Exponential Gaussian approach for spectral modeling: The EGO algorithm I. Band saturation

Curve fitting techniques are a widespread approach to spectral modeling in the VNIR range [Burns, R.G., 1970. Am. Mineral. 55, 1608-1632; Singer, R.B., 1981. J. Geophys. Res. 86, 7967-7982; Roush, T.L., Singer, R.B., 1986. J. Geophys. Res. 91, 10301-10308; Sunshine, J.M., Pieters, C.M., Pratt, S.F., 1990. J. Geophys. Res. 95, 6955-6966]. They have been successfully used to model reflectance spectra of powdered minerals and mixtures, natural rock samples and meteorites, and unknown remote spectra of the Moon, Mars and asteroids. Here, we test a new decomposition algorithm to model VNIR reflectance spectra and call it Exponential Gaussian Optimization (EGO). The EGO algorithm is derived from and complementary to the MGM of Sunshine et al. [Sunshine, J.M., Pieters, C.M., Pratt, S.F., 1990. J. Geophys. Res. 95, 6955-6966]. The general EGO equation has been especially designed to account for absorption bands affected by saturation and asymmetry. Here we present a special case of EGO and address it to model saturated electronic transition bands. Our main goals are: (1) to recognize and model band saturation in reflectance spectra; (2) to develop a basic approach for decomposition of rock spectra, where effects due to saturation are most prevalent; (3) to reduce the uncertainty related to quantitative estimation when band saturation is occurring. In order to accomplish these objectives, we simulate flat bands starting from pure Gaussians and test the EGO algorithm on those simulated spectra first. Then we test the EGO algorithm on a number of measurements acquired on powdered pyroxenes having different compositions and average grain size and binary mixtures of orthopyroxenes with barium sulfate. The main results arising from this study are: (1) EGO model is able to numerically account for the occurrence of saturation effects on reflectance spectra of powdered minerals and mixtures; (2) the systematic dilution of a strong absorber using a bright neutral material is not responsible for band deformation. Further work is still required in order to analyze the behavior of the EGO algorithm with respect to the saturation phenomena using more complex band shapes than pyroxene bands.     

Northwest Africa 6232: Visible–near infrared reflectance spectra variability of an olivine diogenite

Visible and near-infrared (VNIR) reflectance is an important spectroscopic technique to identify minerals, and their associations, on planetary body surfaces. Howardites, eucrites, and diogenites (HED) are a class of igneous-like meteorites whose genetic connection with asteroid 4 Vesta has since long been inferred and recently confirmed by Dawn mission results. Pyroxene and olivine are the two major mafic minerals present in HED which can be identified with VNIR reflectance measurements. Thus, studying the compositional variability of those phases and their mixtures by means of laboratory spectroscopic measurements on different diogenitic or eucritic samples is one of the prime methods to better understand the evolution of 4 Vesta's crust. Here, we report the VNIR reflectance spectral analysis of a harzburgitic olivine diogenite, Northwest Africa 6232 (probably paired with Northwest Africa 5480), containing variable amounts of olivine as small grains or aggregates. We found that the olivine diogenite spectral parameters (e.g., band position) of powdered samples and polished slabs are in agreement. Moreover, the olivine diogenite band position shifts from synthetic orthopyroxene in accordance with the presence of olivine and chromite. In particular, the presence of a large olivine clast permits us to determine a linear variation of the band position from synthetic orthopyroxene and olivine, but underestimates the presence of olivine in the olivine diogenite spot.     

Assessing the spatial variability of the ~3 μm OH/H2O absorption feature in CM2 carbonaceous chondrites

H₂O and OH are readily detected in hydrated minerals in CM chondrites via reflectance spectroscopy due to their characteristic vibration absorptions at infrared wavelengths. Previous spectroscopic work on bulk powdered CM chondrites has shown that spectral parameters, like the wavelength position of the “3 μm absorption feature,” vary systematically with the extent to which the samples have been aqueously altered. However, it is yet unclear how these spectral features may vary across an intact meteorite chip when measured at spatial scales smaller than that of the individual components of the meteorite. Here, we explore the spatial variability of this spectral feature and others on intact CM2 chips which, unlike powders, retain their petrologic and textural characteristics. We also model the modal mineralogy of the bulk meteorite powders and correlate this with key spectral features, demonstrating that microscope Fourier transform infrared spectroscopic mapping provides a powerful, rapid, and non-destructive technique for assessing compositional diversity and variations in water–rock interactions in chondritic planetary materials. In all CM2 chondrites studied here, we find that variations in the position, shape, and strength of the 3 μm absorption feature reveal a single chondrite can exhibit as much spectral variation as the entire suite of CM2 chondrites. The observed variations in the position and shape of the 3 μm feature within individual CM2 chondrite chips suggest a range of alteration products (e.g., Mg-rich to Fe-rich phyllosilicates) are present and record sub-mm scale variations in the amount and/or chemistry of the altering fluids. The samples having experienced the most progressive aqueous alteration show the least amount of variability in features like the 3 μm absorption band minimum position, whereas the least altered samples exhibit the most variability. We also find that the bulk spectral signatures in the least altered samples appear to be biased toward the spectral signatures of clasts versus matrix. By extension, asteroid reflectance spectra exhibiting 3 μm absorption features consistent with those measured here may be interpreted in a similar framework in which the spectrum of what may appear to be the least altered asteroids represents an average that belies the true diversity of mineralogy and chemistry of the body.     

Ultraviolet spectral reflectance properties of common planetary minerals

Ultraviolet spectral reflectance properties (200-400 nm) of a large number of minerals known or presumed to exist on the surfaces of Mars, the Moon, and asteroids, and in many meteorites, were investigated. Ultraviolet reflectance spectra (200-400 nm) of these minerals range from slightly blue-sloped (reflectance decreasing toward longer wavelengths) to strongly red-sloped (reflectance increasing toward longer wavelengths). Most exhibit one or two absorption features that are attributable to FeO charge transfers involving Fe³⁺ or Fe²⁺. The UV region is a very sensitive indicator of the presence of even trace amounts (<0.01 wt%) of Fe³⁺ and Fe²⁺. The major Fe³⁺O absorption band occurs at shorter wavelengths (∼210-230 nm), and is more intense than the major Fe²⁺O absorption band (∼250-270 nm). Ti-bearing minerals, such as ilmenite, rutile and anatase exhibit UV absorption bands attributable to Ti⁴⁺O charge transfers. While the positions of metal-O charge transfer bands sometimes differ for different minerals, the variation is often not diagnostic enough to permit unique mineral identification. However, iron oxides and oxyhydroxides can generally be distinguished from Fe-bearing silicates in the 200-400 nm region on the basis of absorption band positions. Within a given mineral group (e.g., low-calcium pyroxene, olivine, plagioclase feldspar), changes in Fe²⁺ or Fe³⁺ abundance do not appear to result in a measurable change in absorption band minima positions. Absorption band positions can vary as a function of grain size, however, and this variation is likely due to band saturation effects. The intensity of metal-O charge transfers means that some minerals will exhibit saturated UV absorption bands even for fine-grained (<45 μm) powders. In cases where absorption bands are not saturated (e.g., Fe²⁺O bands in some plagioclase feldspars and pyroxenes), changes in Fe²⁺ content do not appear to cause variations in band position. In other minerals (e.g., olivine), changes in band positions are correlated with compositional and/or grain size variations, but this is likely due to increasing band saturation rather than compositional variations. Overall, we find that the UV spectral region is sensitive to different mineral properties than longer wavelength regions, and thus offers the potential to provide complementary capabilities and unique opportunities for planetary remote sensing.     

Spectral reflectance properties of carbonaceous chondrites: 2. CM chondrites

We have examined the spectral reflectance properties and available modal mineralogies of 39 CM carbonaceous chondrites to determine their range of spectral variability and to diagnose their spectral features. We have also reviewed the published literature on CM mineralogy and subclassification, surveyed the published spectral literature and added new measurements of CM chondrites and relevant end members and mineral mixtures, and measured 11 parameters and searched pair-wise for correlations between all quantities. CM spectra are characterized by overall slopes that can range from modestly blue-sloped to red-sloped, with brighter spectra being generally more red-sloped. Spectral slopes, as measured by the 2.4:0.56 μm and 2.4 μm:visible region peak reflectance ratios, range from 0.90 to 2.32, and 0.81 to 2.24, respectively, with values <1 indicating blue-sloped spectra. Matrix-enriched CM spectra can be even more blue-sloped than bulk samples, with ratios as low as 0.85. There is no apparent correlation between spectral slope and grain size for CM chondrite spectra - both fine-grained powders and chips can exhibit blue-sloped spectra. Maximum reflectance across the 0.3-2.5 μm interval ranges from 2.9% to 20.0%, and from 2.8% to 14.0% at 0.56 μm. Matrix-enriched CM spectra can be darker than bulk samples, with maximum reflectance as low as 2.1%. CM spectra exhibit nearly ubiquitous absorption bands near 0.7, 0.9, and 1.1 μm, with depths up to 12%, and, less commonly, absorption bands in other wavelength regions (e.g., 0.4-0.5, 0.65, 2.2 μm). The depths of the 0.7, 0.9, and 1.1 μm absorption features vary largely in tandem, suggesting a single cause, specifically serpentine-group phyllosilicates. The generally high Fe content, high phyllosilicate abundance relative to mafic silicates, and dual Fe valence state in CM phyllosilicates, all suggest that the phyllosilicates will exhibit strong absorption bands in the 0.7 μm region (due to Fe³⁺-Fe²⁺ charge transfers), and the 0.9-1.2 μm region (due to Fe²⁺ crystal field transitions), and generally dominate over mafic silicates. CM petrologic subtypes exhibit a positive correlation between degree of aqueous alteration and depth of the 0.7 μm absorption band. This is consistent with the decrease in fine-grained opaques that accompanies aqueous alteration. There is no consistent relationship between degree of aqueous alteration and evidence for a 0.65 μm region saponite-group phyllosilicate absorption band. Spectra of different subsamples of a single CM can show large variations in absolute reflectance and overall slope. This is probably due to petrologic variations that likely exist within a single CM chondrite, as duplicate spectra for a single subsample show much less spectral variability. When the full suite of available CM spectra is considered, few clear spectral-compositional trends emerge. This indicates that multiple compositional and physical factors affect absolute reflectance, absorption band depths, and absorption band wavelength positions. Asteroids with reflectance spectra that exhibit absorption features consistent with CM spectra (i.e., absorption bands near 0.7 and 0.9 μm) include members from multiple taxonomic groups. This suggests that on CM parent bodies, aqueous alteration resulted in the consistent production of serpentine-group phyllosilicates, however resulting absolute reflectances and spectral shapes seen in CM reflectance spectra are highly variable, accounting for the presence of phyllosilicate features in reflectance spectra of asteroids across diverse taxonomic groups.     

Mid‐infrared spectroscopy of refractory inclusions (CAIs) in CV and CO chondrites

Abstract— We present laboratory mid‐infrared absorption spectra (2.5 urn to 16.0 μm) of powdered calcium‐aluminum‐rich inclusions (CAIs) and matrix separated from the carbonaceous chondrites Allende (CV3.2), Vigarano (CV3.3), and Ornans (C03.3). Two groups of spectra with different features were found for the CAI: in the first group spectra are dominated by spinel, pyroxene, and sodalite ± nepheline, where main features occur at 9.3 μm, 10.3 μm, and 11.3 μm. In the second group, characteristic minerals are spinel and melilite with typical band maxima at 11.0 μm and 12.3 μm, and a broad feature between 14.0 μn and 15.0 μn. The position of the broad spinel feature probably depends on its iron content. Comparison of band positions in spectra from the CAI components to observed circumstellar emission spectra indicates the potential occurrence of CAI‐like material. Pyroxene‐ and spinel‐rich features could occur in spectra of dust around the Herbig Ae star HD104237, the T Tauri star Hen3‐600 and the post‐AGB star R Sge. Melilite‐ and spinel‐rich components possibly appear in the spectrum of HD 104237, Hen3‐600, 04187_1927, R Sge, and the planetary nebula Hb 12. There is also indication for a spinel component in dust from the Herbig Ae/Be star HD 179218. The spectra of the AGB stars R Cas and θ Aps show no features of CAl‐type spinel.     

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.     

Spectral properties of plagioclase and pyroxene mixtures and the interpretation of lunar soil spectra

The effects of particle size and mineral proportions on the spectral characteristics of plagioclase and pyroxene mixtures are investigated. Size separates (<25 μm, 25–45 μm, 45–75 μm, 75–125 μm, 125–250 μm, and 250–500 μm) have been prepared for the following labradorite/enstatite compositional mixtures: 100/0%, 95/5%, 85/15%, 50/50%, and 0/100%. Spectrally, the labradorite and enstatite samples are representative of the plagioclase feldspars and the orthopyroxenes: the labradorite exhibits a weak, broadband centered near 1.25 μm and the enstatite exhibits two well-defined bands centered near 0.9 and 1.9 μm. From analysis of the plagioclase bands of the mixtures, it is found that (1) the amount of plagioclase necessary for the plagioclase band to be observed as a discrete absorption band is dependent on particle size and (2) plagioclase can be detected by flattening of the pyroxene reflectance “peak” between the 0.9- and 1.9-μm absorption bands if significant amounts of plagioclase are present. Analogs for immature and mature lunar highland soils have been created to examine the combined effects of particle size and mineral proportions on spectra of plagioclase and pyroxene mixtures. bidirectional reflectance spectra of these soil analogs are used to examine the detectability of plagioclase in soil-like particle size distributions. Plagioclase in significant amounts is detected by the flattening of the pyroxene reflectance “peak” between the 0.9- and 1.9-μm absorption bands, and the plagioclase absorption band itself is observed with 85% plagioclase present. The soil analogs reveal that particle size accounts for only a minor spectral difference between immature and mature lunar highland soils. From comparisons with spectra of returned lunar samples, agglutinates are found to dominate the spectral variations associated with soil maturity. Spectra of the immature soil analogs can be used to estimate the minimum pyroxene abundance for immature regions observed remotely.     

Spectral reflectance properties of carbonaceous chondrites: 1. CI chondrites

Existing reflectance spectra of CI chondrites (18 spectra of 3 CIs) have been augmented with new (18 spectra of 2 CIs) reflectance spectra to ascertain the spectral variability of this meteorite class and provide insights into their spectral properties as a function of grain size, composition, particle packing, and viewing geometry. Particle packing and viewing geometry effects have not previously been examined for CI chondrites. The current analysis is focused on the 0.3-2.5 μm interval, as this region is available for the largest number of CI spectra. Reflectance spectra of powdered CI1 chondrites are uniformly dark (<10% maximum reflectance) but otherwise exhibit a high degree of spectral variability. Overall spectral slopes range from red (increasing reflectance with increasing wavelength) to blue (decreasing reflectance with increasing wavelength). A number of the CI spectra exhibit weak (<5% deep) absorption bands that can be attributed to both phyllosilicates and magnetite. Very weak absorption bands attributable to other CI phases, such as carbonates, sulfates, and organic matter may be present in one or a few spectra, but their identification is not robust. We found that darker spectra are generally correlated with bluer spectral slopes: a behavior most consistent with an increasing abundance of fine-grained magnetite and/or insoluble organic material (IOM), as no other CI opaque phase appears able to produce concurrent darkening and bluing. Magnetite can also explain the presence of an absorption feature near 1 μm in some CI spectra. The most blue-sloped spectra are generally associated with the larger grain size samples. For incidence and emission angles <60°, increasing phase angle results in darker and redder spectra, particularly below ∼1 μm. At high incidence angles (60°), increasing emission angle results in brighter and redder spectra. More densely packed samples and underdense (fluffed) samples show lower overall reflectance than normally packed and flat-surface powdered samples. Some B-class asteroids exhibit selected spectral properties consistent with CI chondrites, although perfect spectral matches have not been found. Because many CI chondrite spectra exhibit absorption features that can be related to specific mineral phases, the search for CI parent bodies can fruitfully be conducted using such parameters.     

Mechanisms for incorporation of hydrogen in and on terrestrial planetary surfaces

Storage of hydrogen atoms in or on a planetary surface can take place via several different mechanisms. If the hydrogen atom reacts to form a hydroxyl (OH) group or water molecule, an absorption band near 3 μm will be present. Many possible mechanisms for sequestering atomic hydrogen are discussed: internal hydrogen in the form of non-structural OH and H₂O in nominally-anhydrous minerals, structural OH in minerals, structural H₂O in minerals, H₂O in fluid inclusions, and OH and H₂O in glasses; bulk H₂O as either liquid water or ice; and surficial hydrogen that is either physisorbed as H₂O, chemisorbed as an H₂O surface complex, or chemically-bound as an OH group on surface terminal sites and grain boundary regions. Understanding the spectroscopic distinctions among these various phenomena is of critical importance in constraining both the evolution of planetary interiors and the cycling of water on planetary surfaces. Proper interpretation of 3-μm bands in reflectance spectra is shown to depend upon the relative contributions from surficial vs. interior hydrogen, which vary with effective surface area (i.e., the grain size and surface roughness) and the volume sampled by the spectrometer.     

The vibrational overtone spectrum of liquid methane in the visible and near infrared: Applications to planetary studies

The strengths of 10 bands in the absorption spectrum of liquid methane betwen 19 400 and 6190 Å have been measured. After a small correction for the polarizability of the liquid is applied, for the purpose of comparison with similar gas phase measurements, it is found that there is no temperature dependence of the band strengths between 95 and 295°K. Changes of band shape with temperature cause the 95°K laboratory spectra to resemble Saturn more than room temperature observations do. Gas phase absorption clearly dominates the liquid in planetary spectra, so liquid methane cannot be detected in the outer Solar System by Earth-based observations.     

Spectral reflectance properties of carbonaceous chondrites: 6. CV chondrites

Multiple reflectance spectra of 11 CV chondrites have been measured to determine spectral–compositional relationships for this meteorite class and to aid the search for CV parent bodies. The reflectance of CV chondrite spectra is variable, ranging from ～5% to 13% at 0.56 μm, and ～5% to 15% at the 0.7 μm region local reflectance maximum. Overall slopes range from slightly blue to red for powders, while slab spectra are strongly blue-sloped. With increasing average grain size and/or removal of the finest fraction, CV spectra generally become more blue-sloped. CV spectra are characterized by ubiquitous absorption features in the 1 and 2 μm regions. The 1 μm region is usually characterized by a band centered near 1.05–1.08 μm and a band or shoulder near 1.3 μm that are characteristic of Fe-rich olivine. Band depths in the 1 μm region for powdered CVs and slabs range from ～1% to 10%. The 2 μm region is characterized by a region of broad absorption that extends beyond 2 μm and usually includes band minima near 1.95 and 2.1 μm; these features are characteristic of Fe²⁺-bearing spinel. The sample suite is not comprehensive enough to firmly establish whether spectral differences exist between CV_R, CV_OxA, and CV_OxB subclasses, or as a function of metamorphic grade. However, we believe that the mineralogic and petrologic differences that exist between these classes, and with varying petrologic subtype (CV3.0–>3.7), may not be significant enough to result in measurable spectral differences that exceed spectral variations within a subgroup, within an individual meteorite, or as a function of grain size. Terrestrial weathering seems to affect CV spectra most noticeably in the visible region, resulting in more red-sloped spectra for finds as compared to falls. The search for CV parent bodies should focus on the detection of olivine and spinel absorption bands, specifically absorption features near 1.05, 1.3, 1.95, and 2.1 μm, as these are the most commonly seen spectral features of CV chondrites.     

Effects of space weathering on diagnostic spectral features: Results from He+ irradiation experiments

We performed ion irradiation of mineral samples with 50 keV He⁺, aimed to investigate ion irradiation effects on diagnostic spectral features. Reflectance spectra of samples in 0.375–2.5 μm are measured before and after ion irradiation. Silicates, including Luobusha olivine, plagioclase and basaltic glass, have shown reddening and darkening of reflectance spectra at the VIS–NIR range. Olivine is more sensitive to ion irradiation than plagioclase and basaltic glass. Irradiated Panzhihua ilmenite exhibits higher reflectance and stronger absorption features, which is totally different from lunar soil and analog silicate materials in other experiments. Using continuum removal and MGM fit, we extracted and compared absorption features of olivine spectra before and after irradiation. Ion irradiation can induce band strength decrease of olivine but negligible band centers shift. We estimate band centers shift caused by ion irradiation are quite limited, even less than variations due to chemical composition in silicates. It provides one possible explanation for no systematic shift in band positions in lunar soil. Irradiated Luobusha olivine spectrum matches spectra of olivine-dominated asteroids. Our results suggest space weathering should be new clues to explain the subtle difference between A-type asteroid spectra and laboratory spectra of olivine.     

Spectral detectability of Ca- and Mg-sulfates in Martian bright soils in the 4–5 μm wavelength range

Chemical analyses of soil samples performed at different landing sites on Mars suggest the presence of sulfate minerals. These minerals are also thought to be present in the globally mixed Martian bright soils covering large areas of the planet. However, remote soil spectra have so far provided only tentative identification of sulfates regarding mineral types and abundances. This paper concentrates on the detectability of four Ca- and Mg-sulfates (anhydrite, gypsum, kieserite, hexahydrite) in the 4–5 μm range of Martian remote soil spectra. This spectral range is important for sulfate detection as most fine-grained sulfates exhibit significant absorption bands between 4 and 5 μm, independent of the texture of the host soils (e.g., loose powdered or cemented soils). Furthermore, this is the spectral range for which the Planetary Fourier Spectrometer (PFS) and Observatoire pour la Minéralogie, l’Eau, les Glaces, et l’Activité (OMEGA) instruments onboard ESA/Mars Express mission provide high spectral and spatial resolution data. Laboratory near- and mid-IR reflectance spectra of the pure sulfates and their mixtures with a terrestrial Martian soil analog were acquired. The results show that even the smallest amount of admixed sulfate (∼5 wt%) generates significant absorption features in the portion of the 4–5 μm range not covered by the saturated Martian atmospheric CO₂ absorption band between 4.2 and 4.4 μm. Model calculations of the influence of emitted surface radiation on the detectability of sulfate features show that the depth of the features decreases strongly with increasing surface temperature of an observed area resulting in the fact that all sulfates are spectrally hidden at surface temperatures around 270 K even at ∼14 or ∼25 wt% sulfate content in the soils. Sulfates become increasingly detectable depending on the sulfate content if the surface temperature is below 260 K. The outcome of this work helps to constrain the conditions needed for remote detection of sulfates within Martian bright soils in the 4–5 μm range.     

The north-south asymmetry of polarization of Jupiter: The causes of seasonal variations

We present new results obtained from the analysis of the seasonal variations in the asymmetry of polarization of light reflected by Jupiter. From the 23-year set of observations, the anticorrelation between the asymmetries of polarization and insolation has been revealed. The mechanism explaining the observed seasonal variations of polarization has been proposed. The core of this mechanism is the effect of temperature changes in the planetary stratosphere on the processes of the stratospheric aerosol haze formation. Additional irregular factors that may influence the observed polarization asymmetry are considered.     

Distribution of icy particles across Enceladus' surface as derived from Cassini-VIMS measurements

The surface of Enceladus consists almost completely of water ice. As the band depths of water ice absorptions are sensitive to the size of particles, absorptions can be used to map variations of icy particles across the surface. The Visual and Infrared Mapping Spectrometer (VIMS) observed Enceladus with a high spatial resolution during three Cassini flybys in 2005 (orbits EN 003, EN 004 and EN 011). Based on these data we measured the band depths of water ice absorptions at 1.04, 1.25, 1.5, and 2 μm. These band depths were compared to water ice models that represent theoretically calculated reflectance spectra for a range of particle diameters between 2 μm and 1 mm. The agreement between the experimental (VIMS) and model values supports the assumption that pure water ice characterizes the surface of Enceladus and therefore that variations in band depth correspond to variations in water ice particle diameters. Our measurements show that the particle diameter of water ice increases toward younger tectonically altered surface units with the largest particles exposed in relatively “fresh” surface material. The smallest particles were generally found in old densely cratered terrains. The largest particles (∼0.2 mm) are concentrated in the so called “tiger stripes” at the south pole. In general, the particle diameters are strongly correlated with geologic features and surface ages, indicating a stratigraphic evolution of the surface that is caused by cryovolcanic resurfacing and impact gardening.     

Characterising Saturn's vertical temperature structure from Cassini/CIRS

Thermal infrared spectra of Saturn from 10-1400 cm⁻¹ at 15 cm⁻¹ spectral resolution and a spatial resolution of 1°-2° latitude have been obtained by the Cassini Composite Infrared Spectrometer [Flasar, F.M., and 44 colleagues, 2004. Space Sci. Rev. 115, 169-297]. Many thousands of spectra, acquired over eighteen-months of observations, are analysed using an optimal estimation retrieval code [Irwin, P.G.J., Parrish, P., Fouchet, T., Calcutt, S.B., Taylor, F.W., Simon-Miller, A.A., Nixon, C.A., 2004. Icarus 172, 37-49] to retrieve the temperature structure and para-hydrogen distribution over Saturn's northern (winter) and southern (summer) hemispheres. The vertical temperature structure is analysed in detail to study seasonal asymmetries in the tropopause height (65-90 mbar), the location of the radiative-convective boundary (350-500 mbar), and the variation with latitude of a temperature knee (between 150 and 300 mbar) which was first observed in inversions of Voyager/IRIS spectra [Hanel, R., and 15 colleagues, 1981. Science 212, 192-200; Hanel, R., Conrath, B., Flasar, F.M., Kunde, V., Maguire, W., Pearl, J.C., Pirraglia, J., Samuelson, R., Cruikshank, D.P., Gautier, D., Gierasch, P.J., Horn, L., Ponnamperuma, C., 1982. Science 215, 544-548]. Uncertainties due to both the modelling of spectral absorptions (collision-induced absorption coefficients, tropospheric hazes, helium abundance) and the nature of our retrieval algorithm are quantified.Temperatures in the stratosphere near 1 mbar show a 25-30 K temperature difference between the north pole and south pole. This asymmetry becomes less pronounced with depth as the radiative time constant for the atmospheric response increases at deeper pressure levels. Hemispherically-symmetric small-scale temperature structures associated with zonal winds are superimposed onto the temperature asymmetry for pressures greater than 100 mbar. The para-hydrogen fraction in the 100-400 mbar range is greater than equilibrium predictions for the southern hemisphere and parts of the northern hemisphere, and less than equilibrium predictions polewards of 40° N.The temperature knee between 150-300 mbar is larger in the summer hemisphere than in the winter, smaller and higher at the equator, deeper and larger in the equatorial belts and small at the poles. Solar heating on tropospheric haze is proposed as a possible mechanism for this effect; the increased efficiency of ortho- to para-hydrogen conversion in the southern hemisphere is consistent with the presence of larger aerosols in the summer hemisphere, which we demonstrate to be qualitatively consistent with previous studies of Saturn's tropospheric aerosol distribution.     

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.     

Spectral-compositional variations in the constituent minerals of mafic and ultramafic assemblages and remote sensing implications

The 0.3–2.6 m reflectance spectra of most mafic and ultramafic assemblages can best be interpreted by considering the spectra as being composed of mafic silicate spectra modified by the presence of opaques, such as ilmenite or magnetite, and plagioclase feldspar. The systematic spectral-compositional relationships for olivine, orthopyroxene, and clinopyroxene have been examined and it has been determined that absorption band wavelength positions are correlated with ferrous iron content. Binary mafic silicate mixtures are generally less well understood, but certain spectral features such as reflectance maxima and minima wavelength positions and absorption band areas can be used to quantify or at least constrain end member abundances and compositions. The addition of opaques to a mafic silicate assemblage lowers overall reflectance and band depths. This differs from the effects of increasing grain size which are to lower overall reflectance but increase band depths. Plagioclase is relatively transparent compared to mafic silicates and must be present in appreciable amounts (tens of percent) to be spectrally detectable. The reflectance spectra of most mafic and ultramafic assemblages are dominated by mafic silicate absorption features and analysis of their spectra on this basis allows constraints to be placed on properties such as end member abundances and compositions.     

North-south asymmetries of solar particle events in upper stratospheric ozone

Stratospheric ozone depressions, following intense solar particle events (SPE) observed by the backscattered ultraviolet (BUV) experiment on the Nimbus-4 satellite, indicate the existence of distinct asymmetries between the Northern and Southern Hemispheres. These asymmetries are observed in the magnitude of the depressions above the 5-mb level, their temporal variations, and the spatial (i.e., latitude and longitude) dependence of these variations. Possible causes of asymmetries, shown by two events on 4 August 1972 and 25 January 1971, can be attributed to: (1) tilt of the interplanetary magnetic field (IMF) with respect to the Earth's dipole magnetic field which influences the precipitation of energetic solar particles into the polar atmospheres; (2) differences in ozone chemistry caused by the large change in atmospheric temperature between summer and winter hemispheres; (3) seasonal differences of the stratosphere's dynamic states which are affected by upward propagating planetary waves in winter in contrast to the relatively undisturbed zonal flow in summer; (4) topographic asymmetry between Northern and Southern Hemispheres.These effects are shown by three-dimensional plots of the events in geographic coordinates and by color contour plots of the stratospheric ozone distributions in geomagnetic and geographic polar coordinates, respectively.