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1.
At present, there are few laboratory spectra of analogs of astrophysical interest in the far-infrared range (FIR). Laboratory infrared (IR) spectra of simple ices and its mixtures obtained at low temperature and pressure are found mainly up to 25 μm, and few up to 200 μm. On the other hand, there are some spectra for carbonaceous material and silicates up to 2000 μm. Our laboratory is equipped with an IR spectrometer that integrates a Michelson interferometer with a resolution better than 0.25 cm−1 and that operates under vacuum conditions of 10−1 mbar. There is also a silicon bolometer, a very high-sensitivity detector in comparison with the standard deuterated triglycine sulfate (DTGS) detectors. The use of the bolometer and the possibility of working under vacuum conditions inside the optics and the sample compartment of the spectrometer allow obtaining high-sensitivity spectra free from H2O vapor and CO2 gas bands. Those conditions are necessary to obtain high-quality spectra in the FIR where absorption bands are much less intense than those in the mid-IR region. In our laboratory there is also a high-vacuum chamber that allows different studies on ices deposited onto a cold finger. We have already carried out experiments on the study of ice density as a function of temperature, UV irradiation of ices, temperature-programmed desorption (TPD) and UV-vis reflectance. In this work, we present the design of the experimental setup we are building to carry out different experiments simultaneously on the same ice sample, including spectra measurements in the mid-IR range (MIR) and the FIR. This design integrates jointly the IR spectrometer, the high-vacuum chamber and the silicon bolometer. Lastly, we show a spectrum we have obtained of a solid of astrophysical interest such as crystalline forsterite grains by using the polyethylene pellet technique. 相似文献
2.
Near the inner edge of the Edgeworth-Kuiper Belt (EKB) are Pluto and Charon, which are known to have N2- and H2O-dominated surface ices, respectively. Such non-polar and polar ices, and perhaps mixtures of them, also may be present on other trans-Neptunian objects. Pluto, Charon, and all EKB objects reside in a weak, but constant UV-photon and energetic ion radiation environment that drives chemical reactions in their surface ices. Effects of photon and ion processing include changes in ice composition, volatility, spectra, and albedo, and these have been studied in a number of laboratories. This paper focuses on ice processing by ion irradiation and is aimed at understanding the volatiles, ions, and residues that may exist on outer solar system objects. We summarize radiation chemical products of N2-rich and H2O-rich ices containing CO or CH4, including possible volatiles such as alcohols, acids, and bases. Less-volatile products that could accumulate on EKB objects are observed to form in the laboratory from acid-base reactions, reactions promoted by warming, or reactions due to radiation processing of a relatively pure ice (e.g., CO → C3O2). New IR spectra are reported for the 1–5 mu;m region, along with band strengths for the stronger features of carbon suboxide, carbonic acid, the ammonium and cyanate ions, polyoxymethylene, and ethylene glycol. These six materials are possible contributors to EKB surfaces, and will be of interest to observers and future missions. 相似文献
3.
Infrared spectra of Io in the region 2.5-5.0 micrometers, including new observational data, are analyzed using detailed laboratory studies of plausible surface ices. Besides the absorption bands attributable to sulfur dioxide frosts, four infrared spectral features of Io are shown to be unidentified. These unidentified features show spatial and temporal band strength variations. One pair is centered around 3.9 micrometers (3.85 and 3.91 micrometers) and the second pair is centered around 3.0 micrometers (2.97 and 3.15 micrometers). These absorptions fall close to the fundamental stretching modes in H2S and H2O, respectively. The infrared absorption spectra of an extensive set of laboratory ices ranging from pure materials, to binary mixtures of H2S and H2O (either mixed at different concentrations or layered), to H2O:H2S:SO2 mixtures are discussed. The effects of ultraviolet irradiation (120 and 160 nm) and temperature variation (from 9 to 130 K) on the infrared spectra of the ices are examined. This comparative study of Io reflectance spectra with the laboratory mixed ice transmission data shows the following: (1) Io's surface most likely contains H2S and H2O mixed with SO2. The 3.85- and 3.91-micrometers bands in the Io spectra can be accounted for by the absorption of the S-H stretching vibration (nu 1) in H2S clusters and isolated molecules in an SO2-dominated ice. The weak 2.97- and 3.15-micrometers bands which vary spatially and temporally in the Io spectra coincide with the nu 3 and nu 1 O-H stretching vibrations of clusters of H2O molecules complexed, through hydrogen bonding and charge transfer interactions, with SO2. (2) The observations are well matched qualitatively by the transmission spectra of SO2 ices containing about 3% H2S and 0.1% H2O which have been formed by the condensation of a mixture of the gases onto a 100 K surface. (3) No new features are produced in the region 2.5 to 5.0 micrometers in the spectrum of these ices under prolonged ultraviolet irradiation or temperature variation up to 120 K. (4) Comparison of the Io spectra to transmission spectra of both mixed molecular ices and layered ices indicates that only the former can explain the shifts and splitting of the absorption bands seen in the Io spectrum and additionally can account for the fact that solid H2S is observed in the surface material of Io at temperature and pressure conditions above the sublimation point of pure H2S. 相似文献
4.
Despite considerable observational information on infrared absorption and emission spectra of interstellar matter, together
with extensive laboratory data on the spectra of possible constituents, there are presently few firm identifications of individual
chemical components. Simple chemical compounds including H2O, CO and CO2 are detected in dust in dark interstellar clouds, but other molecules such as ethane, acetylene and benzene do not appear
to be present with appreciable abundance. Nevertheless, IR spectra show that hydrocarbons are aubiquitous component of interstellar
matter. The nature of these materials and their relation to specific molecular components is discussed in this paper.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
5.
J. R. Martirosian 《Astrophysics》2007,50(3):301-310
This is a statistical study of galaxies with a UV excess. A sample of 702 Kazarian galaxies (KG) is used. The KGs are identified
with objects from the MAPS, IRAS FSC, IRAS PSC, and NVSS catalogs. The O and E magnitudes are known for more than 92% of the
KGs. It is shown that the KG sample is complete up to 16m.0 in the blue and to 16m.5 in the red. More than 36% of the KGs
are identified with infrared (IR) sources at wavelengths of 12, 25, 60, and 100 m. Calculations of the far IR (FIR) luminosities
show that 4% of the KGs are strong FIR emitters (LFIR ≥ 1011 L⊙). More than 32% of the KGs have been identified as radio sources at a frequency of 1.4 GHz. A determination of the radio
luminosities shows that the sample of KGs with known radial velocities include one powerful, LR ≥ 1025 W/Hz, radio object (Kaz 273) which is a BL Lac object. A close correlation (r=0.93) is bound between the FIR and radio luminosities
for galaxies with a UV excess. An examination of the relationship between the FIR and radio luminosities for galaxies in different
spectral classes shows that the correlation coefficient is higher (r=0.99) and the slope of the fit curve is larger (a=1.18)
for Seyfert galaxies. Calculations of the logarithm of the ratio of the FIR and radio fluxes indicate that the sample includes
4 KGs with a radio excess, while there are none with an IR excess.
__________
Translated from Astrofizika, Vol. 50, No. 3, pp. 369–379 (August 2007). 相似文献
6.
Until recently far infrared (FIR) observations of galaxies were limited to about a dozen bright and/or active galaxies. New photometric data has become available from Infrared Astronomical Satellite (IRAS) on 33 galaxies (most of them faint) from IRAS Circular Nos. 1, 2, and 3 The FIR spectra of these galaxies are similar. The far infrared fluxF
FIR in the wavelength interval 9–118 m of the brighter galaxies is seen to be correlated with the integrated optical magnitudeB
T
0
. The 12 and 25 m fluxes of these galaxies exhibit the same dependence onB
T
0
as the 10 and 21 m fluxes from Seyferts and other emission-line galaxies. This suggests that the galaxies detected by IRAS are some type of active galaxies in accord with the high percentage of these galaxies predicted by Lock and Rowan-Robinson (1983). 相似文献
7.
J. M. Greenberg 《Astrophysics and Space Science》1986,128(1):17-31
The observations of dust gas in diffuse and molecular clouds are shown to reflect not only their current state but their past history. The interpretation of infrared spectra of dust in molecular clouds using appropriate core-mantle grains shows that: (1) the kinds and amounts of ices, (2) the relative proportion of such important interstellar molecules as H2O and CO, (3) the evidence for the less abundant solid species X–CN, COS, H2S, and (4) the thermal history of the dust may all be demonstrated quantitatively from laboratory analog studies of ultraviolet photoprocessing of relevant ices and from theoretical studies of gas-dust interactions. In diffuse clouds the dust is shown to consist predominantly of refractory organic compounds which originate as residues of the photoprocessing of volatile ices in molecular clouds and which undergo further physical and chemical evolution in the diffuse clouds.Review paper presented at a Workshop on The Role of Dust in Dense Regions of Interstellar Matter, held at Georgenthal, G.D.R., in March 1986. 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.
To settle the question of disulfur monoxide and sulfur monoxide deposition and occurrence on Io's surface, we performed series of laboratory experiments reproducing the condensation of S2O at low temperature. Its polymerization has been monitored by recording infrared spectra under conditions of temperature, pressure, mixing with SO2 and UV-visible radiation simulating that of Io's surface. Our experiments show that S2O condensates are not chemically stable under ionian conditions. We also demonstrate that SO and S2O outgassed by Io's volcanoes and condensing on Io's surface should lead to yellow polysulfuroxide deposits or to white deposits of S2O diluted in sulfur dioxide frost (i.e., S2O/SO2 < 0.1%). Thus S2O condensation cannot be responsible for the red volcanic deposits on Io. Comparison of the laboratory infrared spectra of S2O and polysulfuroxide with NIMS/Galileo infrared spectra of Io's surface leads us to discuss the possible identification of polysulfuroxide. We also recorded the visible transmission spectra of sulfur samples resulting from polysulfuroxide decomposition. These samples consist in a mixture of sulfur polymer and orthorhombic sulfur. Using the optical constants extracted from these measurements, we show that a linear combination of the reflectance spectra of our samples, the reflectance spectrum of orthorhombic S8 sulfur and SO2 reflectance spectrum, leads to a very good matching of Io's visible spectrum between 330 and 520 nm. We conclude then that Io's surface is probably mainly composed of sulfur dioxide and a mixture of sulfur S8 and sulfur polymer. Some polysulfuroxide could also co-exist with these dominant components, but is probably restricted to some volcanic areas. 相似文献
10.
A. ROTUNDI J. R. BRUCATO L. COLANGELI G. FERRINI V. MENNELLA E. PALOMBA P. PALUMBO 《Meteoritics & planetary science》2002,37(11):1623-1635
Abstract— The laboratory analyses of cosmic dust analogues—that in the context of this paper include interstellar, circumstellar as well as cometary dust—have a critical role in the study of circumstellar and cometary dust. The morphological, structural and chemical characterization of these analogues are critical for comparisons of their infrared and ultraviolet spectra with those obtained by astronomical observations, as well as for modeling purposes. Besides, the results from these laboratory studies are important to the success of space missions to comets when testing and calibrating the payload instruments. The interpretations of returned scientific data would benefit from the comparison with data recorded by the instruments in a laboratory setting for different classes of previously characterized analogues. We produced various types of condensed samples: (1) Mg,Fe‐silicates, (olivine, pyroxene), (2) carbon‐rich dust, and (3) mixed carbon‐silicate dust. The samples were prepared using different techniques, viz. (1) laser bombardment of solid targets in an Ar and O2 atmosphere, (2) arc discharge in an Ar and H2 atmosphere, and (3) grinding powders of natural minerals. We simulated various post‐condensation processes, such as thermal annealing, ultraviolet irradiation, ion bombardment and exposure to atomic hydrogen. These processes produced compound samples of a wide range of physico‐chemical properties. To identify their textures, morphologies, grain compositions and crystallographic properties we used electron microscopy and far‐ultraviolet to far‐infrared (millimeter range) spectroscopy. 相似文献
11.
Infrared spectra and radiation chemical behavior of N2-dominated ices relevant to the surfaces of Triton and Pluto are presented. This is the first systematic IR study of proton-irradiated N2-rich ices containing CH4 and CO. Experiments at 12 K show that HCN, HNC, and diazomethane (CH2N2) form in the solid phase, along with several radicals. NH3 is also identified in irradiated N2 + CH4 and N2 + CH4 + CO. We show that HCN and HNC are made in irradiated binary ice mixtures having initial N2/CH4 ratios from 100 to 4, and in three-component mixtures have an initial N2/(CH4 + CO) ratio of 50. HCN and HNC are not detected in N2-dominated ices when CH4 is replaced with C2H6, C2H2, or CH3OH.The intrinsic band strengths of HCN and HNC are measured and used to calculate G(HCN) and G(HNC) in irradiated N2 + CH4 and N2 + CH4 + CO ices. In addition, the HNC/HCN ratio is calculated to be ∼1 in both icy mixtures. These radiolysis results reveal, for the first time, solid-phase synthesis of both HCN and HNC in N2-rich ices containing CH4.We examine the evolution of spectral features due to acid-base reactions (acids such as HCN, HNC, and HNCO and a base, NH3) triggered by warming irradiated ices from 12 K to 30-35 K. We identify anions (OCN−, CN−, and N3−) in ices warmed to 35 K. These ions are expected to form and survive on the surfaces of Triton and Pluto. Our results have astrobiological implications since many of these products (HCN, HNC, HNCO, NH3, NH4OCN, and NH4CN) are involved in the syntheses of biomolecules such as amino acids and polypeptides. 相似文献
12.
Identification of astronomical dust composition rests on comparison of Infrared (IR) spectra with standard laboratory spectra; frequently, however, a single mineralogical composition is assumed for spectral matching. Advances in laboratory instrumentation have enabled very precise IR spectra to be measured on single grains and zones within grains; with a more complete set of spectral data for planetary dust, better compositional matches will be achieved for astronomical dust. We have compared several FT-IR spectroscopy techniques (open path transmission spectroscopy and diffuse reflectance spectroscopy of powders; microspectroscopy of single grains and powders and ATR spectroscopy of thin sections) to determine their utility for the direct measurement of the mid-IR spectra of small amounts of extraterrestrial grains. We have focussed our investigation on the spectra of the olivine series of silicates, (Mg,Fe)2SiO4, a species frequently identified as one of the major constituents of interstellar dust. The positions of three characteristic SiO4 stretching bands at ∼10.4, 11.3 and 12 μm were measured for comparison of the techniques. All methods gave satisfactory results, although care must be taken to guard against artefacts from sample thickness and orientation effects. Single grains hand-picked from meteorites can be analysed, but results are inaccurate if the grain size is too large (>1-10 μm). Spectra for single grains also show variations that arise from sample orientation effects. Once the analytical artefacts are taken into account, we found that measurement of powder with a diamond compression cell is best suited for the analysis of small amounts of materials. 相似文献
13.
Optical constants in a broad temperature and wavelength range are important input parameters in radiative transfer models used in studies of planetary atmospheres. In the laboratory, the refractive index values of ices at the HeNe laser wavelength (632.8 nm) are often used to monitor the growth rate and thickness of ice films. In this report we present laboratory measurements determining the refractive index at 632.8 nm of ammonia and hydrocarbon ices in the temperature range 80-100 K. Thin ice films are vapor-deposited on a cryogenically cooled mirror located inside a high-vacuum apparatus. The real component of the refractive index of these ice films is determined by a two-angle interferometric technique. Optical modeling calculations of the transmittance and reflectance through the thin ice films assist in the interpretation of the experimental results. We discuss our results and compare them with other measurements available in the literature. The results reported here are relevant to the spectroscopy of icy objects in the solar system; they are needed to perform laboratory characterization of ices, derive optical constants, and model spectra. 相似文献
14.
K. V. K. Iyengar 《Journal of Astrophysics and Astronomy》1985,6(4):227-231
As dust emission in the far infrared (FIR) is a characteristic property of planetary nebulae we searched the Infrared Astronomical
Satellite (IRAS) point-source catalogue for confirmatory evidence on the two new possible planetary nebulae S 68 and 248 -
5 identified by Fesen, Gull & Heckathorn (1983) and the high-excitation planetary nebula 76 + 36 detected by Sanduleak (1983).
We identify the nebulae 248 - 5 and 76 + 36 with IRAS sources 07404 - 3240 and 17125 + 4919, respectively and have determined
their dust temperature, total FIR emission and optical depth. We also set a lower limit ranging in value from 1.2 × 10-6 to 3.7 × 10-5 forM
dust
/M
bd
of the nebula 248 - 5 depending on whether its grain material is silicate or graphite. S 68 could not be identified with
an IRAS source. 相似文献
15.
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
16.
Both laboratory measurements and theory indicate that CO2 should be a common component in interstellar ices. We show that the exact band position, width, and profile of the solid-state 12CO2 infrared bands near 3705, 3600, 2340, and 660 cm-1 (2.70, 2.78, 4.27, and 15.2 micrometers) and the 13CO2 band near 2280 cm-1 (4.39 micrometers) are dependent on the matrix in which the CO2 is frozen. Measurements of these bands in astronomical spectra can be used to determine column densities of solid-state CO2 and provide important information on the physical conditions present in the ice grains of which the CO2 is a part. Depending on the composition of the ice, the CO2 asymmetric stretching band was observed to vary from 2328.7 to 2346.0 cm-1 and have full widths at half-maxima (FWHMs) ranging from 4.7 to 29.9 cm-1. The other CO2 bands showed similar variations. Both position and width are also concentration dependent. Absorption coefficients were determined for the five CO2 bands. These were found to be temperature independent for CO2 in CO and CO2 matrices but varied slightly with temperature for CO2 in H2O-rich ices. For all five bands this variation was found to be less than 15% from 10 to 150 K, the temperature at which H2O ice sublimes. A number of parameters associated with the physical behavior of CO2 in CO2- and H2O-rich ices were also determined. The CO2-CO2 surface binding energy in pure CO2 ices is found to be (delta Hs/k) = 2690 +/- 50 K. CO2-H2O and CO-H2O surface binding energies were determined to be (delta Hs/k) = 2860 +/- 200 K and 1740 +/- 100 K, respectively. Under our experimental conditions, CO2 condenses in measurable quantities into H2O-rich ices at temperatures up to 100 K, only slightly higher than the temperature at which pure CO2 condenses. Once frozen into an H2O-rich ice, the subsequent loss of CO2 upon warming is highly dependent on concentration. For ices with H2O/CO2 > 20, the CO is physically trapped within the H2O lattice, and little CO2 is lost until the sublimation temperature of the H2O matrix is reached. In contrast, in ices having H2O/CO2 < 5, the CO2 remains only to temperatures of about 90 K. Above this point the CO2 readily diffuses out of the H2O matrix. These results suggest that two different forms of H2O lattice are produced. The implications of these data for cometary models and our understanding of cometary formation are considered. 相似文献
17.
Moroz L. V. Baratta G. Distefano E. Strazzulla G. Starukhina L. V. Dotto E. Barucci M. A. 《Earth, Moon, and Planets》2003,92(1-4):279-289
Trans-Neptunian Objects (TNOs) and Centaurs show remarkable colour variationsin the visual and near-infrared spectral regions. Surface alteration processes such asspace weathering (e.g., bombardment with ions) and impact resurfacingmay play an important role in the colour diversity of such bodies. Ion irradiation ofhydrocarbon ices and their mixtures with water ice transforms neutral (grey) surfacecolours of ices to red and further to grey. Along with the ices, TNOs and Centaursprobably contain complex carbonaceous compounds, in particular, complexhydrocarbons. Unlike ices, such refractory organic materials have originally lowvisual albedos and red colours in the visible and near-infrared ranges. Here wepresent the first results of ion irradiation experiments on asphaltite. Asphaltite isa natural complex hydrocarbon material. The reflectance spectra of asphaltite inthe 0.4–0.8 μm range have been recorded before irradiation and after eachirradiation step. We demonstrate that irradiation of this red dark material with30 keV H+ and 15 keV N+ ions gradually transforms its colour from redto grey as a result of carbonization. A moderate increase in the visual albedo hasbeen observed. These results may imply that the surfaces of primitive red objectsoptically dominated by complex refractory organics may show a similar spaceweathering trend. Our laboratory results were compared with published coloursof TNOs and Centaurs. A broad variety of spectral colours observed for TNOs andCentaurs may be reproduced by various spectra of irradiated organics correspondingto different ion fluences. However, such objects probably also contain ices and silicatecomponents which show different space weathering trends. This fact, together with alack of information about albedos, may explain difficulties to reveal correlations between surface colours within TNO and Centaur populations and their other properties, such as absolute magnitudes and orbital parameters. 相似文献
18.
《Planetary and Space Science》1999,47(10-11):1371-1376
Implantation of reactive ions into targets of planetary interest is a relevant subject to be studied in the laboratory. It could in fact produce new molecular species that are not native to those surfaces. Presented here are new laboratory results obtained by nitrogen implantation (15–30 keV N+) on frozen mixtures of H2O:CH4 (2:1). These species have been chosen in view of their possible presence on the surface of Jovian and Saturnian satellites and rings. In fact these surfaces are exposed to intense irradiation by magnetospheric and/or solar energetic particles. The laboratory investigation utilizes IR spectroscopy. The main objectives of the present study are to identify newly produced species and to verify if these (or at least if the profile of their IR bands) are different from those produced by unreactive ions impinging on targets in which nitrogen is already present, occurring in the form of frozen NH3 (Strazzulla and Palumbo, 1998) or N2 (Palumbo et al., 1999). I find that CN-bearing group is in fact formed and its IR feature has a profile (peak position and band profile) that differs from that obtained after irradiation or frozen gases containing nitrogen. The relevance the results might have to elucidate the origin of some species observed on Jovian icy moons or predicted to be observed on Saturnian satellites are outlined. 相似文献
19.
Dale P. Cruikshank Tobias C. Owen Thomas R. Geballe Catherine de Bergh Francois Poulet Joshua P. Emery 《Icarus》2005,175(1):268-283
We present spectra of Saturn's icy satellites Mimas, Enceladus, Tethys, Dione, Rhea, and Hyperion, 1.0-2.5 μm, with data extending to shorter (Mimas and Enceladus) and longer (Rhea and Dione) wavelengths for certain objects. The spectral resolution (R=λ/Δλ) of the data shown here is in the range 800-1000, depending on the specific instrument and configuration used; this is higher than the resolution (R=225 at 3 μm) afforded by the Visual-Infrared Mapping Spectrometer on the Cassini spacecraft. All of the spectra are dominated by water ice absorption bands and no other features are clearly identified. Spectra of all of these satellites show the characteristic signature of hexagonal H2O ice at 1.65 μm. We model the leading hemisphere of Rhea in the wavelength range 0.3-3.6 μm with the Hapke and the Shkuratov radiative transfer codes and discuss the relative merits of the two approaches to fitting the spectrum. In calculations with both codes, the only components used are H2O ice, which is the dominant constituent, and a small amount of tholin (Ice Tholin II). Tholin in small quantities (few percent, depending on the mixing mechanism) appears to be an essential component to give the basic red color of the satellite in the region 0.3-1.0 μm. The quantity and mode of mixing of tholin that can produce the intense coloration of Rhea and other icy satellites has bearing on its likely presence in many other icy bodies of the outer Solar System, both of high and low geometric albedos. Using the modeling codes, we also establish detection limits for the ices of CO2 (a few weight percent, depending on particle size and mixing), CH4 (same), and NH4OH (0.5 weight percent) in our globally averaged spectra of Rhea's leading hemisphere. New laboratory spectral data for NH4OH are presented for the purpose of detection on icy bodies. These limits for CO2, CH4, and NH4OH on Rhea are also applicable to the other icy satellites for which spectra are presented here. The reflectance spectrum of Hyperion shows evidence for a broad, unidentified absorption band centered at 1.75 μm. 相似文献
20.
M.P. Collings J.W. Dever H.J. Fraser M.R.S. McCoustra 《Astrophysics and Space Science》2003,285(3-4):633-659
The interaction of carbon monoxide (CO) with vapour-deposited water(H2O) ices has been studied using temperature programmed desorption (TPD) and Fourier transform reflection-absorption infrared
spectroscopy (FT-RAIRS) over a range of astrophysically relevant temperatures. Such measurements have shown that CO desorption
from amorphous H2Oices is a much more complex process than current astrochemical models suggest. Re-visiting previously reported laboratory
experiments (Collings et al., 2003), a rate model has been constructed to explain, in a phenomenological manner, the desorption
of CO over astronomically relevant time scales. The model presented here can be widely applied to a range of astronomical
environments where depletion of CO from the gas phase is relevant. The model accounts for the two competing processes of CO
desorption and migration, and also enables the entrapment of some of the CO in the ice matrix and its subsequent release as
the water ice crystallises and then desorbs. The astronomical implications of this model are discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献