Edith Hadamcik

In situ dust measurements from within the coma of 1P/Halley: First-order approximation with a dust dynamical model

In situ measurements of the light locally scattered by cometary dust, as well as the local dust spatial density, are only available for one comet, 1P/Halley. These data were returned from the Optical Probe Experiment (OPE) and the Dust Impact Detection System (DID) aboard the European Space Agency spacecraft Giotto. Due to a detailed calibration of OPE at the time of Giottos encounter with comet 26P/Grigg-Skjellerup, as well as improved analysis techniques, the similarities and correlation between the OPE and DID data sets can be reassessed. In this paper, we utilize this opportunity to compare these unique observations with a cometary coma dynamical model. Our results indicate that, to first order, the data can be fitted by a coma model that incorporates a grain size distribution index (at the nucleus), which need not be time dependent. Further, we find that the general shape of both the OPE and DID data can largely be explained by Keplerian dynamics alone, without recourse to fragmentation processes. The model is used to constrain the cometary dust bulk density, and a likely range of 50 < ρ < 500 kg m-3 is found, although a value of ρ = 100 kg m-3 is favored. In addition, the corresponding favored solution of the geometric albedo Ap(α = 0°) is found to be 0.04. Within the quoted density range, the ratio between density and albedo remains constant at ρ/Ap(α = 0°) ≈ 2500 kg m-3. The modeling also indicates that the grain size distribution power index at the nucleus is constant (in the range 10-12 < m < 10-3 kg) and has a likely value k = -2.6 ± 0.2 (i.e., cumulative mass distribution index b = -0.53 ± 0.1).

Chemical characterization of Titan's tholins: solubility, morphology and molecular structure revisited

In this work Titans atmospheric chemistry is simulated using a capacitively coupled plasma radio frequency discharge in a N(2)-CH(4) stationnary flux. Samples of Titans tholins are produced in gaseous mixtures containing either 2 or 10% methane before the plasma discharge, covering the methane concentration range measured in Titans atmosphere. We study their solubility and associated morphology, their infrared spectroscopy signature and the mass distribution of the soluble fraction by mass spectrometry. An important result is to highlight that the previous Titans tholin solubility studies are inappropriate to fully characterize such a heterogeneous organic matter and we develop a new protocol to evaluate quantitatively tholins solubility. We find that tholins contain up to 35% in mass of molecules soluble in methanol, attached to a hardly insoluble fraction. Methanol is then chosen as a discriminating solvent to characterize the differences between soluble and insoluble species constituting the bulk tholins. No significant morphological change of shape or surface feature is derived from scanning electron microscopy after the extraction of the soluble fraction. This observation suggests a solid structure despite an important porosity of the grains. Infrared spectroscopy is recorded for both fractions. The IR spectra of the bulk, soluble, and insoluble tholins fractions are found to be very similar and reveal identical chemical signatures of nitrogen bearing functions and aliphatic groups. This result confirms that the chemical information collected when analyzing only the soluble fraction provides a valuable insight representative of the bulk material. The soluble fraction is ionized with an atmospheric pressure photoionization source and analyzed by a hybrid mass spectrometer. The congested mass spectra with one peak at every mass unit between 50 and 800 u confirm that the soluble fraction contains a complex mixture of organic molecules. The broad distribution, however, exhibits a regular pattern of mass clusters. Tandem collision induced dissociation analysis is performed in the negative ion mode to retrieve structural information. It reveals that (i) the molecules are ended by methyl, amine and cyanide groups, (ii) a 27 u neutral moiety (most probably HCN) is often released in the fragmentation of tholin anions, and (iii) an ubiquitous ionic fragment at m/z 66 is found in all tandem spectra. A tentative structure is proposed for this negative ion.

Imaging polarimetry of cometary dust: different comets and phase angles

Polarimetric observations of the light scattered by dust in cometary comae have been carried out at Pic-du-Midi Observatory and at Haute-Provence Observatory (France) between 1990 and 2001. The whole coma polarization can easily be inferred from integration of the fluxes on the aperture. Maps are obtained for seven comets by CCD imaging polarimetry, which allow to observe three main regions in a cometary coma: the background coma, a circumnuclear halo and bright structures (jets or arcs) which correspond to different values of the polarization. The polarization maps of short period comets are compared to the maps obtained for comet Hale-Bopp, for phase angles smaller than 50° (α smaller than 15° with negative polarization, α near the inversion angle of approximately 20°, α between 30° and 50° on the linear part of the phase curve). At α≅60° the polarization map of comet C/2000 WM1 is presented. Between 109° and 121° the evolution of brightness images and polarization maps of comet C/1999 S4 is followed during the complete disruption of its nucleus. The polarimetric phase curve for the circumnuclear halo presents highly negative values for α<20°. On the opposite for the same phase angles range, the polarization in jets is positive for the four studied comets. The differences between the regions are discussed in terms of physical properties of the particles (size distribution, packing density, mixtures of materials) by comparison to other observational techniques and laboratory measurements.

Light scattering by irregular dust particles in the solar system: observations and interpretation by laboratory measurements

Results on the polarimetric properties of cometary dust are updated, with emphasis on the phase angle and wavelength dependences. They are compared to results obtained for asteroids and interplanetary dust. Noticeable differences are found between the cometary dust and the S-type asteroids polarization color. Interpretation through laboratory measurements performed under reduced gravity conditions suggests that dust in cometary comae consists of highly porous aggregates. Numerous measurements are expected to be performed with ICAPS on board the ISS, to monitor the evolution of the light scattering properties of the dust particles while they aggregate and, possibly, while ices condense on or evaporate from them under different physical conditions.

Physics and chemistry of icy particles in the universe: answers from microgravity

Abstract During the last century, the presence of icy particles throughout the universe has been confirmed by numerous ground and space based observations. Ultrathin icy layers are known to cover dust particles within the cold regions of the interstellar medium, and drive a rich chemistry in energetic star-forming regions. The polar caps of terrestrial planets, as well as most of the outer-solar-system satellites, are covered with an icy surface. Smaller solar system bodies, such as comets and Kuiper Belt Objects (KBOs), contain a significant fraction of icy materials. Icy particles are also present in planetary atmospheres and play an important role in determining the climate and the environmental conditions on our host planet, Earth. Water ice seems universal in space and is by far the most abundant condensed-phase species in our universe. Many research groups have focused their efforts on understanding the physical and chemical nature of water ice. However, open questions remain as to whether ices produced in Earths laboratories are indeed good analogs for ices observed in space environments. Although temperature and pressure conditions can be very well controlled in the laboratory, it is very difficult to simulate the time-scales and gravity conditions of space environments. The bulk structure of ice, and the catalytic properties of the surface, could be rather different when formed in zero gravity in space. The author list comprises the members of the ESA Topical Team: Physico-chemistry of ices in space. In this paper we present recent results including ground-based experiments on ice and dust, models as well as related space experiments performed under microgravity conditions. We also investigate the possibilities of designing a new infrastructure, and /or making improvements to the existing hardware in order to study ices on the International Space Station (ISS). The type of multidisciplinary facility that we describe will support research in crystal growth of ices and other solid refractory materials, aerosol microphysics, light scattering properties of solid particles, the physics of icy particle aggregates, and radiation processing of molecular ices. Studying ices in microgravity conditions will provide us with fundamental data on the nature of extraterrestrial ices and allow us to enhance our knowledge on the physical and chemical processes prevailing in different space environments.

Statistical analysis of asteroidal and cometary polarization phase curves

We use an empirical polarization phase curve model at various wavelengths to predict some statistical properties of comets and asteroids. First, we show how our model with Bayesian MCMC numerical methods predicts polarization behaviour at larger phase angles when only the smaller angle data are used. Our empirical model incorporates both the phase and colour dependency on the same footing. Second, a comparison between the existing taxonomy of asteroids and the clustering of asteroid classes based on polarization suggests a new method for the classification. We also heavily concentrate on the derivations of quantitative errors in our analysis.

Light scattering by dust particles in microgravity: polarization and brightness imaging with the new version of the PROGRA2 instrument

A new version of the PROGRA2 instrument, dedicated to measuring the polarization phase function of various kinds of solid particles, allows obtaining maps of polarization and brightness with a spatial resolution of a few tens of micrometers. The measurements are conducted in microgravity during parabolic flights to ensure random distribution and orientation of the particles. The results of the first two sessions are presented. Comparison between measurements and Mie theory modeling for glass spheres shows that the instrument works well and that accurate results can be obtained even at small phase angles. Results for irregularly shaped particles are also presented.

Polarimetric observations of comet 67P/Churyumov-Gerasimenko during its 2008–2009 apparition

Remote observations of the light scattered by comet 67P/Churyumov-Gerasimenko dust coma are of major importance for determining the physical properties of the particles and prepare the rendezvous with the ESA/Rosetta spacecraft in 2014. Aims. Light scattering and especially linear polarization observations allow comparison between different coma regions and different comets, including comets that have been studied by space probes. Our aim is to retrieve physical properties of the dust particles and to characterize their evolution around perihelion passage. Methods. Recent imaging polarimetric observations were conducted at the Haute-Provence observatory (France) on 2009 March 17-19 at 35° phase angle and at IUCAA Girawali observatory (India) on 2008 December 25-27 at 36° phase angle and on 2009 April 30-May 1 at 29° phase angle. With the imaging technique, the intensity and linear polarization variations are studied through the various coma regions. These observations are compared to other cometary data (e.g. Jupiter family comets) and to numerical and experimental simulations. Results. The decrease in intensity as a function of the distance to nucleus in log-log scale is on average close to -1, although important variations with values down to -1.5 are noticed, in agreement with previous observations in 1982-83 and 1995-96. The intensity along the tailward direction decreases with a slope between -1.2 two months before perihelion (2009 February 28) to -1.0 two months after perihelion, and the decrease is more pronounced in the sunward direction. Before perihelion, aperture polarization values are comparable to polarization values measured on other comets at similar phase angles. The sharp decrease in intensity and the feature in the tailward direction, without any difference in polarization in the coma before perihelion, could suggest the presence of large dark particles. The post-perihelion increase in intensity and in polarization suggests that an outburst has occurred. The freshly ejected dust polarizes more the scattered light and is more sensitive to the solar radiation pressure, suggesting small micron- or submicron-sized grains. Conclusions. Polarization and intensity variations in the coma of 67P/Churyumov-Gerasimenko are reminiscent of those noticed for some comets such as comet 81P/Wild 2 and comet 9P/Tempel 1. The presence of rather large particles can thus be suggested before and just after perihelion and the ejection of post-perihelion smaller grains, eventually in fluffy aggregates. A strong seasonal effect related to the obliquity of the comet suggests that the different grains originate in different hemispheres of the nucleus.

Laboratory light scattering measurements on natural particles with the PROGRA2 experiment: an overview

Abstract Polarimetric phase curves were obtained with the PROGRA2 instrument for different particles: glass beads, polyhedral solids, rough particles, dense aggregates and aggregates with porosity higher than 90%. The main purpose of these measurements is to build a large database, which allows interpreting remote sensing observations of solar system bodies. For some samples numerical or experimental models (i.e. DDA, stochastically built particles, microwave analogue) and laboratory experiments are compared to better disentangle the involved physical properties. This paper gives some main results of the experiment, and their applications to Earth atmosphere, comets and asteroids.

Light scattering by dust particles in microgravity: The PROGRA2 achievements and results

The PROGRA2 instrument has been developed to constitute the largest possible database of polarimetric phase functions, to compare with laboratory measurements and remote sensing observations. 24 such phase curves and corresponding polarimetric parameters have presently been obtained. This paper will describe results on basaltic glass, volcanic black sand, and lunar soil simulant samples. A trend seems to be valid for all observed samples: the phase angle of maximum polarization is invariably lower when measured under microgravity conditions as compared to ground-based measurements.