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1.
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
2.
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
3.
Robert Hodyss Paul V. Johnson Grazyna E. Orzechowska Jay D. Goguen Isik Kanik 《Icarus》2008,194(2):836-842
The mid-infrared spectra of mixed vapor deposited ices of CO2 and H2O were studied as a function of both deposition temperature and warming from 15 to 100 K. The spectra of ices deposited at 15 K show marked changes on warming beginning at 60 K. These changes are consistent with CO2 segregating within the ice matrix into pure CO2 domains. Ices deposited at 60 and 70 K show a greater degree of segregation, as high as 90% for 1:4 CO2:H2O ice mixtures deposited at 70 K. As the ice is warmed above 80 K, preferential sublimation of the segregated CO2 is observed. The kinetics of the segregation process is also examined. The segregation of the CO2 as the ice is warmed corresponds to temperatures at which the structure of the water ice matrix changes from the high density amorphous phase to the low density amorphous phase. We show how these microstructural changes in the ice have a profound effect on the photochemistry induced by ultraviolet irradiation. These experimental results provide a framework in which observations of CO2 on the icy bodies of the outer Solar System can be considered. 相似文献
4.
Recent detection of methane (CH4) on Mars has generated interest in possible biological or geological sources, but the factors responsible for the reported variability are not understood. Here we explore one potential sink that might affect the seasonal cycling of CH4 on Mars - trapping in ices deposited on the surface. Our apparatus consisted of a high-vacuum chamber in which three different Mars ice analogs (water, carbon dioxide, and carbon dioxide clathrate hydrates) were deposited in the presence of CH4 gas. The ices were monitored for spectroscopic evidence of CH4 trapping using transmission Fourier-Transform Infrared (FT-IR) spectroscopy, and during subsequent sublimation of the ice films the vapor composition was measured using mass spectrometry (MS). Trapping of CH4 in water ice was confirmed at deposition temperatures <100 K which is consistent with previous work, thus validating the experimental methods. However, no trapping of CH4 was observed in the ice analogs studied at warmer temperatures (140 K for H2O and CO2 clathrate, 90 K for CO2 snow) with approximately 10 mTorr CH4 in the chamber. From experimental detection limits these results provide an upper limit of 0.02 for the atmosphere/ice trapping ratio of CH4. If it is assumed that the trapping mechanism is linear with CH4 partial pressure and can be extrapolated to Mars, this upper limit would indicate that less than 1% is expected to be trapped from the largest reported CH4 plume, and therefore does not represent a significant sink for CH4. 相似文献
5.
Distributions of H2O and CO2 ices on Ariel, Umbriel, Titania, and Oberon from IRTF/SpeX observations
We present 0.8-2.4 μm spectral observations of uranian satellites, obtained at IRTF/SpeX on 17 nights during 2001-2005. The spectra reveal for the first time the presence of CO2 ice on the surfaces of Umbriel and Titania, by means of 3 narrow absorption bands near 2 μm. Several additional, weaker CO2 ice absorptions have also been detected. No CO2 absorption is seen in Oberon spectra, and the strengths of the CO2 ice bands decline with planetocentric distance from Ariel through Titania. We use the CO2 absorptions to map the longitudinal distribution of CO2 ice on Ariel, Umbriel, and Titania, showing that it is most abundant on their trailing hemispheres. We also examine H2O ice absorptions in the spectra, finding deeper H2O bands on the leading hemispheres of Ariel, Umbriel, and Titania, but the opposite pattern on Oberon. Potential mechanisms to produce the observed longitudinal and planetocentric distributions of the two ices are considered. 相似文献
6.
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
7.
Cassini VIMS detected carbon dioxide on the surface of Iapetus during its insertion orbit. We evaluated the CO2 distribution on Iapetus and determined that it is concentrated almost exclusively on Iapetus’ dark material. VIMS spectra show a 4.27-μm feature with an absorption depth of 24%, which, if it were in the form of free ice, requires a layer 31 nm thick. Extrapolating for all dark material on Iapetus, the total observable CO2 would be 2.3 × 108 kg.Previous studies note that free CO2 is unstable at 10 AU over geologic timescales. Carbon dioxide could, however, be stable if trapped or complexed, such as in inclusions or clathrates. While complexed CO2 has a lower thermal volatility, loss due to photodissociation by UV radiation and gravitational escape would occur at a rate of 2.6 × 107 kg year−1. Thus, Iapetus’ entire inventory of surface CO2 could be lost within a few decades.The high loss/destruction rate of CO2 requires an active source. We conducted experiments that generated CO2 by UV radiation of simulated icy regolith under Iapetus-like conditions. The simulated regolith was created by flash-freezing degassed water, crushing it into sub-millimeter sized particles, and then mixing it with isotopically labeled amorphous carbon (13C) dust. These samples were placed in a vacuum chamber and cooled to temperatures between 50 K and 160 K. The samples were irradiated with UV light, and the products were measured using a mass spectrometer, from which we measured 13CO2 production at a rate of 2.0 × 1012 mol s−1. Extrapolating to Iapetus and adjusting for the solar UV intensity and Iapetus’ surface area, we calculated that CO2 production for the entire surface would be 1.1 × 107 kg year−1, which is only a factor of two less than the loss rate. As such, UV photochemical generation of CO2 is a plausible source of the detected CO2. 相似文献
8.
We present a detailed study of an Iapetus mosaic of VIMS data with high spatial resolution (0.5 × 0.5° or ∼6.4 km/pixel). The spectra were taken in August 2007 and provide the highest VIMS spatial resolution data for this object during Cassini’s primary mission. We analyze this set of data using a statistical clustering approach to reduce the analysis of a large number of data (∼104 spectra from 0.35 to 5.10 μm) to the study of seven representative groups accounting for 99.6% of the surface covered by the original sample. We analyze the spectral absorption bands in the spectra of the different clusters indicative of different composition over the observed surface. We find coherence between the distribution of the clusters and the geographical features on the surface. We give special attention to the study of the water ice and CO2 bands. We find that CO2 is widespread over the entire surface being studied, including the bright and dark areas on Iapetus’ surface, and is probably trapped at the molecular level with other materials. The strength of the CO2 band in the areas where both, H2O- and carbon-bearing materials exist, gives support to the hypothesis that this volatile is formed on the surface of Iapetus as a product of irradiation of these two components. Finally, we also compare the Iapetus CO2 with that on other satellites confirming, that there are evident differences on the center, depth and width of the band on Iapetus and Phoebe, where CO2 has been suggested to be endogenous. 相似文献
9.
Studies of impacts (impactor velocity about 5 km s−1) on icy targets were performed. The prime goal was to study the response of solid CO2 targets to impacts and to find the differences between the results of impacts on CO2 targets with those on H2O ice targets. The crater dimensions in CO2 ice were found to scale with impact energy, with little dependence on projectile density (which ranged from nylon to copper, i.e., 1150-8930 kg m−3). At equal temperatures, craters in CO2 ice were the same diameter as those in water ice, but were shallower and smaller in volume. In addition, the shape of the radial profiles of the craters was found to depend strongly on the type of ice and to change with impact energy. The impact speed of the data is comparable to that for impacts on many types of icy bodies in the outer Solar System (e.g., the satellites of the giant planets, the cometary nuclei and the Kuiper Belt objects), but the size and thus energy of the impactors is lower. Scaling with impact energy is demonstrated for the impacts on CO2 ice. The issue of impact disruption (rather than cratering) is discussed by analogy with that on water ice. Expressions for the critical energy density for the onset of disruption rather than cratering are established for water ice as a function of porosity and silicate content. Although the critical energy density for disruption of CO2 ice is not established, it is argued that the critical energy to disrupt a CO2 ice body will be greater than that for a (non-porous) water ice body of the similar mass. 相似文献
10.
Carbon dioxide has been detected associated with Iapetus' dark material by the Cassini spacecraft. This CO2 may be primordial and/or resulting from ongoing production by photolysis of water-ice in the presence of carbonaceous material [Allamandola, L.J., Sandford, S.A., Valero, G.J., 1988. Icarus 76, 225-252]. Although any primordial CO2 would likely be complexed with the dark material and thus stable against thermal transport to Iapetus' poles [Buratti, B.J., and 28 colleagues, 2005. Astrophys. J. 622, L149-L152], active production of CO2 would result in some fraction of the CO2 being mobile enough to allow the accumulation of CO2 at Iapetus' poles. We develop a computer model to simulate ballistic transport of CO2 ice on Iapetus, accounting for Iapetus' gravitational binding energy and polar cold traps. We find that the residence time of CO2 ice outside the polar regions is very short; a sheet of CO2 ice near the equator of Iapetus decreases in thickness at a rate of 50 mm year−1. The sublimated CO2 will ballistically move around Iapetus until it reaches the polar cold traps where it can be sequestered for up to 15 years. If the total surface inventory of CO2 exceeds 3×107 kg, the polar ice cap will be permanent. While CO2 is moving around the surface, a small percentage will eventually reach escape velocity and be lost from the system. As such, a seasonal polar cap is lost at rate of 12% every solar orbit as the CO2 moves between the two polar cold traps. 相似文献
11.
Galina M. Chaban 《Icarus》2007,187(2):592-599
An absorption band at ∼4.26 μm wavelength attributed to the asymmetric stretching mode of CO in CO2 has been found on two satellites of Jupiter and several satellites of Saturn. The wavelength of pure CO2 ice determined in the laboratory is 4.2675 μm, indicating that the CO2 on the satellites occurs either trapped in a host material, or in a chemical or physical complex with other materials, resulting in a blue shift of the wavelength of the band. In frequency units, the shifts in the satellite spectra range from 3.7 to 11.3 cm−1. We have performed ab initio quantum chemical calculations of CO2 molecules chemically complexed with one, two, and more H2O molecules and molecules of CH3OH to explore the possibility that the blue shift of the band is caused by chemical complexing of CO2 with other volatile materials. Our computations of the harmonic and anharmonic vibrational frequencies using high levels of theory show a frequency shift to the blue by 5 cm−1 from pure CO2 to CO-H2O, and an additional 5 cm−1 from CO2-H2O to CO2-2H2O. Complexing with more than two H2O molecules does not increase the blue shift. Complexes of CO2 with one molecule of CH3OH and with one CH3OH plus one H2O molecule produce smaller shifts than the CO2-2H2O complex. Laboratory studies of CO2:H2O in a solid N2 matrix also show a blue shift of the asymmetric stretching mode. 相似文献
12.
In this paper we present the results of new experiments of ion irradiation of water ice deposited on top of a solid sulfurous residue to study the potential formation of SO2 at the interface ice/refractory material and discuss the possibility that this mechanism accounts for the sulfur dioxide ice detected on the surfaces of the Galilean satellites. In situ infrared spectroscopy was the used experimental technique. We have irradiated a thin film of H2O frost on a sulfurous layer with 200 keV of He+ at 80 K. The used sulfurous residue was obtained by irradiation of frozen SO2 at 16 K and it is used as a template of sulfur bearing solid materials. We have not found evidences of the efficient formation of SO2 after irradiation of H2O ice on top of the sulfurous residue. An upper limit to the production yield of SO2, of interface area for each 100 eV of energy absorbed in 1 cm3 of ice-covered residue, has been estimated. These results have relevance in the context of the surfaces of the icy Galilean satellites in which SO2 was detected. Our results show that radiolysis of mixtures of water ice and refractory sulfurous materials is not the primary formation mechanism responsible for the SO2 present on the surfaces of the Galilean satellites. 相似文献
13.
We present eight new 0.8 to 2.4 μm spectral observations of Neptune's satellite Triton, obtained at IRTF/SpeX during 2002 July 15-22 UT. Our objective was to determine how Triton's near-infrared spectrum varies as Triton rotates, and to establish an accurate baseline for comparison with past and future observations. The most striking spectral change detected was in Triton's nitrogen ice absorption band at 2.15 μm; its strength varies by about a factor of two as Triton rotates. Maximum N2 absorption approximately coincides with Triton's Neptune-facing hemisphere, which is also the longitude where the polar cap extends nearest Triton's equator. More subtle rotational variations are reported for Triton's CH4 and H2O ice absorption bands. Unlike the other ices, Triton's CO2 ice absorption bands remain nearly constant as Triton rotates. Triton's H2O ice is shown to be crystalline, rather than amorphous. Triton's N2 ice is confirmed to be the warmer, hexagonal, β N2 phase, and its CH4 is confirmed to be highly diluted in N2 ice. 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.
Interfacial liquid water has been hypothesized to form during the seasonal evolution of the dark dune spots observed in the high latitudes of Mars. In this study we assess the presence, nature and properties of ices - in particular water ice - that occur within these spots using HIRISE and CRISM observations, as well as the LMD Global Climate Model. Our studies focus on Richardson crater (72°S, 179°E) and cover southern spring and summer (LS=175-341°). Three units have been identified of these spots: dark core, gray ring and bright halo. Each unit show characteristic changes as the season progress. In winter, the whole area is covered by CO2 ice with H2O ice contamination. Dark spots form during late winter and early spring. During spring, the dark spots are located in a 10 cm thick depression compared to the surrounding bright ice-rich layer. They are spectrally characterized by weak CO2 ice signatures that probably result from spatial mixing of CO2 ice-rich and ice-free regions within pixels, and from mixing of surface signatures due to aerosols scattering. The bright halo shaped by winds shows stronger CO2 absorptions than the average ice-covered terrain, which is consistent with a formation process involving CO2 re-condensation. According to spectral, morphological and modeling considerations, the gray ring is composed of a thin layer of a few tens of μm of water ice. Two sources/processes could participate to the enrichment of water ice in the gray ring unit: (i) water ice condensation at the surface in early fall (prior to the condensation of a CO2-rich winter layer) or during wintertime (due to cold trapping of the CO2 layer) and (ii) ejection of dust grains surrounded by water ice by the geyser activity responsible for the dark spot. In any case, water ice remains longer in the gray ring unit after the complete sublimation of the CO2. Finally, we also looked for liquid water in the near-IR CRISM spectra using linear unmixing modeling but found no conclusive evidence for it. 相似文献
16.
The nature of cometary volatile materials is subject to debate. Theoretical models of cometary nuclei and laboratory studies suggest that these objects could be made of amorphous water ice in addition to other volatile molecules and refractory grains. This water ice structure has the ability to encapsulate the gases of surrounding environment, reflecting the physical and chemical conditions during their deposition. Therefore, the knowledge of the chemical composition of volatile molecules trapped in amorphous water ice provides a tool for probing the formation environment of cometary ice grains. Experimental studies of gas trapping efficiency in amorphous water ice have been previously conducted mostly under kinetic conditions, where dynamic pumping and temperature gradients prevented rigorous calibrations. In this work, we investigated the trapping efficiencies of Ar, CO, CH4, Kr and N2 by depositing water vapor as ice in the presence of trace gases in a volume submerged in liquid nitrogen at 77 K. The gas trapping efficiencies were determined simply by monitoring the pressure difference of the trace gases before and after the deposition of a known amount of water molecules as amorphous ice.Our results show that the trapped gas to water molecule ratio in amorphous ice is controlled primarily by the partial pressure of the gas during water ice deposition, and is independent of the ice deposition rate as well as the gas to water ratio in the vapor phase. The trapping efficiencies of gases decrease in the order of Kr > CH4 > CO > Ar > N2 in accordance with previous studies. Assuming that the water ice structure of comets is at least partially amorphous water ice at the time of their formation, these results suggest that the total pressure and composition of the surrounding environment of amorphous ice formation are significant controlling factors of trace gas concentrations in cometary ice. This further indicates that the evolution of the solar nebula and timing of cometary ice condensation can also be important parameters in linking the volatile contents of comets and their formation process. 相似文献
17.
New 0.8- to 2.4-μm spectral observations of the leading and trailing hemispheres of the uranian satellite Ariel were obtained at IRTF/SpeX during 2002 July 16 and 17 UT. The new spectra reveal contrasts between Ariel’s leading and trailing hemispheres, with the leading hemisphere presenting deeper H2O ice absorption bands. The observed dichotomy is comparable to leading-trailing spectral asymmetries observed among jovian and saturnian icy satellites. More remarkably, the trailing hemisphere spectrum exhibits three narrow CO2 ice absorption bands near 2 μm. This discovery of CO2 ice on one hemisphere of Ariel is its first reported detection in the uranian system. 相似文献
18.
We report a study on the broadband ultraviolet photolysis of methane-water ice mixtures, at low methane concentrations and temperatures relevant to the icy satellites of the outer Solar System. The photochemistry of these mixtures is dominated by the action of hydroxyl radicals on methane and the resulting products. This implies that, given sufficient exposure time, the methane will eventually be completely oxidized to carbon dioxide. The presence of methane inhibits the formation of hydrogen peroxide by serving as a trap for hydroxyl radicals. The distribution of photochemical products is broadly similar to that previously conducted using ion and electron sources, with some differences possibly attributable to the difference in radiation source. The results are applicable to a variety of icy bodies in the Solar System. On Enceladus, where methane mixed with water is measured in the plumes, methane in the surface ices is subject to oxidation and will eventually be converted to CO2. The CH stretch feature detected in the VIMS spectra of the Enceladus surface ice suggests that methane is currently being supplied to the surface ice, likely from re-condensation of the plume gas. 相似文献
19.
Using new laboratory spectra, we have calculated the real and imaginary parts of the index of refraction of amorphous and crystalline H2O-ice from 20 to 150 K in the frequency range 9000-3800 cm−1 (1.1-2.6 μm) at a spectral resolution of 1 cm−1. These optical constants can be used to create model spectra for comparison to spectra from Solar System objects. We also analyzed the differences between the amorphous and crystalline H2O-ice spectra, including weakening of bands and shifting of bands to shorter wavelength in amorphous H2O-ice spectra. We have also observed two spectrally distinct phases of amorphous H2O-ice. 相似文献
20.
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