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1.
We report observations of Titan's high-altitude exosphere detected out to about 50,000 km altitude. The observations were made by the Ion Neutral Camera (INCA) on board the Cassini spacecraft. INCA detects energetic neutral atoms (ENA) that are formed when the ambient magnetospheric ions charge exchange with Titan's neutral atmosphere and exosphere. We find that Titan's exospheric H2 distribution follows closely a full Chamberlain distribution including ballistic, escaping and satellite distributions. As expected, neutral densities are dominated by a satellite distribution above about 10,000 km. The maximum detectable extent of the exosphere (～50,000 km) coincides with the radius of the Hill sphere of gravitational influence from Saturn. While we find no direct indications of a neutral Titan torus with densities greater than about 1000 cm?3, we observe interesting asymmetries in the distribution that warrants further investigation. Based on these findings we compute the average precipitating ENA flux to be about 5×106 keV/(cm2 s), or 8×10?3 erg/(cm2 s), which is directly comparable to that of precipitating energetic ions (Sittler, et al., 2009) and slightly higher than that of solar EUV (Tobiska, 2004). Thus, the energy deposited by precipitating ENAs must also be taken into consideration when studying the energy balance of Titan's thermosphere.  相似文献   

2.
The Visible and Infra-Red Thermal Imaging Spectrometer (VIRTIS) instrument on board the Venus Express spacecraft has measured the O2(a1Δ) nightglow distribution at 1.27 μm in the Venus mesosphere for more than two years. Nadir observations have been used to create a statistical map of the emission on Venus nightside. It appears that the statistical 1.6 MR maximum of the emission is located around the antisolar point. Limb observations provide information on the altitude and on the shape of the emission layer. We combine nadir observations essentially covering the southern hemisphere, corrected for the thermal emission of the lower atmosphere, with limb profiles of the northern hemisphere to generate a global map of the Venus nightside emission at 1.27 μm. Given all the O2(a1Δ) intensity profiles, O2(a1Δ) and O density profiles have been calculated and three-dimensional maps of metastable molecular and atomic oxygen densities have been generated. This global O density nightside distribution improves that available from the VTS3 model, which was based on measurements made above 145 km. The O2(a1Δ) hemispheric average density is 2.1 × 109 cm?3, with a maximum value of 6.5 × 109 cm?3 at 99.2 km. The O density profiles have been derived from the nightglow data using CO2 profiles from the empirical VTS3 model or from SPICAV stellar occultations. The O hemispheric average density is 1.9 × 1011 cm?3 in both cases, with a mean altitude of the peak located at 106.1 km and 103.4 km, respectively. These results tend to confirm the modeled values of 2.8 × 1011 cm?3 at 104 km and 2.0 × 1011 cm?3 at 110 km obtained by Brecht et al. [Brecht, A., Bougher, S.W., Gérard, J.-C., Parkinson, C.D., Rafkin, S., Foster, B., 2011a. J. Geophys. Res., in press] and Krasnopolsky [Krasnopolsky, V.A., 2010. Icarus 207, 17–27], respectively. Comparing the oxygen density map derived from the O2(a1Δ) nightglow observations, it appears that the morphology is very different and that the densities obtained in this study are about three times higher than those predicted by the VTS3 model.  相似文献   

3.
Vladimir Krasnopolsky 《Icarus》2012,219(1):244-249
To search for DCl in the Venus atmosphere, a spectrum near the D35Cl (1–0) R4 line at 2141.54 cm?1 was observed using the CSHELL spectrograph at NASA IRTF. Least square fitting to the spectrum by a synthetic spectrum results in a DCl mixing ratio of 17.8 ± 6.8 ppb. Comparing to the HCl abundance of 400 ± 30 ppb (Krasnopolsky [2010a] Icarus, 208, 314–322), the DCl/HCl ratio is equal to 280 ± 110 times the terrestrial D/H = 1.56 × 10?4. This ratio is similar to that of HDO/H2O = 240 ± 25 times the terrestrial HDO/H2O from the VEX/SOIR occultations at 70–110 km. Photochemistry in the Venus mesosphere converts H from HCl to that in H2O with a rate of 1.9 × 109 cm?2 s?1 (Krasnopolsky [2012] Icarus, 218, 230–246). The conversion involves photolysis of HCl; therefore, the photochemistry tends to enrich D/H in HCl and deplete in H2O. Formation of the sulfuric acid clouds may affect HDO/H2O as well. The enriched HCl moves down by mixing to the lower atmosphere where thermodynamic equilibriums for H2 and HCl near the surface correspond to D/H = 0.71 and 0.74 times that in H2O, respectively. Time to establish these equilibriums is estimated at ～3 years and comparable to the mixing time in the lower atmosphere. Therefore, the enriched HCl from the mesosphere gives D back to H2O near the surface. Comparison of chemical and mixing times favors a constant HDO/H2O up to ～100 km and DCl/HCl equal to D/H in H2O times 0.74.Ammonia is an abundant form of nitrogen in the reducing environments. Thermodynamic equilibriums with N2 and NO near the surface of Venus give its mixing ratio of 10?14 and 6 × 10?7, respectively. A spectrum of Venus near the NH3 line at 4481.11 cm?1 was observed at NASA IRTF and resulted in a two-sigma upper limit of 6 ppb for NH3 above the Venus clouds. This is an improvement of the previous upper limit by a factor of 5. If ammonia exists at the ppb level or less in the lower atmosphere, it quickly dissociates in the mesosphere and weakly affects its photochemistry.  相似文献   

4.
We present observations of the O2(a1Δg) nightglow at 1.27 μm on Mars using the SPICAM IR spectrometer onboard of the Mars Express orbiter. In contrast to the O2(a1Δg) dayglow that results from the ozone photodissociation, the O2(a1Δg) nightglow is a product of the recombination of O atoms formed by CO2 photolysis on the dayside at altitudes higher than 80 km and transported downward above the winter pole by the Hadley circulation. The first detections of the O2(a1Δg) nightglow in 2010 indicate that it is about two order of magnitude less intense than the dayglow (Bertaux, J.-L., Gondet, B., Bibring, J.-P., Montmessin, F., Lefèvre, F. [2010]. Bull. Am. Astron. Soc. 42, 1040; Clancy et al. [2010]. Bull. Am. Astron. Soc. 42, 1041). SPICAM IR sounds the martian atmosphere in the near-IR range (1–1.7 μm) with the spectral resolution of 3.5 cm?1 in nadir, limb and solar occultation modes. In 2010 the vertical profiles of the O2(a1Δg) nightside emission have been obtained near the South Pole at latitudes of 82–83°S for two sequences of observations: Ls = 111–120° and Ls = 152–165°. The altitude of the emission maximum varied from 45 km on Ls = 111–120° to 38–49 km on Ls = 152–165°. Averaged vertically integrated intensity of the emission at these latitudes has shown an increase from 0.22 to 0.35 MR. Those values of total vertical emission rate are consistent with the OMEGA observations on Mars-Express in 2010. The estimated density of oxygen atoms at altitudes from 50 to 65 km varies from 1.5 × 1011 to 2.5 × 1011 cm?3. Comparison with the LMD general circulation model with photochemistry (Lefèvre, F., Lebonnois, S., Montmessin, F., Forget, F. [2004]. J. Geophys. Res. 109, E07004; Lefèvre et al. [2008]. Nature 454, 971–975) shows that the model reproduces fairly well the O2(a1Δg) emission layer observed by SPICAM when the large field of view (>20 km on the limb) of the instrument is taken into account.  相似文献   

5.
To ascertain the importance of sputtering by solar wind ions on the formation of a sodium exosphere around Mercury and the Moon, we have irradiated with 4 keV He ions, the Na bearing tectosilicates: albite, labradorite, and anorthoclase, as well as adsorbed Na layers deposited on albite and on olivine (a neosilicate that does not contain Na). Sodium at the surface and near surface (<40 Å) was quantified with X-ray photoelectron spectroscopy before and after each irradiation to determine the depletion cross section. We measured a cross section for sputtering of Na adsorbed on mineral surfaces, σs ≈ 1 × 10?15 cm2 atom?1. In addition, mass spectrometric analyses of the sputtered flux show that a large fraction of the Na is sputtered as ions rather than as neutral atoms. These results have strong implications for modeling the sodium population within the mercurian and the lunar exospheres.  相似文献   

6.
C. Plainaki  A. Milillo  S. Orsini 《Icarus》2010,210(1):385-395
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   

7.
《Planetary and Space Science》2007,55(11):1494-1501
In this work, we calculate the neutral Na production rates on the Moon and Mercury, as due to the impacts of meteoroids having an impact probability on the surface that can influence the daily observations of the exosphere: the meteoroids radius range considered for the Moon and Mercury are 10−8–0.15 and 10−8–0.10 m, respectively. We also estimate the mass of meteoroids that has impacted the surfaces of the Moon and Mercury in the last 3.8 Gy (after the end of the Late Heavy Bombardment).The results of our model are that (i) the Na production rates are ∼(3–4.9)×104 and ∼(1.8–2.3)×106 atoms cm−2 s−1, for Moon and Mercury, respectively, and (ii) in the last 3.8 Gy, the mass of meteoroids that has impacted the whole surface of the Moon and Mercury has been 8.86×1018 and 2.66×1019 g, respectively.  相似文献   

8.
We present spectral and spatial information for major volatile species in Comet 10P/Tempel 2, based on high-dispersion infrared spectra acquired on UT 2010 July 26 (heliocentric distance Rh = 1.44 AU) and September 18 (Rh = 1.62 AU), following the comet’s perihelion passage on UT 2010 July 04. The total production rate for water on July 26 was (1.90 ± 0.12) × 1028 molecules s?1, and abundances of six trace gases (relative to water) were: CH3OH (1.58% ± 0.23%), C2H6 (0.39% ± 0.04%), NH3 (0.83% ± 0.20%), and HCN (0.13% ± 0.02%). A detailed analysis of intensities for water emission lines provided a rotational temperature of 35 ± 3 K. The mean OPR is consistent with nuclear spin populations in statistical equilibrium (OPR = 3.01 ± 0.18), and the (1σ) lower bound corresponds to a spin temperature >38 K. Our measurements were contemporaneous with a jet-like feature observed at optical wavelengths. The spatial profiles of four primary volatiles display strong enhancements in the jet direction, which favors release from a localized vent on the nucleus. The measured IR continuum is much more sharply peaked and is consistent with a dominant contribution from the nucleus itself. The peak intensities for H2O, CH3OH, and C2H6 are offset by ～200 km in the jet direction, suggesting the possible existence of a distributed source, such as the release of icy grains that subsequently sublimed in the coma. On UT September 18, no obvious emission lines were present in our spectra, nevertheless we obtained a 3σ upper limit Q(H2O) < 2.86 × 1027 molecules s?1.  相似文献   

9.
We present the two-dimensional distribution of the O2 a1Δ–X3Σ (0–0) band at 1.27 μm and the OH Δv = 1 Meinel airglow measured simultaneously with the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on board Venus Express. We show that the two emissions present very similar spatial structures. A cross-correlation analysis indicates that the highest level of correlation is reached with only very small relative shifts of the pairs of images. In spite of the strong spatial correlation between the morphology of the bright spots in the two emissions, we also show that their relative intensity is not constant, in agreement with earlier statistical studies of their limb profiles. We conclude that the two emissions have a common precursor that controls the production of both excited species. We argue that atomic oxygen, which produces O2 (1Δ) molecules by three-body recombination and is the precursor of ozone formation, also governs to a large extent the OH airglow morphology through the H + O3 → OH* + O2 reaction.  相似文献   

10.
We observed the products C4H5, C4H4, C3H3 and CH3 of the C(3P) + C3H6 reaction using product time-of-flight spectroscopy and selective photoionization. The identified species arise from the product channels C4H5 + H, C4H4 + 2H and C3H3 + CH3. Product isomers were identified via measurements of photoionization spectra and calculations of adiabatic ionization energy. Product C4H5 probably involves three isomers HCCCHCH3, H2CCCCH3 and H2CCCHCH2. In contrast, products C4H4 and C3H3 involve exclusively HCCCHCH2 and H2CCCH, respectively. Reaction mechanisms are unraveled with crossed-beam experiments and quantum-chemical calculations. The 3P carbon atom attacks the π orbital of propene (C3H6) to form a cyclic complex c-H2C(C)CHCH3 that rapidly opens the ring to form H2CCCHCH3 followed by decomposition to HCCCHCH3/H2CCCCH3/H2CCCHCH2 + H and H2CCCH + CH3; the corresponding branching ratios are 7:5:10:78 predicted with RRKM calculations at collision energy 4 kcal mol?1. Nascent C4H5 with enough internal energy further decomposes to HCCCHCH2 + H. Ratios of products C4H5, C4H4 and C3H3 are experimentally evaluated to be 17:8:75. This work provides a comprehensive look at product channels of the title reaction and gives implications for the formation of hydrocarbons in extra-terrestrial environments such as Titan and carbon-rich interstellar media. We suggest that the title reaction, hitherto excluded in any chemical networks, needs to be taken into account at least in the atmosphere of Titan and carbon-rich molecular clouds where rapid neutral–neutral reactions are dominant and carbon atoms and propene are abundant.  相似文献   

11.
The model is intended to respond to the recent findings in the Venus atmosphere from the Venus Express and ground-based submillimeter and infrared observations. It extends down to 47 km for comparison with the kinetic model for the lower atmosphere (Krasnopolsky, V.A. [2007]. Icarus 191, 25–37) and to use its results as the boundary conditions. The model numerical accuracy is significantly improved by reduction of the altitude step from 2 km in the previous models to 0.5 km. Effects of the NUV absorber are approximated using the detailed photometric observations at 365 nm from Venera 14. The H2O profile is not fixed but calculated in the model. The model involves odd nitrogen and OCS chemistries based on the detected NO and OCS abundances. The number of the reactions is significantly reduced by removing of unimportant processes. Column rates for all reactions are given, and balances of production and loss may be analyzed in detail for each species.The calculated vertical profiles of CO, H2O, HCl, SO2, SO, OCS and of the O2 dayglow at 1.27 μm generally agree with the existing observational data; some differences are briefly discussed. The OH dayglow is ～30 kR, brighter than the OH nightglow by a factor of 4. The H + O3 process dominates in the nightglow excitation and O + HO2 in the dayglow, because of the reduction of ozone by photolysis. A key feature of Venus’ photochemistry is the formation of sulfuric acid in a narrow layer near the cloud tops that greatly reduces abundances of SO2 and H2O above the clouds. Delivery of SO2 and H2O through this bottleneck determines the chemistry and its variations above the clouds. Small variations of eddy diffusion near 60 km result in variations of SO2, SO, and OCS at and above 70 km within a factor of ～30. Variations of the SO2/H2O ratio at the lower boundary have similar but weaker effect: the variations within a factor of ～4 are induced by changes of SO2/H2O by ±5%. Therefore the observed variations of the mesospheric composition originate from minor variations of the atmospheric dynamics near the cloud layer and do not require volcanism. NO cycles are responsible for production of a quarter of O2, SO2, and Cl2 in the atmosphere. A net effect of photochemistry in the middle atmosphere is the consumption of CO2, SO2, and HCl from and return of CO, H2SO4, and SO2Cl2 to the lower atmosphere. These processes may be balanced by thermochemistry in the lower atmosphere even without outgassing from the interior, though the latter is not ruled out by our models. Some differences between the model and observations and the previous models are briefly discussed.  相似文献   

12.
We report sensitive Chandra X-ray non-detections of two unusual, luminous Iron Low-Ionization Broad Absorption Line Quasars (FeLoBALs). The observations do detect a non-BAL, wide-binary companion quasar to one of the FeLoBAL quasars. We combine X-ray-derived column density lower limits (assuming solar metallicity) with column densities measured from ultraviolet spectra and CLOUDY photoionization simulations to explore whether constant-density slabs at broad-line region densities can match the physical parameters of these two BAL outflows, and find that they cannot. In the “overlapping-trough” object SDSS J0300+0048, we measure the column density of the X-ray absorbing gas to be NH ? 1.8 × 1024 cm?2. From the presence of Fe ii UV78 absorption but lack of Fe ii UV195/UV196 absorption, we infer the density in that part of the absorbing region to be ne ? 106 cm?3. We do find that a slab of gas at that density might be able to explain this object’s absorption. In the Fe iii-dominant object SDSS J2215–0045, the X-ray absorbing column density of NH ? 3.4 × 1024 cm?2 is consistent with the Fe iii-derived NH ? 2 × 1022 cm?2 provided the ionization parameter is log U > 1.0 for both the ne = 1011 cm?3 and ne = 1012 cm?3 scenarios considered (such densities are required to produce Fe iii absorption without Fe iiabsorption). However, the velocity width of the absorption rules out its being concentrated in a single slab at these densities. Instead, this object’s spectrum can be explained by a low density, high ionization and high temperature disk wind that encounters and ablates higher density, lower ionization Fe iii-emitting clumps.  相似文献   

13.
Hubble Space Telescope/Wide Field and Planetary Camera 2 (HST/WFPC2) images of Io obtained between 1995 and 2007 between 0.24 and 0.42 μm led to the detection of the Pele plume in reflected sunlight in 1995 and 1999; imaging of the Pele plume via absorption of jovian light in 1996 and 1999; detection of the Prometheus-type Pillan plume in reflected sunlight in 1997; and detection of the 2007 Pele-type Tvashtar plume eruption in reflected sunlight and via absorption of jovian light. Based on a detailed analysis of these observations we characterize and compare the gas and dust properties of each of the detected plumes. In each case, the brightness of the plumes in reflected sunlight is less at 0.26 μm than at 0.33 μm. Mie scattering analysis of the wavelength dependence of each plume’s reflectance signature suggests that range of particle sizes within the plumes is quite narrow. Assuming a normal distribution of particle sizes, the range of mean particle sizes is ～0.035–0.12 μm for the 1997 Pillan eruption, ～0.05–0.08 μm for the 1999 Pele and 2007 Tvasthar plumes, and ～0.05–0.11 μm for the 1995 Pele plume, and in each case the standard deviation in the particle size distribution is <15%. The Mie analysis also suggests that the 2007 Tvashtar eruption released ～109 g of sulfur dust, the 1999 Pele eruption released ～109 g of SO2 dust, the 1997 Pillan eruption released ～1010 g of SO2 dust, and the 1995 Pele plume may have released ～1010 g of SO2 dust. Analysis of the plume absorption signatures recorded in the F255W filter bandpass (0.24–0.28 μm) indicates that the opacity of the 2007 Tvashtar plume was 2× that of the 1996 and 1999 Pele plume eruptions. While the sulfur dust density estimated for the Tvashtar from the reflected sunlight data could have produced 61% of the observed plume opacity, <10% of the 1999 Pele F255W plume opacity could have resulted from the SO2 dust detected in the eruption. Accounting for the remaining F255W opacity level of the Pele and Tvasthar plumes based on SO2 and S2 gas absorption, the SO2 and S2 gas density inferred for each plume is almost equivalent corresponding to ～2–6 × 1016 cm?2 and 3–5 × 1015 cm?2, respectively, producing SO2 and S2 gas resurfacing rates ～0.04–0.2 cm yr?1 and 0.007–0.01 cm yr?1; and SO2 and S2 gas masses ～1–4 × 1010 g and ～2–3 × 109 g; for a total dust to gas ratio in the plumes ～10?1–10?2. The 2007 Tvashtar plume was detected by HST at ～380 ± 40 km in both reflected sunlight and absorbed jovian light; in 1999, the detected Pele plume altitude was 500 km in absorbed jovian light, but in reflected sunlight the detected height was ～2× lower. Thus, for the 1999 Pele plume, similar to the 1979 Voyager Pele plume observations, the most efficient dust reflections occurred in the region closest to the plume vent. The 0.33–0.42 μm brightness of the 1997 Pillan plume was 10–20× greater than the Pele or Tvashtar plumes, exceeding by a factor of 3 the average brightness levels observed within 200 km of 1979 Loki eruption vent. But, the 0.26 μm brightness of the 1997 Pillan plume in reflected sunlight was significantly lower than would be predicted by the dust scattering model. Presuming that the 0.26 μm brightness of the 1997 Pillan plume was attenuated by the eruption plume’s gas component, then an SO2 gas density ～3–6 × 1018 cm?2 is inferred from the data (for S2/SO2 ratios ?4%), comparable to the 0.3–2 × 1018 cm?2 SO2 density detected at Loki in 1979 (Pearl, J.C. et al. [1979]. Nature 280, 755; Lellouch et al., 1992), and producing an SO2 gas mass ～3–8 × 1011 g and an SO2 resurfacing rate ～8–23 cm yr?1. These results confirm the connection between high (?1017 cm?2) SO2 gas content and plumes that scatter strongly at nearly blue wavelengths, and it validates the occurrence of high density SO2 gas eruptions on Io. Noting that the SO2 gas content inferred from a spectrum of the 2003 Pillan plume was significantly lower ～2 × 1016 cm?2 (Jessup, K.L., Spencer, J., Yelle, R. [2007]. Icarus 192, 24–40); and that the Pillan caldera was flooded with fresh SO2 frost/slush just prior to the 1997 Pillan plume eruption (Geissler, P., McEwen, A., Phillips, C., Keszthelyi, L., Spencer, J. [2004a]. Icarus 169, 29–64; Phillips, C.B. [2000]. Voyager and Galileo SSI Views of Volcanic Resurfacing on Io and the Search for Geologic Activity at Europa. Ph.D. Thesis, Univ. of Ariz., Tucson); we propose that the density of SO2 gas released by this volcano is directly linked to the local SO2 frost abundance at the time of eruption.  相似文献   

14.
A series of observations of the venusian hydrogen corona made by SPICAV on Venus Express are analyzed to estimate the amount of hydrogen in the exosphere of Venus. These observations were made between November 2006 and July 2007 at altitudes from 1000 km to 8000 km on the dayside. The Lyman-α brightness profiles derived are reproduced by the sum of a cold hydrogen population dominant below ～2000 km and a hot hydrogen population dominant above ～4000 km. The temperature (～300 K) and hydrogen density at 250 km (～105 cm?3) derived for the cold populations, near noon, are in good agreement with previous observations. Strong dawn–dusk exospheric asymmetry is observed from this set of observations, with a larger exobase density on the dawn side than on the dusk side, consistent with asymmetry previously observed in the venusian thermosphere, but with a lower dawn/dusk contrast. The hot hydrogen density derived is very sensitive to the sky background estimate, but is well constrained near 5000 km. The density of the hot population is reproduced by the exospheric model from Hodges (Hodges, R.R. [1999]. J. Geophys. Res. 104, 8463–8471) in which the hot population is produced by neutral–ions interactions in the thermosphere of Venus.  相似文献   

15.
From a comparison between the different observations of Martian methane existing today, including the new TES methane maps (Fonti and Marzo, 2010), we show that all sets of data are globally consistent with each other, and that a well definite seasonal cycle of methane has been at work for at least 10 yr. With a simple model of the balance between the loss fluxes of H and O, using up-to-date values of the escape fluxes, we show that the long-standing enigma of the imbalance between H and O escape fluxes may be solved by assuming that the missing sink of oxygen is the oxidation of methane. If no H2 is released together with CH4, a good agreement is found between the present CH4 flux and the value imposed by the balance between H and O escape fluxes, an average over the last ≈103 yr. If H2 is released together with CH4, as expected if CH4 originates in serpentinization, the average level of CH4 during the last 103 yr should have been at least ten times lower than the present one. The lack of present H2 release could suggest a long-term storage of methane in the subsurface under the form of clathrates, whereas H2 has been lost to the atmosphere shortly after being produced. We suggest that the thin layer of CO2 ice covering the permanent southern polar cap could result from the release of methane since the end of the last obliquity transition (time scale: 1 Myr), at an average rate of 0.1 Mt yr?1, consistent with the values derived from: (i) the present observations of methane (time scale: 10 yr), (ii) the estimate from the observed imbalance between the H and O escape fluxes (time scale: 1 kyr). If so, the present release of methane from subsurface clathrates would have acted at a similar rate since at least 3 Myr.  相似文献   

16.
An automated cloud tracking algorithm is applied to Cassini Imaging Science Subsystem high-resolution apoapsis images of Saturn from 2005 and 2007 and moderate resolution images from 2011 and 2012 to define the near-global distribution of zonal winds and eddy momentum fluxes at the middle troposphere cloud level and in the upper troposphere haze. Improvements in the tracking algorithm combined with the greater feature contrast in the northern hemisphere during the approach to spring equinox allow for better rejection of erroneous wind vectors, a more objective assessment at any latitude of the quality of the mean zonal wind, and a population of winds comparable in size to that available for the much higher contrast atmosphere of Jupiter. Zonal winds at cloud level changed little between 2005 and 2007 at all latitudes sampled. Upper troposphere zonal winds derived from methane band images are ～10 m s?1 weaker than cloud level winds in the cores of eastward jets and ～5 m s?1 stronger on either side of the jet core, i.e., eastward jets appear to broaden with increasing altitude. In westward jet regions winds are approximately the same at both altitudes. Lateral eddy momentum fluxes are directed into eastward jet cores, including the strong equatorial jet, and away from westward jet cores and weaken with increasing altitude on the flanks of the eastward jets, consistent with the upward broadening of these jets. The conversion rate of eddy to mean zonal kinetic energy at the visible cloud level is larger in eastward jet regions (5.2 × 10?5 m2 s?3) and smaller in westward jet regions (1.6 × 10?5 m2 s?3) than the global mean value (4.1 × 10?5 m2 s?3). Overall the results are consistent with theories that suggest that the jets and the overturning meridional circulation at cloud level on Saturn are maintained at least in part by eddies due to instabilities of the large-scale flow near and/or below the cloud level.  相似文献   

17.
《Planetary and Space Science》2007,55(12):1741-1756
The dynamics of Venus’ mesosphere (70–110 km) is characterized by the superposition of two different wind regimes: (1) Venus’ retrograde superrotation; (2) a sub-solar to anti-solar (SS–AS) flow pattern, driven by solar EUV heating on the sunlit hemisphere. Here, we report on new ground-based velocity measurements in the lower part of the mesosphere. We took advantage of two essentially symmetric Venus elongations in 2001 and 2002 to perform high-resolution Doppler spectroscopy (R=120,000) in 12C16O2 visible lines of the 5ν3 band and in a few solar Fraunhofer lines near 8700 Å. These measurements, mapped over several points on Venus’ illuminated hemisphere, probe the region of cloud tops. More precisely, the solar Fraunhofer lines sample levels a few kilometers below the UV features (i.e. near ∼67 km), while the CO2 lines probe an altitude higher by about 7 km. The wind field over Venus’ disk is retrieved with an rms uncertainty of 15–25 m s−1 on individual measurements. Kinematical fit to a one- or two-component circulation model indicates the dominance of the zonal retrograde flow with a mean equatorial velocity of ∼75 m s−1, exhibiting very strong day-to-day variations (±65 m s−1). Results are very consistent for the two kinds of lines, suggesting a negligible vertical wind shear over 67–74 km. The SS–AS flow is not detected in single-day observations, but combining the results from all data suggests that this component may invade the lower mesosphere with a ∼40 m s−1 velocity.  相似文献   

18.
We present an analysis of 19 μm spectra of Io’s SO2 atmosphere from the TEXES mid-infrared high spectral resolution spectrograph on NASA’s Infrared Telescope Facility, incorporating new data taken between January 2005 and June 2010 and a re-analysis of earlier data taken from November 2001 to January 2004. This is the longest set of contiguous observations of Io’s atmosphere using the same instrument and technique thus far. We have fitted all 16 detected blended absorption lines of the ν2 SO2 vibrational band to retrieve the subsolar values of SO2 column abundance and the gas kinetic temperature. By incorporating an existing model of Io’s surface temperatures and atmosphere, we retrieve sub-solar column densities from the disk-integrated data. Spectra from all years are best fit by atmospheric temperatures <150 K. Best-fit gas kinetic temperatures on the anti-Jupiter hemisphere, where SO2 gas abundance is highest, are low and stable, with a mean of 108 (±18) K. The sub-solar SO2 column density between longitudes of 90–220° varies from a low of 0.61 (±0.145) × 10?17 cm?2, near aphelion in 2004, to a high of 1.51 (±0.215) × 1017 cm?2 in 2010 when Jupiter was approaching its early 2011 perihelion. No correlation in the gas temperature was seen with the increasing SO2 column densities outside the errors.Assuming that any volcanic component of the atmosphere is constant with time, the correlation of increasing SO2 abundance with decreasing heliocentric distance provides good evidence that the atmosphere is at least partially supported by frost sublimation. The SO2 frost thermal inertias and albedos that fit the variation in atmospheric density best are between 150–1250 W m?2 s?1/2 K?1 and 0.613–0.425 respectively. Photometric evidence favors albedos near the upper end of this range, corresponding to thermal inertias near the lower end. This relatively low frost thermal inertia produces larger amplitude seasonal variations than are observed, which in turn implies a substantial additional volcanic atmospheric component to moderate the amplitude of the seasonal variations of the total atmosphere on the anti-Jupiter hemisphere. The seasonal thermal inertia we measure is unique both because it refers exclusively to the SO2 frost surface component, and also because it refers to relatively deep subsurface layers (few meters) due to the timescales of many years, while previous studies have determined thermal inertias at shallower levels (few centimeters), relevant for timescales of ～2 h (eclipse) or ～2 days (diurnal curves).  相似文献   

19.
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   

20.
《Planetary and Space Science》2007,55(10):1328-1345
The planetary fourier spectrometer (PFS) for the Mars express mission (MEX) is an infrared spectrometer operating in the wavelength range from 1.2 to 45 μm by means of two spectral channels, called SWC (short wavelength channel) and LWC (long wavelength channel), covering, respectively, 1.2–5.5 and 5.5–45 μm.The middle-spring Martian north polar cap (Ls∼40°) has been observed by PFS/MEX in illuminated conditions during orbit 452. The SWC spectra are here used to study the cap composition in terms of CO2 ice, H2O ice and dust content. Significant spectral variation is noted in the cap interior, and regions of varying CO2 ice grain sizes, water frost abundance, CO2 ice cover and dust contamination can be distinguished. In addition, we correlate the infrared spectra with an image acquired during the same orbit by the OMEGA imaging spectrometer and with the altimetry from MOLA data. Many of the spectra variations correlate with heterogeneities noted in the image, although significant spectral variations are not discernible in the visible. The data have been divided into five regions with different latitude ranges and strong similarities in the spectra, and then averaged. Bi-directional reflectance models have been run with the appropriate lighting geometry and used to fit the observed data, allowing for CO2 ice and H2O ice grain sizes, dust and H2O ice contaminations in the form of intimate granular mixtures and spatial mixtures.A wide annulus of dusty water ice surrounds the recessing CO2 seasonal cap. The inner cap exhibits a layered structure with a thin CO2 layer with varying concentrations of dark dust, on top of an H2O ice underneath ground. In the best-fits, the ices beneath the top layer have been considered as spatial mixtures. The results are still very good everywhere in the spectral range, except where the CO2 ice absorption coefficients are such that even a thin layer is enough to totally absorb the incoming radiation (i.e. the band is saturated). This only happens around 3800 cm−1, inside the strong 2.7-μm CO2 ice absorption band. The effect of finite snow depth has been investigated through a layered albedo model. The thickness of the CO2 ice deposits increases with latitude, ranging from 0.5–1 g cm−2 within region II to 60–80 g cm−2 within the highest-latitude (up to 84°N) region V.Region I is at the cap edge and extends from 65°N to 72°N latitude. No CO2 ice is present in this region, which consists of relatively large grains of water ice (20 μm), highly contaminated by dust (0.15 wt%). The adjacent region II is a narrow region [76–79°N] right at the edge of the north residual polar cap. This region is very distinct in the OMEGA image, where it appears to surround the whole residual cap. The CO2 ice features are barely visible in these spectra, except for the strong saturated 2.7 μm band. It basically consists of a thin layer of 5-mm CO2 ice on top of an H2O ice layer with the same composition as region I. A third interesting region III is found all along the shoulder of the residual cap [79–81°N]. It extends over 1.5 km in altitude and over only 2° of latitude and consists of CO2 ice with a large dust content. It is an admixture of CO2 ice (3–4 mm), with several tens of ppm by mass of water ice and more than 2 ppt by mass of dust. The surface temperatures have been retrieved from the LWC spectra for each observation. We found an increase in the surface temperature in this region, indicating a spatial mixture of cold CO2 ice and warmer dust/H2O ice. Region IV is close to the top of the residual cap [81–84°N]; it is much brighter than region III, with a dust content 10 times lower than the latter. The CO2 grain size is 3 mm and strong CO2 ice features are present in the data, indicating a thicker CO2 ice layer than in region II (1–2 g cm−2). The final region V is right at the top of the residual cap (⩾84°N). It is “pure” CO2 ice (no dust) of 5 mm grain sizes, with 30 ppm by weight of water ice. The CO2 ice features are very pronounced and the 2.7 μm band is saturated. The optical thickness is close to the semi-infinite limit (30–40 g cm−2). Assuming a snowpack density of 0.5 g cm−3, we get a minimum thickness of 1–2 cm for the top-layer of regions II and III, 4–10 cm for region IV, and ⩾60–80 cm thickness for region V. These values are in close agreement with several recent results for the south seasonal polar cap.These results should provide new, useful constraints in models of the Martian climate system and volatile cycles.  相似文献   

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