We present a spectrophotometric analysis of the surface areas of asteroid (21) Lutetia that were imaged during the Rosetta flyby on July 2010 by the OSIRIS camera in the wavelength range 269 to ...989 nm. A spectral slope change with respect to phase angle is found and interpreted as being due to areal diversity of Lutetia's surface. We applied Minnaert and Hapke models to disk-integrated and disk-resolved data of Lutetia. Except for the albedo in both models, the Minnaert (limb darkening k) and Hapke (roughness) parameter display no dependence on wavelength. The single scattering albedo, ω, shows a relative flat spectrum across the wavelength range with a slope of (1.65±0.29)% per 100 nm. The Hapke roughness parameter of Lutetia is constrained to be 28°± 1° in average for all wavelengths. We generated three different color ratio maps to investigate the overall color variation across the surface of Lutetia. Apart from differences on local scale no significant color variations are found across the regions imaged on Lutetia's surface (Achaia, Noricum, Narbonensis, Baetica and Etruria). Based on (931.9-nm/269.3-nm) color ratio map, the typical color contrast is estimated to be on the order of 10%. We note that, compared to other asteroids visited by spacecraft, the subtle color variation in order of 10% or less is observed for E-type asteroid (2867) Steins and the C-type asteroid (253) Mathilde.
•We build the visible disk-integrated spectrum of asteroid Lutetia at two different phase angles.•We applied Minnaert and Hapke models using OSIRIS images in the wavelength range 269 to 989 nm.•Apart from albedo parameter, no wavelength dependency is found for the resulting photometeric parameters.•We generated three different color ratio maps to explore the overall color variation.•The typical color contrast across the surface of Lutetia is estimated to be on the order of 10 percent.
Context. Comet 2P/Encke, a short-period comet, is a favorable target for studies of light scattering by bare cometary nuclei. These studies enable assessment of the nucleus size, the material albedo, ...and the light-scattering properties of the cometary surface. Observations of 2P/Encke were completed between October to December 2006 when the comet approached the Sun between 2.7 and 2.1 AU. Aims. We characterize the activity of the cometary coma and the physical properties of the nucleus of 2P/Encke such as its size, albedo, colors, spectral slope, surface roughness, porosity, and single-particle properties. Methods. Broadband imaging photometry and broadband and narrowband linear polarimetry is measured for the nucleus of 2P/Encke over the phase-angle range 4-28 deg. Results. An analysis of the point spread function of the comet in polarimetric images reveal only weak coma activity in 2P/Encke, corresponding to dust production of the order of 0.05 kg/s. Over the measured phase-angle range the nucleus displays a color independent photometric phase function of almost linear slope (β = 0.050 ± 0.004 mag/deg). The absolute R filter magnitude at zero phase angle is 15.05 ± 0.05 mag and corresponds to an equivalent radius for the nucleus of 2.43 ± 0.06 km (adopted albedo of 0.047). The nucleus color $V-R$ was measured to be 0.47 ± 0.07, suggesting a spectral slope S' of 11 ± 8%/100 nm. The phase function of linear polarimetry in the V and R filter shows a widely color independent linear increase with phase angle (0.12 ± 0.02%/deg). The test of the empirical albedo-polarization relationship for asteroids reveals unreasonably high albedo values for the nucleus. We find discrepancies in the photometric and polarimetric parameters between 2P/Encke and other minor bodies in the solar system, which may indicate significant differences in the surface material properties and light-scattering behavior of the bodies. Conclusions. The linear polarimetric phase function of 2P/Encke presented here is the first ever measured for a cometary nucleus. It provides encouragement to future studies of cometary nuclei in order to characterize the light-scattering behavior of comets on firm empirical grounds and provide suitable input to a comprehensive modeling of the light scattering by cometary surfaces.
Photometric measurements of 33 transneptunian objects (TNOs) and Centaurs from the target list of the Herschel Key program ‘TNOs are cool’ are presented. Broadband filter observations of 5 Plutinos, ...14 classical disk objects (CDOs), 5 scattered disk objects (SDOs), 5 detached disk objects (DDOs) and 4 Centaurs are used to determine absolute magnitudes, broadband colours and spectral gradients in the visible wavelength range. The diameters of the objects estimated with assumed average albedo values fall in the typical range for the various dynamical populations. Deviations between our and published measurements of the photometric brightnesses for three objects indicate larger lightcurve amplitudes (0.4–0.8 mag) due to non-spherical shape and/or albedo. A statistical analysis of photometric population properties using our data and those of the MBOSS2 database by Hainaut et al. (A&A 546:A115,
2012
) supports the results and conclusion of this group of authors, namely it shows that dynamically cold CDOs are disjunct for their visible colours from the other TNO populations and Centaurs. Six objects (2002 GV31, 2003 AZ84, 2003 MW12, 2003 OP32, 2003 UZ117, 2005 RM43) with neutral to bluish spectral gradients were found, of which 2002 GV31 shows the smallest spectral slope among the dynamically cold CDOs known so far. Three very red objects (2002 KY14, 2004 GV9, 2007 OR10) with spectral gradients above 40 %/100 nm were found of which 2007 OR10 is by far the reddest DDO measured so far.
Context. One of the main scientific objectives of NASA's New Horizons mission is to map the icy surface compositions of Pluto and its moon Charon. The encounter will be in 2015. Meanwhile remote ...observations from earth and space are the most suitable means to enhance further our knowledge of the Pluto/Charon system. Aims. We intend to assist the New Horizons mission by improving our knowledge of Pluto's and Charon's surface compositions. Specifically, we extend the wavelength coverage of the surface spectroscopy beyond the K band, with the goal to detect further surface ice absorption bands predicted from the models that are based on the available JHK spectra, and to search for signatures of yet unknown ices. In particular we aim to resolve the binary system Pluto/Charon and to obtain, for the first time, spectra up to 5 μm of the two objects resolved. Methods. Spectroscopic measurements of Pluto/Charon taken with the adaptive optics instrument NACO at the ESO VLT in the interval 3–7 August 2005 were obtained. The nature and properties of the compounds present on the surface of Pluto and Charon are investigated by applying a Hapke radiative transfer model to the measured spectra. Results. We present Pluto's reflectance spectrum in the wavelength range (1–5) μm. Apart from known and expected absorption bands of methane ice, our Pluto spectrum reveals a new absorption band centered near 4.6 μm, not previously detected. This absorption band could be related to the presence of CO and nitriles (compounds of C and N connected with a triple bond). A geographic mixture of pure methane ice with two different grain sizes, methane and CO ice diluted in nitrogen, CH2CHCN and titan tholin gives the best fit to Pluto's spectrum, although not in all details. Differences compared to published Pluto spectra from 2001 taken at similar longitude could be due to a different surface coverage in latitude or to a possible resurfacing process on Pluto. Charon's spectrum is measured in the wavelength range (1–4) μm. The surface of Charon can be modeled by pure water ice darkened by a spectrally neutral continuum absorber.
Context. Photometric surveys of transNeptunian objects (TNOs) and Centaurs have suggested possible correlations between some orbital parameters and surface colors of classical objects, scattered disk ...objects (SDOs), and Centaurs. However, larger sample sizes are needed in order to corroborate or rule out the possible correlations and find some possible new ones. The implications from these correlations for the formation and evolution of classical TNOs, SDOs, and Centaurs are important to understand possible physico-chemical coloring mechanisms and their influence on the surfaces of the TNOs and Centaurs, as well as their evolutionary history. Aims. We aim to obtain a sufficiently large photometric dataset in order to carry out a more significant statistical analysis, with emphasis on improving the sample size of observed objects, in particular for classical TNOs and SDOs. Methods. We use VLT-FORS images through BVRI filters of 32 Kuiper Belt Objects (KBOs) and obtain their colors after proper reduction and calibration. We study the possible correlations (Peixinho et al. 2004, Icarus, 170, 153; Doressoundiram et al. 2005, Icarus, 174, 90) merging these new measurements with the VLT published results from the ESO large program and with the latest published results of the Meudon Multicolor Survey via non-parametric statistical tests. Results. We obtain a large dataset of 116 objects (classical, SDOs and Centaurs) and, in addition to confirming most of the correlations and conclusions reached in the literature, some possible new correlations are found. The most interesting ones are some correlations of color vs. orbital parameters for the different dynamical groups. We find that some correlations in the classical group, as well as the (dynamically) cold and hot subgroups depend on the size of the objects. As a by-product of our study, we were able to identify new candidates for light curve studies and found that ~55% of the objects showed variability above 0.15 mag. This is a higher value than what is found in other studies. Since our sample contains smaller objects than samples from other studies, this result might be an indication that the smaller TNOs are more elongated than the larger ones.
During its sungrazing perihelion passage, comet ISON appeared in the field of view of the SUMER spectrometer and allowed unique observations at far-ultraviolet wavelengths with high spatial and ...temporal resolution. We report results of these observations completed on November 28, 2013, when the comet was only 2.82 Rʘ away from the Sun. Our data show the arrow-shaped dust tail in Ly-α emission trailing behind the predicted position of the nucleus, but offset from the trajectory. We interpret the emission as sunlight that is scattered at μm-sized dust particles. We modeled the dust emission and dynamics to reproduce the appearance of the tail. We were unable to detect any signature of cometary gas or plasma around the expected position of the nucleus and conclude that the outgassing processes must have stopped before the observation started. Moreover, the model we used to reproduce the observed dust tail needs a sharp fall-off of the dust production hours before perihelion transit. We compare the radiances of the disk and the dust tail for an estimate of the dust column density and tail mass.
Context.The Jupiter family comet 73P/Schwassmann-Wachmann 3 (SW3) split into several pieces in 1995. Some of the original fragments were observed during the next apparition of the comet in 1999-2001. ...The last return of the comet in 2005-2006 was accompanied by tremendous further splitting of some SW3 components – in particular component B – into a large number of subfragments. Aims.We present observations of components B and C during their closest approach to Earth in the first half of May 2006. These results aim at characterizing the properties of dust particles released from the fragments of comet SW3 and at identifying dusty and gaseous structures in the comae and tail regions of components B and C, which could be useful for conclusions about the presence of active regions and break-up events of the components. Methods.We used narrowband imaging polarimetry and an analysis of the broadband surface brightness for the characterization of the dust comae of the SW3 components. Coma and tail structures in components B and C were enhanced by numerical methods. Results.The degree of linear polarization of components B and C follows the empirical phase-angle dependence as derived from other comets. Fragment B was found in outburst on May 8. The brightness outburst was accompanied by changes in the shape of the coma. During the outburst, the spatial distribution of the linear polarization showed some peculiarities that had disappeared again on May 9. Arc-like signatures of a temporary nature, typical of fragmentation events, are seen in broadband filter images of component B on 3, 8, and 9 May 2006. The secondary dust tail, seen in component B on 3 and 4 May 2006, marks the synchrone of an earlier splitting event around 25 April 2006. Fan-like coma structures are found in the sunward coma hemisphere of components B and C that might be related to localized enhanced activity on the rotating nuclei.
Context. Comet 67P/Churyumov-Gerasimenko is the main target of ESA’s Rosetta mission and will be encountered in May 2014. Because the spacecraft will orbit the comet nucleus before and after release ...of the lander Philae, it is necessary to know the conditions in the coma. Aims. We study the dust environment, including the dust production rate and its variations along the comet’s pre-perihelion orbit. Methods. The comet was observed during its approach to the Sun on four epochs between early-June 2008 and mid-January 2009 over a large range of heliocentric distances that will be covered by the mission in 2014. Results. An anomalous enhancement of the coma dust density was measured toward the comet nucleus. The scale length of this enhancement increased with decreasing heliocentric distance of the comet. We interpret this as a result of an unusually slow expansion of the dust coma. Assuming a spherically symmetric coma, we derive the average amount of dust as well as its ejection velocity. The latter increases exponentially with decreasing heliocentric distance (rh), ranging from about 1 m/s at 3 AU to about 25–35 m/s at 1.4 AU. Based on these results we describe the dust environment at those nucleocentric distances at which the spacecraft will presumably be in orbit.
Polarization of Saturn’s moon Iapetus Ejeta, C; Muinonen, K; Boehnhardt, H ...
Astronomy and astrophysics (Berlin),
06/2013, Letnik:
554
Journal Article
Recenzirano
Context. Like many other atmosphereless solar system bodies, Iapetus exhibits a phenomenon of negative polarization at small phase angles, which can be modeled using theoretical approaches that ...consider interaction of light with a complex medium. Aims. To retrieve information on the nature of Iapetus' surface material, we carried out theoretical modeling analyses for the observed polarization of its two sides. Methods. We applied two light-scattering models. The first modeling approach is based on the utilization of the phenomenological single-particle scattering matrix parametrization using the double Henyey-Greenstein (2HG) scattering phase function to characterize the resulting multiple scattering by a medium composed of such discrete scatterers. With this approach we carried out radiative-transfer coherent-backscattering (RT-CB) computations for a random medium composed of phenomenological fundamental scatterers. The second model, called the multiple sphere T-matrix method, is based on the exact solutions of the Maxwell equations. Employing this method, we carried out simulations of the scattering and absorption properties of light by a medium represented by a spherical volume of randomly positioned monodisperse particles. The modeling entails physical characteristics of the particulate surface, such as the porosity of the medium; the number of constituent particles; the size, and optical properties of the scatterers. Results. While our RT-CB model suggests geometric albedo values in the neighborhood of 0.40 for Iapetus' trailing side and ~0.10 for the leading one, our T-matrix model retrieves particles of radius ~0.10 < or = r < or = 0:20 mu m for both Iapetus' leading and trailing surface materials.
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