The NASA-ARC 10/20 micron camera

A new infrared camera (AIR Camera) has been developed at NASA-Ames Research Center for observations from ground-based telescopes. The heart of the camera is a Hughes 58 × 62 pixel Arsenic-doped Silicon detector array that has the spectral sensitivity range to allow observations in both the 10 and 20 micron atmospheric windows.The U.S. Government right to retain a non-exclusive, royalty-free licence in and to any copyright is acknowledged.     

Transient Effects in ISOPHOT Data: Status of Modelling and Correction Procedures

The calibration of ISOPHOT observations is hampered in many cases by the transient response of the detectors. Here we outline thebehaviour observed in flight for the different ISOPHOT detectors. We also present some of the procedures which are currently used to correct transienteffects and their implementation in the ISOPHOT Interactive Analysis.     

FT-IR measurements of cold C3H8 cross sections at 7–15 μm for Titan atmosphere

We present absorption cross sections of propane (C₃H₈) at temperatures from 145 K to 297 K in the 690–1550 cm⁻¹ region. Pure and N₂-broadened spectra were measured at pressures from 3 Torr to 742 Torr using a Bruker IFS125 FT-IR spectrometer at JPL. The gas absorption cell, developed at Connecticut College, was cooled by a closed-cycle helium refrigerator. The cross sections were measured and compiled for individual spectra recorded at various experimental conditions covering the planetary atmosphere and Titan. In addition to the cross sections, a propane pseudoline list with a frequency grid of 0.005 cm⁻¹, was fitted to the 34 laboratory spectra. Line intensities and lower state energies were retrieved for each line, assuming a constant width. Validation tests showed that the pseudoline list reproduces discrete absorption features and continuum, the latter contributed by numerous weak and hot band features, in most of the observed spectra within 3%. Based on the pseudoline list, the total intensity in the 690–1550 cm⁻¹ region was determined to be 52.93 (±3%) × 10⁻¹⁹ cm⁻¹/(molecule cm⁻²) at 296 K; this value is within 3% of the average from four earlier studies. Finally, the merit of the pseudoline approach is addressed for heavy polyatomic molecules in support of spectroscopic observation of atmospheres of Titan and other planets. The cold cross sections will be submitted to the HITRAN database (hitran.harvard.edu), and the list of C₃H₈ pseudolines will be available from a MK-IV website of JPL (http://mark4sun.jpl.nasa.gov/data/spec/Pseudo).     

H2O2 production by high-energy electrons on icy satellites as a function of surface temperature and electron flux

Chemistry on the icy surface of Europa is heavily influenced by the incident energetic particle flux from the jovian magnetosphere. The majority (>75%) of this energy is in the form of high energy electrons (extending to >10 MeV). We have simulated the electron irradiation environment of Europa with a vacuum system containing a high-energy electron gun for irradiation of ice samples formed on a gold mirror cooled with a cryostat. Pure water films of ∼2.6 μm thickness were grown at 100 K and then either cooled (to 80 K), warmed (to 120 K) or left at 100 K and subsequently irradiated with 10 keV electrons. The production of hydrogen peroxide (H₂O₂) was monitored by observation of the 2850 cm⁻¹ (3.5 μm) band. Equilibrium concentrations of H₂O₂, in units of percent by number H₂O₂ relative to water, were found to be 0.043% (80 K), 0.029% (100 K), and 0.0063% (120 K). These values are 33%, 22%, and 5%, respectively, that of the reported surface concentration on the leading hemisphere of Europa (Carlson, R.W., Anderson, M.S., Johnson, R.E., Smythe, W.D., Hendrix, A.R., Barth, C.A., et al. [1999]. Science 283(5410), 2062-2064) and less than the equilibrium concentrations formed by ion irradiation. In addition to the ice film temperature, the current of electrons was varied between different experiments to determine the production and destruction of H₂O₂ as a function of both electron flux and ice temperature. Variation in current was found to have little effect on the results other than accelerating arrival at radiolytic equilibrium.     

CO clathrate hydrate: Near to mid-IR spectroscopic signatures

Carbon monoxide is the second most abundant molecule after H₂ in the molecular universe, and as such an abundant constituent of interstellar and Solar System ices. To trace the possibility of this molecule to be found in a clathrate hydrate inclusion compound, its pure phase FTIR spectrum is investigated. We confirm the formation of a type I clathrate structure whereas simple guest size estimates would favour a type II clathrate hydrate, revealing interactions of this molecule with its water network during clathrate formation. The observed cage vibrational downshift with respect to pure CO ice is within 5 cm⁻¹. The temperature dependent wavenumber separation between the two enclathrated CO vibrational transitions in the two distinct type I clathrate cages is less than a wavenumber below 140 K, implying that the spectral simplification for detailed spectroscopic analysis of the individual profiles is a difficult task. The dynamics of the CO molecules in its cage change considerably from 5 K to 140 K. At temperatures above 30 K, the molecule is extremely mobile in the cages, as revealed by the infrared profile, significantly different from CO entrapped in water ice and different from observed profiles in astrophysical objects.     

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

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.     

Calculation of single-scattering albedos: Comparison of Mie results with Hapke approximations

The grain size of water ice can be determined from its near-infrared spectrum, which has numerous diagnostic absorption bands of different opacities. Models that have been used to determine water ice grain size from infrared spectra of icy outer Solar System objects have shown discrepancies in modeled grain size of a factor of two or more. Here the single-scattering albedo calculated using the commonly used Hapke model given by Roush [Roush, T.L., 1994. Icarus 108, 243-254] is compared with the exact calculation for spheres from a Mie series. An earlier approximation of single-scattering albedo called the Hapke “slab” model is also used in the comparison. All three models are implemented using the same optical constants for water ice at ∼110 K. Results are displayed for a large range of grain sizes from 1 μm to 1 mm. In general neither Hapke model can mimic the Rayleigh effects from particles sized near the wavelength of light that the Mie model predicts. For 10 μm particles, the slab model matches the Mie calculation quite well, but larger sizes are more discrepant. The Hapke/Roush model grain size needs to be ∼2.5 times larger to mimic the Mie results, and there are additional discrepancies in the continuum levels and band strengths. The Mie calculation for spheres is recommended for analysis of unknown remote sensing measurements, as it can mimic the spectra of oblate, prolate, and hollow particles given by equivalent sphere theories.     

Infrared astronomy: From one to one million pixels

In less than a decade, infrared array detectors have revolutionized infrared astronomy. Most of us remember using single element photometers in the early eighties, yet today, most of us are using 256×256 pixel arrays. At this meeting we have heard of plans to fabricate 1024×1024 arrays in the near infrared. From one to one million pixels in such a short period of time is amazing. The new array technology has also stimulated the development of many varieties of infrared cameras and spectrometers. At the UCLA Infrared Imaging Detector Lab we have commissioned two near infrared imaging systems (KCam and Gemini) based on 256×256 arrays and a spectrometer design study is in progress for 1024×1024 arrays. Performance of these cameras on the telescope will be reported briefly.     

Modelling the dust around Sakurai's Object

We follow the evolution of the infrared excess and the dustshell of Sakurai's Object by modelling the dust emission. The optical depth,dust temperature, and shell thickness parameters from the models are presented.Fits of DUSTY models to infrared spectra between 1997 May and 1999September illustrate the development of the dust from small grains to a rangeof grain sizes with an almost constant 60% graphite – 40% amorphous carbonmixture. The size of the infrared emitting region continues to growand may now be big enough to resolve. This may help to answerquestions such as the morphology of the dust shell, and whether the dustshell is `blobby' or smooth.     

Refractive index measurements of ammonia and hydrocarbon ices at 632.8 nm

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.     

Analysis of Near Infrared Observations of the Symbiotic Mira RR Tel

JHKL photometry of the symbiotic Mira RR Tel over 25 years shows variable fading in all bands, which can be explained by variable obscuration due to dust. The evolutionary track of the star is presented in a two-colour diagram together with the corresponding mean values for normal Miras with thick and thin dust shells and pulsation periods between 350 and 600 days. The observed colours for RR Tel are significantly shifted from the range shown by normal unreddened Miras. The distance of RR Tel is estimated from its K magnitude and the Mira period luminosity relation after correcting for reddening. The dust envelope is discussed with respect to a simple model involving two blackbodies. There appears to be rather little dust emission even at 3 μm, but considerable extinction.     

Photochemistry of methane-water ices

We report a study on the broadband ultraviolet photolysis of methane-water ice mixtures, at low methane concentrations and temperatures relevant to the icy satellites of the outer Solar System. The photochemistry of these mixtures is dominated by the action of hydroxyl radicals on methane and the resulting products. This implies that, given sufficient exposure time, the methane will eventually be completely oxidized to carbon dioxide. The presence of methane inhibits the formation of hydrogen peroxide by serving as a trap for hydroxyl radicals. The distribution of photochemical products is broadly similar to that previously conducted using ion and electron sources, with some differences possibly attributable to the difference in radiation source. The results are applicable to a variety of icy bodies in the Solar System. On Enceladus, where methane mixed with water is measured in the plumes, methane in the surface ices is subject to oxidation and will eventually be converted to CO₂. The CH stretch feature detected in the VIMS spectra of the Enceladus surface ice suggests that methane is currently being supplied to the surface ice, likely from re-condensation of the plume gas.     

The coating hypothesis for ammonia ice particles in Jupiter: Laboratory experiments and optical modeling

We report laboratory experiments and modeling calculations investigating the effect of a hydrocarbon coating on ammonia ice spectral signatures. Observational evidence and thermochemical models indicate an abundance of ammonia ice clouds in Jupiter's atmosphere. However, spectrally identifiable ammonia ice clouds are found covering less than 1% of Jupiter's atmosphere, notably in areas of strong vertical transport, indicating a short lifetime for the signature of ammonia absorption on condensed ammonia particles [Baines, K.H., Carlson, R.W., Kamp, L.W., 2002. Icarus 159, 74-94]. Current literature has suggested coating of ammonia ice particles by a hydrocarbon haze as a possible explanation for this paradox. The work presented here supports the inference of a coating effect that can alter or suppress ammonia absorption features. In the experiments, thin films of ammonia ices are deposited in a cryogenic apparatus, coated with hydrocarbons, and characterized by reflection-absorption infrared spectroscopy. We have observed the effects on the ammonia ice absorption features near 3 and 9 μm with coverage by thin layers of hydrocarbons. Modeling calculations of these multilayer thin films assist in the interpretation of the experimental results and reveal the important role of optical interference in altering the aforementioned ammonia spectral features. Mie and T-matrix scattering calculations demonstrate analogous effects for ammonia ice particles and investigate the relative effects of ammonia ice particle size, shape, and coating layer thickness on the ice particle spectral signatures.     

Infrared instruments for the Gemini 8-M telescopes

The Gemini Telescopes are being built to exploit the splendid infrared sites of Mauna Kea in Hawaii and Cerro Pachon in Chile. Both telescopes are being designed to deliver 0.1 arcsecond images at 2.2 m to the focal plane. This image size includes all tracking and enclosure effects. To exploit the superb infrared characteristics of the sites and telescopes we will require a new generation of IR instruments which will challenge both instrument designers and infrared array technologies.     

Unusual infrared fading of CH Cygni in 2006

Infrared observations of the unique symbiotic system CH Cyg in 2003–2006 are presented. Analysis of the observations has shown that a fairly dense dust structure (a cloud or a shell) appeared on the line of sight in August–November 2006. The dust grains in the new shell are similar in optical properties to graphite ones and their sizes are mostly within the range 0.14–0.16 μm. The dust shell is optically thick and its optical depth at 2.2 μm is τ(2.2) ≈ 0.97. The dust shell mass is M _d(06) ≈ 8 × 10⁻⁶ M _⊙ and the rate of matter flow into the shell has reached ∼2 × 10⁻⁵ M _⊙ yr⁻¹. Original Russian Text ? O.G. Taranova, V.I. Shenavrin, 2007, published in Pis’ma v Astronomicheskiĭ Zhurnal, 2007, Vol. 33, No. 8, pp. 598–603.     

Cassini CIRS update on stratospheric ices at Titan's winter pole

A strong, broad spectral emission feature at 85° N latitude centered at 221 cm⁻¹ remains unidentified after candidate ices of H₂O and pure crystalline CH₃CH₂CN are unambiguously ruled out. A much shallower weak emission feature starts at 160 cm⁻¹ and blends into the strong feature at ∼190 cm⁻¹. This feature is consistent with one formed by an HCN ice cloud composed of ?5 μm radius particles that resides in the lower stratosphere somewhere below an altitude of 160 km. Titan's stratospheric aerosol appears to have a spectral emission feature at about 148 cm⁻¹. The aerosol abundance at 85° N is about a factor 2.2 greater than at 55° S.     

Ion irradiation of H2O ice on top of sulfurous solid residues and its relevance to the Galilean satellites

In this paper we present the results of new experiments of ion irradiation of water ice deposited on top of a solid sulfurous residue to study the potential formation of SO₂ at the interface ice/refractory material and discuss the possibility that this mechanism accounts for the sulfur dioxide ice detected on the surfaces of the Galilean satellites. In situ infrared spectroscopy was the used experimental technique. We have irradiated a thin film of H₂O frost on a sulfurous layer with 200 keV of He⁺ at 80 K. The used sulfurous residue was obtained by irradiation of frozen SO₂ at 16 K and it is used as a template of sulfur bearing solid materials. We have not found evidences of the efficient formation of SO₂ after irradiation of H₂O ice on top of the sulfurous residue. An upper limit to the production yield of SO₂, of interface area for each 100 eV of energy absorbed in 1 cm³ of ice-covered residue, has been estimated. These results have relevance in the context of the surfaces of the icy Galilean satellites in which SO₂ was detected. Our results show that radiolysis of mixtures of water ice and refractory sulfurous materials is not the primary formation mechanism responsible for the SO₂ present on the surfaces of the Galilean satellites.     

First experiences with ARNICA,the Arcetri observatory imaging camera

ARNICA (ARcetri Near Infrared CAmera) is the imaging camera for the near infrared bands between 1.0 and 2.5 m that Arcetri Observatory has designed and built as a common use instrument for the TIRGO telescope (1.5 m diameter, f/20) located at Gornergrat (Switzerland). The scale is 1 per pixel, with sky coverage of more than 4×4 on the NICMOS 3 (256×256 pixels, 40 m side) detector array. The optical path is compact enough to be enclosed in a25.4 cm diameter dewar; the working temperature of detector and optics is 76 K. We give an estimate of performance, in terms of sensitivity with an assigned observing time, along with some preliminary considerations on photometric accuracy.     

A NICMOS-3 based spectrograph for near infrared astronomy

We describe a design for a compact cooled spectrograph which incorporates two cameras and two collimators to achieve high throughput from 0.8 to 2.5m. The spectrograph utilizes multiple NICMOS 3 arrays, specially mounted to minimize their separation in the spectral dimension. This accommodates the broad wavelength range while providing extensive coverage in the spatial dimension.     

Eclipse reappearances of Io: Time-resolved spectroscopy (1.9-4.2 μm)

We obtained time-resolved, near-infrared spectra of Io during the 60-90 min following its reappearance from eclipse by Jupiter on five occasions in 2004. The purpose was to search for spectral changes, particularly in the well-known SO₂ frost absorption bands, that would indicate surface-atmosphere exchange of gaseous SO₂ induced by temperature changes during eclipse. These observations were a follow-on to eclipse spectroscopy observations in which Bellucci et al. [Bellucci et al., 2004. Icarus 172, 141-148] reported significant changes in the strengths of two strong SO₂ bands in data acquired with the VIMS instrument aboard the Cassini spacecraft. One of the bands (4.07 μm [ν₁ + ν₃]) observed by Bellucci et al. is visible from ground-based observatories and is included in our data. We detected no changes in Io’s spectrum at any of the five observed events during the approximately 60-90 min during which spectra were obtained following Io’s emergence from Jupiter’s shadow. The areas of the three strongest SO₂ bands in the region 3.5-4.15 μm were measured for each spectrum; the variation of the band areas with time does not exceed that which can be explained by the Io’s few degrees of axial rotation during the intervals of observation, and in no case does the change in band strength approach that seen in the Cassini VIMS data. Our data are of sufficient quality and resolution to show the weak 2.198 μm (4549.6 cm⁻¹) 4ν₁ band of SO₂ frost on Io for what we believe is the first time. At one of the events (June 22, 2004), we began the acquisition of spectra ∼6 min before Io reappeared from Jupiter’s shadow, during which time it was detected through its own thermal emission. No SO₂ bands were superimposed on the purely thermal spectrum on this occasion, suggesting that the upper limit to condensed SO₂ in the vertical column above Io’s surface was ∼4 × 10⁻⁵ g cm⁻².