Water Ice Absorption Feature Research Articles

Ice, Ice, Maybe? Investigating 46P/Wirtanen’s Inner Coma for Icy Grains

The release of volatiles from comets is usually from direct sublimation of ices on the nucleus, but for very or hyperactive comets other sources have to be considered to account for the total production rates. In this work, we present new near-IR (NIR) imaging and spectroscopic observations of 46P/Wirtanen taken during its close approach to Earth on 2018 December 19 with the MMIRS instrument at the MMT Observatory to search for signatures of icy or ice-rich grains in its inner coma that might explain its previously reported excess water production. The morphology of the images does not suggest any change in grain properties within the field of view, and the NIR spectra do not show the characteristic absorption features of water ice. Using a new Markov Chain Monte Carlo–based implementation of the spectral modeling approach of Protopapa et al., we estimate the areal water ice fraction of the coma to be <0.6%. When combined with slit-corrected Af ρ values for the J, H, and K bands and previously measured dust velocities for this comet, we estimate an icy grain production rate of less than 4.6 kg s−1. This places a strict constraint on the water production rate from pure icy grains in the coma, and in turn we find that for the 2018–2019 apparition approximately 64% of 46P’s surface was actively sublimating water near perihelion. We then discuss 46P’s modern properties within the context of other (formerly) hyperactive comets to understand how these complex objects evolve.

Complex Water-ice Mixtures on NII Nereid: Constraints from NIR Reflectance

Abstract Nereid, Neptune’s third-largest satellite, lies in an irregular orbit and is the only outer satellite in the system (apart from Triton) that can be spectroscopically characterized with the current generation of Earth-based telescopes. We report our results on the spectral characterization of Nereid using its reflectance spectrum from 0.8 to 2.4 μm, providing the first measurements over the range of 0.8–1.4 μm. We detect spectral absorption features of crystalline water ice in close agreement with previous measurements. We show that model fits of simple intimate mixtures including water ice do not provide simultaneous matches to absorption band depths at 1.5 and 2.0 μm when accounting for the spectral continuum. Possible solutions include invoking a more complex continuum, including both crystalline and amorphous water ice, and allowing for submicron-sized grains. We show that mixtures including magnetite and the CM2 chondrite Murchison provide a flexible framework for interpreting spectral variation of bodies with neutral-sloped spectra like that of Nereid. Magnetite in particular provides a good match to the spectral continuum without requiring the presence of tholin-like organics. We note that carbonaceous chondrites and their components may be useful analogs for the non-ice components of outer solar system bodies, consistent with recent findings by Fraser et al. Comparison to spectra of large trans-Neptunian objects and satellites of Uranus show that Nereid’s low albedo, deep water bands, and neutral color is distinct from many other icy objects, but such comparisons are limited by an incomplete understanding of spectral variability among ∼100 km-sized icy bodies.

Searching for water ice in the coma of interstellar object 2I/Borisov

Aims.Interstellar objects passing through our Solar System offer a rare opportunity to probe the physical and chemical processes involved in solid body and planet formation in extrasolar systems. The main objective of our study is to search for diagnostic absorption features of water ice in the near-infrared (NIR) spectrum of the second interstellar object 2I/2019 Q4 (Borisov) and compare its ice features to those of the Solar System icy objects.Methods.We observed 2I in the NIR on three separate occasions. The first observation was made on 2019 September 19 UT using the SpeX spectrograph at the 3m IRTF and again on September 24 UT with the GNIRS spectrograph at the 8m GEMINI telescope; the last observation was made on October 09 UT with IRTF.Results.The spectra obtained from all three nights appear featureless. No absorption features associated with water ice are detected. Spectral modeling suggests that water grains, if present, comprise no more than 10% of the coma cross section. The comet consistently exhibits a red D-type like spectrum with a spectral slope of about 6% per 1000 Å, which is similar to that of 1I/’Oumuamua and is comparable to Solar System comets.

Direct evidence of surface exposed water ice in the lunar polar regions.

Water ice may be allowed to accumulate in permanently shaded regions on airless bodies in the inner solar system such as Mercury, the Moon, and Ceres [Watson K, et al. (1961) J Geophys Res 66:3033-3045]. Unlike Mercury and Ceres, direct evidence for water ice exposed at the lunar surface has remained elusive. We utilize indirect lighting in regions of permanent shadow to report the detection of diagnostic near-infrared absorption features of water ice in reflectance spectra acquired by the Moon Mineralogy Mapper [M (3)] instrument. Several thousand M (3) pixels (∼280 × 280 m) with signatures of water ice at the optical surface (depth of less than a few millimeters) are identified within 20° latitude of both poles, including locations where independent measurements have suggested that water ice may be present. Most ice locations detected in M (3) data also exhibit lunar orbiter laser altimeter reflectance values and Lyman Alpha Mapping Project instrument UV ratio values consistent with the presence of water ice and also exhibit annual maximum temperatures below 110 K. However, only ∼3.5% of cold traps exhibit ice exposures. Spectral modeling shows that some ice-bearing pixels may contain ∼30 wt % ice that is intimately mixed with dry regolith. The patchy distribution and low abundance of lunar surface-exposed water ice might be associated with the true polar wander and impact gardening. The observation of spectral features of H2O confirms that water ice is trapped and accumulates in permanently shadowed regions of the Moon, and in some locations, it is exposed at the modern optical surface.

NEAR-INFRARED SPECTRA OF HIGH-ALBEDO OUTER MAIN-BELT ASTEROIDS

Most outer main-belt asteroids have low albedos because of their carbonaceouslike bodies. However, infrared satellite surveys have revealed that some asteroids have high albedos, which may suggest the presence of unusual surface minerals for those primitive objects. We present new near-infrared (1.1–2.5 μm) spectra of four outer main-belt asteroids with albedos ≥ 0.1. The C-complex asteroids (555) Norma and (2542) Calpurnia are featureless and have (50%–60%) amorphous Mg pyroxenes that might explain the high albedos. Asteroids (701) Oriola (which is a C-complex asteroid) and (2670) Chuvashia (a D/T-type or M-type asteroid) show possible broad absorption bands (1.5–2.1 μm). The feature can be reproduced by either Mg-rich amorphous pyroxene (with 50%–60% and 80%–95% Mg, respectively) or orthopyroxene (crystalline silicate), which might be responsible for the high albedos. No absorption features of water ice (near 1.5 and 2.0 μm) are detected in the objects. We discuss the origin of high albedo components in the outer main-belt asteroids and their physical relations to comets.

ADAPTIVE OPTICS OBSERVATIONS OF 3 μm WATER ICE IN SILHOUETTE DISKS IN THE ORION NEBULA CLUSTER AND M43

We present the near-infrared images and spectra of four silhouette disks in the Orion Nebula Cluster (ONC; M42) and M43 using the Subaru Adaptive Optics system. While d053-717 and d141-1952 show no water ice feature at 3.1 micron, a moderately deep (tau~0.7) water ice absorption is detected toward d132-1832 and d216-0939. Taking into account the water ice so far detected in the silhouette disks, the critical inclination angle to produce a water ice absorption feature is confirmed to be 65-75deg. As for d216-0939, the crystallized water ice profile is exactly the same as in the previous observations taken 3.63 years ago. If the water ice material is located at 30AU, then the observations suggest it is uniform at a scale of about 3.5AU.

Ices in the Galactic Center?

We present for the first time, an infrared data-cube of the central parsec of our Galaxy in the 2.8 to 4.2 micron range. This spectral band (the L-band) harbors important signatures of the interstellar and circumstellar medium, since the water ice absorption feature at 3 micron traces the dense medium and the hydrocarbon absorption at 3.4 micron is only observed in the diffuse gas.Thanks to a calibrator spectrum of the foreground extinction in the L-band derived in a previous paper, we corrected our data-cube for the line of sight extinction. Our observations performed with ISAAC spectrograph at the VLT-ESO telescope suggest that part of the absorption features takes place in the local environment of the Galactic center. This induces the presence of very low temperatures in the central parsec.

THE SIZE, DENSITY, AND FORMATION OF THE ORCUS-VANTH SYSTEM IN THE KUIPER BELT

The Kuiper belt object Orcus and its satellite Vanth form an unusual system in the Kuiper belt. Orcus is amongst the largest objects known in the Kuiper belt, but the relative size of Vanth is much larger than that of the tiny satellites of the other large objects. From Hubble Space Telescope observations we find that Orcus and Vanth have different visible colors and that Vanth does not share the water ice absorption feature seen in the infrared spectrum of Orcus. We also find that Vanth has a nearly face-on circular orbit with a period of 9.5393 +-0.0001 days and semimajor axis of 8980+-20 km, implying a system mass of 6.32+- 0.01 X 10^20 kg or 3.8% the mass of dwarf planet Eris. From Spitzer Space Telescope observations we find that the thermal emission is consistent with a single body with diameter 940+-70 km and a geometric albedo of 0.28+-0.04. Assuming equal densities and albedos, this measurements implies sizes of Orcus and Vanth of 900 and 280 km, respectively, and a mass ratio of 33. Assuming a factor of 2 lower albedo for the non-icy Vanth, however, implies sizes of 820 and 640 km and a mass ratio of 2. The measured density depends on the assumed albedo ratio of the two objects but is approximately 1.5+-0.3 g cm^-3$, midway between typical densities measured for larger and for smaller objects. The orbit and mass ratio is consistent with formation from a giant impact and subsequent outward tidal evolution and even consistent with the system having now achieved a double synchronous state. The system can equally well be explained, however, by initial eccentric capture, Kozai cycling which increases the eccentricity and decreases the pericenter of the orbit of Vanth, and subsequent tidal evolution inward.

Far ultraviolet spectral properties of Saturn’s rings from Cassini UVIS

Spectra taken by the Cassini Ultraviolet Imaging Spectrograph (UVIS) of Saturn’s C ring, B ring, Cassini Division, and A ring have been analyzed in order to characterize ring particle surface properties and water ice abundance in the rings. UVIS spectra sense the outer few microns of the ring particles. Spectra of the normalized reflectance ( I/ F) in all four regions show a characteristic water ice absorption feature near 165 nm. Our analysis shows that the fractional abundance of surface water ice is largest in the outer B ring and decreases by over a factor of 2 across the inner C ring. We calculate the mean path length of UV photons through icy ring particle regolith and the scattering asymmetry parameter using a Hapke reflectance model and a Shkuratov reflectance model to match the location of the water ice absorption edge in the data. Both models give similar retrieved values of the photon mean length, however the retrieved asymmetry ( g) values are different. The photon mean path lengths are nearly uniform across the B and A rings. Shortward of 165 nm the rings exhibit a slope that turns up towards shorter wavelengths, while the UV slope of 180/150 nm (reflectance outside the water absorption ratioed to that inside the absorption band) tracks I/ F with maxima in the outer B ring and in the central A ring. Retrieved values of the scattering asymmetry parameter show the regolith grains to be highly backscattering in the FUV spectral regime.

Detection of Additional Members of the 2003 EL61 Collisional Family via Near-Infrared Spectroscopy

We have acquired near-infrared spectra of Kuiper Belt objects 2003 UZ117, 2005 CB79, and 2004 SB60 with NIRC on the Keck I telescope. These objects are dynamically close to the core of the 2003 EL61 collisional family and were suggested to be potential fragments of this collision by Ragozzine and Brown. We find that the spectra of 2003 UZ117 and 2005 CB79 both show the characteristic strong water ice absorption features seen exclusively on 2003 EL61, its largest satellite, and the six other known collisional fragments. In contrast, we find that the near-infrared spectrum of 2004 SB60 is essentially featureless with a fraction of water ice of less than 5%. We discuss the implications of the discovery of these additional family members for understanding the formation and evolution of this collisional family in the outer solar system.

On the crystallinity of silicate dust in the interstellar medium

Abstract An accurate knowledge of the mineralogy (chemical composition and crystal structure) of the silicate dust in the interstellar medium (ISM) is crucial for understanding its origin in evolved stars, the physical and chemical processing in the ISM, and its subsequent incorporation into protostellar nebulae, protoplanetary discs and cometary nuclei where it is subjected to further processing. While an appreciable fraction of silicate dust in evolved stars, in protoplanetary discs around pre-main-sequence stars, in debris discs around main-sequence stars and in cometary nuclei is found to be in crystalline form, very recent infrared spectroscopic studies of the dust along the sightline toward the Galactic Centre source Sgr A* placed an upper limit of ∼1.1 per cent on the silicate crystalline fraction, well below the previous estimates of ∼5 or ∼60 per cent derived from the observed 10-μm absorption profile for the local ISM toward Cyg OB2 No. 12. Since the sightline toward Sgr A* contains molecular cloud materials as revealed by the detection of the 3.1- and 6.0-μm water ice absorption features, we argue that by taking into account the presence of ice mantles on silicate cores, the upper limit on the degree of silicate crystallinity in the ISM is increased to ∼3–5 per cent.

Detection of Methane on Kuiper Belt Object (50000) Quaoar

The near-infrared spectrum of (50000) Quaoar obtained at the Keck Observatory shows distinct absorption features of crystalline water ice, solid methane, and ethane, and possibly other higher order hydrocarbons. Quaoar is only the fifth Kuiper Belt object (KBO) on which volatile ices have been detected. The small amount of methane on an otherwise water ice-dominated surface suggests that Quaoar is a transition object between the dominant volatile-poor small KBOs and the few volatile-rich large KBOs such as Pluto and Eris.

The albedo dichotomy of Iapetus measured at UV wavelengths

The dramatic hemispheric dichotomy in albedo displayed by Saturn's moon Iapetus has intrigued astronomers for centuries. Here we report on far-ultraviolet observations of Iapetus' bright and dark terrains from Cassini. We compare the reflectance spectra of Iapetus's dark terrain, Hyperion and Phoebe and find that both Phoebe and Hyperion are richer in water ice than Iapetus' dark terrain. Spectra of the lowest latitudes of the dark terrain display the diagnostic water ice absorption feature; water ice amounts increase within the dark material away from the apex (at 90° W longitude, the center of the dark leading hemisphere), consistent with thermal segregation of water ice. The water ice in the darkest, warmest low latitude regions is not expected to be stable and may be a sign of ongoing or recent emplacement of the dark material from an exogenic source.

Cloud property estimates from zenith spectral measurements of scattered sunlight between 0.9 and 1.7 μm

A theoretical approach is used to quantify the information available to retrieve cloud physical properties from data taken by a ground‐based spectrometer measuring scattered sunlight in the near‐infrared wavelength region. Three wavelength regions between 0.9 and 1.7 μm, each containing water vapor, liquid, and ice absorption features, are examined using a differential optical absorption spectroscopy optimal estimation retrieval technique. Cloud properties that can be retrieved include path‐integrated liquid water path and path‐integrated ice water path (PLWP and PIWP), cloud liquid and ice temperatures, and the second moment of the photon path distribution. The accuracy of these cloud property retrievals is estimated for a variety of simulated conditions, with key analysis assumptions identified. The sensitivity of the measurements in the longest wavelength region to liquid water and ice is high, allowing for accurate estimates of PLWP and PIWP under optically thin clouds, while the shorter two wavelength bands provide more information under optically thicker clouds. Observations of mixed‐phase clouds over Barrow, Alaska, are used to illustrate the practicality of the technique. Retrieved LWP values (inferred from PLWP) are compared to LWP estimates from a microwave radiometer and an atmospheric emitted radiance interferometer; PIWP estimates are compared to IWP estimates from a millimeter‐wave cloud radar. Cloud liquid temperature and photon path distribution information retrieved from these data are also presented. Furthermore, we suggest a technique for combining near‐infrared spectral PLWP measurements with microwave radiometer observations to estimate cloud droplet effective radius.

Simultaneous visible-near IR photometric study of Kuiper Belt Object surfaces with the ESO/Very Large Telescopes

We present simultaneous visible-near-IR (BVRIJHK) photometry of 8 Kuiper belt objects (KBOs) and 4 Centaurs. The observations were carried out using two 8m-units of the ESO/Very Large Telescope operated simultaneously on November 29 and 30, 2000. The KBOs displayed linear reflectivity spectra in the optical with a wide range of slopes from neutral (solar) to very red. In most cases, the reflectivity spectra are linear from B to J bands. We notice a change of regime in the spectra which flatten toward the H and K bandpasses. (33128) 1998 BU 48 even has a significantly negative H - K value. Only (54598) 2000 QC 243 does not flatten in the infra-red. (24835) 1995 SM 55 has a noticeably blue spectrum in the near-lR which might be indicative of water ice absorption features. We also present an empirical model for KBO surface evolution, based on models from Luu & Jewitt (1996) and Stern (1995), in which surface colors evolve because of collisions. In the model presented here we propose that cometary activity can refresh the surface. This model predicts that (1) small (e.g. «80 km in radius) KBOs should have color variations with rotation, (2) the larger objects should display uniform colors, (3) there should be no color dependence with inclination for objects > 100 km in radius, and (4) very distant KBOs are fully irradiated.

The Composition of Saturn's Rings

A composite spectrum between 0.30 and 4.05 μm of Saturn's rings is analyzed using the Shkuratov scattering theory (Shkuratov et al. 1999, Icarus 137, 235–246). Several types of surface and composition are discussed. We demonstrate that both the strong reddening over the interval 0.3–0.7 μm and the water ice absorption features are well reproduced by an intimate (“salt-and-pepper”) mixture of four coarse particles of two different materials: 93% are grains (typical sizes of 10, 200, and 2000 μm) of water ice containing a few percent of refractory organic solid (tholin) impurities within their bulk, and 7% are coarse grains of a dark material (amorphous carbon). The cosmogenic implications of the inferred composition are discussed.

The Near Infrared Spectrum of Miranda: Evidence of Crystalline Water Ice

A spectrum from 1.2 to 2.5 μm of Uranus' small satellite Miranda obtained in June 1999 reveals strong water-ice signatures. It confirms the existence of a 2.0-μm water feature previously detected on Miranda and shows a strong second broad 1.5-μm water-ice absorption feature. The spectra also reveal a weak absorption band at 1.65 μm that is indicative of crystalline water ice. Reflectance models which combine the new spectra with new photometry indicate that the spectra are characteristic of a mostly water-ice surface, with a large fraction of carbonaceous or silicate contaminates, and the possible presence of ammonia hydrate, as implied by an apparent weak feature near 2.2 μm. The possible presence of other volatiles is also investigated.

A Detection of Water Ice on Jupiter with Voyager IRIS

Possible water ice spectral signatures have been detected on Jupiter using Voyager Infrared Interferometer Spectrometer and Radiometer (IRIS) spectra taken near closest approach of the Voyager 1 and 2 missions. Comparison with simultaneous imaging data shows that these IRIS footprints approximately correspond to locations on the planet where deep convection is suspected. Of the sample of 7500+ spectra, approximately 1% show a water ice absorption feature near 44 μm. The infrared optical depths of the absorption features, between 0.18 and 0.44, indicate that large amounts of ice particles need not be present to produce the observed spectral signature. The water ice clouds would have visible optical depths on the order of unity and would not be distinguishable above a thick ammonia cloud deck, which can have much larger visible optical depths. These thin clouds yield about 1 precipitable micrometer of water, comparable to Martian Tharsis Ridge clouds.

Viking era water‐ice clouds

We have spatially and temporally mapped water ice clouds on Mars for 1.25 Martian years using the Viking infrared thermal mapper (IRTM) data. Our technique compares brightness temperatures in the 11 and 20 μm IRTM channels, utilizing the 11 μm water ice absorption feature. A complication arises because of the surface nonunit and wavelength‐dependent emissivities. We developed a technique for removal of this effect. Using a surface thermal model, we calculated brightness temperatures and their differences for the IRTM channels resulting from the surface emissivity effect alone. These were then subtracted from the measured brightness temperatures, yielding brightness temperature differences dominated by atmospheric effects. The ability to identify water ice clouds in the infrared provides potential new information about particle size and physical processes by comparing these clouds to those seen in visible wavelengths. We found that water ice clouds were more widespread and frequent during the Viking period than had been previously recognized, with the northern spring and summer being the cloudiest periods on Mars. We interpret some of the identified cloudy zones as the southern and northern solstice season upwelling branches of the Hadley cell, although these were shifted 15°–20° southward from model predictions. Additionally, the transition between the two branches occurred later in time than in the model predictions. We observed the extension of the north polar hood below 60°N in longitudes 120°–200°. We did not find evidence for a south polar hood north of 60°S nor any evidence for interannual variability within our limited data set.

Detection of Water Ice on 2060 Chiron

We present a reflectance spectrum of 2060 Chiron over the wavelength range 1.6–2.2 μm. The 2-μm water ice absorption feature seen in several other Centaurs at λ=2.03 μm is identified at the 5% level below the continuum.