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1.
C.Y. Robert WuD.L. Judge Bing-Ming ChengWen-Hao Shih Tai-Sone YihW.H. Ip 《Icarus》2002,156(2):456-473
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
2.
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
3.
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
4.
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
5.
The neutral gas environment of a comet is largely influenced by dissociation of parent molecules created at the surface of the comet and collisions of all the involved species. We compare the results from a kinetic model of the neutral cometary environment with measurements from the Neutral Mass Spectrometer and the Dust Impact Detection System onboard the Giotto spacecraft taken during the fly-by at Comet 1P/Halley in 1986. We also show that our model is in good agreement with contemporaneous measurements obtained by the International Ultraviolet Explorer, sounding rocket experiments, and various ground based observations.The model solves the Boltzmann equation with a Direct Simulation Monte Carlo technique (Tenishev, V., Combi, M., Davidsson, B. [2008]. Astrophys. J. 685, 659-677) by tracking trajectories of gas molecules and dust grains under the influence of the comet’s weak gravity field with momentum exchange among particles modeled in a probabilistic manner. The cometary nucleus is considered to be the source of dust and the parent species (in our model: H2O, CO, H2CO, CO2, CH3OH, C2H6, C2H4, C2H2, HCN, NH3, and CH4) in the coma. Subsequently our model also tracks the corresponding dissociation products (H, H2, O, OH, C, CH, CH2, CH3, N, NH, NH2, C2, C2H, C2H5, CN, and HCO) from the comet’s surface all the way out to 106 km.As a result we are able to further constrain cometary the gas production rates of CO (13%), CO2 (2.5%), and H2CO (1.5%) relative to water without invoking unknown extended sources. 相似文献
6.
Mikio Shimizu 《Astrophysics and Space Science》1975,36(2):353-361
Dirty ice of a second kind (major components, H2O, CO, and N2; minor components less than several percents, NH3, CH4, and other organic substances such as HCN, CH3CN etc.) is assumed for the composition of volatiles in the cometary nucleus. The consistency with the observations of molecular ions and daughter molecules in the cometary atmosphere is argued by taking into account various ion-molecular reactions and dissociative recombinations. There is a satisfactory agreement for the second kind of dirty-ice model, but the presence of large amounts of CH4 and NH3 is found to be rather in contradiction with observational evidence. A velocity of 8 km s?1 for the hydrogen atoms, derived from analysis of the hydrogen Lyman-alpha corona around comets, is found from the dissociative recombination of H3O+, the dominant constituent of cometary ionosphere, in accordance with H3O++e ?→OH+H+H. 相似文献
7.
High resolution infrared spectra of Comet C/1995 O1 (Hale-Bopp) were obtained during 2-5 March 1997 UT from the NASA Infrared Telescope Facility on Mauna Kea, Hawaii, when the comet was at r≈1.0 AU from the Sun pre-perihelion. Emission lines of CH4, C2H6, HCN, C2H2, CH3OH, H2O, CO, and OH were detected. The rotational temperature of CH4 in the inner coma was Trot=110±20 K. Spatial profiles of CH4, C2H6, and H2O were consistent with release solely from the nucleus. The centroid of the CO emission was offset from that of the dust continuum and H2O. Spatial profiles of the CO lines were much broader than those of the other molecules and asymmetric. We estimate the CO production rate using a simplified outflow model: constant, symmetric outflow from the peak position. A model of the excitation of CO that includes optical depth effects using an escape probability method is presented. Optical depth effects are not sufficient to explain the broad spatial extent. Using a parent+extended-source model, the broad extent of the CO lines can be explained by CO being produced mostly (∼90% on 5 March) from an extended source in the coma. The CO rotational temperature was near 100 K. Abundances relative to H2O (in percent) were 1.1±0.3 (CH4), 0.39±0.10 (C2H6), 0.18±0.04 (HCN), 0.17±0.04 (C2H2), 1.7±0.5 (CH3OH), and 37-41 (CO, parent+extended source). These are roughly comparable to those obtained for other long-period comets also observed in the infrared, though CO appears to vary. 相似文献
8.
In this paper we study the effect of shock waves on the chemical structure of the interstellar clouds. A model of molecular cloud has been assumed. The chemistry is investigated in a time dependent model. Our chemical network contains 56 species in 251 reactions to including molecules of the elements H, O, C, N, S, and Si.The results indicate that the calculated fractional abundance of the molecules NS, H2O, CN, NH, CO, and SO agrees well with the observations. The molecules OH, H2S, CS, H2CS, HS, NO, SiO, CH, CH2, CH3, HCO, C2, and HCN reach high post-shock abundances. 相似文献
9.
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
10.
We present near-IR spectra of solid CO2 in H2O and CH3OH, and find they are significantly different from that of pure solid CO2. Peaks not present in either pure H2O or pure CO2 spectra become evident when the two are mixed. First, the putative theoretically forbidden CO2 (2ν3) overtone near 2.134 μm (4685 cm−1), that is absent from our spectrum of pure solid CO2, is prominent in the spectra of H2O/CO2=5 and 25 mixtures. Second, a 2.74-μm (3650 cm−1) dangling OH feature of H2O (and a potentially related peak at 1.89 μm) appear in the spectra of CO2-H2O ice mixtures, but are probably not diagnostic of the presence of CO2. Other CO2 peaks display shifts in position and increased width because of intermolecular interactions with H2O. Warming causes some peak positions and profiles in the spectrum of a H2O/CO2=5 mixture to take on the appearance of pure CO2. Absolute strengths for absorptions of CO2 in solid H2O are estimated. Similar results are observed for CO2 in solid CH3OH. Since the CO2 (2ν3) overtone near 2.134 μm (4685 cm−1) is not present in pure CO2 but prominent in mixtures, it may be a good observational (spectral) indicator of whether solid CO2 is a pure material or intimately mixed with other molecules. These observations may be applicable to Mars polar caps as well as outer Solar System bodies. 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.
H. Levy 《Planetary and Space Science》1973,21(4):575-591
Altitude profiles for the number densities of NO, NO2, NO3, N2O5, HNO2, CH3O, CH3O2, H2CO, OH, and HO2 are calculated as a function of time of day with a steady-state photochemical model in which the altitude profiles for the number densities of H2O, CH4, H2, CO, O3, and the sum of NO and NO2 are fixed at values appropriate to a summer latitude of 34°. Average daily profiles are calculated for the long-lived species, HNO3, H2O2, and CH3O2H.The major nitrogen compound HNO3 may have a number density approaching 5 × 1011 molecules cm?3 at the surface, although an effective loss path due to collisions with particulates could greatly reduce this value.The number density of OH remains relatively unchanged in the first 6 km and reaches 1 × 107 molecules cm?3 at noon, while the number density of HO2 decreases throughout the lower troposphere from its noontime value of 8 × 108 molecules cm?3 at the surface.H2O2 and H2CO both have number densities in the ppb range in the lower troposphere.Owing to decreasing temperature and water concentration, the production of radicals and their steady-state number densities decrease with altitude, reaching a noontime minimum of 1 × 108 molecules cm?3 for OH and 3 × 107 molecules cm?3 for HO2 at the tropopause. The related minor species show even sharper decreases with increasing altitude.The primary path for interconverting OH and HO2 serves as the major sink for CO and leads to a tropospheric lifetime for CO of ~0.1 yr.Another reaction cycle, the oxidation of CH4, is quite important in the lower troposphere and leads to the production of H2CO along with the destruction of CH4 for which a tropospheric lifetime of ~2 yr is estimated.The destruction of H2CO that was produced in the CH4 oxidation cycle provides the major source of CO and H2 in the atmosphere. 相似文献
13.
Karen Magee-Sauer Michael J. Mumma Neil Dello Russo Boncho P. Bonev G.L. Villanueva 《Icarus》2008,194(1):347-356
We used the NIRSPEC instrument on the Keck-2 telescope atop Mauna Kea, HI to observe Comet C/2001 A2 (LINEAR) in a Target of Opportunity campaign on UT 2001 July 9.5, 10.5 August 4.4, 10.5. We measured seven organic parent volatiles (C2H6, C2H2, HCN, CH4, CO, CH3OH, H2CO) simultaneously with H2O. We obtained absolute production rates and relative abundances for parent volatiles, and also measured rotational temperatures for several of these species. The chemical composition of C/2001 A2 differs substantially from any comet we have observed to date. The abundances we measure (relative to H2O) for C2H6, C2H2, HCN, and CH3OH are enriched by a factor of ∼2 to 3 in C/2001 A2 compared with most comets in our database. Other molecular species were detected within the typical range of measured abundances. C/2001 A2 presented a unique opportunity to study the chemistry of a fragmenting comet where pristine areas are exposed to the Sun. 相似文献
14.
Synthetic spectra of Comet Halley between 2.5 and 15 μm are calculated on the basis of current cometary models. This study shows that molecules which are most likely detectable in the infrared range include H2O, CO, CO2, CH4, NH3, N2H4, and H2CO. The flux emitted by these molecules, mostly due to resonant scattering, should be sufficient for detection by a flyby mission. 相似文献
15.
A. G. Yeghikyan 《Astrophysics》2011,54(1):87-99
The fluxes of cosmic rays inside typical molecular clouds are calculated. Protons and a-particles with energies of 1 MeVd
≤ ? ≤ 10 GeV penetrate deeply enough to produce irradiation doses in the ice mantle of dust particles on the order of 0.1-1
eV/amu over the 10–50 million year lifetime of clouds with and without star formation regions. The possible use of these results
for interpreting laboratory experiments on the irradiation of ice mixtures of the type H2O:CH3OH:NH3:CO is discussed. Complex organic radiolysis products may play an important role in the prebiological evolution of the dust
component of molecular clouds. 相似文献
16.
One-dimensional radial models of the chemistry in cometary comae have been constructed for heliocentric distances ranging from 2 to 0.125 AU. The coma's opacity to solar radiation is included and photolytic reaction rates are calculated. A parent volatile mixture similar to that found in interstellar molecular clouds is assumed. Profiles through the coma of number density and column density are presented for H2O, OH, O, CN, C2, C3, CH, and NH2. Whole-coma abundances are presented for NH2, CH, C2, C3, CN, OH, CO+, H2O+, CH+, N2+, and CO2+. 相似文献
17.
Nalin R. Mukherjee 《Earth, Moon, and Planets》1975,14(1):169-186
The solar wind interacts directly with the lunar surface material resulting in an essentially complete absorption of the corpuscles producing no upstream bowshock but a cavity downstream from the Moon. The main source of most neutral species of the atmosphere, except probably40Ar, is the solar-wind interaction products. The other sources which appear to be minor contributors to the atmosphere are the interaction products of cosmic rays, planetary degassing, effects of meteorite impacts and radioactive decays. Most of the hydrogen atoms derived from the solar-wind protons contribute to the atmosphere as hydrogen molecules rather than atoms. Only on the basis of the solar-wind protons, alpha particles and ions of oxygen and carbon, the atmospheric species concentration (cm–3) near the lunar surface at 300K are as follows: H2 3.3 to 9.9 × 103; He 2.4 to 4.7 × 103; H 3.7; OH 0.25; H2O 0.24; and O2, O, CO, CO2 and CH4 in concentrations smaller than H2. Whatever the source, the OH and H2O concentrations in the atmosphere are about the same. The calculated concentrations are in good agreement with the observations by the Apollo 17 lunar surface mass spectrometer and the Apollo 17 orbital UV spectrometer. At the time of sample collection from the Moon, the hydrogen content in the trapped gas layer of the lunar surface material was partly as hydrogen atoms and partly as hydrogen molecules, but at the time of sample analysis hydrogen was mostly in molecular form. The H2O content at the time of sample analysis was only a few parts per million by weight.Paper presented at the Conference on Interactions of the Interplanetary Plasma with the Modern and Ancient Moon, sponsored by the Lunar Science Institute, Houston, Texas and held at the Lake Geneva Campus of George Williams College, Wisconsin, between September 30 and October 4, 1974. 相似文献
18.
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
19.
《Physics and Chemistry of the Earth, Part C: Solar, Terrestrial & Planetary Science》1999,24(5):427-434
SCIAMACHY (SCanning Imaging Absorption spectrometer for Atmospheric CHartographY) is a space based spectrometer designed to measure sunlight transmitted, reflected and scattered by the Earth atmosphere or surface. It is a contribution to the Envisat-1 satellite to be launched in late 1999.SCIAMACHY measurements will provide amounts and distribution of 03, BrO, OCl0, ClO, S02, H2CO, N02, CO, CO2, CH4, H2O, N20, pressure, temperature, aerosol, radiation, cloud cover and cloud top height from atmospheric measurements in nadir, limb and occultation geometry.By the combination of the near simultaneous limb and nadir observations SCIAMACHY is one of a limited number of instruments which is able to detect tropospheric column amounts of 03, N02, CO, CH4, H2O, N20, S02, H2CO, and BrO down to the planetary boundary layer under cloud free conditions. 相似文献
20.
A detailed theoretical study of the potential energy surface of H2CO3 is explored at the CCSD(T)//B3LYP/aug-cc-pVTZ level. On the potential energy surface, 12 isomers of H2CO3 are located. Their molecular properties such as geometries, vibrational frequencies, rotational constants, dipole moments, gas-phase acidities, and relative energies are calculated. Various reaction pathways and decomposition products have also been discussed. Among these products, CO2 and H2O are definitely the most favorable products with predominant abundance. Large energy barriers are predicted for other dissociation channels leading to the formation of oxygen, formaldehyde, and so on. These high energy channels are not important thermodynamically and kinetically, but they might occur in the presence of cosmic rays in astronomic environments. From the present work we suggest that chemical reactions between CO2 and H2O at the polar ice caps could be a potential source of H2CO and O2, in addition to the previously proposed mechanisms, i.e., the oxidation of methane and cosmic-ray-mediated production through the intermediate H2CO3. The results of the present work may provide useful data to improve our understanding of icy chemistry at the polar caps on Mars. 相似文献