A new method to predict meteor showers. I. Description of the model

The original publication is available in Astronomy & Astrophysics at www.aanda.org.International audienceObservations of meteor showers allow us to constrain several cometary parameter and to retrieve useful parameters on cometary dust grains, for instance the dust size distribution index s. In this first paper, we describe a new model to compute the time and level of a meteor shower whose parent body is a known periodic comet. The aim of our work was to use all the available knowledge on cometary dust to avoid most of the "a priori" hypotheses of previous meteoroid stream models. The ejection velocity is based on a hydrodynamic model. Because of the large amount of particles released by the comet, it is impossible to compute the orbits of all of them. Instead, we link each computed particle with the real number of meteoroids ejected in the same conditions, through a "dirty snowball" cometary model calibrated with the [A f ρ] parameter. We used a massive numerical integration for all the particles without hypotheses about size distribution. The time of maximum is evaluated from the position of the nodes of impacting meteoroids. The model allows us to compute ephemerides of meteors showers and the spatial density of meteors streams, from which a ZHR can be estimated. At the end a fit of our predictions with observations allows us to compute the dust size distribution index. We used 2002 and 2003 leonid meteor showers to illustrate our method. The application of our model to the Leonid meteor shower from 1833 to 2100 is given in Paper II

A new method to predict meteor showers. II. Application to the Leonids

The original publication is available in Astronomy & Astrophysics at www.aanda.org.International audienceOur model of meteor shower forecasting (described in Paper I) is applied to the Leonid shower. This model is based on the "dirty snowball" model of comets, and on heavy numerical simulation of the generation and evolution of meteoroid streams. The amount of dust emitted by comet 55P/Tempel-Tuttle is computed. A statistical weight is associated to each simulated particle. This weight represents the real amount of meteoroids released by the comet. Particles close to the Earth are examined. There is no unique set of initial conditions (velocity and angle of ejection) for them to reach the Earth at the time of the shower. The shape of the metoroid stream projected on the ecliptic is not elliptical, due to non-gravitational forces and ejection processes. The mixing of particles is revealed to be very efficient. A comparison between observations and predictions of Leonid meteor showers is done. The time of maximum is found to be very accurate, except for certain years (1999 in particular). This problem is common to all models. The level of the predicted shower is obtained through a fit of the size distribution index s = 2.4±0.1. This model provides a unique opportunity to derive cometary parameters from meteor shower observations. Leonid meteor shower forecasts are provided for years up to 2100. The next storm is expected in 2034

Gas flow in near surface comet like porous structures: Application to 67P/Churyumov-Gerasimenko

We performed an investigation of a comet like porous surface to study how sub-surface sublimation with subsequent flow through the porous medium can lead to higher gas temperatures at the surface. A higher gas temperature of the emitted gas at the surface layer, compared to the sublimation temperature, will lead to higher gas speeds as the gas expands into the vacuum thus altering the flow properties on larger scales (kilometres away from the surface). Unlike previous models that have used modelled artificial structures, we used Earth rock samples with a porosity in the range 24 – 92 % obtained from X-ray micro computed tomography (micro-CT) scans with resolution of some μm. Micro-CT scanning technology provides 3D images of the pore samples. The direct simulation Monte Carlo (DSMC) method for the rarefied gas dynamics is directly applied on the digital rock samples in an unstructured mesh to determine the gas densities, temperatures and speeds within the porous medium and a few centimetres above the surface. The thicknesses of the rock samples were comparable to the diurnal thermal skin depth (5cm). H2O was assumed to be the outgassing species. We correlated the coma temperatures and other properties of the flow with the rock porosities. The results are discussed as an input to analysis of data from the Microwave Instrument on Rosetta Orbiter (MIRO) on the 67P/Churyumov-Gerasimenko

Évolution paléogéographique de la plaine<br />du Roussillon au cours de la seconde moitié<br />de l'Holocène : implication sur la répartition<br />des sites néolithiques

International audienceA 3D database of Holocene deposits in the Roussillon basin is used to compute a scenarioof the post-glacial marine transgression in the near shore area. The effects of this evolutionon Early to Middle Neolithic site locations are discussed.À partir d'une base de données 3D des dépôts holocènes intégrée sous SIG, l'évolutiongéodynamique de la plaine du Roussillon a été constituée. Les modalités de la transgressionpost-glaciaire et son influence sur la répartition des sites du Néolithique sontdiscutées

Nouvelles recherches sur l'ensemble paléochrétien et médiéval d'Ereruyk en Arménie

Premiers résultats des campagnes d'investigations menées en 2009-2011 sur le site paléochrétien et médiéval d'Ereruyk, dans le nord-ouest de la république d'Arménie, par une équipe du LA3M (UMR 7298, Aix-Marseille Université / CNRS)

Gas and dust productions of Comet 103P/Hartley 2 from millimetre observations: Interpreting rotation-induced time variations

Comet 103P/Hartley 2 made a close approach to the Earth in October 2010. It was the target of an extensive observing campaign including ground- and orbit-based observatories and was visited by the Deep Impact spacecraft in the framework of its mission extension EPOXI. We present observations of HCN and CH_3OH emission lines conducted with the IRAM Plateau de Bure interferometer on 22–23, 28 October and 4, 5 November 2010 at 1.1, 1.9 and 3.4 mm wavelengths. The thermal emission from the dust coma and nucleus is detected simultaneously. Interferometric images with unprecedented spatial resolution of ∼100 to ∼500 km are obtained. A sine–wave like variation of the thermal continuum is observed in the 23 October data, that we associate with the nucleus thermal light curve. The nucleus contributes up to 30–55% of the observed continuum emission. The dust thermal emission is used to measure the dust production rate. The inferred large dust-to-gas ratio (in the range 2–6) can be explained by the unusual activity of the comet for its size, which allows decimeter size particles and large boulders to be entrained by the gas due to the small nucleus gravity. The rotational temperature of CH_3OH is measured with beam radii from ∼150 km to ∼1500 km. We attribute the increase from ∼35 K to ∼46 K with increasing beam size to radiative processes. The HCN production rate displays strong rotation-induced temporal variations, varying from ∼0.3 × 10^(25) s^(−1) to ∼2.0 × 10^(25) s^(−1) in the 4–5 November period. The HCN production curve, as well as the CO_2 and H_2O production curves measured by EPOXI, are interpreted with a geometric model which takes into account the complex rotational state of 103P/Hartley 2 and its shape. The HCN and H_2O production curves are in phase, showing that these molecules have common sources. The ∼1.7 h delay, in average, of the HCN and H_2O production curves with respect to the CO_2 production curve suggests that HCN and H_2O are mainly produced by subliming icy grains. The scale length of production of HCN is determined to be on the order of 500–1000 km, implying a mean velocity of 100–200 m s^(−1) for the icy grains producing HCN. From the time evolution of the insolation of the nucleus, we show that the CO_2 production is modulated by the insolation of the small lobe of the nucleus. The three-cycle pattern of the production curves reported earlier is best explained by an overactivity of the small lobe in the longitude range 0–180°. The good correlation between the insolation of the small lobe and CO_2 production is consistent with CO_2 being produced from small depths below the surface. The time evolution of the velocity offset of the HCN lines, as well as the displacement of the HCN photocenter in the interferometric maps, are overall consistent with this interpretation. Other localized sources of gas on the nucleus surface are also suggested

Imaging Asteroid 4 Vesta Using the Framing Camera

The Framing Camera (FC) onboard the Dawn spacecraft serves a dual purpose. Next to its central role as a prime science instrument it is also used for the complex navigation of the ion drive spacecraft. The CCD detector with 1024 by 1024 pixels provides the stability for a multiyear mission and its high requirements of photometric accuracy over the wavelength band from 400 to 1000 nm covered by 7 band-pass filters. Vesta will be observed from 3 orbit stages with image scales of 227, 63, and 17 m/px, respectively. The mapping of Vesta s surface with medium resolution will be only completed during the exit phase when the north pole will be illuminated. A detailed pointing strategy will cover the surface at least twice at similar phase angles to provide stereo views for reconstruction of the topography. During approach the phase function of Vesta was determined over a range of angles not accessible from earth. This is the first step in deriving the photometric function of the surface. Combining the topography based on stereo tie points with the photometry in an iterative procedure will disclose details of the surface morphology at considerably smaller scales than the pixel scale. The 7 color filters are well positioned to provide information on the spectral slope in the visible, the depth of the strong pyroxene absorption band, and their variability over the surface. Cross calibration with the VIR spectrometer that extends into the near IR will provide detailed maps of Vesta s surface mineralogy and physical properties. Georeferencing all these observation will result in a coherent and unique data set. During Dawn s approach and capture FC has already demonstrated its performance. The strong variation observed by the Hubble Space Telescope can now be correlated with surface units and features. We will report on results obtained from images taken during survey mode covering the whole illuminated surface. Vesta is a planet-like differentiated body, but its surface gravity and escape velocity are comparable to those of other asteroids and hence much smaller than those of the inner planets o

Thermal fracturing on comets: Applications to 67P/Churyumov-Gerasimenko

We simulate the stresses induced by temperature changes in a putative hard layer near the surface of comet 67P/Churyumov-Gerasimenko with a thermo-viscoelastic model. Such a layer could be formed by the recondensation or sintering of water ice (and dust grains), as suggested by laboratory experiments and computer simulations, and would explain the high compressive strength encountered by experiments on board the Philae lander. Changes in temperature from seasonal insolation variation penetrate into the comet’s surface to depths controlled by the thermal inertia, causing the material to expand and contract. Modelling this with a Maxwellian viscoelastic response on a spherical nucleus, we show that a hard, icy layer with similar properties to Martian permafrost will experience high stresses: up to tens of MPa, which exceed its material strength (a few MPa), down to depths of centimetres to a metre. The stress distribution with latitude is confirmed qualitatively when taking into account the comet’s complex shape but neglecting thermal inertia. Stress is found to be comparable to the material strength everywhere for sufficient thermal inertia (≳ 50 J m−2 K−1 s−1∕2) and ice content (≳ 45% at the equator). In this case, stresses penetrate to a typical depth of ~0.25 m, consistent with the detection of metre-scale thermal contraction crack polygons all over the comet. Thermal fracturing may be an important erosion process on cometary surfaces which breaks down material and weakens cliffs

Bilobate comet morphology and internal structure controlled by shear deformation

Bilobate comets—small icy bodies with two distinct lobes—are a common configuration among comets, but the factors shaping these bodies are largely unknown. Cometary nuclei, the solid centres of comets, erode by ice sublimation when they are sufficiently close to the Sun, but the importance of a comet’s internal structure on its erosion is unclear. Here we present three-dimensional analyses of images from the Rosetta mission to illuminate the process that shaped the Jupiter-family bilobate comet 67P/Churyumov–Gerasimenko over billions of years. We show that the comet’s surface and interior exhibit shear-fracture and fault networks, on spatial scales of tens to hundreds of metres. Fractures propagate up to 500 m below the surface through a mechanically homogeneous material. Through fracture network analysis and stress modelling, we show that shear deformation generates fracture networks that control mechanical surface erosion, particularly in the strongly marked neck trough of 67P/Churyumov–Gerasimenko, exposing its interior. We conclude that shear deformation shapes and structures the surface and interior of bilobate comets, particularly in the outer Solar System where water ice sublimation is negligible.Additional co-authors: M. A. Barucci, J.-L. Bertaux, I. Bertini, D. Bodewits, G. Cremonese, V. Da Deppo, S. Debei, M. De Cecco, J. Deller, S. Fornasier, M. Fulle, P. J. Gutiérrez, C. Güttler, W.-H. Ip, H. U. Keller, L. M. Lara, F. La Forgia, M. Lazzarin, A. Lucchetti, J. J. López-Moreno, F. Marzari, M. Massironi, S. Mottola, N. Oklay, M. Pajola, L. Penasa, F. Preusker, H. Rickman, F. Scholten, X. Shi, I. Toth, C. Tubiana & J.-B. Vincen