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1.
We have observed the coma of Comet 9P/Tempel 1, the target of the Deep Impact mission, by the polarization imaging technique, before and after the impact event (−32, −7, +43 and +65 h). Our observations were conducted in the red wavelength domain from Haute-Provence Observatory (France), with the 80-cm telescope. The overall polarization of 9P/Tempel 1, as obtained near 41° phase angle, is monitored and compared to data from other (active and less active) comets studied by the same technique. The linear polarization of the dust ejected by the impact is compared to previous observations of dust present in jets, ejected during outbursts or released when comets happen to split. At phase angles of about 41°, the difference in polarization between the comets with a low maximum in polarization and the comets with a high maximum in polarization is about 1%; it may thus be difficult to conclude about the classification. Nevertheless, the overall polarization after the impact rapidly reached a value corresponding to the high polarization class of comets, and later progressively decreased to its initial value. The polarization was measured to be slightly lower (about 1%) before the impact than after it in a 26,000-km aperture. The plume formed from dust ejected by the impact was still present 65 h after it. The variations of the intensity and the polarization in the coma provide some clues to variations of the physical properties of the particles; comparison with other techniques corroborates the presence of large particles and of submicron-sized grains in aggregates. 相似文献
2.
The knowledge of the physical and dynamical properties, distribution, formation, and evolution of small bodies is fundamental
to understand how planet formation occurred and, even more importantly, if and how these objects have played a role in the
apparition of life on Earth. In the last century, asteroids began to no longer appear as starlike points of light in our telescopes,
but to be resolved worlds with distinctly measurable sizes, shapes, and surface morphologies. Only in the last 25 years, the
exploration of small bodies by spacecraft has begun and revealed objects widely diverse in formation region, evolution and
properties (e.g. shape, albedo density, gravity, regolith size distribution, and porosity). In this paper we will provide
a chronological analysis of comet nuclei and asteroids as revealed by space missions. The real breakthrough began with the
ESA Giotto mission in 1986 to the comet Halley, while the latest JAXA Hayabusa mission was devoted to hover above the small
asteroid Itokawa with a touch-and-go for a sample return of asteroidal regolith. Comet and asteroid science stands at the
threshold of a new exceptional era, with many new missions to be devoted to these widely diverse and still poorly known small
bodies. 相似文献
3.
Irina N. Belskaya Anny-Chantal Levasseur-Regourd Yuriy S. Efimov Edith Hadamcik 《Icarus》2009,199(1):97-105
New UBVRI polarimetric observations of ten asteroids, including space mission targets 1 Ceres and 21 Lutetia, are presented. These observations were obtained with the 1.25-m telescope of the Crimean Astrophysical Observatory and have been used to study the wavelength dependence of polarization for a sample of asteroids belonging to the M and low albedo classes. A more general analysis including also a larger data set of UBVRI polarimetric observations available in the literature for more than 50 main belt asteroids belonging to different taxonomic classes shows that the variation of the polarization degree Pr as a function of wavelength is generally well described by a linear trend. It typically does not exceed 0.2% in the studied spectral range 0.37-0.83 microns and tends to increase for increasing phase angle. Asteroids belonging to the S and M classes are found to exhibit a deeper negative branch and smaller positive polarization for increasing wavelength (negative sign of the slope of ΔPr/Δλ). Since the objects belonging to these classes are known to exhibit reddish reflectance spectra, the observed wavelength behavior of negative polarization contradicts the well-known inverse correlation of Pmin and albedo. Low albedo asteroids show larger dispersion of spectral slopes, but the overall trend is characterized by a shallower negative branch and a larger positive polarization for increasing wavelength (positive sign of the slope of ΔPr/Δλ). A few exceptions from this general trend are discussed. The observed variety in the wavelength dependence of asteroid polarization seems to be mainly attributed to surface composition. 相似文献
4.
5.
There is no direct evidence about the internal structure of cometary nuclei, which are mostly hidden by their gas and dust comae, and have not yet been orbited by any spacecraft. Their densities are low, typically of about 400 kg m−3 for 9P/Tempel 1 (that was impacted by the Deep Impact probe) and 67P/Churyumov-Gerasimenko (that is the target of the Rosetta mission). Such low densities are in favour of a high macro-porosity, or a high micro-porosity, or both. Observations of disruption or splitting of nuclei indeed suggest that some huge sub-nuclei or some meter-sized fragments could be the building blocks of comets. Analysis, from in-situ measurements and from remote light scattering observations, of the structure of the dust particles, which significantly consist of fluffy aggregates of submicron-sized grains, could be in favour of a fractal structure. However, the presence of huge icy grains in the innermost coma, and of flat layers on the surface of 9P/Tempel 1, are clues to the complexity of these objects, which have suffered drastic erosion phenomena on their elongated orbits. It is expected that the Rosetta mission will provide a fair understanding of the structure of the deep interior of the nucleus of 67P/Churyumov-Gerasimenko, thanks to the on-board CONSERT experiment. 相似文献
6.
Cometary particles mainly consist of silicates and carbon compounds; they seem to be fluffy aggregates of tiny grains, as found in some IDPs. The linear polarization of the scattered light is an efficient method to characterize their physical properties. Laboratory simulations of light scattering by cometary analog particles help to disentangle different physical parameters by comparison with observational data. We present here polarization laboratory results with nine samples levitating particles: five samples of vapor-condensed magnesiosilica, one ferrosilica smoke, a mixture of magnesio-ferrosilica smokes, one mixture of ferrosilica with carbon and one mixture of magnesio-ferrosilica with carbon. The phase curves are bell-shaped with a maximum polarization at a phase range of (80°-100°). A shallow negative branch can be present at phase angles smaller than 20°. The different characteristics of the phase curves are discussed considering the size and the structure of the constituent grains and the size of the particles. For the five magnesiosilica samples, the maximum in polarization is in the 40% range (close to cometary values), and no wavelength dependence is detected; the negative branch, whose presence seems to be linked to the presence of large aggregates of fine silica (SiO2) grains, does not always exist. For the ferrosilica smoke, the maximum in polarization is about 30% in red light (632.8 nm) and 40% in green light (543.5 nm); the negative branch occurs for phase angles smaller than 20°. For the two mixtures with carbon black, the polarization spectral gradient is positive, as expected for cometary analog particles. Finally, the phase curves obtained for agglomerates of magnesio-ferrosilica and carbon (expected to be the main components of cometary particles) are comparable to those obtained by remote observations of dust in cometary comae. 相似文献
7.
P. Rousselot A. C. Levasseur-Regourd K. Muinonen J.-M. Petit 《Earth, Moon, and Planets》2005,97(3-4):353-364
The Kuiper-Belt Object (29981) 1999 TD10, classified as a Scattered-Disk Object, has been observed at three different phase angles with the ESO 8.2-m VLT and FORS 1 instrument in polarimetric mode in November and December 2003. These observations have been used to compute the Stokes parameter q, which represents the linear polarization degree. We have also used the previously published photometric observations to improve the R-band phase function. The main conclusions are as follows: (i) a negative linear polarization degree decreasing with phase angle α up to, at least, α=3°, (ii) for α=3°, (iii) a possible color effect between the R and V band, the polarization degree being more negative in R. The R-band polarimetric observations can be explained by the coherent-backscattering mechanism and fitted by a two-component Rayleigh-scatterer model for a spherical small body. The rotation period of 15.382±0.001 h published by Mueller et al. (2004, Icarus
171, 506–515) and Choi et al. (2003, Icarus
165, 101–111) is confirmed. The R-band phase curve provides H=8.35±0.02 and G=−0.25±0.022 parameters with the IAU H–G formalism.Based on observations obtained at the Cerro Paranal observatory of the European Southern Observatory (ESO) in Chile. 相似文献
8.
This work was carried out with the PROGRA2 experiment developed to measure the angular dependence of the polarization of light scattered by dust particles. The dust samples are fluffy aggregates (size range 0.01-1 mm) with constituent grains of about 10 nm. Various setups were used: samples deposited on surfaces, the same samples lifted under the effect of a draft, and particles levitating in microgravity conditions on board the CNES dedicated aircraft. For deposited particles, the maximum value of polarization (Pmax) follows the Umov law. For a cloud of particles (Pmax) near 100° phase angle decreases when: (i) multiple scattering between the particles—or between the grains inside the particles—increases, or (ii) the real part of the refractive index of the materials increases, or (iii) the size parameter of the constituent grains increases between 0.05 and 0.5. A negative branch in the polarization phase curve is found for deposited samples. For levitating particles made of a single material and a single size distribution, a positive increase of polarization appears at phase angles smaller than 20°; for mixtures of these materials the polarization is negative at the same phase angles. These results are compared to modeling results as well as to polarimetric observations of comets. 相似文献
9.
Levasseur-Regourd A.C. Cabane M. Haudebourg V. Worms J.C. 《Earth, Moon, and Planets》1998,80(1-3):343-368
Most of our knowledge on heterogeneous media in the Universe comes from the light they scatter. This light is mainly linearly
polarized, and the polarization phase curves contain information about the properties of the scattering dust. In the solar
system, the dust seems to be made of irregular aggregates with a size greater than a few microns and a fractal structure.
Many constraints appear in the scattering computations, due to the trickiness of the mathematical calculations, and to our
ignorance of the precise structure of the dust. This leads to the necessity to perform light scattering measurements on characteristic
aggregates, built under low velocity ballistic collisions. Microgravity is a sensible way to achieve such measurements on
a cloud of levitating and aggregating dust particles. A first step has been the PROGRA2 experiment, which operates during parabolic flights on an aircraft. The instrument is a polar nephelometer measuring successively
the light scattered by a dust sample at various angles; it is fully operational, and will provide a data base of polarization
phase curves. A second step is the CODAG-SR experiment, which uses the duration of a rocket flight to build up dust aggregates.
The instrument measures simultaneously the light scattered at numerous phase angles; it is now space qualified, and should
provide in a near future a monitoring of the intensity and polarization phase curves while the aggregation processes are taking
place.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
10.
The analysis of the polarized light scattered by cometary dust particles provides information on the physical properties of the solid component of cometary comae for C/1995 O1 Hale-Bopp and 1P/Halley. A model of light scattering by a size distribution of aggregates of up to 256 submicron-sized grains (spherical or spheroidal) mixed with single spheroidal particles has been developed, with its parameters adjusted to fit the phase angle and wavelength dependence of the polarization observations. The particles are built of two materials: a non-absorbing silicates-type material and a more absorbing organic-type material. The model reproduces accurately the inversion angle and the positive branch of the polarization phase curves from the visible to the near-infrared spectral domains. A negative branch of the polarization phase curves appears in our model, although the negative branch is not deep enough to reproduce accurately the observations. Significant differences are shown between the two comets, with dominance of small grains in the coma of Comet C/1995 O1 Hale-Bopp, well fitted by a distribution of the volume-equivalent diameter, a, following a−3.0 with a lower cutoff around 0.20 μm and an upper cutoff of at least 40 μm. For 1P/Halley, the size distribution follows a−2.8 with a lower cutoff around 0.26 μm and an upper cutoff of about 38 μm. The relative amount of organic-type particles is larger for 1P/Halley while the amount of aggregates, significant for both comets, is larger for C/1995 O1 Hale-Bopp. 相似文献