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We present here the Raman spectra of 9 different carbonaceous materials of astronomical relevance, including single polycyclic aromatic hydrocarbons and mixtures. Eight of 9 samples show two main bands falling respectively at around 1600 and 1350 cm −1 which imply that all the materials are composed by randomly oriented structural units varying from 50 to 80 A in size. Further bands and/or shoulders appear in some cases between 800 and 1700 cm −1 (6 and 12 μm) and are attributed to CC or CH n ( n = 1,2,3) functional groups. The coronene spectrum is the only one which does not present any significant band although it shows a very high level of fluorescence. These results provide information about candidate materials which may account for unidentified IR bands observed in astronomical sources.

The Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on board the ESA/Venus Express mission has technical specifications well suited for many science objectives of Venus exploration. VIRTIS will both comprehensively explore a plethora of atmospheric properties and processes and map optical properties of the surface through its three channels, VIRTIS-M-vis (imaging spectrometer in the 0.3-1 μm range), VIRTIS-M-IR (imaging spectrometer in the 1-5 μm range) and VIRTIS-H (aperture high-resolution spectrometer in the 2-5 μm range). The atmospheric composition below the clouds will be repeatedly measured in the night side infrared windows over a wide range of latitudes and longitudes, thereby providing information on Venus's chemical cycles. In particular, CO, H2O, OCS and SO2 can be studied. The cloud structure will be repeatedly mapped from the brightness contrasts in the near-infrared night side windows, providing new insights into Venusian meteorology. The global circulation and local dynamics of Venus will be extensively studied from infrared and visible spectral images. The thermal structure above the clouds will be retrieved in the night side using the 4.3 μm fundamental band of CO2. The surface of Venus is detectable in the short-wave infrared windows on the night side at 1.01, 1.10 and 1.18 μm, providing constraints on surface properties and the extent of active volcanism. Many more tentative studies are also possible, such as lightning detection, the composition of volcanic emissions, and mesospheric wave propagation. © 2007 Elsevier Ltd. All rights reserved.

There is much evidence about the ancient presence of water on Mars and it is reasonable to suppose that simple forms of life may have developed during the geological evolution of the planet. In such a case traces of this extinct life could still be present on the planet in form of microfossils included into some geological layer. The rover payloads planned for the next decades will include spectrometers in order to accomplish various scientific tasks. In this respect, we have developed a quantitative model for microfossil inclusions into a crystalline matrix. Such a method foresees some visible effects on measurements obtained via spectroscopic techniques such as infrared reflectance and Raman spectroscopy. In this work we present the quantitative model of the fossilization process and the effects that the microfossil inclusions should have in the real spectra, evaluated by means of computer simulations. Preliminary measurements, in order to provide examples of future model testing, have been performed on samples of homogeneous composition, but with detectable microfossils content, collected at the K–T sequence placed near Gubbio (Italy). The preliminary results are presented and discussed in order to investigate the potential application of such spectroscopic techniques for the detection of extinct life.

The study of Mars linked to its climatic history, the presence of water and possible forms of life on the planet, are becoming more and more important in recent years. In this paper we present laboratory reflection measurements on olivine samples which are believed to be the product of alteration induced by their coexistence with water. The reflection spectra have been obtained in the wavelength range from 0.2 to 2.5 μ m and for different grain sizes; they exhibit, among various bands, some of which due to H 2 O and OH - , a characteristic absorption feature at about 0.64 μ m , whose emission wings at 0.56 and near 0.7 μ m have relative strengths which are size dependent and well correlated. The identification of such feature in the observed spectra of Mars would provide useful information about the grain size of the Martian regolith and, in association with other bands, also about the possible presence of olivine altered by water in the past.

[1] In this work, an innovative approach for remote sensing data analysis is presented. A statistical multivariate approach, applied to spectroscopic data, is able to reduce and explore the large amount of data collected during planetary missions. The multivariate statistical approach implemented is a cluster analysis method together with a criterium able to identify the natural number of clusters present in the spectral data set. An evaluation of the statistical analysis methods has been developed, implemented, and applied to analyze Mars thermal emission data. We find the statistical approach readily identifies spurious data. The resulting number of clusters provides ≥105 reduction in data volume. This allows a focusing of scientific interest onto a limited number of statistically significant groups. A comparison of the results of the statistical approach to previous expert analysis of Mars thermal emission data, for the Sinus Meridiani region where a hematite-rich area of Mars has been previously detected, is provided. We find that several of the clusters reproduce the results of the expert analyses of the Sinus Meridiani hematite distribution. The current approach has the additional advantage of eliminating the time-consuming techniques of atmospheric correction, when surface features are to be investigated.

Various indications seem to suggest that in the past the Martian climate allowed the presence of liquid water on the surface of the planet. It is therefore reasonable to think that bodies of standing water were once present in basins such as craters or depressions and that evaporite deposits of sulfates could have formed in these basins. The identification of such deposits, which might have survived in some regions of the planet, could confirm the present hypothesis about the ancient climate on Mars. Recent analyses of Mars spectra, obtained by ground-based observations, show bands consistent with sulfate features. If such data have to be interpreted with the aid of synthetic spectra, the knowledge of the complex refractive index of some sulfate is necessary. In this work we present the optical constants of submicron particles of gypsum (a typical hydrate–sulfate very common on Earth), derived by means of the dispersion theory from transmission spectra. We have performed the calculation of the optical constants in the wavelength range 2–70 μm, also in view of the wide spectral range covered by the Planetary Fourier Spectrometer (PFS) on board of the ESA space mission Mars Express. The data have been compared with optical constants obtained by other authors from reflectance measurements of particulate samples.

The modelling and the interpretation of infrared spectra exhibited by astronomical dusty objects require fair acquain- tance with complex refractive indices, the so-called "optical constants", of cosmic analog materials. It turns out that the spectra of the latter, in case of a crystalline granular material, depend on the size and the shape of the grains and may dier from the spectra of the same material but in bulk form. This phenomenon can be very elegantly accounted for by considering optical lattice excitations specific to small particles, the so-called "surface modes". We present a study of the spectral behaviour, in the 1.5-62.5m range, of the optical constants of a particulate sample of limestone, a typical carbonate material mainly composed of calcite (CaCO3). Shape eects have been accounted for by considering a collection of randomly oriented ellipsoids with var- ious continuous distributions of shape parameters. It is shown that in the spectral region around the bands at 32 m and 44m, whose assignment to surface modes raises no doubt, the optical constants derived for various shape distributions are markedly dierent from each other. We find that the best agreement between laboratory and theoretical spectra is obtained for spheres while for two continuous distributions of ellipsoids the fits are slightly worse. In other words the optical constants, that describe best the interaction between electromagnetic radiation and our limestone sample, are those derived by using Mie theory (valid for spheres); this is in agreement with SEM analysis which indicates a spheroidal shape of the particles. Such conclusions, valid for limestone particles, cannot be extrapolated directly to other particles and/or materials, since every case has to be treated independently. They should nevertheless be helpful in avoiding the possible error of interpreting absorption spectra of particulate crystalline stus without taking into account the eects of particle size and shape.

The aim of this work is to present some preliminary results of a research program, carried out in our laboratory, in order to develop a reliable method for determining the spectral emissivity of a particulate sample. The mid-infrared region of the spectrum is very important in planetary studies since in this region, usually dominated by thermal emission, are present the spectral features of many materials of great interest for the knowledge of most objects belonging to our solar system. Therefore, laboratory measurements of spectral emissivity are required in order to simulate the observed spectra, mainly those originated by present and future space missions. In this paper we present preliminary results and describe the experimental procedure used to obtain the spectral emissivity, based on the measurements of the sample and two reference black bodies kept at different temperatures.

Theoretical studies and numerical models of the atmosphere and surface of Mars need the knowledge of the optical constants of candidate materials for Martian dust, in a wavelength range as wide as possible. Limestone, as a carbonate-bearing material, is commonly considered a likely component particularly important for its links with the climate evolution and water resources on Mars. In a previous paper, we reported the complex indices of refraction of submicron particles of limestone, derived from laboratory transmission spectra in the wavelength range 3.5– 20 μm by means of the dispersion theory. In this work we present the same quantities in a wider spectral range extending from 1.5 up to 62.5 μm . Such an extension is particularly important since limestone, as well as other carbonates, has some of its characteristic features between 20 and 50 μm . Our data are discussed and compared with those given by other authors and obtained with a different method (i.e. reflection measurements) on samples consisting of pressed pellets of randomly oriented calcite grains. As in the case of the previous work, our results point out, again, that the extinction spectra of submicron particles can be better reproduced using optical constants directly obtained in the laboratory for particulate materials. Finally, the extinction and emission properties of limestone grains are used in order to tentatively identify carbonates on Mars and to understand whether these materials are present in the Martian aerosol or on the surface. Our results seem to suggest that, limestone particles can be a minor component of the atmospheric dust rather than of the surface regolith, provided that some bands observed in the Martian spectra will be finally ascribed to such material.

The evidence of the water erosion on Mars is particularly interesting since present climatic conditions are such that liquid water cannot exist at the surface. But, if water was present on the planet in the past, there may have been life, too. Since the discovery of carbonates on Mars also may have very important implications on the possibility that life developed there, we are studying minerals that can have biotic or abiotic origin: calcite (CaCO(3)) and aragonite, a metastable state of calcite.We have analysed biomineral aragonite, in the form of recent sea shells, as well as crystals of mineral aragonite. Infrared spectroscopy in the 2-25 mum wavelength range reveals that, after thermal processing, the biotic samples have a different spectral behaviour from the abiotic ones. As a result, it is possible to distinguish abiotic mineral aragonite from aragonite of recent biological origin.Obviously, if life existed in the past on the Red Planet, we could expect to find "ancient" biotic carbonates, which should therefore be investigated, in order to search for a way of discriminating them from abiotic minerals. For this reason, at the beginning we have considered samples of crushed fossil shells of aragonite composition. Afterwards, in order to take into account that fossilization processes almost always produce a transformation of metastable form (aragonite) into more stable form (calcite), we also studied samples of mineral calcite and different types of fossils completely transformed into calcite. All these biotic fossil samples show the same spectral behaviour as the fresh biotic material after thermal annealing at 485 degrees C. Instead, the calcite behaves like abiotic aragonite.Furthermore, it is known that seashells and other biominerals are formed through an intimate association of inorganic materials with organic macromolecules. The macromolecules control the nucleation, structure, morphology, crystal orientation and spatial confinement of the inorganic phase: this differentiates biominerals from minerals. Analysing the aragonite or calcite fossils with a Scanning Electron Microscope, we found that the fossilization process did not modify the structure of the biominerals which maintain their microscopic characteristics. Looking at the morphology of fossil biominerals, it is evident that the crystals are arranged in complex architectures compared with the compact structure of the mineral crystals. In conclusion, the properties and structure of the biominerals are different from those of the minerals. The rapid increase of the crystalline structure developed under biotic conditions makes these minerals less resistant to thermal treatments, compared with samples of abiotic origin. This result holds both for recent shells as well as all fossil samples. The spectroscopic behaviour of all analysed calcium carbonates of biotic origin is different from that of the abiotic one. Therefore, the infrared spectroscopy is a valid technique to discern the origin of the samples and a powerful tool for analysing in-situ and "sample-return" Mars missions specimens. Also Optical and Scanning Electron Microscopy can be useful to support this type of studies.

The main constituents of the dust, produced around late-type carbon stars, are thought to be carbon and silicon carbide (SiC), although their exact nature is not yet well established. This subject has been addressed by several authors and good fits of a large number of IRAS sources (chosen among the carbon stars exhibiting the SiC feature at about 11.3 pm) were obtained. In this work we use the same procedure on a limited number of objects, of the same type, taking into account the variability of such sources and the changes induced in their IR spectra. For this purpose, the chosen stars have been observed with a spectral resolution higher than that available for the IRAS data (λ/Δλ ≅50), using the CGS3 instrument of the UKIRT telescope, both in the low (λ/Δλ ≅ 160) and in the high (λ/Δλ ≅ 500) resolution configuration. The results are discussed and some preliminary conclusions are drawn.

The spectra of 26 IRAS sources have been fitted using experimental optical data obtained in our laboratory for different types of carbon and silicon carbide (SiC) submicronic particles. Laboratory data are the main input for a radiative transfer model, which, taking into account several important parameters of the circumstellar matter and of the central star, evaluates the temperature gradient across the envelope and hence the emission best-fit spectrum. In this way it has been possible to fit, with good accuracy, most of the examined IRAS spectra in the range 8 — 100 μm. The sources have been chosen among those showing SiC feature around 11 pm and the best fits have been obtained by carefully assessing the amount and the type (α & β) of SiC particles present in the grain mixtures.

Recent IR observations of Comets Halley, Wilson, and Bradfield have shown the existence of an emission feature falling at around 3.4 microns. In this paper, it is shown that a good fit of both the IR continuum and the band can be obtained by assuming a simple thermal emission model based upon the optical properties of hydrogenated amorphous carbon (HAC) grains, measured in laboratory. The best fits of P/Halley's spectra are obtained for HAC dust amounts which appear consistent with the constraints imposed by the observed production rates. The presence of amorphous carbon solid particles is also supported on the basis of in situ mass spectrometry measurements performed by the Vega 1/2 and Giotto spacecraft in the environment of P/Halley. At present, it appears reasonable to suggest that amorphous carbon grains are able to match the observations and can be considered among likely candidates for cometary materials. 51 refs.

We present here an updated review of laboratory results obtained from different carbonaceous materials in grain and molecular forms. Temperature behaviour of bands falling between 3 µm and 13 µm is reported for some PAHs. FIR spectra are measured for ten different solids. Finally, we have also identified several IR space sources which can be studied by means of ISO instruments to contribute a possible clarification on UIBs.

Les molecules d'hydrocarbures aromatiques polycycliques (HAP) sont considerees parmi les candidats les plus probables pour expliquer les bandes IR non identifiees. Jusqu'a present, seules les proprietes d'absorption a temperature ambiante de quelques especes HAP neutres ont ete utilisees pour simuler les spectres d'emission observes et pour deduire les proprietes physiques et chimiques des especes HAP interstellaires, sous l'hypothese ou les proprietes optiques des HAP ne dependent pas de la temperature. Cette hypothese a ete testee en mesurant, dans l'etendue des temperatures 300-520 K, la transmittance IR du coronene (C 24 H 12 ), du chrysene (C 18 H 12 ), et du 1-methylcoronene (C 25 H 14 ) plonges dans des matrices de KBr et CsI.

Recent exploration of comet Halley by space missions and ground-based observations have allowed to acquire a large amount of new information on cometary materials. In particular, IR spectroscopy has evidenced two pronounced and structured emission features at around 3.28 and 3.37 μm. Different kinds of gaseous molecules and/or solid grains including CH-X bonds have been proposed as possible carriers of the bands. In the past years, laboratory experiments have allowed to characterize the physical and optical properties of different classes of carbonaceous materials. The absorption properties measured for some of them are here used to fit the cometary features and to identify - if possible - the most probable carriers of these bands.

In this paper we present the results of a simplified model to determine the flux emerging from dust envelopes around cool stars. The model proposed holds under the hypotheses of negligible scattering effects and spherical geometry of the dust cloud.

Three different kinds of α-SiC and one β-SiC sample which have undergone the same treatment procedures have been studied morphologically. Their absorption spectra have also been measured in the range 2.5–300 μm. Outside the typical 11.5 μm absorption band, all spectra show an FIR trend which can be fitted by a power law λ -γ . The steepness is seen to increase as a function of sample purity. For the highest purity, 99.3%, γ = 1.4. The lowest values are interpreted as being due to the presence of some contaminants in the samples, metallic Fe mainly and also free C and free SiO. β-SiC also shows a λ -γ spectrum with γ ≅ 1.0, while that measured by Tanabe et al. 7 is found to be characterized by γ = 1.4 again. This difference is tentatively interpreted as being due to a higher purity of the Japanese group sample. The results of our analysis seem to suggest that α-SiC and β-SiC differ morphologically, while their FIR spectra follow a similar λ -1.4 trend. Astronomical observations at these long wavelengths do not allow any identification of the actual SiC type existing in space which, on the contrary, can probably be identified by analysing the 11.5 μm band.

We present here Raman spectra of different samples of materials, potentially candidates of interstellar dust, namely amorphous carbon submicron particles and polycyclic aromatic hydrocarbon collections (PAH). The carbon particles show spectra characterized by two peaks, falling respectively at 1365 cm −1 and 1590 cm −1 , while PAH spectra seem to be characterized by a single line at 1590 cm −1 .These preliminary results confirm that PAH collections alone cannot account for the astronomical unidentified infrared bands (UIR), while mixtures containing also hydrogenated amorphous carbon (HAC) produce a better match.

Interstellar grains represent an important constituent of matter in space. They are necessary to the cooling and heating mechanisms and play an important role in the gas phase composition of molecular clouds. In addition, metallic components are important because they carry the charge inside dense clouds regulating the ion-molecule chemistry in these regions. Despite these facts and active research which is presently lasting since over 50 years, some major questions remain still controversial. According Mathis (1986a) it is crucial to note that the words “interstellar dust” refer to different materials as soon as we consider different space regions as the diffuse ISM, the outer edges of dense clouds, the dark central regions of these clouds.

The VIRTIS scientific and technical teams will take advantage of their previous experience in the design and development of spectrometers for space applications. In fact, the various groups contributing to the VIRTIS experiment, from Italy, France and Germany, have been deeply involved in the CASSINI mission, with the experiments VIMS and CIRS. The targets of the ROSETTA mission are the most primi- tive solar system bodies : comets and asteroids. ROSETTA will study in detail a comet nucleus, the prime target of the mission, and will fly by one or two asteroids. The small bodies of the solar system are of great interest for planetary science and their study is crucial to understand the solar system formation. In fact it is believed that comets and, to a lesser extent, asteroids underwent a moderate evolution so that they preserve some pristine solar system material. Comets and asteroids are in close relationship with the plan- etesimals, which formed from the solar nebula 4.6 billion years ago. The global characterisation of one comet nucleus and one or two asteroids will provide basic information on the origin of the solar system and on the interrelation between the solar system and the interstellar dust environment. The ROSETTA mission is designed to obtain the above mentioned scientific goals by : (a) in situ analysis of comet material ; (b) long period of remote sensing of the comet. The combination of remote sensing and in situ measurements will increase the scientific return of the mission. In fact, the “in situ” measurements will give relevant “ground-truth” for the remote sensing information and, in turn, the locally collected data will be interpreted in the appropriate scenario provided by remote sensing investigation. The scientific payload of ROSETTA includes a Visual InfraRed Spectral and Thermal Spectrometer (VIRTIS) among the instrument on board the spacecraft orbiting around the comet. This instrument is fundamental to detect and study the evolution of specific fingerprints—such as the typical spectral bands of minerals and molecules—arising from surface components and from materials dispersed in the coma. Their identification is a primary goal of the ROSETTA mission as it will allow us to identify the nature of the main constituent of the comets. Moreover, the surface thermal evolution during comet approach to Sun is important information that can be obtained by means of spectroscopic observation. The VIRTIS design and its detailed science goals are reported hereafter.

There are reasons to expect that Mars is surrounded by a region of dust, similar to rings, originating from the bombardment of Phobos and Deimos by meteroids. Using a simple radiative transfer model, we have investigated the angular distribution and the absolute values of the solar radiance scattered by such a dust region, to the purpose of assessing the possibilities and limitations of future photometric searches after the circummartian dust. Our model values of the number density of the dust grains in the space around Mars and of their size distribution have been derived from the results obtained by other authors. The single-scattering albedo of the dust grains has been deduced from the reflectance spectra of Phobos, taken by the spacecraft Phobos 2. Calculations, carried out for a few phenomenological phase functions, have shown that in the visible the radiance scattered by the rings is well within the detectability range of a modern sensible photometer, so that the prospectives for photometric search for the Martian dust rings are optimistic. Furthermore, our results confirm that the dust region could not be observed by the Viking cameras and this supports o our assumptions regarding the optical properties of the circummartian grains.

Amorphous carbon grains are discussed as possible candidates for cosmic dust. Particles obtained in the laboratory do not correctly reproduce the portion of the interstellar extinction curve commonly attributed to graphite. Amorphous carbon grains with a 40-A mean radius respect the carbon cosmic abundance constraints; however, they cannot explain the interstellar UV extinction curve because they show a peak at 2350 A and give a contribution which is too high to the visual extinction. The present experimental results have been used to construct an interstellar amorphous carbon (IAC) whose properties are able to resolve the above-mentioned difficulties. Extrapolations of the laboratory data show that a size distribution with a 10-A average radius has an extinction efficiency with the peak at the right position, satisfactorily matching in shape the interstellar hump. The peak to visual extinction ratio, Gamma = 3.8, justifies the existence of other dielectric materials which may account for the observed linear polarization. IAC requires no more than 18 percent of the available carbon to produce the hump. The results are discussed in terms of evolution of carbon grains in space showing that, in this picture, the presence of graphite is no longer necessary to account for most ofmore » the solid carbonaceous material. 39 references.« less