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1.
There is a significant progress in the observational data relevant to Mars photochemistry in the current decade. These data are not covered by and sometimes disagree with the published models. Therefore we consider three types of models for Mars photochemistry. A steady-state model for global-mean conditions is currently the only way to calculate the abundances of long living species (H2, O2, and CO). However, our model does not fit the observed CO abundance using gas-phase chemistry and reasonable values of heterogeneous loss of odd hydrogen on the water ice aerosol. The second type of the calculated models is steady-state models for local conditions. The MGS/TES data on temperature profiles, H2O, and dust are input parameters for these models. The calculations have been made for nine seasonal points spread over the martian year and for twelve latitudes with a step of 10° for each season. The only adopted heterogeneous reaction is a weak loss of H2O2 on water ice with probability of 5×10−4. The results are in good agreement with the recent observations of the O2 dayglow at 1.27 μm and the O3 and H2O2 abundances. Global maps of the seasonal and latitudinal behavior of these species have been made. The third type of models is a time-dependent model for local conditions. These models show that odd hydrogen quickly converts to H2O2 at the nighttime and the chemistry is switched off while the association of O, the heterogeneous loss of H2O2, and eddy diffusion continue. This requires significant changes in the global-mean and local steady-state models discussed above, and these changes have been properly done. The calculated diurnal variations of Mars photochemistry are discussed. The martian photochemistry at low and middle latitudes is significantly different in the aphelion period at LS=10°-130° from that in the remaining part of the year.  相似文献   

2.
Experiments on the hydrogenation of CO on pure CO and CO-H2O mixed ice have been performed at temperatures between 8 and 20 K. We obtained temperature and compositional dependence of the effective reaction rate constants. Results indicate that hydrogenation proceeds efficiently on pure solid CO and CO-H2O mixed ice at temperatures below 10 and 20 K, respectively. Rate constants for pure CO decreased significantly at 12 K compared to those obtained with CO-H2O mixed ice. Hydrogenation of CO at temperatures greater than 12 K were catalyzed by the H2O adjacent to the CO. The importance of the experimental results for some relevant astrophysical environments has also been outlined.  相似文献   

3.
The trapping and release of H2, CO, CO2, CH4, Ar, Ne, and N2 by amorphous water ice was studied experimentally under dynamic conditions, at low temperatures starting at 16°K, with gas pressure of 5 × 10?8?10?6 Torr. CO, CH4, Ar, and N2 were found to be released in three or four distinct temperature ranges, each resulting from a different trapping mechanism: (a) 30–55°K, where the gas frozen on the water ice evaporates; (b) 135–155°K, where gas is squeezed out of the water ice during the transformation of amorphous ice to cubic ice; (c) 165–190°K, where gas and water are released simultaneously, probably by the evaporation of a clathrate hydrate, and, occasionally (d) 160–175°K, where deeply buried gas is released during the transformation of cubic ice to hexagonal ice. If the third range is indeed due to clathrate formation, CO was found to form this compound. CO2 did not form a clathrate under the experimental conditions. Excess hydrogen did not affect the occlusion of other gases. Hydrogen itself was trapped only at 16°K. Neon was not trapped at 25°K. With cubic ice, the only trapping mechanism is freezing of gas on the ice surface. No fractionation between the gas phase and the ice was observed with a mixture of CO and Ar. Massive ejection of ice grains was observed during the evaporation of the gas in three (a,c,d) out of the four ranges. The experimental results are used to explain several cometary phenomena, especially those occurring at large heliocentric distances, and are applied also to Titan's atmospheric composition and to the possible ejection of ice grains from Enceladus.  相似文献   

4.
The discovery of C/1995 O1 (Hale-Bopp) at 7 AU from the Sun provided the first opportunity to follow the activity of a bright comet over a large range of heliocentric distances rh. Production rates of a number of parent molecules and daughter species have been monitored both pre- and postperihelion. CO was found to be the major driver of the activity far from the Sun, surpassed by water within 3 AU whose production rate reached 1031 s−1 at perihelion. Gas production curves obtained for various species show several behaviours with rh. Gas production curves contain important information concerning the physical state of cometary ices, the structure of the nucleus and all the processes taking place inside the nucleus leading to outgassing. They are relevant to the study of several other phenomena such as the sublimation from icy grains, dust mantling or seasonal effects. For some species, such as H2CO or HNC, they permit to constrain their origin in the coma. We discuss models of subsurface gas production in distant comets and predictions of how such a source may vary as the comet moves along its orbit, approaching perihelion and receding again. Features in the observed gas production curves of comet Hale-Bopp are generally interpretable in terms of either subsurface production (typical example: CO at large rh) or free sublimation (typical example: H2O). Possible implications for the vertical stratification of the cometary ices are reviewed, and preference is found for a model with crystallization of amorphous ice close to the nuclear surface. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

5.
The Mars Express spacecraft has a highly inclined orbit around Mars and so has been able to observe the south pole of Mars in illuminated conditions at the end of the southern summer (Ls=330). Spectra from the planetary Fourier spectrometer (PFS) short wavelength (SW) channel were recorded over the permanent ice cap to study its composition in terms of CO2 ice and H2O ice. Models are fitted to the observed data, which include a spatial mixture of soil (not covered by ice) and CO2 frost (with a specific grain size and a small amount of included dust and H2O ice). Two different kinds of spectra were observed: those over the permanent polar cap with almost pure CO2 ice, negligible water ice, no soil fraction required, and bright; and those over mixed terrain (at the edge of the cap or near troughs) containing a significant soil spatial fraction, more water ice and smaller CO2 grain size. The amount of water ice given by fits to scaled albedo models is less than 10 ppm by weight. When using multi-stream reflectance models with the appropriate lighting geometry, the water amount must be 2-5 times greater than the albedo fit (less than 50 ppm). At the periphery of the residual polar cap, we found a region almost completely covered by water frost, modeled as a mixture of micron-sized and sub-mm sized grains. Our result using a granular mixture of micron-sized grains of water ice and dust with the CO2 grains is different from the modeling of OMEGA polar cap observations using molecular mixtures.  相似文献   

6.
It has been suggested that inclusions of CO2 or CO2 clathrate hydrates may comprise a portion of the polar deposits on Mars. Here we present results from an experimental study in which CO2 molecules were trapped in water ice deposited from CO2/H2O atmospheres at temperatures relevant for the polar regions of Mars. Fourier-Transform Infrared spectroscopy was used to monitor the phase of the condensed ice, and temperature programmed desorption was used to quantify the ratio of species in the generated ice films. Our results show that when H2O ice is deposited at 140-165 K, CO2 is trapped in large quantities, greater than expected based on lower temperature studies in amorphous ice. The trapping occurs at pressures well below the condensation point for pure CO2 ice, and therefore this mechanism may allow for CO2 deposition at the poles during warmer periods. The amount of trapped CO2 varied from 3% to 16% by mass at 160 K, depending on the substrate studied. Substrates studied were a tetrahydrofuran (C4H8O) base clathrate and Fe-montmorillonite clay, an analog for Mars soil. Experimental evidence indicates that the ice structures are likely CO2 clathrate hydrates. These results have implications for the CO2 content, overall composition, and density of the polar deposits on Mars.  相似文献   

7.
We have quantitatively studied, by infrared absorption spectroscopy, the CO/CO2 molecular number ratio after ion irradiation of ices and mixtures containing astrophysically relevant species such as CO, CO2, H2O, CH4, CH3OH, NH3, O2, and N2 at 12–15 K. The ratios have also been measured after warm up to temperatures between 12 and 200 K. As a general result we find that the CO/CO2 ratio decreases with the irradiation dose (amount of energy deposited on the sample). In all of the studied mixtures, as expected, it decreases with increasing temperature because of CO sublimation. However the temperature where CO sublimes strongly depends on the initial mixture, remaining at a temperature over 100 K in some cases. Our results might be relevant to interpret the observed CO/CO2 ratio in several astrophysical scenarios such as planetary icy surfaces and ice mantles on grains in the interstellar medium. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

8.
This report arises from an ongoing program to monitor Neptune’s largest moon Triton spectroscopically in the 0.8 to 2.4 μm range using IRTF/SpeX. Our objective is to search for changes on Triton’s surface as witnessed by changes in the infrared absorption bands of its surface ices N2,CH4,H2O, CO, and CO2. We have recorded infrared spectra of Triton on 53 nights over the ten apparitions from 2000 to 2009. The data generally confirm our previously reported diurnal spectral variations of the ice absorption bands (Grundy and Young, 2004). Nitrogen ice shows a large amplitude variation, with much stronger absorption on Triton’s Neptune-facing hemisphere. We present evidence for seasonal evolution of Triton’s N2 ice: the 2.15 μm absorption band appears to be diminishing, especially on the Neptune-facing hemisphere. Although it is mostly dissolved in N2 ice, Triton’s CH4 ice shows a very different longitudinal variation from the N2 ice, challenging assumptions of how the two ices behave. Unlike Triton’s CH4 ice, the CO ice does exhibit longitudinal variation very similar to the N2 ice, implying that CO and N2 condense and sublimate together, maintaining a consistent mixing ratio. Absorptions by H2O and CO2 ices show negligible variation as Triton rotates, implying very uniform and/or high latitude spatial distributions for those two non-volatile ices.  相似文献   

9.
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   

10.
We present infrared absorption studies on the effects of 50-100 keV Ar+ and 100 keV H+ ion irradiation of water ice films at 20-120 K. The results support the view that energetic ions can produce hydrogen peroxide on the surface of icy satellites and rings in the outer Solar System, and on ice mantles on interstellar grains. The ion energies are characteristic of magnetospheric ions at Jupiter, and therefore the results support the idea that radiolysis by ion impact is the source of the H2O2 detected on Europa by the Galileo infrared spectrometer. We found that Ar+ ions, used to mimic S+ impacts, are roughly as efficient as H+ ions in producing H2O2, and that 100 keV H+ ions can produce hydrogen peroxide at 120 K. The synthesized hydrogen peroxide remained stable while warming the ice film after irradiation; the column density of the formed H2O2 is constant until the ice film begins to desorb, but the concentration of H2O2 increases with time during desorption because the water sublimes at a faster rate. Comparing the shape of the 3.5-μm absorption feature of H2O2 to the one measured on Europa shows excellent agreement in both shape and position, further indicating that the H2O2 detected on Europa is likely caused by radiolysis of water ice.  相似文献   

11.
M.H. Moore  R.L. Hudson 《Icarus》2007,189(2):409-423
Spectra of Europa, Ganymede, and Callisto reveal surfaces dominated by frozen water, hydrated materials, and minor amounts of SO2, CO2, and H2O2. These icy moons undergo significant bombardment by jovian magnetospheric radiation (protons, electrons, and sulfur and oxygen ions) which alters their surface compositions. In order to understand radiation-induced changes on icy moons, we have measured the mid-infrared spectra of 0.8 MeV proton-irradiated SO2, H2S, and H2O-ice mixtures containing either SO2 or H2S. Samples with H2O/SO2 or H2O/H2S ratios in the 3-30 range have been irradiated at 86, 110, and 132 K, and the radiation half-lives of SO2 and H2S have been determined. New radiation products include the H2S2 molecule and HSO−3, HSO−4, and SO2−4 ions, all with spectral features that make them candidates for future laboratory work and, perhaps, astronomical observations. Spectra of both unirradiated and irradiated ices have been recorded as a function of temperature, to examine thermal stability and phase changes. The formation of hydrated sulfuric acid in irradiated ice mixtures has been observed, along with the thermal evolution of hydrates to form pure sulfuric acid. These laboratory studies provide fundamental information on likely processes affecting the outer icy shells of Europa, Ganymede, and Callisto.  相似文献   

12.
Abstract— The reaction between kamacite grains and H2 + CO gas mixture has been tested in the laboratory under experimental conditions presumed for interplanetary dust particle (IDP) formation in a nebular-type environment (H2:CO = 250:1; 5 × 10?4 atm total pressure, and 473 K). Carbon deposition, hydrocarbon production in the C1–C4 range, and the formation of an ?-carbide phase occur when well-defined model FeNi bcc alloy (kamacite) particles are exposed to a mixture of H2 + CO during 103 h. These results strongly support the idea that gas-solid reactions in the solar nebula during CO hydrogenation represent a plausible scenario for the formation of carbides and carbonaceous materials in IDPs, as well as for the production of hydrocarbons through Fischer-Tropsch-type reactions.  相似文献   

13.
The distributions of various kinds of molecular ions in the atmospheres of new and old comets made up from dirty ice of the second kind (H2O ice and hydrate clathrates of CO and N2) have been computed at various heliocentric distances, by taking into account photoionization, ionmolecular reactions, electron-ion recombinations, and some transport effects. The results have been compared with observations and other computations. It is argued that dirty ice of the second kind model will impose a restriction on the theory of the origin of the solar system.  相似文献   

14.
We present new experimental results on impact shock chemistry into icy satellites of the outer planets. Icy mixtures of pure water ice with CO2, Na2CO3, CH3OH, and CH3OH/(NH4)2SO4 at 77 K were ablated with a powerful pulsed laser—a new technique used to simulate shock processes which can occur during impacts. New products were identified by GC-MS and FTIR analyses after laser ablation. Our results show that hydrogen peroxide is formed in irradiated H2O/CO2 ices with a final concentration of 0.23%. CO and CH3OH were also detected as main products. The laser ablation of frozen H2O/Na2CO3 generates only CO and CO2 as destruction products from the salt. Pulsed irradiation of water ice containing methanol leads also to the formation of CO and CO2, generates methane and more complex molecules containing carbonyl groups like acetaldehyde, acetone, methyl formate, and a diether, dimethyl formal. The last three compounds are also produced when adding ammonium sulfate to H2O/CH3OH ice, but acetone is more abundant. The formation of two hydrocarbons, CH4 and C2H6 is observed as well as the production of three nitrogen compounds, nitrous oxide, hydrogen cyanide, and acetonitrile.  相似文献   

15.
Radiation synthesis has been proposed as a mechanism for changing the nature of the outer few meters of ice in a comet stored 4.6 billion years in the Oort cloud and may explain some of the differences observed between new and more evolved comets. Cometary-type ice mixtures were studied in a laboratory experiment designed to approximately simulate the expected temperature, pressure, and radiation environment of the interstellar Oort cloud region. The 2.5- to 15-μm infrared absorption features of thin ice films were analyzed near 20°K before and after 1 MeV proton irradiation. Various ice mixtures included the molecules H2O, NH3, CH4, N2, C3H8, CO, and CO2. All experiments confirm the synthesis of new molecular species in solid phase mixtures at 20°K. The synthesized molecules, identified by their infrared signatures, are C2H6, CO2, CO, N2O, NO, and CH4 (weak). Synthesized molecules, identified by gas chromatographic (GC) analysis of the volatile fraction of the warmed irradiated ice mixture, are C2H4 or C2H6, and C3H8. When CH4 is present in the irradiated ice mixture, long-chained volatile hydrocarbons and CO2 are synthesized along with high-molecular-weight carbon compounds present in the room temperature residue. Irradiated mixtures containing CO and H2O synthesize CO2 and those CO2 and H2O synthesize CO. Due to radiation synthesis, ～1% of the ice was converted into a nonvolatile residue containing complicated carbon compounds not present in blank samples. These results suggest that irrespective of the composition of newly accreted comets, initial molecular abundances can be altered and new species created as a result of radiation synthesis. Irradiated mixtures exhibited thermoluminescence and pressure enhancements during warming; these phenomena suggest irradiation synthesis of reactive species. Ourbursts in new comets resulting from similar radiation induced exothermic activity would be expected to occur beginning at distances of the order of 100 AU.  相似文献   

16.
CO2 ices are known to exist in different astrophysical environments. In spite of this, its physical properties (structure, density, refractive index) have not been as widely studied as those of water ice. It would be of great value to study the adsorption properties of this ice in conditions related to astrophysical environments. In this paper, we explore the possibility that CO2 traps relevant molecules in astrophysical environments at temperatures higher than expected from their characteristic sublimation point. To fulfil this aim we have carried out desorption experiments under High Vacuum conditions based on a Quartz Crystal Microbalance and additionally monitored with a Quadrupole Mass Spectrometer. From our results, the presence of CH4 in the solid phase above the sublimation temperature in some astrophysical scenarios could be explained by the presence of several retaining mechanisms related to the structure of CO2 ice.  相似文献   

17.
《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   

18.
Experimental results on the spectral identification of new infrared absorption features and the changes of their absorbances produced through vacuum ultraviolet-extreme ultraviolet (VUV-EUV) photon-induced chemical reactions in the C2H2-H2O mixed ices at 10 K are obtained. To the best of our knowledge, this is the first time that EUV photons have been employed in the study of the photolysis of ice analogues. Two different compositions, i.e., C2H2:H2O=1:4 and 1:1, were investigated in this work. A tunable intense synchrotron radiation light source available at the Synchrotron Radiation Research Center, Hsinchu, Taiwan, was employed to provide the required VUV-EUV photons. In this study, the photon wavelengths selected to irradiate the icy samples corresponded to the prominent solar hydrogen, helium, and helium ion lines at 121.6 nm, 58.4 nm, and 30.4 nm, respectively. The photon dosages used were typically in the range of 1×1015 to 2×1017 photons. Molecular species produced and identified in the ice samples at 10 K resulting from VUV-EUV photon irradiation are mainly CO, CO2, CH4, C2H6, CH3OH, and H2CO. In addition to several unidentified features, we have tentatively assigned several absorption features to HCO, C3H8, and C2H5OH. While new molecular species were formed, the original reactants, i.e., H2O and C2H2, were detectably depleted due to their conversion to other species. The new chemical species produced by irradiation of photons at 30.4 nm and 58.4 nm can be different from those produced by the 121.6-nm photolysis. In general, the product column density of CO reaches saturation at a lower photon dosage than that of CO2. Furthermore, the production yield of CO is higher than that of CO2 in the photon irradiation. In the present study, we also observe that the photon-induced chemical reaction yields are high using photons at 30.4 and 58.4 nm. The results presented in this work are essential to our understanding of chemical synthesis in ice analogues, e.g., the cometary-type ices and icy satellites of planetary systems.  相似文献   

19.
Abstract— Fischer‐Tropsch catalysis, by which CO and H2 are converted to CH4 on the surface of transition metals, has been considered to be one of the most important chemical reactions in many planetary processes, such as the formation of the solar and circumplanetary nebulae, the expansion of vapor clouds induced by cometary impacts, and the atmospheric re‐entry of vapor condensate due to asteroidal impacts. However, few quantitative experimental studies have been conducted for the catalytic reaction under conditions relevant to these planetary processes. In this study, we conduct Fischer‐Tropsch catalytic experiments at low pressures (1.3 times 10?4 bar ≤ P ≤ 5.3 times 10?1 bar) over a wide range of H2/CO ratios (0.25–1000) using pure iron, pure nickel, and iron‐nickel alloys. We analyze what gas species are produced and measure the CH4 formation rate. Our results indicate that the CH4 formation rate for iron catalysts strongly depends on both pressure and the H2/CO ratio, and that nickel is a more efficient catalyst at lower pressures and lower H2/CO ratios. This difference in catalytic properties between iron and nickel may come from the reaction steps concerning disproportionation of CO, hydrogenation of surface carbon, and the poisoning of the catalyst. These results suggest that nickel is important in the atmospheric re‐entry of impact condensate, while iron is efficient in circumplanetary subnebulae. Our results also indicate that previous numerical models of iron catalysis based on experimental data at 1 bar considerably overestimate CH4 formation efficiency at lower pressures, such as the solar nebula and the atmospheric re‐entry of impact condensate.  相似文献   

20.
O. Gomis  G. Strazzulla 《Icarus》2005,177(2):570-576
In this work we report on new experiments of ion irradiation of water ice deposited on top of solid carbonaceous materials to study the production of CO2 at the interface ice/refractory material and discuss the possibility that this mechanism accounts for the quantity of CO2 ice detected on the surfaces of the Galilean satellites. The used experimental technique has been in situ infrared spectroscopy. We have irradiated thin films of H2O frost on carbonaceous layers with 200 keV of He+ and Ar+, and 30 keV of He+ at 16 and 80 K. The used carbonaceous layers have been asphaltite, a natural bitumen, and solid organic residues obtained by irradiation of frozen benzene. In both cases the results show that CO2 is produced very efficiently after irradiation obtaining a maximum quantity of the order of . These results are, also quantitatively similar, to those recently obtained for water ice deposited on amorphous carbon films [Mennella, V., Palumbo, M.E., Baratta, G.A., 2004. Formation of CO and CO2 molecules by ion irradiation of water ice covered hydrogenated carbon grains. Astrophys. J. 615, 1073-1080]. Thus we suggest that, whatever is the carbonaceous residue, CO2 will be produced efficiently by the studied process. These results have interest in the context of the surfaces of the icy Galilean satellites in which CO2 has been detected mainly trapped in the non-ice material, not in the pure water ice. We suggest that radiolysis of mixtures of water ice and refractory carbonaceous materials is the primary formation mechanism responsible for the CO2 formation on the surfaces of the Galilean satellites.  相似文献   

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