Search for relations among a sample of 460 asteroids with featureless spectra

We present an analysis of 460 featureless asteroid spectra in the range 0.5-0.92 μm obtained in the Small Solar System Objects Spectroscopic Survey. The spectra are described in terms of the continuum steepness (cSlope), its concavity (RRE), and the blue wing of drop in the UV reflectance (BD). Comparison with meteorite spectra confirms the link between CM meteorites and asteroids with asteroids with 0.7 μm band. Also, it is found that asteroids with extreme negative slope values may be related to CK chondrites and that asteroids with pronounced concave-down curvature are related to CO chondrites. An analysis of the distribution of the spectral parameters with semimajor axis, diameter, and albedo is performed.     

Estimating the water content of hydrated minerals using reflectance spectroscopy: I. Effects of darkening agents and low-albedo materials

The apparent strength of absorptions due to H₂O near 1.9 and 3 μm in reflectance spectra is strongly affected by sample albedo. This study uses experimental and analytical approaches to quantify the effects of albedo on estimating the water content of hydrated minerals using various band parameters. We compare spectral band parameters for a series of low-albedo physical and numerical mixtures to measured water contents. Physical experiments consist of montmorillonite, clinoptilolite, and palagonite mixed with lesser amounts of carbon black and ilmenite, whereas numerical mixtures are composed of these host minerals mixed with a material of constant, low albedo. We find the effective single-particle absorption-thickness parameter provides the best correlation to water content, independent of composition and albedo, when derived from continuum-removed single scattering albedo spectra. Uncertainties in estimated water content are on the order of ±1 wt% using this method. The normalized optical path length parameter provides the best correlation to water content when using reflectance spectra, yielding estimates within ±1.6 wt% H₂O. The accuracy of these models is related to the physical nature of the darkening material. Scattering and absorption efficiencies are easier to model for intimate mixtures containing relatively large, dark grains than mixtures dominated by coatings of a fine-grained, strongly absorbing material. This suggests the physical properties that give rise to the albedo of a material are an important factor for accurate estimates of absolute water content.     

Lunar occultation of Saturn: III. How big is Iapetus?

By considering both the orbital lightcurve of Iapetus and data obtained during the March 30, 1974, occultation of the satellite by the Moon, we obtain information about the brightness distribution on the bright face of Iapetus and derive an accurate value for the satellite's radius. From the observed orbital lightcurve we find that the trailing face of Iapetus must consist predominantly of a single bright material with an effective limb-darkening parameter of k = 0.62_?0.12^0.10. Given this result the occultation observations imply a radius of 718_?78⁺⁸⁷ km. If the patchy albedo model proposed by Morrison et al. represents the surface of Iapetus accurately (as far as the relative albedo distribution is concerned) then the radius of Iapetus is 724 ± 60 km. Both estimates are consistent with the radiometric radius of 835 (+50, ?75) km derived by Morrison et al. Combining our results with the value of 0.60 ± 0.14 for the normal reflectance (in V) of the material at the center of the bright face derived by Elliot et al. we find that the normal reflectance of the dark side material is 0.11_?0.03^+0.04. These values are higher than the corresponding values of 0.35 and 0.05 quoted by Morrison et al.     

Spectral reflectivities of the Galilean satellites and Titan, 0.32 to 0.86 micrometers

A spectrophotometric observational study of the Galilean satellites and Titan was carried out at 0.004-μm (40-Å) resolution over the spectral range 0.32 to 0.86 μm. A standard lunar area was used as a primary spectroscopic standard to establish the relative reflection spectra of the objects by ratioing the sky-corrected satellite spectra to the standard area on the Moon. J1 (Io) is found to have a spectral edge at 0.33 μm that has not been previously reported. The increase in reflectivity from 0.4 to 0.5 μm and the band at 0.56 μm are confirmed. A weak band at 0.56 μm is probable on J2 (Europa) and possible on J3 (Ganymede). J4 (Callisto) shows no spectral features that have not been previously reported. On Titan, no temporal variations in the methane bands greater than 2% were found, indicating that the effective path length in the Titan atmosphere did not change over the 3-month period of this study. A new absorption band of methane at 0.68 μm was found on Titan. We propose an extension of the evaporite model of Fanale et al. (1974, 1977) and the sulfur mixing models of Wamsteker et al. (1974) in which the primary constituent of the surface of J1 is elemental sulfur sublimated onto the surface by photodissociation of hydrogen sulfide outgassing from the interior. The sulfur is continually renewed by sublimation, sputtering, and redeposition. At low temperatures irradiation produces stable S₂, S₃, S₄, S₆, and long chain polymers. Some of these allotropes have an edge at 0.33 μm, a rising reflectance between 0.4 and 0.5 μm a band at 0.56 μm. All of these features are found in the spectrum of J1. We conclude that the lunar ratioing technique used in this study is well suited for determining the relative reflection spectra of solar system objects.     

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

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.     

The radius and albedo of hyperion

The radius and surface geometric albedo of Hyperion are calculated using the photometric/ radiometric method and a new measurement of the 20-μm thermal flux of the satellite. The results are R = 112 ± 15 km and p_v = 0.47 ± 0.11.     

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.     

Near-infrared spectral monitoring of Triton with IRTF/SpeX I: establishing a baseline for rotational variability

We present eight new 0.8 to 2.4 μm spectral observations of Neptune's satellite Triton, obtained at IRTF/SpeX during 2002 July 15-22 UT. Our objective was to determine how Triton's near-infrared spectrum varies as Triton rotates, and to establish an accurate baseline for comparison with past and future observations. The most striking spectral change detected was in Triton's nitrogen ice absorption band at 2.15 μm; its strength varies by about a factor of two as Triton rotates. Maximum N₂ absorption approximately coincides with Triton's Neptune-facing hemisphere, which is also the longitude where the polar cap extends nearest Triton's equator. More subtle rotational variations are reported for Triton's CH₄ and H₂O ice absorption bands. Unlike the other ices, Triton's CO₂ ice absorption bands remain nearly constant as Triton rotates. Triton's H₂O ice is shown to be crystalline, rather than amorphous. Triton's N₂ ice is confirmed to be the warmer, hexagonal, β N₂ phase, and its CH₄ is confirmed to be highly diluted in N₂ ice.     

The diameter and thermal inertia of 433 Eros

Radiometry of Eros at 10 and 20 μm demonstrates that the thermal conductivity of the upper centimeter of the surface is approximately as low as that of the Moon, suggesting that the asteroid has a regolith of highly porous rocky material. When combined with photoelectric photometry, these infrared measurements yield an effective diameter of Eros at maximum light of D₀ = 22 ± 2 km and a geometric albedo of p_v = 0.18 ± 0.03.     

Accurate and approximate calculations of Raman scattering in the atmosphere of Neptune

Raman scattering by H₂ in Neptune's atmosphere has significant effects on its reflectivity for λ<0.5 μm, producing baseline decreases of ∼20% in a clear atmosphere and ∼10% in a hazy atmosphere. However, few accurate Raman calculations are carried out because of their complexity and computational costs. Here we present the first radiation transfer algorithm that includes both polarization and Raman scattering and facilitates computation of spatially resolved spectra. New calculations show that Cochran and Trafton's (1978, Astrophys. J. 219, 756-762) suggestion that light reflected in the deep CH₄ bands is mainly Raman scattered is not valid for current estimates of the CH₄ vertical distribution, which implies only a 4% Raman contribution. Comparisons with IUE, HST, and groundbased observations confirm that high altitude haze absorption is reducing Neptune's geometric albedo by ∼6% in the 0.22-0.26 μm range and by ∼13% in the 0.35-0.45 μm range. A sample haze model with 0.2 optical depths of 0.2-μm radius particles between 0.1 and 0.8 bars fits reasonably well, but is not a unique solution. We used accurate calculations to evaluate several approximations of Raman scattering. The Karkoschka (1994, Icarus 111, 174-192) method of applying Raman corrections to calculated spectra and removing Raman effects from observed spectra is shown to have limited applicability and to undercorrect the depths of weak CH₄ absorption bands. The relatively large Q-branch contribution observed by Karkoschka is shown to be consistent with current estimates of Raman cross-sections. The Wallace (1972, Astrophys. J. 176, 249-257) approximation, produces geometric albedo ∼5% low as originally proposed, but can be made much more accurate by including a scattering contribution from the vibrational transition. The original Pollack et al. (1986, Icarus 65, 442-466) approximation is inaccurate and unstable, but can be greatly improved by several simple modifications. A new approximation based on spectral tuning of the effective molecular single scattering albedo provides low errors for zenith angles below 70° in a clear atmosphere, although intermediate clouds present problems at longer wavelengths.     

Estimating the water content of hydrated minerals using reflectance spectroscopy: II. Effects of particle size

Visible-near infrared reflectance spectra for five particle size fractions of a Hawaiian palagonite (HWMK101) and a nontronite (ferruginous smectite, Clay Minerals Society source clay SWa-1) were measured under ambient, purged, and heated conditions to characterize the effects of surface and volume scattering on the relationship between absolute H₂O content and the strength of the 3 μm absorption feature. Both materials were ground and dry sieved to particle sizes of <25, 25-45, 45-75, 75-125, and 125-250 μm. Particles of the bulk palagonite have an approximate bimodal distribution consisting of small, amorphous particles <5 μm in diameter mixed with crystalline and glass particles <1 mm in diameter, whereas the nontronite particles are polycrystalline aggregates. We find that band parameters value relating the strength of the 3 μm hydration feature to water content increase with particle size for a given water content, regardless of whether reflectance or single scattering albedo spectra are used. Spectra generally increase in reflectance as particle size decreases, a result of the relative increase in volume to surface scattering. Spectra of large particles are commonly saturated in the 3 μm region due to an increase in optical path length, making an accurate estimate of water content indeterminate until the samples dehydrate to the volume-scattering regime. We find that the presence of fines in several of the size fractions of palagonite cause their spectra to be representative of the finest fraction rather than the mean particle size. The nontronite spectra appear to be representative of an effective particle size within the range of the sieved size fractions. Many planetary surfaces are expected to have a large number of small particles which can dominate their spectral signature. Our results for particles <45 μm provide a reasonable model for estimating the H₂O content of hydrated asteroids and regions of Mars.     

Phosphine absorption in the 5-μm window of Jupiter

Since the original suggestion by Gillett et al. (1969) it has generally been assumed that the region of partial transparency near 5 μm in Jupiter's atmosphere (the 5-μm window) is bounded by the v₄ NH₃ at 6.1 μm and the v₃ CH₄ band at 3.3 μm. New measurements of Jupiter and of laboratory phosphine (PH₃) samples show that PH₃ is a significant contributor to the continuum opacity in the window and in fact defines its short-wavelength limit. This has important implications for the use of 5-mu;m observations as a means to probe the deep atmospheric structure of Jupiter. The abundance of PH₃ which results from a comparison of Jovian and laboratory spectra is about 3 to 5 cm-am. This is five to eight times less than that found by Larson et al. [Astrophys. J. (1977) 211, 972–979] in the same spectral region, but is in good agreement with the result of Tokunaga et al. [Astrophys. J. (1979) 232, 603–615] from 10-μm observations.     

The Temperature-Dependent Spectrum of Methane Ice I between 0.7 and 5 μm and Opportunities for Near-Infrared Remote Thermometry

New infrared absorption coefficient spectra of pure methane ice I were measured at temperatures between 30 and 90 K, over wavelengths from 0.7 to 5 μm, along with spectra of methane ice II at 20 K and liquid methane at 93 K. The spectra were derived from transmission measurements through monocrystalline samples grown in a series of closed cells having interior dimensions ranging from 100 μm to 1 cm. The thicker samples permitted measurement of extremely weak absorption bands, with absorption coefficients as small as 0.003 cm⁻¹. We report 14 new absorption bands, which we tentatively assign to specific vibrational transitions. Two of the new bands are attributed to CH₃D. Measurements of the weaker CH₄ bands are particularly needed for interpreting spectral observations of Pluto and Triton, where a number of weak CH₄-ice absorption bands have been observed. The data presented in this paper complement studies of spectral transmission by thin films of methane ice, which are most suitable for measuring the stronger absorption bands. Temperature-dependent spectral features revealed by the new data offer the opportunity to determine CH₄-ice temperatures remotely, via near-infrared reflectance spectroscopy. This approach could prove particularly valuable for future spacecraft exploration of Pluto.     

Hydrocarbons on Saturn's satellites Iapetus and Phoebe

Material of low geometric albedo (pV?0.1) is found on many objects in the outer Solar System, but its distribution in the saturnian satellite system is of special interest because of its juxtaposition with high-albedo ice. In the absence of clear, diagnostic spectral features, the composition of this low-albedo (or “dark”) material is generally inferred to be carbon-rich, but the form(s) of the carbon is unknown. Near-infrared spectra of the low-albedo hemisphere of Saturn's satellite Iapetus were obtained with the Visible-Infrared Mapping Spectrometer (VIMS) on the Cassini spacecraft at the fly-by of that satellite of 31 December 2004, yielding a maximum spatial resolution on the satellite's surface of ∼65 km. The spectral region 3-3.6 μm reveals a broad absorption band, centered at 3.29 μm, and concentrated in a region comprising about 15% of the low-albedo surface area. This is identified as the CH stretching mode vibration in polycyclic aromatic hydrocarbon (PAH) molecules. Two weaker bands attributed to CH₂ stretching modes in aliphatic hydrocarbons are found in association with the aromatic band. The bands most likely arise from aromatic and aliphatic units in complex macromolecular carbonaceous material with a kerogen- or coal-like structure, similar to that in carbonaceous meteorites. VIMS spectra of Phoebe, encountered by Cassini on 11 June 2004, also show the aromatic hydrocarbon band, although somewhat weaker than on Iapetus. The origin of the PAH molecular material on these two satellites is unknown, but PAHs are found in carbonaceous meteorites, cometary dust particles, circumstellar dust, and interstellar dust.     

A 0.4-2.5 μm spectroscopic investigation of Triton's two faces

We have obtained new observations of Triton with the ESO New Technology Telescope (La Silla, Chile) in October 2002. Using the high quality of NTT instrumentation, we were able to cover the entire 0.4-2.5 μm spectral range in a single night. We applied this procedure for two nights, well selected along the orbit of Triton, in order to cover essentially the trailing side one night, and the leading one the other night, obtaining the first face-resolved 0.4-2.4 μm spectra of Triton. We discuss here the spectra and the differences between the two faces, and the implications of these new results for a better understanding of the surface composition of Triton. In particular we found possible clues for the presence of rocky materials on Triton's surface.     

2060 Chiron: Visual and thermal infrared observations

Five-color (λλ = 0.36?0.85 μm) and thermal infrared (λ = 22.5 μm) photometric observations of the unusual asteroid 2060 Chiron were made. Between 0.36 and 0.85 μm, Chiron's reflectance spectrum is similar to those of C-class asteroids as well as Saturn's satellite Phoebe. However, the thermal IR measurements imply an albedo≥0.05 (i.e., a diameter ≤250 kmat the level 2σ level) that is probably higher than those of C-class asteroids or Phoebe.     

Global color variations on the Martian surface

Surface materials exposed throughout the equatorial region of Mars have been classified and mapped on the basis of spectral reflectance properties determined by the Viking II Orbiter vidicon cameras. Frames acquired at each of three wavelengths (0.45 ± 0.03 μm, 0.53 ± 0.05 μm, and 0.59 ± 0.05 μm) during the approach of Viking Orbiter II in Martian summer (L_s = 105°) were mosaicked by computer. The mosaics cover latitudes 30°N to 63°S for 360° of longitude and have resolutions between 10 and 20 km per line pair. Image processing included Mercator transformation and removal of an average Martian photometric function to produce albedo maps at three wavelengths. The classical dark region between the equator and ～30°S in the Martian highlands is composed of two units: (i) and ancient unit consisting of topographic highs (ridges, crater rims, and rugged plateaus riddled with small dendritic channels) which is among the reddest on the planet $\text{(0.59/0.45 μ}\text{m}\text{? 3)}$; and (ii) intermediate age, smooth, intercrater volcanic plains displaying numerous mare ridges which are among the least red on Mars $\text{(0.59/0.45 μ}\text{m}\text{? 2)}$. The relatively young shield volcanoes are, like the oldest unit, dark and very red. Two probable eolian deposits are recognized in the intermediate and high albedo regions. The stratigraphically lower unit is intermediate in both color $\text{(0.59/ 0.45 μ}\text{m}\text{? 2.5)}$ and albedo. The upper unit has the highest albedo, is very red $\text{(0.59/0.45 μ}\text{m}\text{? 3)}$, and is apparently the major constituent of the annual dust storms as its areal extent changes from year to year. The south polar ice cap and condensate clouds dominate the southernmost part of the mosaics.     

Vesta: The first pyroxene band from new spectroscopic measurements

New spectral reflectance measurements of asteroid 4 Vesta were obtained using a silicon vidicon spectrometer with a resolution of 0.002–0.004 μm. The major absorption band in the near infrared has a minimum at 0.924 ± 0.004 μm with a bandwidth of 0.18 μm full width at half power (fwhp). The band represents a 30% absorption relative to peak reflectance at 0.75 μm. The absorption band has been interpreted to be due to electronic absorptions in ferrous iron in sixfold coordination in the pyroxene, pigeonite. The increased spectral resolution of these observations compared to earlier spectrophotometry enables us to refine the pyroxene composition, from the position of the Fe²⁺ absorption band, and arrive at a relative calcium content [Ca/(Mg + Fe + Ca)] of 10–12%. The absorption band is symmetric about its center, implying the presence of little or no olivine. The existence of the 2.0-μm pyroxene band which was verified by Larson and Fink (1975) confirms the interpretation based on the 1.0-μm band.     

Nitrogen on Triton

The near-infrared spectrum of Triton is characterized by strong absorption bands of methane, probably in the solid state. An additional absorption band at 2.16 μm is tentatively identified as the density-induced (2-0) band of molecular nitrogen in the liquid state. The fundamental overtones of this band system cannot presently be observed because of limitations of the terrestrial atmosphere or spectral signal precision. Using the absorption coefficient for this band derived from laboratory observations and from the literature, it is calculated that Triton must have a layer of nitrogen at least tens of centimeters deep over much of its surface; this quantity is plausible in terms of the cosmic abundance of nitrogen and by comparison with Titan where a massive atmosphere of nitrogen exists. The Triton spectrum has been modeled with liquid nitrogen and solid methane, and it is found that the shape of the continuum in two spectral regions can be properly accounted for by adding a spectral component corresponding to fine-grained water frost. It is speculated that yet another component, a dark, solid, photochemical derivative of methane, may occur as a trace contaminant of the surface materials. If much of the surface of Triton is liquid, the radiometric observations of the satellite must be reinterpreted to derive the radius and surface albedo. If there is liquid nitrogen exposed on the surface, the atmosphere of Triton is probably dominated by nitrogen rather than methane because of the much higher vapor pressure of the former. At the calculated subsolar temperature of Triton, the vapor pressure of nitrogen implies a surface atmospheric pressure in the range 0.13 to 0.30 atm.