Discovery of co₂ ice and leading-trailing spectral asymmetry on the uranian satellite ariel     

W.M Grundy  L.A Young  E.F Young 《Icarus》2003,162(1):222-229

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   

The Uranian satellites: Surface compositions and opposition brightness surges     

Robert Hamilton Brown  Dale P. Cruikshank 《Icarus》1983,55(1):83-92

High-precision spectrophotometry at 5% resolution has been obtained for the Uranian satellites Ariel, Umbriel, Titania, and Oberon. These spectra cover the wavelength region 1.43–2.57 μm and represent a substantial improvement in precision or wavelength coverage over previous studies. The presence of a spectrally dominant water-ice component in the surface of Ariel, Umbriel, Titania, and Oberon is confirmed. The 1.5- and 2.0-μm water absorption band depths and the continuum reflectance (as defined by the reflectance at 1.78 and 2.25 μm) indicate significant differences among the surface compositional properties of the four satellites. Comparisons of the new spectra to those of other solar system bodies, and to laboratory spectra of water ice of various degrees of purity, indicate that these satellites have a significant non-water-ice component on/in their surfaces. The lack of spectral absorptions at 5% resolution attributable to anything other than water ice suggests that the non-water-ice component is a roughly neutral reflector in the 1.5- to 2.5- μm region. The nature of the non-water-ice component cannot be uniquely determined from these data, but it is relatively dark and has spectral similarities to substances such as carbon black, the dark substance covering one face of Iapetus, or other neutral-color, low-reflectance materials. Finally, preliminary measurements of the near-infrared opposition brightness surges of Ariel, Titania, and Oberon show them to be among the largest in the solar system.  相似文献   

The Uranian satellites: Water ice on Ariel and Umbriel     

Dale P. Cruikshank  R. Hamilton Brown 《Icarus》1981,45(3):607-611

New near-infrared reflectance spectra are presented for Ariel and Umbriel. Water ice on the surface of Ariel and Umbriel is verified from spectral signatures in the 2-μm region. However, the weaker bands in Umbriel's spectrum indicate that its surface is significantly different from Ariel either in degree of contamination with dark material or in microstate. Umbriel may have a lower albedo than Ariel, Titania, and Oberon and, therefore, may have a diameter comparable to that of Titania.  相似文献   

Photometric study of the major satellites of Uranus     

V. V. Avramchuk  V. K. Rosenbush  T. P. Bul’ba 《Solar System Research》2007,41(3):186-202

In this paper, we analyze the results of ground-based and space-born photometric observations of the major satellites of Uranus—Miranda, Ariel, Umbriel, Titania, and Oberon. All sets of photometric observations of the satellites available in the literature were examined for uniformity and systematic differences and summarized to a unified set by wavelength ranging from 0.25 to 2.4 μm. This set covers the interval of phase angles from 0.034° to 35°. The compound phase curves of brightness of the satellites in the spectral bands at 0.25, 0.41, 0.48, 0.56, 0.75, 0.91, 1.4, and 1.8 μm, which include a pronounced opposition surge and linear part, were constructed. For each satellite, the geometric albedo was found in different spectral bands taking into account the brightness opposition effect, and its spectral dependence was studied. It has been shown that the reflectance of the satellites linearly depends on the wavelength at different phase angles, but has different spectral gradients. The parameters of the phase functions of brightness, including the amplitude and the angular width of the brightness opposition surge, the phase coefficient, and the phase angle at which the nonlinear increase in brightness starts, were determined and their dependences on wavelength and geometric albedo were analyzed. Our investigations show that, in their optical properties, the satellites Miranda and Ariel, Titania and Oberon, and Umbriel present three types of surfaces. The observed parameters of the brightness opposition effect for the Uranian satellites, some ice satellites of Jupiter and Saturn, and the E-and S-type asteroids are analyzed and compared within the framework of the coherent backscattering and mutual shadowing mechanisms.  相似文献   

Near-infrared laboratory spectra of solid H₂O/CO₂ and CH₃OH/CO₂ ice mixtures     

Max P. Bernstein  Dale P. Cruikshank  Scott A. Sandford 《Icarus》2005,179(2):527-534

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   

Narrow-band spectrophotometry of Ariel,Umbriel, Titania,Oberon, and Triton     

Paul E. Johnson  Thomas F. Greene  Richard W. Shorthill 《Icarus》1978,36(1):75-81

The spectral reflectances of Ariel, Umbriel, Titania, Oberon, and Triton were measured in 28 bandpasses between λ326 and λ976 nm on the night of 28/29 June 1974. These observations were made with the 200-in. Hale telescope and multichannel spectrometer. Bandpasses of 8 nm from λ326 to λ566 nm and 16 nm from λ592 to λ976 nm were employed. The spectral reflectances of Ariel, Oberon, and Titania increase from λ342 to λ534 nm and are relatively flat from λ550 to λ976 nm. Umbriel's reflectance decreases monotonically with increasing wavelength through the entire range of measured wavelengths. Triton is found to have a constant spectral reflectance.  相似文献   

A spectroscopic study of the surfaces of Saturn's large satellites: H₂O ice, tholins, and minor constituents     

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   

Carbon dioxide segregation in 1:4 and 1:9 CO₂:H₂O ices     

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   

Near-infrared spectral monitoring of Triton with IRTF/SpeX II: Spatial distribution and evolution of ices     

W.M. Grundy  L.A. Young  J.A. Stansberry  C.B. Olkin  E.F. Young 《Icarus》2010,205(2):594-604

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   

Occurrence of central peak craters on the Uranian satellites: Implications for surface structure and comparison with the Jovian and Saturnian systems     

Kent D. Trego 《Earth, Moon, and Planets》1987,38(3):299-303

Craters with central peaks occur on the Uranian satellites Ariel, Umbriel, Titania, and Oberon; but do not occur on Miranda. The inelastic surface of Miranda is apparently due to the heavy tectonic reworking of its surface. A theory of expansion/contraction is proposed to explain the tectonic history of Miranda. The existence of central peak craters on the four largest satellites of Uranus implies that they have surface strengths similar to those of the Saturnian satellites and silicate bodies of the inner solar system which all have central peak craters. The absence of central peak craters on Miranda implies that it has an inelastic surface similar to those of the Jovian ice satellites Ganymede and Callisto whose surfaces do not contain central peak craters.  相似文献   

Polarimetry of major Uranian moons at the 6-m telescope     

V. L. Afanasiev  V. K. Rosenbush  N. N. Kiselev 《Astrophysical Bulletin》2014,69(2):211-223

We present the results of polarimetric observations of the icymoons of Uranus (Ariel, Titania, Oberon, and Umbriel) performed at the 6-m BTA telescope of the SAO RAS with the SCORPIO-2 focal reducer within the phase angle range of $0_.^ \circ 06 - 2_.^ \circ 37$ . The parameters of the negative polarization branch (referred to the scattering plane) are obtained in the V filter: for Ariel the maximum branch depth of P _min ≈ ?1.4% is reached at the phase angle of α _min ≈ 1°; for Titania P _min ≈ ?1.2%, $\alpha _{\min } \approx 1_.^ \circ 4$ ; for Oberon P _min ≈ ?1.1%, $\alpha _{\min } \approx 1_.^ \circ 8$ . For Umbriel the polarization minimum was not reached: for the last measurement point at $\alpha _{\min } \approx 2_.^ \circ 4$ , polarization amounts to ?1.7%. The declining P _min and shifting α_min towards larger phase angles correlate with a decrease of the geometric albedo of the Uranian moons. There is no longitudinal dependence of polarization for the moons within the observational errors which indicates a similarity in the physical properties of the leading and trailing hemispheres. The phase-angle dependences of polarization for the major moons of Uranus are quite close to those observed in the group of small trans-Neptunian objects (Ixion, Huya, Varuna, 1999 DE9, etc.), which are characterized by a large gradient of negative polarization, about ?1% per degree in the phase-angle range of $0_.^ \circ 1 - 1^ \circ$ .  相似文献   

Near-infrared spectra of laboratory H₂O-CH₄ ice mixtures     

Max P. Bernstein  Dale P. Cruikshank  Scott A. Sandford 《Icarus》2006,181(1):302-308

We present 1.25-19 μm infrared spectra of pure solid CH₄ and H₂O/CH₄=87, 20, and 3 solid mixtures at temperatures from 15 to 150 K. We compare and contrast the absorptions of CH₄ in solid H₂O with those of pure CH₄. Changes in selected peak positions, profiles, and relative strength with temperature are presented, and absolute strengths for absorptions of CH₄ in solid H₂O are estimated. Using the two largest (ν₃+ν₄) and (ν₁+ν₄) near-IR absorptions of CH₄ at 2.324 and 2.377 μm (4303 and 4207 cm⁻¹), respectively, as examples, we show that peaks of CH₄ in solid H₂O are at slightly shorter wavelength (higher frequency) and broader than those of pure solid CH₄. With increasing temperature, these peaks shift to higher frequency and become increasingly broad, but this trend is reversible on re-cooling, even though the phase transitions of H₂O are irreversible. It is to be hoped that these observations of changes in the positions, profiles, and relative intensities of CH₄ absorptions with concentration and temperature will be of use in understanding spectra of icy outer Solar System bodies.  相似文献   

A simple two-layer model for the Uranus satellites     

J. Leliwa-Kopystynski 《Earth, Moon, and Planets》1987,39(3):215-223

A two-layer model of a satellite interior with a rocky core with a density 3–3.4 g cm^-3 and with a H₂O mantle with a density 0.94–1.2 g cm^-3 is applied for the icy satellites. The case of Mimas is discussed separately. A comparison of the results with these obtained for more complicated models as applied for Jupiter and Saturn icy satellites has been carried out. This comparison shows that the two-layer model offers a reasonable approximation and, therefore, it can be applied for the satellites of Uranus. We obtained the dimensionless core radii 0.55–0.74, 0.45–0.68, 0.59–0.67, 0.55–0.65, and dimensionless core masses 0.42–0.72, 0.26–0.63, 0.47–0.61, 0.41–0.57, for Ariel, Umbriel, Titania, and Oberon, respectively.Institute of Geophysics of Warsaw University, Warszawa, Poland.  相似文献   

Resolving dynamic parameters of the August 2007 Titania and Ariel occultations by Umbriel     

C. Miller  N.J. Chanover 《Icarus》2009,200(1):343-346

We observed the 15 August 2007 occultation of the uranian satellite Titania and the 19 August 2007 occultation of Ariel by Umbriel using the Agile high-speed photometer on the APO 3.5 m telescope. We find that the Titania event midpoint occurred at 09:16:39.20 UT and the Ariel event midpoint at 07:59:49.4 UT, which was 26.2 s and 37.4 s later than predicted, respectively. Our best fit impact parameter was 71.0 km for the Titania occultation and 476.9 km for the Ariel event, both of which were less than predicted.  相似文献   

Near-infrared spectra of the leading and trailing hemispheres of Enceladus     

Anne J. Verbiscer  Dawn E. Peterson  Michael Cushing  Matthew J. Nelson  John C. Wilson 《Icarus》2006,182(1):211-223

We present individual spectra 0.8-2.5 μm of the leading and trailing hemispheres of Enceladus obtained with the CorMASS spectrograph on the 1.8 m Vatican Advanced Technology Telescope (VATT) at the Mount Graham International Observatory. While the absorption bands of water ice dominate the spectrum of both hemispheres, most of these bands are stronger on the leading hemisphere than the trailing hemisphere. In addition, longward of 1 μm, the continuum slope is greater on the leading hemisphere than the trailing hemisphere. These differences could be produced by the presence of particles on the trailing side that are smaller and/or microstructurally more complex than those on the leading side, consistent with the preferential erosion or structural degradation of regolith particle grains on the trailing side by magnetospheric sweeping. We also explore compositional differences between the two hemispheres by applying Hapke spectrophotometric mixture models to the spectra whose components include water ice and ammonia hydrate (1% NH₃⋅H₂O). We find that spectral models which include as much as 25% by weight ammonia hydrate intimately mixed with water ice and covering 80% of the illuminated area of the satellite fit the observed spectrum of both the leading and trailing hemispheres. Areal (checkerboard) mixing models of ammonia hydrate and water ice fit the leading hemisphere with 15% of the surface comprised of ammonia hydrate and the trailing hemisphere with 10% ammonia hydrate. Therefore, while these spectral data do not contain an unambiguous detection of ammonia hydrate on Enceladus, our spectral models do not preclude the presence of a modest amount of 1% NH₃⋅H₂O on both hemispheres. We examine spectral differences and similarities between both hemispheres and the tenuous E ring within which Enceladus orbits. The spectral resolution (R=λ/Δλ) of these CorMASS data (R∼300) is comparable to but nevertheless higher than that of the Visual-Infrared Mapping Spectrometer (VIMS) (R=225) onboard the Cassini spacecraft.  相似文献   

Spatially resolved SO₂ ice on Io, observed in the near IR     

Conor Laver  Imke de Pater 《Icarus》2008,195(2):752-757

We present equivalent width maps of the 1.98 and 2.13 μm SO₂ ice absorption bands on the surface of Io. The data were taken on 17 April 2006 with the near-infrared mapping spectrometer, OSIRIS at the W.M. Keck Observatory, Hawaii. The maps show significant regional enhancements of SO₂ ice over the Bosphoros, Media, Tarsus and Chalybes Regiones.  相似文献   

Iapetus surface variability revealed from statistical clustering of a VIMS mosaic: The distribution of CO₂     

Noemi Pinilla-Alonso  Ted L. Roush  Dale P. Cruikshank 《Icarus》2011,215(1):75-82

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   

High-resolution spectroscopy of Saturn at 3 microns: CH₄, CH₃D, C₂H₂, C₂H₆, PH₃, clouds, and haze     

Joo Hyeon Kim  Sang J. Kim  Sungsoo S. Kim 《Icarus》2006,185(2):476-486

We report observation and analysis of a high-resolution 2.87-3.54 μm spectrum of the southern temperate region of Saturn obtained with NIRSPEC at Keck II. The spectrum reveals absorption and emission lines of five molecular species as well as spectral features of haze particles. The ν₂+ν₃ band of CH₃D is detected in absorption between 2.87 and 2.92 μm; and we derived from it a mixing ratio approximately consistent with the Infrared Space Observatory result. The ν₃ band of C₂H₂ also is detected in absorption between 2.95 and 3.05 μm; analysis indicates a sudden drop in the C₂H₂ mixing ratio at 15 mbar (130 km above the 1 bar level), probably due to condensation in the low stratosphere. The presence of the ν₃+ν₉+ν₁₁ band of C₂H₆ near 3.07 μm, first reported by Bjoraker et al. [Bjoraker, G.L., Larson, H.P., Fink, U., 1981. Astrophys. J. 248, 856-862], is confirmed, and a C₂H₆ condensation altitude of 10 mbar (140 km) in the low stratosphere is determined. We assign weak emission lines within the 3.3 μm band of CH₄ to the ν₇ band of C₂H₆, and derive a mixing ratio of 9±4×¹⁰⁻⁶ for this species. Most of the C₂H₆ 3.3 μm line emission arises in the altitude range 460-620 km (at ∼μbar pressure levels), much higher than the 160-370 km range where the 12 μm thermal molecular line emission of this species arises. At 2.87-2.90 μm the major absorber is tropospheric PH₃. The cloud level determined here and at 3.22-3.54 is 390-460 mbar (∼30 km), somewhat higher than found by Kim and Geballe [Kim, S.J., Geballe, T.R., 2005. Icarus 179, 449-458] from analysis of a low resolution spectrum. A broad absorption feature at 2.96 μm, which might be due to NH₃ ice particles in saturnian clouds, is also present. The effect of a haze layer at about 125 km (∼12 mbar level) on the 3.20-3.54 μm spectrum, which was not apparent in the low resolution spectrum, is clearly evident in the high resolution data, and the spectral properties of the haze particles suggest that they are composed of hydrocarbons.  相似文献   

Clouds, haze, and CH₄, CH₃D, HCN, and C₂H₂ in the atmosphere of Titan probed via 3 μm spectroscopy     

Sang J. Kim  T.R. Geballe  Regis Courtin 《Icarus》2005,173(2):522-532

Using synthetic spectra derived from an updated model atmosphere together with a continuum model that includes contributions from haze, cloud and ground, we have re-analyzed the recently published (Geballe et al., 2003, Astrophys. J. 583, L39-L42) high-resolution 3 μm spectrum of Titan which contains newly-detected bands of HCN (in emission) and C₂H₂ and CH₃D (in absorption), in addition to previously detected bands of CH₄. In the 3.10-3.54 μm interval the analysis yields strong evidence for the existence of a cloud deck or optically thick haze layer at about the 10 mbar (∼ 100 km) level. The haze must extend well above this altitude in order to mask the strong CH₄ lines at 3.20-3.50 μm. These cloud and haze components must be transparent at 2.87-2.92 μm, where analysis of the CH₃D spectrum demonstrates that Titan's surface is glimpsed through a second cloud deck at about the 100 mbar (∼ 50 km) level. Through a combination of areal distribution and optical depth this cloud deck has an effective transmittance of ∼ 20%. The spectral shape of Titan's continuum indicates that the higher altitude cloud and haze particles responsible for suppressing the CH₄ absorptions have a largely organic make-up. The rotational temperature of the HCN ranges from 140 to 180 K, indicating that the HCN emission occurs over a wide range of altitudes. This emission, remodeled using an improved collisional deactivation rate, implies mesospheric mixing ratio curves that are consistent with previously predictions. The stratospheric and mesospheric C₂H₂ mixing ratios are ∼¹⁰⁻⁵, considerably less than previous model predictions (Yung et al., 1984), but approximately consistent with recent observational results. Upper limits to mixing ratios of HC₃N and C₄H₂ are derived from non-detections of those species near 3.0 μm.  相似文献   

Asymmetries in the distribution of H₂O and CO₂ in the inner coma of Comet 9P/Tempel 1 as observed by Deep Impact     

L.M. Feaga  M.F. A'Hearn  J.M. Sunshine  O. Groussin  T.L. Farnham 《Icarus》2007,190(2):345-356

Prior to the impact event, Deep Impact monitored the ambient inner coma of Comet 9P/Tempel 1 at high spatial resolution in July 2005. Gaseous H₂O and CO₂ are unambiguously detected in the infrared spectra collected with the HRI-IR spectrometer aboard Deep Impact. Detailed distribution maps of these volatiles in the inner coma, within 60 km from the nucleus, are produced from the integrated emission bands of H₂O (2.66 μm) and CO₂ (4.26 μm). Uncorrelated asymmetries are determined in the spatial distribution of both species indicating chemical heterogeneities within the nucleus. Although present at some abundance surrounding the entire nucleus, H₂O has a pronounced enhancement in abundance in the sunward direction rotational phases, evidence that the dominant process of subliming water ice from the nucleus is solar heating. In contrast, CO₂ is enhanced in the regions near the negative rotational pole of the nucleus, suggesting localized outgassing there. Both species show an increase in radiance above the limb of the nucleus toward Ecliptic North. The distribution maps also suggest that the process of dust removal from the nucleus is strongly connected to the outgassing of volatiles. Detailed study of these coma asymmetries gives insight to the relative abundances of the dominant molecular components of the inner coma, source regions of the native volatiles, anisotropic outgassing of the nucleus, and the formation and evolution of the nucleus. A quiescent water production rate for Tempel 1 on July 4, 2005, is estimated to be .  相似文献