal maximum during the mid-afternoon. At higher latitudes and shadowed latitudes (smaller ionizing fluxes), the diurnal maximum retreats towards noon, and the ratio of [H+]/[H+3] decreases, with H+3 becoming the dominant ion at altitudes near the peak (∼1200-1600 km) for noon-time hours. Shadowing from the rings leads to attenuation of solar flux, the magnitude and latitudinal structure of which is seasonal. During solstice, the season for the Cassini spacecraft's encounter with Saturn, attenuation has a maximum of two orders of magnitude, causing a reduction in modeled peak electron densities and total electron column contents by as much as a factor of three. Calculations are performed that explore the parameter space for charge-exchange reactions of H+ with vibrationally excited H2, and for different influxes of H2O, resulting in a maximum diurnal variation in electron density much weaker than the diurnal variations inferred from Voyager's Saturn Electrostatic Discharge (SED) measurements. Peak values of height-integrated Pedersen conductivities at high latitudes during solar maximum are modeled to be ∼42 mho in the summer hemisphere during solstice and ∼18 mho during equinox, indicating that even without ionization produced by auroral processes, magnetosphere-ionosphere coupling can be highly variable. 相似文献
11.
A summary is presented of our spectroscopic survey of comets extending for roughly 19 years from 1985 to 2004 comprising data for 92 comets of which 50 showed good emissions. All data were re-analyzed using consistent reduction techniques. Our observations of comets over several apparitions and comets observed over an extended period indicate no major changes in compositional classification. To our regret, no major unidentified cometary features were found in our surveyed spectral region of 5200-10400 Å. Absolute production rates for the dominant parent molecule H2O and the daughter species C2, NH2 and CN are determined within the limits of the Haser model as are values for the dust continuum, Afρ. From these data, production rate ratios are calculated for C2/H2O, NH2/H2O, CN/H2O and Afρ/H2O. Excluding the odd Comets Yanaka (1988r), 43P/Wolf-Harrington and 19P/Borrelly, with unusual spectra, our set of comets exhibited relatively uniform composition. Detailed analyses of our data resulted in four taxonomic classes:
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- Comets of typical composition (∼70%); exhibiting typical ratios with respect to water of C2, NH2, and CN.
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- Tempel 1 type (∼22%); having a deficiency in C2 but normal NH2 abundance.
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- G-Z type (∼6%); having both low C2 and NH2 ratios.
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- The unusual object Yanaka (1988r) (∼2%?); no detectable C2 or CN emission but normal NH2.
12.
The goal of this study was to explore prebiotic chemistry in a range of plausible early Earth and Mars atmospheres. To achieve this laboratory continuous flow plasma irradiation experiments were performed on N2/H2/CO/CO2 gas mixtures chosen to represent mildly reducing early Earth and Mars atmospheres derived from a secondary volcanic outgassing of volatiles in chemical equilibrium with magmas near present day oxidation state. Under mildly reducing conditions (91.79% N2, 5.89% H2, 2.21% CO, and 0.11% CO2), simple nitriles are produced in the gas phase with yield (G in molecules per 100 eV), for the key prebiotic marker molecule HCN at G∼1×10−3 (0.1 nmol J−1). In this atmosphere localized HCN concentrations possibly could approach the 10−2 M needed for HCN oligomerization. Yields under mildly oxidizing conditions (45.5% N2, 0.1% H2, 27.2% CO, 27.2% CO2) are significantly less as expected, with HCN at G∼3×10−5 (). Yields in a Triton atmosphere which can be plausibly extrapolated to represent what might be produced in trace CH4 conditions (99.9% N2, 0.1% CH4) are significant with HCN at G∼1×10−2 (1 nmol J−1) and tholins produced. Recently higher methane abundance atmospheres have been examined for their greenhouse warming potential, and higher abundance hydrogen atmospheres have been proposed based on a low early Earth exosphere temperature. A reducing (64.04% N2, 28.8% H2, 3.60% CO2, and 3.56% CH4), representing a high CH4 and H2 abundance early Earth atmosphere had HCN yields of G∼5×10−3 (0.5 nmol J−1). Tholins generated in high methane hydrogen gas mixtures is much less than in a similar mixture without hydrogen. The same mixture with the oxidizing component CO2 removed (66.43% N2, 29.88% H2, 0% CO2, and 3.69% CH4) had HCN yields of G∼1×10−3 (0.1 nmol J−1) but more significant tholin yields. 相似文献
13.
Vladimir A. Krasnopolsky 《Icarus》2010,209(2):314-322
While CO, HCl, and HF, that were considered in the first part of this work, have distinct absorption lines in high-resolution spectra and were detected four decades ago, the lines of HDO, OCS, and SO2 are either very weak or blended by the telluric lines and have not been observed previously by ground-based infrared spectroscopy at the Venus cloud tops. The H2O abundance above the Venus clouds is typically below the detection limit of ground-based IR spectroscopy. However, the large D/H ratio on Venus facilitates observations of HDO. Converted to H2O with D/H ≈ 200, our observations at 2722 cm−1 in the Venus afternoon show a H2O mixing ratio of ∼1.2 ppm at latitudes between ±40° increasing to ±60° by a factor of 2. The observations in the early morning reveal the H2O mixing ratio that is almost constant at 2.9 ppm within latitudes of ±75°. The measured H2O mixing ratios refer to 74 km. The observed increase in H2O is explained by the lack of photochemical production of sulfuric acid in the night time. The recent observations at the P-branch of OCS at 4094 cm−1 confirm our detection of OCS. Four distributions of OCS along the disk of Venus at various latitudes and local times have been retrieved. Both regular and irregular components are present in the variations of OCS. The observed OCS mixing ratio at 65 km varies from ∼0.3 to 9 ppb with the mean value of ∼3 ppb. The OCS scale height is retrieved from the observed limb darkening and varies from 1 to 4 km with a mean value of half the atmospheric scale height. SO2 at the cloud tops has been detected for the first time by means of ground-based infrared spectroscopy. The SO2 lines look irregular in the observed spectra at 2476 cm−1. The SO2 abundances are retrieved by fitting by synthetic spectra, and two methods have been applied to determine uncertainties and detection limits in this fitting. The retrieved mean SO2 mixing ratio of 350 ± 50 ppb at 72 km favors a significant increase in SO2 above the clouds since the period of 1980-1995 that was observed by the SOIR occultations at Venus Express. Scale heights of OCS and SO2 may be similar, and the SO2/OCS ratio is ∼500 and may be rather stable at 65-70 km under varying conditions on Venus. 相似文献
14.
The neutral gas environment of a comet is largely influenced by dissociation of parent molecules created at the surface of the comet and collisions of all the involved species. We compare the results from a kinetic model of the neutral cometary environment with measurements from the Neutral Mass Spectrometer and the Dust Impact Detection System onboard the Giotto spacecraft taken during the fly-by at Comet 1P/Halley in 1986. We also show that our model is in good agreement with contemporaneous measurements obtained by the International Ultraviolet Explorer, sounding rocket experiments, and various ground based observations.The model solves the Boltzmann equation with a Direct Simulation Monte Carlo technique (Tenishev, V., Combi, M., Davidsson, B. [2008]. Astrophys. J. 685, 659-677) by tracking trajectories of gas molecules and dust grains under the influence of the comet’s weak gravity field with momentum exchange among particles modeled in a probabilistic manner. The cometary nucleus is considered to be the source of dust and the parent species (in our model: H2O, CO, H2CO, CO2, CH3OH, C2H6, C2H4, C2H2, HCN, NH3, and CH4) in the coma. Subsequently our model also tracks the corresponding dissociation products (H, H2, O, OH, C, CH, CH2, CH3, N, NH, NH2, C2, C2H, C2H5, CN, and HCO) from the comet’s surface all the way out to 106 km.As a result we are able to further constrain cometary the gas production rates of CO (13%), CO2 (2.5%), and H2CO (1.5%) relative to water without invoking unknown extended sources. 相似文献
15.
We measured the chemical composition of Comet C/2007 W1 (Boattini) using the long-slit echelle grating spectrograph at Keck-2 (NIRSPEC) on 2008 July 9 and 10. We sampled 11 volatile species (H2O, OH∗, C2H6, CH3OH, H2CO, CH4, HCN, C2H2, NH3, NH2, and CO), and retrieved three important cosmogonic indicators: the ortho-para ratios of H2O and CH4, and an upper-limit for the D/H ratio in water. The abundance ratios of almost all trace volatiles (relative to water) are among the highest ever observed in a comet. The comet also revealed a complex outgassing pattern, with some volatiles (the polar species H2O and CH3OH) presenting very asymmetric spatial profiles (extended in the anti-sunward hemisphere), while others (e.g., C2H6 and HCN) showed particularly symmetric profiles. We present emission profiles measured along the Sun-comet line for all observed volatiles, and discuss different production scenarios needed to explain them. We interpret the emission profiles in terms of release from two distinct moieties of ice, the first being clumps of mixed ice and dust released from the nucleus into the sunward hemisphere. The second moiety considered is very small grains of nearly pure polar ice (water and methanol, without dark material or apolar volatiles). Such grains would sublimate only very slowly, and could be swept into the anti-sunward hemisphere by radiation pressure and solar-actuated non-gravitational jet forces, thus providing an extended source in the anti-sunward hemisphere. 相似文献
16.
Jochen Jänchen Richard V. Morris David L. Bish Mareike Janssen Udo Hellwig 《Icarus》2009,200(2):463-467
The recent detection of up to ∼10 wt% water-equivalent H heterogeneously distributed in the upper meter of the equatorial regions of the martian surface and the presence of the 3-μm hydrations feature across the entire planet raises the question whether martian surficial dust can account for this water-equivalent H. We have investigated the H2O and CO2 adsorption properties of palagonitic dust (<5 μm size fraction of phyllosilicate-poor palagonitic tephra HWMK919) as a martian dust analog and two smectites under simulated martian equatorial surface conditions. Our results show that the palagonitic dust, which contains hydrated and hydroxylated volcanic glass of basaltic composition, accommodates significantly more H2O under comparable humidity and temperature conditions than do the smectites nontronite and montmorillonite. 相似文献
17.
H. Levy 《Planetary and Space Science》1973,21(4):575-591
Altitude profiles for the number densities of NO, NO2, NO3, N2O5, HNO2, CH3O, CH3O2, H2CO, OH, and HO2 are calculated as a function of time of day with a steady-state photochemical model in which the altitude profiles for the number densities of H2O, CH4, H2, CO, O3, and the sum of NO and NO2 are fixed at values appropriate to a summer latitude of 34°. Average daily profiles are calculated for the long-lived species, HNO3, H2O2, and CH3O2H.The major nitrogen compound HNO3 may have a number density approaching 5 × 1011 molecules cm?3 at the surface, although an effective loss path due to collisions with particulates could greatly reduce this value.The number density of OH remains relatively unchanged in the first 6 km and reaches 1 × 107 molecules cm?3 at noon, while the number density of HO2 decreases throughout the lower troposphere from its noontime value of 8 × 108 molecules cm?3 at the surface.H2O2 and H2CO both have number densities in the ppb range in the lower troposphere.Owing to decreasing temperature and water concentration, the production of radicals and their steady-state number densities decrease with altitude, reaching a noontime minimum of 1 × 108 molecules cm?3 for OH and 3 × 107 molecules cm?3 for HO2 at the tropopause. The related minor species show even sharper decreases with increasing altitude.The primary path for interconverting OH and HO2 serves as the major sink for CO and leads to a tropospheric lifetime for CO of ~0.1 yr.Another reaction cycle, the oxidation of CH4, is quite important in the lower troposphere and leads to the production of H2CO along with the destruction of CH4 for which a tropospheric lifetime of ~2 yr is estimated.The destruction of H2CO that was produced in the CH4 oxidation cycle provides the major source of CO and H2 in the atmosphere. 相似文献
18.
The newly detected oxygen atmosphere of Europa is modeled by invoking charged particle sputtering with H2O and O2molecules as the main ejecta. The magnetospheric corotating ions could provide the required source strength (∼3 × 1026sec−1) of O2molecules if a fraction (∼20%) of the exospheric ions were recycled to Europa's surface where they produce additional sputtering product. Two exospheric components are expected to form: an extended corona with a size of a few satellite radii which is composed of sputtered molecules in ballistic motion, and a thermal population with a surface density of 108–109cm−3and a scale height of about 20 km. The electron impact ionization of this exosphere would lead to an Io-like interaction with the jovian magnetosphere with a field-aligned Birkeland current of about 5 × 105A. 相似文献
19.
Using a one-dimensional model, we investigate the hydrogen budget and escape to space in Titan’s atmosphere. Our goal is to study in detail the distributions and fluxes of atomic and molecular hydrogen in the model, while identifying sources of qualitative and quantitative uncertainties. Our study confirms that the escape of atomic and molecular hydrogen to space is limited by the diffusion through the homopause level. The H distribution and flux inside the atmosphere are very sensitive to the eddy diffusion coefficient used above altitude 600 km. We chose a high value of this coefficient 1 × 108 cm2 s−1 and a homopause level around altitude 900 km. We find that H flows down significantly from the production region above 500 km to the region [300-500] km, where it recombines into H2. Production of both H and H2 also occurs in the stratosphere, mostly from photodissociation of acetylene. The only available observational data to be compared are the escape rate of H deduced from Pioneer 11 and IUE observations of the H torus 1-3 × 109 cm−2 s−1 and the latest retrieved value of the H2 mole fraction in the stratosphere: (1.1 ± 0.1) × 10−3. Our results for both of these values are at least 50-100% higher, though the uncertainties within the chemical schemes and other aspects of the model are large. The chemical conversion from H to H2 is essentially done through catalytic cycles using acetylene and diacetylene. We have studied the role of this diacetylene cycle, for which the associated reaction rates are poorly known. We find that it mostly affects C4 species and benzene in the lower atmosphere, rather than the H profile and the hydrogen budget. We have introduced the heterogenous recombination of hydrogen on the surface of aerosol particles in the stratosphere, and this appears to be a significant process, comparable to the chemical processes. It has a major influence on the H distribution, and consequently on several other species, especially C3H4, C4H2 and C6H6. Therefore, this heterogenous process should be taken into account when trying to understand the stratospheric distribution of these hydrocarbons. 相似文献
20.
Vladimir A. Krasnopolsky 《Icarus》2009,201(1):226-1163
A global-mean model of coupled neutral and ion chemistry on Titan has been developed. Unlike the previous coupled models, the model involves ambipolar diffusion and escape of ions, hydrodynamic escape of light species, and calculates the H2 and CO densities near the surface that were assigned in some previous models. We tried to reduce the numbers of species and reactions in the model and remove all species and reactions that weakly affect the observed species. Hydrocarbon chemistry is extended to C12H10 for neutrals and C10H+11 for ions but does not include PAHs. The model involves 415 reactions of 83 neutrals and 33 ions, effects of magnetospheric electrons, protons, and cosmic rays. UV absorption by Titan's haze was calculated using the Huygens observations and a code for the aggregate particles. Hydrocarbon, nitrile, and ion chemistries are strongly coupled on Titan, and attempt to calculate them separately (e.g., in models of ionospheric composition) may result in significant error. The model densities of various species are typically in good agreement with the observations except vertical profiles in the stratosphere that are steeper than the CIRS limb data. (A model with eddy diffusion that facilitates fitting to the CIRS limb data is considered as well.) The CO densities are supported by the O+ flux from Saturn's magnetosphere. The ionosphere includes a peak at 80 km formed by the cosmic rays, steplike layers at 500-700 and 700-900 km and a peak at 1060 km (SZA = 60°). Nighttime densities of major ions agree with the INMS data. Ion chemistry dominates in the production of bicyclic aromatic hydrocarbons above 600 km. The model estimates of heavy positive and negative ions are in reasonable agreement with the Cassini results. The major haze production is in the reactions C6H + C4H2, C3N + C4H2, and condensation of hydrocarbons below 100 km. Overall, precipitation rate of the photochemical products is equal to 4-7 kg cm−2 Byr−1 (50-90 m Byr−1 while the global-mean depth of the organic sediments is ∼3 m). Escape rates of methane and hydrogen are 2.9 and 1.4 kg cm−2 Byr−1, respectively. The model does not support the low C/N ratio observed by the Huygens ACP in Titan's haze. 相似文献