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1.
More than 500 images of Io in eclipse were acquired by the Cassini spacecraft in late 2000 and early 2001 as it passed through the jovian system en route to Saturn (Porco et al., 2003, Science 299, 1541-1547). Io's bright equatorial glows were detected in Cassini's near-ultraviolet filters, supporting the interpretation that the visible emissions are predominantly due to molecular SO2. Detailed comparisons of laboratory SO2 spectra with the Cassini observations indicate that a mixture of gases contribute to the equatorial emissions. Potassium is suggested by new detections of the equatorial glows at near-infrared wavelengths from 730 to 800 nm. Neutral atomic oxygen and sodium are required to explain the brightness of the glows at visible wavelengths. The molecule S2 is postulated to emit most of the glow intensity in the wavelength interval from 390 to 500 nm. The locations of the visible emissions vary in response to the changing orientation of the external magnetic field, tracking the tangent points of the jovian magnetic field lines. Limb glows distinct from the equatorial emissions were observed at visible to near-infrared wavelengths from 500 to 850 nm, indicating that atomic O, Na, and K are distributed across Io's surface. Stratification of the atmosphere is demonstrated by differences in the altitudes of emissions at various wavelengths: SO2 emissions are confined to a region close to Io's surface, whereas neutral oxygen emissions are seen at altitudes that reach up to 900 km, or half the radius of the satellite. Pre-egress brightening demonstrates that light scattered into Jupiter's shadow by gases or aerosols in the giant planet's upper atmosphere contaminates images of Io taken within 13 minutes of entry into or emergence from Jupiter's umbra. Although partial atmospheric collapse is suggested by the longer timescale for post-ingress dimming than pre-egress brightening, Io's atmosphere must be substantially supported by volcanism to retain auroral emissions throughout the duration of eclipse. 相似文献
2.
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. 相似文献
3.
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
4.
Andrew C. Walker Sergey L. Gratiy Chris H. Moore Laurence M. Trafton Bénédicte Stewart 《Icarus》2010,207(1):409-432
Io’s sublimation-driven atmosphere is modeled using the direct simulation Monte Carlo (DSMC) method. These rarefied gas dynamics simulations improve upon earlier models by using a three-dimensional domain encompassing the entire planet computed in parallel. The effects of plasma heating, planetary rotation, inhomogeneous surface frost, molecular residence time of SO2 on the exposed (non-volatile) rocky surface, and surface temperature distribution are investigated. Circumplanetary flow is predicted to develop from the warm dayside toward the cooler nightside. Io’s rotation leads to a highly asymmetric frost surface temperature distribution (due to the frost’s high thermal inertia) which results in circumplanetary flow that is not axi-symmetric about the subsolar point. The non-equilibrium thermal structure of the atmosphere, specifically vibrational and rotational temperatures, is also examined. Plasma heating is found to significantly inflate the atmosphere on both the dayside and nightside. The plasma energy flux causes high temperatures at high altitudes but plasma energy depletion through the dense gas column above the warmest frost permits gas temperatures cooler than the surface at low altitudes. A frost map (Douté, S., Schmitt, B., Lopes-Gautier, R., Carlson, R., Soderblom, L., Shirley, J., and the Galileo NIMS Team [2001]. Icarus 149, 107-132) is used to control the sublimated flux of SO2 which can result in inhomogeneous column densities that vary by nearly a factor of four for the same surface temperature. A short residence time for SO2 molecules on the “rock” component is found to smooth lateral atmospheric inhomogeneities caused by variations in the surface frost distribution, creating an atmosphere that looks nearly identical to one with uniform frost coverage. A longer residence time is found to agree better with mid-infrared observations (Spencer, J.R., Lellouch, E., Richter, M.J., López-Valverde, M.A., Jessup, K.L, Greathouse, T.K., Flaud, J. [2005]. Icarus 176, 283-304) and reproduce the observed anti-jovian/sub-jovian column density asymmetry. The computed peak dayside column density for Io assuming a surface frost temperature of 115 K agrees with those suggested by Lyman-α observations (Feaga, L.M., McGrath, M., Feldman, P.D. [2009]. Icarus 201, 570-584). On the other hand, the peak dayside column density at 120 K is a factor of five larger and is higher than the upper range of observations (Jessup, K.L., Spencer, J.R., Ballester, G.E., Howell, R.R., Roesler, F., Vigel, M., Yelle, R. [2004]. Icarus 169, 197-215; Spencer et al., 2005). 相似文献
5.
Using the Hubble Space Telescope's Space Telescope Imaging Spectrograph we have obtained for the first time spatially resolved 2000-3000 Å spectra of Io's Prometheus plume and adjoining regions on Io's anti-jovian hemisphere in the latitude range 60° N-60° S, using a 0.1″ slit centered on Prometheus and tilted roughly 45° to the spin axis. The SO2 column density peaked at 1.25×1017 cm−2 near the equator, with an additional 5×1016 cm−2 enhancement over Prometheus corresponding to a model volcanic SO2 output of 105 kg s−1. Apart from the Prometheus peak, the SO2 column density dropped fairly smoothly away from the subsolar point, even over regions that included potential volcanic sources. At latitudes less than ±30°, the dropoff rate was consistent with control by vapor pressure equilibrium with surface frost with subsolar temperature 117.3±0.6 K, though SO2 abundance was higher than predicted by vapor pressure control at mid-latitudes, especially in the northern hemisphere. We conclude that, at least at low latitudes on the anti-jovian hemisphere where there are extensive deposits of optically-thick SO2 frost, the atmosphere is probably primarily supported by sublimation of surface frost. Although the 45° tilt of our slit prevents us from separating the dependence of atmospheric density on solar zenith angle from its dependence on latitude, the pattern is consistent with a sublimation atmosphere regardless of which parameter is the dominant control. The observed drop in gas abundance towards higher latitudes is consistent with the interpretation of previous Lyman alpha images of Io as indicating an atmosphere concentrated at low latitudes. Comparison with previous disk-resolved UV spectroscopy, Lyman-alpha images, and mid-infrared spectroscopy suggests that Io's atmosphere is denser and more widespread on the anti-jovian hemisphere than at other longitudes. SO2 gas temperatures were in the range of 150-250 K over the majority of the anti-jovian hemisphere, consistent with previous observations. SO was not definitively detected in our spectra, with upper limits to the SO/SO2 ratio in the range 1-10%, roughly consistent with previous observations. S2 gas was not seen anywhere, with an upper limit of 7.5×1014 cm−2 for the Prometheus plume, confirming that this plume is significantly poorer in S2 than the Pele plume (S2 /SO2<0.005, compared to 0.08-0.3 at Pele). In addition to the gas absorption signatures, we have observed continuum emission in the near ultraviolet (near 2800 Å) for the first time. The brightness of the observed emission was directly correlated with the SO2 abundance, strongly peaking in the equatorial region over Prometheus. Emission brightness was modestly anti-correlated with the jovian magnetic latitude, decreasing when Io intersected the torus centrifugal equator. 相似文献
6.
John R. Spencer Emmanuel Lellouch Miguel A. López-Valverde Thomas K. Greathouse Jean-Marie Flaud 《Icarus》2005,176(2):283-304
We have observed about 16 absorption lines of the ν2 SO2 vibrational band on Io, in disk-integrated 19-μm spectra taken with the TEXES high spectral resolution mid-infrared spectrograph at the NASA Infrared Telescope Facility in November 2001, December 2002, and January 2004. These are the first ground-based infrared observations of Io's sunlit atmosphere, and provide a new window on the atmosphere that allows better longitudinal and temporal monitoring than previous techniques. Dramatic variations in band strength with longitude are seen that are stable over at least a 2 year period. The depth of the strongest feature, a blend of lines centered at 530.42 cm−1, varies from about 7% near longitude 180° to about 1% near longitude 315° W, as measured at a spectral resolution of 57,000. Interpretation of the spectra requires modeling of surface temperatures and atmospheric density across Io's disk, and the variation in non-LTE ν2 vibrational temperature with altitude, and depends on the assumed atmospheric and surface temperature structure. About half of Io's 19-μm radiation comes from the Sun-heated surface, and half from volcanic hot spots with temperatures primarily between 150 and 200 K, which occupy about 8% of the surface. The observations are thus weighted towards the atmosphere over these low-temperature hot spots. If we assume that the atmosphere over the hot spots is representative of the atmosphere elsewhere, and that the atmospheric density is a function of latitude, the most plausible interpretation of the data is that the equatorial atmospheric column density varies from about 1.5×1017 cm−2 near longitude 180° W to about 1.5×1016 cm−2 near longitude 300° W, roughly consistent with HST UV spectroscopy and Lyman-α imaging. The inferred atmospheric kinetic temperature is less than about 150 K, at least on the anti-Jupiter hemisphere where the bands are strongest, somewhat colder than inferred from HST UV spectroscopy and millimeter-wavelength spectroscopy. This longitudinal variability in atmospheric density correlates with the longitudinal variability in the abundance of optically thick, near-UV bright SO2 frost. However it is not clear whether the correlation results from volcanic control (regions of large frost abundance result from greater condensation of atmospheric gases supported by more vigorous volcanic activity in these regions) or sublimation control (regions of large frost abundance produce a more extensive atmosphere due to more extensive sublimation). Comparison of data taken in 2001, 2002, and 2004 shows that with the possible exception of longitudes near 180° W between 2001 and 2002, Io's atmospheric density does not appear to decrease as Io recedes from the Sun, as would be expected if the atmosphere were supported by the sublimation of surface frost, suggesting that the atmosphere is dominantly supported by direct volcanic supply rather than by frost sublimation. However, other evidence such as the smooth variation in atmospheric abundance with latitude, and atmospheric changes during eclipse, suggest that sublimation support is more important than volcanic support, leaving the question of the dominant atmospheric support mechanism still unresolved. 相似文献
7.
We present a Monte Carlo (MC) model of [OI] 6300 Å and [SII] 6716 Å emission from Io entering eclipse. The simulation accounts for the 3-D distribution of SO2, O, SO, S, and O2 in Io’s atmosphere, several volcanic plumes, and the magnetic field around Io. Thermal electrons from the jovian plasma torus are input along the simulation domain boundaries and move along the magnetic field lines distorted by Io, occasionally participating in collisions with neutrals. We find that the atmospheric asymmetry resulting from varying degrees of atmospheric collapse across Io (due to eclipse ingress) and the presence of volcanoes contributes significantly to the unique morphology of the [OI] 6300 Å emission. The [OI] radiation lifetime of ∼134 s limits the emission to regions that have a sufficiently low neutral density so that intermolecular collisions are rare. We find that at low altitudes (typically <40 km) and in volcanic plumes (Pele, Prometheus, etc.) the number density is large enough (>4 × 109 cm−3) to collisionally quench nearly all (>95%) of the excited oxygen for reasonable quenching efficiencies. Upstream (relative to the plasma flow), Io’s perturbation of the jovian magnetic field mirrors electrons with high pitch angles, while downstream collisions can trap the electrons. This magnetic field perturbation is one of the main physical mechanisms that results in the upstream/downstream brightness asymmetry in [OI] emission seen in the observation by Trauger et al. (Trauger, J.T., Stapelfeldt, K.R., Ballester, G.E., Clarke, J.I., 1997. HST observations of [OI] emissions from Io in eclipse. AAS-DPS Abstract (1997DPS29.1802T)). There are two other main causes for the observed brightness asymmetry. First, the observation’s viewing geometry of the wake spot crosses the dayside atmosphere and therefore the wake’s observational field of view includes higher oxygen column density than the upstream side. Second, the phased entry into eclipse results in less atmospheric collapse and thus higher collisional quenching on the upstream side relative to the wake. We compute a location (both in altitude and latitude) for the intense wake emission feature that agrees reasonably well with this observation. Furthermore, the peak intensity of the simulated wake feature is less than that observed by a factor of ∼3, most likely because our model does not include direct dissociation-excitation of SO2 and SO. We find that the latitudinal location of the emission feature depends not so much on the tilt of the magnetic field as on the relative north/south flux tube depletion that occurs due to Io’s changing magnetic latitude in the plasma torus. From 1-D simulations, we also find that the intensity of [SII] 6716 and 6731 Å emission is much weaker than that of [OI] even if the [SII] excitation cross section is 103 times larger than excitation to [OI]. This is because the density of S+ is much less than that of O and because the Einstein-A coefficient of the [SII] emission is a factor of ∼10 smaller than that of [OI]. 相似文献
8.
G. Bellucci E. D'Aversa D. Cruikshank R.N. Clark D. Matson T.B. McCord P.D. Nicholson the Cassini VIMS Team 《Icarus》2004,172(1):141-148
During the Cassini-Jupiter flyby, VIMS observed Io at different phase angles, both in full sunlight and in eclipse. By using the sunlight measurements, we were able to produce phase curves in the visual through all the near infrared wavelengths covered by the VIMS instrument (0.85-5.1 μm). The phase angle spanned from ∼2° to ∼120°. The measurements, done just after Io emerged from Jupiter's shadow, show an increase of about 15% in Io's reflectance with respect to what would be predicted by the phase curve. This behavior is observed at wavelengths >1.2 μm. Moreover, just after emergence from eclipse an increase of about 25% is observed in the depth of SO2 frost bands at 4.07 and 4.35 μm. At 0.879<λ<1.04 μm the brightening is 10-24%. Below λ=0.879 μm the brightening, if present, should be less than the precision of our measurements (∼5%). Apparently, these observations are not explained neither by a diverse spatial distribution of SO2 on the Io' surface nor by atmospheric SO2 condensation on the surface during the eclipse. 相似文献
9.
Vladimir A. Krasnopolsky 《Icarus》2011,215(1):197-203
The vertical profile of H2SO4 vapor is calculated using current atmospheric and thermodynamic data. The atmospheric data include the H2O profiles observed at 70-112 km by the SOIR solar occultations, the SPICAV-UV profiles of the haze extinction at 220 nm, the VeRa temperature profiles, and a typical profile of eddy diffusion. The thermodynamic data are the saturated vapor pressures of H2O and H2SO4 and chemical potentials of these species in sulfuric acid solutions. The calculated concentration of sulfuric acid in the cloud droplets varies from 85% at 70 km to a minimum of 70% at 90 km and then gradually increasing to 90-100% at 110 km. The H2SO4 vapor mixing ratio is ∼10−12 at 70 and 110 km with a deep minimum of 3 × 10−18 at 88 km. The H2O-H2SO4 system matches the local thermodynamic equilibrium conditions up to 87 km. The column photolysis rate of H2SO4 is 1.6 × 105 cm−2 s−1 at 70 km and 23 cm−2 s−1 at 90 km. The calculated abundance of H2SO4 vapor at 90-110 km and its photolysis rate are smaller than those presented in the recent model by Zhang et al. (Zhang, X., Liang, M.C., Montmessin, F., Bertaux, J.L., Parkinson, C., Yung, Y.L. [2010]. Nat. Geosci. 3, 834-837) by factors of 106 and 109, respectively. Assumptions of 100% sulfuric acid, local thermodynamic equilibrium, too warm atmosphere, supersaturation of H2SO4 (impossible for a source of SOX), and cross sections for H2SO4·H2O (impossible above the pure H2SO4) are the main reasons of this huge difference. Significant differences and contradictions between the SPICAV-UV, SOIR, and ground-based submillimeter observations of SOX at 70-110 km are briefly discussed and some weaknesses are outlined. The possible source of high altitude SOX on Venus remains unclear and probably does not exist. 相似文献
10.
Chemical kinetic model for the lower atmosphere of Venus 总被引：1，自引：0，他引：1
Vladimir A. Krasnopolsky 《Icarus》2007,191(1):25-37
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, Sx), thermochemistry in the lowest 10 km, and photolysis of S3. The sulfur bonds in OCS and Sx 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 Sx, and a minor photochemical cycle which forms CO and Sx from OCS. The net effect of thermochemistry in the lowest 10 km is formation of OCS from CO and Sx. 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 Sx. The geological cycle of sulfur remains unchanged. 相似文献
11.
Europa is bombarded by intense radiation that erodes the surface, launching molecules into a thin “atmosphere” representative of surface composition. In addition to atoms and molecules created in the mostly water ice surface such as H2O, O2, H2, the atmosphere is known to have species representative of trace surface materials. These trace species are carried off with the 10-104 H2O molecules ejected by each energetic heavy ion, a process we have simulated using molecular dynamics. Using the results of those simulations, we found that a neutral mass spectrometer orbiting ∼100 km above the surface could detect species with surface concentrations above ∼0.03%. We have also modeled the atmospheric spatial structure of the volatile species CO2 and SO2 under a variety of assumptions. Detections of these species with moderate time and space resolution would allow us to constrain surface composition, chemistry and to study space weathering processes. 相似文献
12.
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
13.
We present an updated model for the photochemistry of Io's atmosphere and ionosphere and use this model to investigate the sensitivity of the chemical structure to vertical transport rates. SO2is assumed to be the dominant atmospheric gas, with minor molecular sodium species such as Na2S or Na2O released by sputtering or venting from the surface. Photochemical products include SO, O2, S, O, Na, NaO, NaS, and Na2. We consider both “thick” and “thin” SO2atmospheres that encompass the range allowed by recent HST and millimeter-wave observations, and evaluate the possibility that O2and/or SO may be significant minor dayside constituents and therefore likely dominant nightside gases. The fast reaction between S and O2limits the column abundance of O2to ∼104less than that calculated by Kumar (J. Geophys. Res.87, 1677–1684, 1982; 89(A9), 7399–7406, 1984) for a pure sulfur/oxygen atmosphere. If a significant source of NaO2or Na2O were supplied by the surface and mixed rapidly upward, then oxygen liberated in the chemical reactions which also liberate free Na would provide an additional source of O2. Fast eddy mixing will enhance the transport of molecular sodium species to the exobase, in addition to increasing the vertical transport rate of ions. Ions produced in the atmosphere will be accelerated by the reduced corotation electric field penetrating the atmosphere. These ions experience collisions with the neutral gas, leading to enhanced vertical ion diffusion. The dominant ion, Na+, is lost primarily by charge exchange with Na2O and/or Na2S in the lower atmosphere and by diffusion through the ionopause in the upper atmosphere. The atmospheric column abundance of SO, O2, and the upper atmosphere escape rates of Na, S, O, and molecular sodium species are all strong functions of the eddy mixing rate. Most atmospheric escape, including that of molecular sodium species, probably occurs from the low density “background” SO2atmosphere, while a localized high density “volcanic” SO2atmosphere can yield an ionosphere consistent with that detected by the Pioneer 10 spacecraft. 相似文献
14.
Many of the key properties of Io’s atmosphere, such as its spatial distribution, temperature, column density and composition, are still not fully assessed despite decades of extensive observations. The contribution of the possible gas sources to the atmospheric replenishment are then still unclear.This paper presents disk-resolved observations performed with the Submillimeter Array (SMA) at 345 GHz of atmospheric rotational lines of the main atmospheric species SO2, and, for the first time, of the minor species SO and NaCl. All these species appear concentrated on the anti-jovian hemisphere, but do not share the same spatial distribution. The obtained maps and line-averaged fluxes are compared to realistic models simulating gas sources including volcanic plume outgassing, SO2 frost sublimation and photolysis. Arguments in favor of each sources are examined and compared to observations, putting constraints on their relative roles for each species.While sublimation clearly appears as the favored major source for SO2, SO2 photolysis may account for most of the production of SO. Using constraints on the volcanic plumes distribution from Galileo results, we find that direct volcanic input can only contribute for a minor fraction of atmospheric SO2, but represent a more significant source for SO atmosphere, and is likely to be the only source for NaCl. Temperature and column densities findings are also presented for SO2, and compare well to previously published observations and atmospheric models. 相似文献
15.
We study the morphology of Io’s aurora by comparing simulation results of a three-dimensional (3D) two-fluid plasma model to observations by the high-resolution Long-Range Reconnaissance Imager (LORRI) on-board the New Horizons spacecraft and by the Hubble Space Telescope Advanced Camera for Surveys (HST/ACS). In 2007, Io’s auroral emission in eclipse has been observed simultaneously by LORRI and ACS and the observations revealed detailed features of the aurora, such as a huge glowing plume at the Tvashtar paterae close to the North pole. The auroral radiation is generated in Io’s atmosphere by collisions between impinging magnetospheric electrons and various neutral gas components. We calculate the interaction of the magnetospheric plasma with Io’s atmosphere-ionosphere and simulate the auroral emission. Our aurora model takes into account not only the direct influence of the atmospheric distribution on the morphology and intensity of the emission, but also the indirect influence of the atmosphere on the plasma environment and thus on the exciting electrons. We find that the observed morphology in eclipse can be explained by a smooth (non-patchy) equatorial atmosphere with a vertical column density that corresponds to ∼10% of the column density of the sunlit atmosphere. The atmosphere is asymmetric with two times higher density and extension on the downstream hemisphere. The auroral emission from the Tvashtar volcano enables us to constrain the plume gas content for the first time. According to our model, the observed intensity of the Tvashtar plume implies a mean column density of ∼5 × 1015 cm−2 for the plume region. 相似文献
16.
The dramatic growth and evolution of the 2001 martian global dust storm were captured using the Submillimeter Wave Astronomy Satellite (SWAS). While the lower and middle atmosphere (pressures greater than 50 μbar, up to ∼45 km altitude) showed rapid heating of up to 40 K, the average surface brightness temperature plummeted by ∼20 K at the peak of the storm. The storm appears to have had little impact on the global temperature structure at altitudes above ∼ 10 μbar (∼ 60 km). 相似文献
17.
Chemical reactions and volatile supply through hypervelocity impacts may have played a key role for the origin and evolution of both planetary and satellite atmospheres. In this study, we evaluate the role of impact-induced N2 production from reduced nitrogen-bearing solids proposed to be contained in Titan’s crust, ammonium sulfate ((NH4)2SO4), for the replenishment of N2 to the atmosphere in Titan’s history. To investigate the conversion of (NH4)2SO4 into N2 by hypervelocity impacts, we measured gases released from (NH4)2SO4 that was exposed to hypervelocity impacts created by a laser gun. The sensitivity and accuracy of the measurements were enhanced by using an isotope labeling technique for the target. We obtained the efficiency of N2 production from (NH4)2SO4 as a function of peak shock pressure ranging from ∼8 to ∼45 GPa. Our results indicate that the initial and complete shock pressures for N2 degassing from (NH4)2SO4 are ∼10 and ∼25 GPa, respectively. These results suggest that cometary impacts on Titan (i.e., impact velocity vi > ∼8 km/s) produce N2 efficiently; whereas satellitesimal impacts during the accretion (i.e., vi < 4 km/s) produce N2 only inefficiently. Even when using the proposed small amount of (NH4)2SO4 content in the crust (∼4 wt.%) (Fortes, A.D. et al., 2007. Icarus 188, 139-153), the total amount of N2 provided through cometary impacts over 4.5 Ga reaches ∼2-6 times the present atmospheric N2 (i.e., ∼7 × 1020-2 × 1021 [mol]) based on the measured production efficiency and results of a hydrodynamic simulation of cometary impacts onto Titan. This implies that cometary impacts onto Titan’s crust have the potential to account for a large part of the present N2 through the atmospheric replenishment after the accretion. 相似文献
18.
Shingo Kosuge Kazuo Aoki Takeshi Inoue David B. Goldstein Philip L. Varghese 《Icarus》2012,221(2):658-669
The behavior of Io’s atmosphere during and after eclipse is investigated on the basis of kinetic theory. The atmosphere is mainly composed of sulfur dioxide (SO2) gas, which condenses to or sublimates from the frost of SO2 on the surface depending on the variation of surface temperature (～90–114 K). The atmosphere may also contain a noncondensable gas, such as sulfur monoxide (SO) or oxygen (O2), as a minor component. In the present study, an accurate numerical analysis for a model Boltzmann equation by a finite-difference method is performed for a one-dimensional atmosphere, and the detailed structure of unsteady gas flows caused by the phase transition of SO2 is clarified. For instance, the following scenario is obtained. The condensation of SO2 on the surface, starting when eclipse begins, gives rise to a downward flow of the atmosphere. The falling atmosphere then bounces upward when colliding with the lower atmosphere but soon falls again. This process of falling and bounce back of the atmosphere repeats during the eclipse, resulting in a temporal oscillation of the macroscopic quantities, such as the velocity and temperature, at a fixed altitude. For a pure SO2 atmosphere, the amplitude of the oscillation is large because of a fast downward flow, but the oscillation decays rapidly. In contrast, for a mixture, the downward flow is slow because the noncondensable gas adjacent to the surface hinders the condensation of SO2. The oscillation in this case is weak but lasts much longer than in the case of pure SO2. The present paper is complementary to the work by Moore et al. (Moore, C.H., Goldstein, D.B., Varghese, P.L., Trafton, L.M., Stewart, B. [2009]. Icarus 201, 585–597) using the direct simulation Monte Carlo (DSMC) method. 相似文献
19.
T.T. Koskinen R.V. Yelle D.S. Snowden P. Lavvas B.R. Sandel F.J. Capalbo Y. Benilan R.A. West 《Icarus》2011,216(2):507-534
Stellar occultations observed by the Cassini/UVIS instrument provide unique data that probe the mesosphere and lower thermosphere of Titan at altitudes between 400 and 1400 km. This region is a site of complex photochemistry that forms hydrocarbon and nitrile species, and plays a crucial role in the formation of the organic hazes observed in the stratosphere, but has yet to be adequately characterized. We analyzed publicly available data obtained between flybys Tb in December 2004 and T58 in July 2009, with an emphasis on two stable occultations obtained during flybys T41 and T53. We derived detailed density profiles for CH4, C2H2, C2H4, C4H2, HCN, HC3N and C6H6 between ∼400 and 1200 km and extinction coefficients for aerosols between 400 and 900 km. Our analysis reveals the presence of extinction layers in the occultation data that are associated with large perturbations in the density profiles of the gaseous species and extinction profiles of the aerosols. These relatively stable features vary in appearance with location and change slowly over time. In particular, we identify a sharp extinction layer between 450 and 550 km that coincides with the detached haze layer. In line with recent images obtained by Cassini/ISS, the altitude of this layer changes rapidly around the equinox in 2009. Our results point to unexpected complexity that may have significant consequences for the dynamics and physical processes taking place in the upper atmosphere of Titan. 相似文献
20.
To determine how active volcanism might affect the standard picture of sulfur dioxide photochemistry on Io, we have developed a one-dimensional atmospheric model in which a variety of sulfur-, oxygen-, sodium-, potassium-, and chlorine-bearing volatiles are volcanically outgassed at Io's surface and then evolve due to photolysis, chemical kinetics, and diffusion. Thermochemical equilibrium calculations in combination with recent observations of gases in the Pele plume are used to help constrain the composition and physical properties of the exsolved volcanic vapors. Both thermochemical equilibrium calculations (Zolotov and Fegley 1999, Icarus141, 40-52) and the Pele plume observations of Spencer et al. (2000; Science288, 1208-1210) suggest that S2 may be a common gas emitted in volcanic eruptions on Io. If so, our photochemical models indicate that the composition of Io's atmosphere could differ significantly from the case of an atmosphere in equilibrium with SO2 frost. The major differences as they relate to oxygen and sulfur species are an increased abundance of S, S2, S3, S4, SO, and S2O and a decreased abundance of O and O2 in the Pele-type volcanic models as compared with frost sublimation models. The high observed SO/SO2 ratio on Io might reflect the importance of a contribution from volcanic SO rather than indicate low eddy diffusion coefficients in Io's atmosphere or low SO “sticking” probabilities at Io's surface; in that case, the SO/SO2 ratio could be temporally and/or spatially variable as volcanic activity fluctuates. Many of the interesting volcanic species (e.g., S2, S3, S4, and S2O) are short lived and will be rapidly destroyed once the volcanic plumes shut off; condensation of these species near the source vent is also likely. The diffuse red deposits associated with active volcanic centers on Io may be caused by S4 radicals that are created and temporarily preserved when sulfur vapor (predominantly S2) condenses around the volcanic vent. Condensation of SO across the surface and, in particular, in the polar regions might also affect the surface spectral properties. We predict that the S/O ratio in the torus and neutral clouds might be correlated with volcanic activity—during periods when volcanic outgassing of S2 (or other molecular sulfur vapors) is prevalent, we would expect the escape of sulfur to be enhanced relative to that of oxygen, and the S/O ratio in the torus and neutral clouds could be correspondingly increased. 相似文献