Fractal aggregates in reduced gravity experiments and numerical simulations to characterize cometary material properties

Abstract : In situ missions have shown that cometary dust particles have low densities and are easily fragmenting aggregates [1]. The linear polarization of the solar light scattered by cometary dust corresponds to bell-shaped (with a small negative branch and a maximum below 30%) phase curves with a quasi-linear increase with the wavelength between 30° and 50° phase angle [2]. Such physical properties of the cometary material are reconciled by a fractal model of cometary dust and comet nuclei as formed by aggregation in reduced gravity as studied by laboratory experiments and numerical simulations. Reduced gravity light scattering experiments: The CODAG-LSU experiment (1999) gave the first indication of the light scattering properties transition between single particles and low dimensions fractal aggregates (D 1.3) [3, 4]. Such studies will be pursued on board the ISS with the ICAPS precursor experiment. The PROGRA2 experiment is designed to study the light scattering properties of particles levitated during dedicated microgravity flights or with ground-based configurations [5]. The material properties are chosen so as to be relevant in the context of cosmic dust from cometary and asteroidal origins. It is especially useful to disentangle the effects of varying albedos of constitutive materials [6], shape and size of constitutive grains [7]. Some of the results are interpreted in terms of fractal aggregates growth. Light scattering numerical simulations Based on numerical simulations and in coherence with the experimental results, a model of cometary coma by a mixture of fractal aggregates of up to 256 sub-micron sized spheroidal grains and compact spheroidal particles is shown to reproduce the polarimetric observations of comets such as 1P/Halley or C/1995 O1 Hale-Bopp [8]. Physical parameters of the size distribution of particles (minimum and maximum size, shape of the size distribution and quantity and location of absorbing and non-absorbing particles) can be retrieved to determine cometary dust properties useful to interpret observations and to prepare on-going missions. Cometary primordial aggregation Recent in situ studies have unveiled surface features hinting at a pervasive internal layered structure on 9P/Tempel 1 [9]. Assuming reduced gravity aggregation of cometesimals and taking into account sintering processes, quantitative internal properties of the nucleus can be obtained [10]. At large dissipation of the kinetic energy during impact, possible fractal structure disappears and is replaced by a density, cohesive strength and porosity layering of the material. The cohesive strength of the nuclei obtained (¡104 N.m-2 ) compares well with values deduced from disruption events observations and laboratory impact experiments. During an impact, if the ejected material is lost, the internal structure of the nucleus can be erased. This suggests that either the structures observed are not primordial (e.g. cryovolcanism resurfacing [11]) or that secondary mechanisms, such as the re-accretion of fragments after destructive collision, have occurred during the accretion process [12]. Support from LPI, LATMOS/IPSL, CNES for PROGRA2 and CNES and ESA for the micro-gravity flights are acknowledged. [1] Levasseur-Regourd et al, PSS 2009 [2] Levasseur-Regourd and Hadamcik, JQSRT 2003; [3] Levasseur-Regourd et al., ESA-SP 2001; [4] Blum et al., ASR 2002; [5] Renard et al., AO 2002; [6] Hadamcik et al., JQSRT 2006; [7] Hadamcik et al., JQSRT 2007; [8] Lasue et al., Icarus (2009a) ; [9] Thomas et al., Icarus 2007 ; [10] Lasue et al., Icarus (2009b) ; [11] Belton et al., Icarus (2008); [12] Blum and Wurm, Annu. Rev. Astro. Astrophys. (2008)
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Jérémie Lasue, Anny Chantal Levasseur-Regourd, Edith Hadamcik, Robert Botet, Jean-Baptiste Renard. Fractal aggregates in reduced gravity experiments and numerical simulations to characterize cometary material properties. 38th COSPAR Scientific Assembly, Jul 2010, Bremen, Germany. ⟨hal-00793212⟩

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