Modelling The 5–30 μm Spectrum of Comet Halley

The 5–30 μm spectrum of Comet P/Halley is modelled for various grain compositions on the basis of an observationally determined distribution of grain sizes. We compute the distribution function of grain temperatures and fluxes arising from (1) a mineral grain model, and (2) an organic grain model comprised of a diatom/POM mixture. The organic/POM model yields excellent accord with the cometary observations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Mid-IR (8–13 μm) images of the 21 μm,carbon rich proto-planetary nebulae

We present 9.7 and 11.8 m narrow band (/=10%) images of three carbon (C-) rich proto-planetary nebulae with an unusual 21 m feature: IRAS 07134+ 1005, IRAS 22272+5435, and IRAS 04296+3429. The images were taken at UKIRT using the Berkeley/IGPP/LEA mid-IR camera. All three objects have a bipolar shape adding to the existing evidence that C-rich PPNe are by nature bipolar. Furthermore, we find the same bipolar morphology in a previous study of the C-rich, young planetary nebula, IRAS 21282+5050. We believe these four objects form an evolutionary sequence which links the C-rich asymptotic giant branch (AGB) stars with the C-rich planetary nebulae (PNe). From this evolutionary sequence, we conclude that bipolarity in C-rich PNe begins on the AGB and that the dynamical ages of these PPNe are in fair agreement with theoretical ages for a 0.6 M hydrogen burning core star.     

A Model of The 2–4 μm Spectrum of Comet Halley

Recent observations of Halley's Comet show a broad absorption band centred at 3.4 μm and which can be explained on the basis of a bacterial grain model. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Galilean satellites: New near-infrared spectral reflectance measurements (0.65–2.5 μm) and a 0.325–5 μm summary

New near-infrared (0.65–2.5 μm) reflectance spectra of the Galilean satellites with 1.5% spectral resolution and ≈2% intensity precision are presented. These spectra more precisely define the water ice absorption features previously identified on Europa, Ganymede, and Callisto at 1.55 and 2.0 μm. In addition, previously unreported spectral features due to water ice are seen at 1.25, 1.06, 0.90, and 0.81 μm on Europa, and at 1.25, 1.04, and possibly 0.71 μm on Ganymede. Unreported absorption features in Callisto's spectrum occur at 1.2 μm, probably due to H₂O, and a weak, broad band extending from 0.75 to 0.95 μm, due possibly to other minerals. The spectrum of Io has only weak absorption features at 1.15 μm and between 0.8 and 1.0 μm. No water absorptions are positively identified in the Io spectra, indicating an upper limit of areal water frost coverage of 2% (leading and trailing sides). It is found for Callisto, Ganymede, and Europa that the water ice absorption features are due to free water and not to water bound or absorbed onto minerals. The areal coverage of water frost is ≈ 100% on Europa (trailing side), ≈65% on Ganymede (leading side), and 20–30% on Callisto (leading side). An upper limit of ≈5% bound water (in addition to the 20–30% ice) may be present on Callisto, based on the strong 3-μm band seen by other investigators. A summary of spectra of the satellites from 0.325 to about 5 μm to aid in laboratory and interpretation studies is also presented.     

Infrared diffuse reflectances (2.5–25 μm) of some meteorites

We measured emissionless infrared diffuse reflectances of some meteorites by using a Fourier transform infrared spectrophotometer to obtain additional information on identification of asteroidal surface materials. Although the spectral features of diffuse reflectances are apparently different from those of transmission spectra, the absorption bands of constituent minerals can be detected. C2 carbonaceous chondrite materials can be distinguished from C3 materials by the depth of their hydration bands around 3 and 6 μm. These hydration bands do not lose contrast by pulverization of the sample. Acid dissolution experiment shows that the 6.8-μm band in the spectra of the Murchison (C2) meteorite is probably due to carbonates. The enstatite meteorite examined (Norton County) shows absorption bands around 10 μm caused by pyroxene unlike the iron meteorite examined (Mundrabilla). Because these latter two meteorite types give similar spectra without strong absorption bands in the UV-Visible-near IR region, middle infrared spectra should be helpful in interpreting asteroidal surface materials when combined with the UV-Visible-near IR spectra.     

Correlation of simultaneous ultraviolet (0.36 μm) and infrared (8 to 14 μm) images of Venus

We present evidence for a correlation between features observed in simultaneous infrared (8 to 14 μm) and ultraviolet (0.36 μm) images obtained during Venus' 1975 and 1977 apparitions. The sense of the observed correlation is such that bright uv markings correspond to bright (warm) ir features, and similarly, dark uv markings correspond to dark (cool) ir features.     

The spectra of Uranus and Neptune at 8–14 and 17–23 μm

An array spectrometer was used on the nights of 1985 May 30–June 1 to observe the disks of Uranus and Neptune in the spectral regions 7–14 and 17–23 μm with effective resolution elements ranging from 0.23 to 0.87 μm. In the long-wavelength region, the spectra are relatively smooth with the broad S(1) H₂ collision-induced rotation line showing strong emission for Neptune. In the short-wavelength spectrum of Uranus, an emission feature attributable to C₂H₂ with a maximum stratospheric mixing ratio of 9 × 10⁻⁹ is apparent. An upper limit of 2 × 10⁻⁸ is placed on the maximum stratospheric mixing ratio of C₂H₆. The spectrum of Uranus is otherwise smooth and quantitatively consistent with the opacity provided by H₂ collision-induced absorption and spectrally continuous stratospheric emission, as would be produced by aerosols. Upper limits to detecting the planet near 8 μm indicate a CH₄ stratospheric mixing ratio of 1 × 10⁻⁵ or less, below a value consistent with saturation equilibrium at the temperature minimum. In the short-wavelength spectrum of Neptune, strong emission features of CH₄ and C₂H₆ are evident and are consistent with local saturation equilibrium with maximum stratospheric mixing ratios of 0.02 and 6 × 10⁻⁶, respectively. Emission at 8–10 μm is most consistent with a [CH₃D]/[CH₄] volume abundance ratio of 5 × 10⁻⁵. The spectrum of Neptune near 13.5 μm is consistent with emission by stratospheric C₂H₂ in local saturation equilibrium and a maximum mixing ratio of 9 × 10⁻⁷. Radiance detected near 10.5 μm could be attributed to stratospheric C₂H₄ emission for a maximum mixing ratio of approximately 3 × 10⁻⁹. Quantitative results are considered preliminary, as some absolute radiance differences are noted with respect to earlier observations with discrete filters.     

2.4 μm emission from the galactic centre region

It is shown that the 2.6 mm CO emission profile in the regions of two unidentified 2.4 m features observed near the galactic centre is consistent with an explanation of these features in terms of inhomogeneities in interstellar extinction. From our observations we estimate the mass to luminosity ratio of the galactic central bulge to beM/L _v=4.     

8–13 μm spectrophotometry of Comet Kohoutek

Spectrophotometry of Comet Kohoutek (1973f) covering the wavelength range 8–13 μm is presented. The spectral shape of the derived flux excess above a blackbody closely resembles that seen in circumstellar and interstellar dust and generally attributed to metallic silicates.     

Infrared spectrum of Io, 2.8–5.2 μm

The reflectance spectrum of Io is presented from 2.8 to 5.2 μm, extending the earlier results of D. P. Cruikshank, T. J. Jones, and C. B. Pilcher (1978, Astrophys. J. 225, L89–L92), and demonstrating the full extent of the broad and deep spectral absorption between 3.5 and 4.8 μm. Laboratory spectra of nitrates and carborates diluted with sulfur do not satisfactorily reproduce the Io spectrum, but new information based on the recently discovered volcanic activity on the satellite lead to consideration of other classes of compounds as reported in a companion paper (F. P. Fanale, R. H. Brown, D. P. Cruikshank, and R. N. Clark, 1979, Nature280, 761–763).     

The vertical cloud structure of Jupiter from 5 μm measurements

From 5 μm high-spatial-resolution images of Jupiter, flux-frequency histograms of the equatorial region show a trimodal distribution of brightness temperatures. Combined with limbdarkening measurements, a three-layer cloud model for Jupiter is developed. The highest, coldest clouds, apparently homogeneous and displaying relatively little limb darkening, cover the zones. These clouds are not present over the belts, allowing observational access to deeper regions. The belts appear heterogeneous: small, localized hot areas show enhanced limb darkening, while much of the belt is distinctly cooler and exhibits shallower limb darkening. These belt properties can be explained by a cool emitting layer superposed upon a hot, dense cloud deck.     

The 1.95-2.50 μm Spectrum of J6 Himalia

The reflectance spectrum of Jupiter's sixth satellite, Himalia, is featureless in the wavelength region 1.95-2.50 μm as seen at a spectral resolution of 0.005 μm, with no absorptions deeper than a few percent. From model calculations we establish an upper limit of 10% by weight of H₂O (30-μm grains) mixed intimately in the soil of Himalia, or alternatively 0.3% of the surface covered by exposures of H₂O ice spatially segregated from the darker soil. For CH₄ and CO₂ ices the upper limits in spatially segregated models are both 0.3%.     

Asteroid (101955) 1999 RQ36: Spectroscopy from 0.4 to 2.4 μm and meteorite analogs

We present reflectance spectra from 0.4 to 2.4 μm of Asteroid (101955) 1999 RQ36, the target of the OSIRIS-REx spacecraft mission. The visible spectral data were obtained at the McDonald Observatory 2.1-m telescope with the ES2 spectrograph. The infrared spectral data were obtained at the NASA Infrared Telescope Facility using the SpeX instrument. The average visible spectrum is combined with the average near-infrared wavelength spectrum to form a composite spectrum. We use three methods to constrain the compositional information in the composite spectrum of Asteroid (101955) 1999 RQ36 (hereafter RQ36). First, we perform a least-squares search for meteorite spectral analogs using 15,000 spectra from the RELAB database. Three most likely meteorite analogs are proposed based on the least-squares search. Next, six spectral parameters are measured for RQ36 and their values are compared with the ranges in parameter values of the carbonaceous chondrite meteorite classes. A most likely meteorite analog group is proposed based on the depth of overlap in parameter values. The results of the least-squares search and the parametric comparisons point to CIs and/or CMs as the most likely meteorite analogs for RQ36, and COs and CHs as the least likely. RQ36 has a spectrally “blue” continuum slope that is also observed in carbonaceous chondrites containing magnetite. We speculate that RQ36 is composed of a “CM1”-like material. Finally, we compare RQ36 to other B-type asteroids measured by Clark et al. (Clark, B.E. et al. [2010]. J. Geophys. Res. 115, E06005). The results of this comparison are inconclusive. RQ36 is comparable to Themis spectral properties in terms of its albedo, visible spectrum, and near-infrared spectrum from 1.1 to 1.45 μm. However, RQ36 is more similar to Pallas in terms of its near-infrared spectrum from 1.6 to 2.3 μm. Thus it is possible that B-type asteroids form a spectral continuum and that RQ36 is a transitional object, spectrally intermediate between the two end-members. This is particularly interesting because Asteroid 24 Themis was recently discovered to have H₂O ice on the surface (Rivkin, A., Emery, J. [2010]. Nature 464, 1322–1323; Campins, H. et al. [2010a]. Nature 464, 1320–1321).     

Observations of Jupiter and Saturn at 5–8 μm

Moderate-resolution spectrophotometry (Δλ/λ～0.015) has shown the effects of known atmospheric constituents (NH₃, CH₄, C₂H₆) on the 5–8 μm spectrum of Jupiter. Broadband observations of Saturn at 6.5 μm are also reported.     

The rings of Saturn: New near-infrared reflectance measurements and a 0.326–4.08 μm summary

A new high photometric precision reflectance spectrum of Saturn's rings covering the spectral region 0.65 to 2.5-μm is presented and three previously unreported absorption features at 1.25, 0.85, and probably 1.04 μm are identified. The 1.25- and 1.04-μm absorptions are due to water ice. The 0.85-μm feature may be due to a combination of 0.81- and 0.90-μm ice absorptions but this feature appears too strong relative to the 1.04-μm band to be completely explained by weater ice. Another possibility is that the 0.85-μm band is due to Fe³⁺-bearing minerals in an ice-mineral mixture. This explanation could also account for the drop in the visible and ultraviolet reflectance and the rise in reflectance around 3.6 μm. Finally, a composite spectrum from 0.325 to 4.08 μm is presented which will be useful for future analysis and laboratory studies.     

Spectral reflectance of solid sulfur trioxide (0.25–5.2 μm): Implications for Jupiter's satellite Io

We have measured the reflection spectrum of solid sulfur trioxide and we have compared this spectrum to the spectral geometric albedo of Jupiter's satellite Io. We find that the laboratory spectrum of solid SO₃ has very strong absorption features at 3.38, and 4.08 μm. The 3.38- and 3.70-μm absorptions are present very weakly (if indeed at all) in the spectral geometric albedo of Io. This suggests that solid SO₃, if present at all, could exist only as a very minor component of Io's surface. We note that studies involving particle bombardment of SO₂ (a known Io surface constituent) produce SO₃ (Moore, 1984, Icarus 31, 40–80). Sulfur trioxide, once formed on Io's surface, would be extremely stable; however, it would not be expected to accumulate to levels detectable from Earth-based instruments. While it may be possible that the constant resurfacing of Io by volcanic ejecta may cover any SO₃ formed, the area subject to such extensive resurfacing on short time scales (∼ 1 year) is at best ∼10%. Therefore, we would expect that condensed SO₂ remote from volcanos should develop a small but significant SO₃ concentration that could be detected by instruments such as the near-infrared mapping spectrometer on the Galileo spacecraft.     

Infrared Spectroscopy Over The 2.9–3.9 μm Waveband in Biochemistry and Astronomy

The infrared spectrum of the galactic centre source GC-IRS 7 over the 2.9–3.9 μm waveband is interpreted as strong evidence for bacterial grains. This revised version was published online in July 2006 with corrections to the Cover Date.     

The spectrum of comet C/2009 R1 (McNaught) in 4140–5240 Å wavelength region

The spectroscopic observations of comet C/2009 R1 (McNaught) were carried out with the 2 m Zeiss-RCC Telescope of Pik Terskol Observatory operated by the International Center for Astronomical and Medico-Ecological Research (Ukraine, Russia). The Multi Mode Cassegrain spectrometer was used to obtain spectra of moderate spectral resolving power with a wavelength coverage from 4140 to 5240 Å. The spectrum is characterized by the extremely low continuum level and strong molecular features. 192 emission lines of C₂, CN, CH, NH₂, CO⁺, and CH⁺ have been identified by the comparison of the observed and theoretical spectra of the molecules. The ratios of the gas production rates of Q(C₂)/Q(CN)=1.32, Q(CH)/Q(CN)=0.49, and Q(NH₂)/Q(CN)=0.81 were derived using a Haser model.     

Visible spectral reflectance measurements (0.33–1.1 μm) of the Galilean satellites at many orbital phase angles

One hundred eighty-seven reflectance spectra (0.33–1.10 μm) of the Galilean satellites have been obtained. Solar phase angle color correction coefficients were derived and the spectra corrected to a solar phase of 6°. Solar phase angle coefficients beyond 0.55 μm are presented for the first time. The spectra as a function of orbital phase angle are presented in the form of images to display hemispheric spectral variations. Io and Europa are redder on their trailing hemispheres while Callisto is redder on its leading hemisphere. Ganymede shows small longitudinal color variations despite the complex albedo structure visible in Voyager images. Comparisons of these data with previous measurements reveal that most differences can be attributed to the solar calibration. Reflectance measurements of Io at 0.73 μm observed 8.5 years apart show a 6% global reflectance decrease. However, it is difficult to unambigously attribute this particular decrease in reflectance to a change in Io's surface composition.     

10 μm Imaging of the Blue Compact Galaxy HE 2–10

We present the first results of a survey of Blue Compact Galaxies with a 10 m array. Blue Compact Galaxies (BCGs) are dwarf galaxies experiencing an intense burst of star formation. As dwarf systems, their main characteristics is a low heavy-elements abundance. Although dust is thought to be less abundant in these galaxies than in normal spirals, the presence of a compact starburst region favors a detection in the infrared, and 10 m imaging is perfectly suited to star formation studies in BCGs since it focuses on the hottest dust inside the star-forming regions.