139 research outputs found
Infrared reflectance spectra of Martian analogues
No full textReflectance spectroscopy in the infrared is a powerful tool to characterise the optical properties of analogue materials for Mars. In this work we present laboratory studies of four Martian analogues: calcite, montmorillonite, palagonite and andesite. Diffuse and specular reflectance and transmittance spectra in the IR are shown. The spectral effect produced by small grains clung to the larger ones is analysed for calcite. Significant variations in the diffuse reflectance spectrum, especially in the reststrahlen region, are observed. The diffuse reflectance spectra of the other materials show the shift of the reststrahlen features with Si content in the materials. Optical constants of andesite are retrieved, by using the Lorentz model, from the specular reflectance spectrum. These values are fundamental in performing modelling of the radiance coming from the planet Mars
Dehydrogenation study of cosmic-dust analogue grains
No full textNew infrared and optical-absorption measurements performed on hydrogenated amorphous-carbon grains are presented. In particular, the spectral modifications produced by dehydrogenation of the samples are analysed. We interpret the observed variations in terms of an increasing dimension of the graphitic clusters disordely arranged to form carbon grains. The results support the hypothesis that the behaviour of the interstellar extinction in the ultraviolet can be attributed to dehydrogenated amorphous-carbon grains. © 1993 Società Italiana di Fisica
Simulation of the cometary 10 Î1⁄4m band by laboratory data. II. Extension to spectra available for different comets
No full textRecent observations have shown that the presence of emission bands at 9.8 and 11.3 Î1⁄4m is a characteristic of many comets. Although their attribution to silicates is commonly accepted, a wide debate is open to identify the actual species responsible for the two bands. In a previous paper (Colangeli et al. 1995a) we fitted emission spectra from comets P/Halley 1986 III and Levy 1990 XX in the 10 Î1⁄4m band region by using spectroscopic data obtained in our laboratory on various silicate samples. In the present work we extend the simulation to available cometary spectra showing similar bands. We show that combinations of pyroxene-type and olivine-type submicron grains produce satisfactory fits. Despite the limited number of observational data available in the literature, our results evidence that spectral variations can be interpreted in terms of intrinsic and/or evolutionary differences among the comets
Interstellar extinction: a parametrical study by using laboratory data
No full textIn this paper we present a comparative study between laboratory data, astronomical observations and theoretical models in order to provide a contribution to the characterization of cosmic-dust properties. In particular, we propose a parametrization of the amorphous-carbon extinction curve. The laboratory extinction measurements, performed in the extreme ultraviolet spectral region, are compared to the UV interstellar extinction curves provided by the IUE satellite. The aim of our work was to define a model that could describe both observational and laboratory data. To this purpose we referred to the model developed by Fitzpatrick and Massa. The second step of the study was the determination of extinction coefficients, for two different size distributions of amorphous-carbon grains, by means of the optical constants, n and k, derived by Rouleau and Martin. © 1993 Società Italiana di Fisica
Simulation of the dust flux on the ROSETTA probe during the orbiting phase around comet 46P/Wirtanen
No full textWe present a probabilistic model of the dust mass, flux and fluence which will be collected by the ROSETTA probe while orbiting around comet 46P/Wirtanen. The dust environment of the target comet is simulated according to the most recent data available in the literature. Best fits of the DIDSY-GIOTTO data collected during the fly-by of comet 1P/Halley have shown that the probabilistic properties of dust ejection from the inner coma are crucial (Fulle et al. 1995). Therefore, we pay particular attention to the dust ejection velocity, which is assumed to have a wide distribution around the most probable values, and the dust ejection distribution, which is assumed to have a strong anisotropy peaked towards the sun. To compute the impact velocity in the probe reference frame, the rigorous keplerian orbit of each grain is considered taking into account aberrations due to the probe orbital velocity. We analyse the dependence of the results on the probe orbit parameters, such as true anomaly, probe-nucleus distance, orbit node and inclination. Computations are performed for the six main directions of the probe reference frame and for different values of the acceptance angle. The only way to collect direct grains is to point towards the nucleus; the mass collected in this direction is almost independent of the acceptance angle and of the time evolution of dust loss rate. A strong dependence of the collected dust mass on node and inclination is evidenced. By assuming an acceptance angle of 40°, the flux of reflected grains received in the two directions perpendicular to the probe orbit is higher than that in the nucleus direction, for 42% of randomly oriented probe orbits. The value increases up to 56% when the acceptance angle in the directions perpendicular to the probe orbit is increased up to 80°. The dust ejection anisotropy produces a strong dependence of the fluxes on the probe anomaly. For reflected grains, the fluences show relevant depletions at the largest masses, due to dust orbital effects, and the collected masses strongly depend on the acceptance angle and on the time evolution of the dust loss rate. The total dust fluxes are evaluated by assuming a half sphere field of view (corresponding to an acceptance angle of 180°)
Performance of micro-balances for dust flux measurement
No full textMicro-balances have been used in the past for volatile deposition monitoring in laboratory and in space environment. In order to determine their suitability to measure mass deposition in the form of solid particles, some topical aspects must be characterised, such as the sensitivity versus temperature and grain mass and the sticking efficiency versus grain speed. These parameters have been retrieved for different sensor configurations, i.e. with and without an adhesive coating, used in the perspective of improving the sensor particle collection efficiency. Our studies show that the adhesive coating improves the sensor sticking efficiency only for fast (100-400 m s-1) grains. However, the stability of the output signal with temperature is worse in the coated configuration by a factor of about ten. These results provide important inputs in the view of using micro-balances for dust monitoring. In particular, they have been carefully considered for the selection of the configuration of micro-balances, included as sub-systems of the GIADA experiment onboard the ESA ROSETTA mission and aimed at studying flux and dynamic properties of cometary grains
Extinction signatures of amorphous carbon grains from the vacuum UV to the far-IR
No full textAmong the various components of cosmic dust, amorphous carbon grains play a relevant role. Together with other carbon-based materials. they have been invoked to interpret a number of spectral features observed in the interstellar and circumstellar environments. In order to validate, on a more quantitative basis, the reliability of such hypotheses. it is necessary to analyse in detail the optical properties of "analogue" grains produced in the laboratory. In the present paper, we report, for the first time, a complete spectral analysis of the extinction properties measured on various kinds of amorphous carbon grains, from the vacuum UV to the far-IR. These data can be complemented with other experimental results obtained on the same samples by means of different analytical techniques. This approach allows us to propose a self-consistent interpretation about the actual physico-chemical properties of our cosmic dust analogues. The resulting scenario provides a powerful tool to interpret astronomical observations concerning cosmic dust. © 1995
Interstellar-dust properties as deduced from FIR and millimetric observations
No full textWe have considered some millimetric observations of our Galaxy and of the Large Magellanic Cloud (LMC) taken at the Italian Base in Antarctica. Using other submm data and fitting all the observations with a modified Planck curve, we have obtained useful information about the temperaure T and the spectral index α of the dust grains both for the Galaxy and the LMC. For the galactic spectrum two different couples of T and α match the detected fluxes:a)T=7K, α=2;b)T=15K, α=1.1. We have tried to identify the nature of the dust grains responsible for the submm and mm emission by comparing these results with the spectral trend of some cosmic analogues studied in laboratory. © 1993 Società Italiana di Fisica
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