Space station impact experiments

Four processes serve to illustrate potential areas of study and their implications for general problems in planetary science. First, accretional processes reflect the success of collisional aggregation over collisional destruction during the early history of the solar system. Second, both catastrophic and less severe effects of impacts on planetary bodies survivng from the time of the early solar system may be expressed by asteroid/planetary spin rates, spin orientations, asteroid size distributions, and perhaps the origin of the Moon. Third, the surfaces of planetary bodies directly record the effects of impacts in the form of craters; these records have wide-ranging implications. Fourth, regoliths evolution of asteroidal surfaces is a consequence of cumulative impacts, but the absence of a significant gravity term may profoundly affect the retention of shocked fractions and agglutinate build-up, thereby biasing the correct interpretations of spectral reflectance data. An impact facility on the Space Station would provide the controlled conditions necessary to explore such processes either through direct simulation of conditions or indirect simulation of certain parameters

Collisional dust avalanches in debris discs

We quantitatively investigate how collisional avalanches may developin debris discs as the result of the initial break-up of a planetesimal or comet-like object, triggering a collisional chain reaction due to outward escaping small dust grains. We use a specifically developed numerical code that follows both the spatial distribution of the dust grains and the evolution of their size-frequency distribution due to collisions. We investigate how strongly avalanche propagation depends on different parameters (e.g., amount of dust released in the initial break-up, collisional properties of dust grains and their distribution in the disc). Our simulations show that avalanches evolve on timescales of ~1000 years, propagating outwards following a spiral-like pattern, and that their amplitude exponentially depends on the number density of dust grains in the system. We estimate a probability for witnessing an avalanche event as a function of disc densities, for a gas-free case around an A-type star, and find that features created by avalanche propagation can lead to observable asymmetries for dusty systems with a beta Pictoris-like dust content or higher. Characteristic observable features include: (i) a brightness asymmetry of the two sides for a disc viewed edge-on, and (ii) a one-armed open spiral or a lumpy structure in the case of face-on orientation. A possible system in which avalanche-induced structures might have been observed is the edge-on seen debris disc around HD32297, which displays a strong luminosity difference between its two sides.Comment: 18 pages, 19 figures; has been accepted for publication in Astronomy and Astrophysics, section 6. Interstellar and circumstellar matter. The official date of acceptance is 29/08/200

Collisional processes and size distribution in spatially extended debris discs

We present a new multi-annulus code for the study of collisionally evolving extended debris discs. We first aim to confirm results obtained for a single-annulus system, namely that the size distribution in "real" debris discs always departs from the theoretical collisional equilibrium dN\proptoR^{-3.5}dR power law, especially in the crucial size range of observable particles (<1cm), where it displays a characteristic wavy pattern. We also aim at studying how debris discs density distributions, scattered light luminosity profiles, and SEDs are affected by the coupled effect of collisions and radial mixing due to radiation pressure affected small grains. The size distribution evolution is modeled from micron-sized grains to 50km-sized bodies. The model takes into account the crucial influence of radiation pressure-affected small grains. We consider the collisional evolution of a fiducial a=120AU radius disc with an initial surface density in $\Sigma(a)\propto a^{\alpha}$. We show that the system's radial extension plays a crucial role: in most regions the collisional and size evolution of the dust is imposed by small particles on eccentric or unbound orbits produced further inside the disc. The spatial distribution of small grains strongly departs from the initial profile, while the bigger objects, containing most of the system's mass, still follow the initial distribution. This has consequences on the scattered--light radial profiles which get significantly flatter, and we propose an empirical law to trace back the distribution of large unseen parent bodies from the observed profiles. We finally provide empirical formula for the collisional size distribution and collision timescale that can be used for future debris disc modeling.Comment: Accepted for publication in Astronomy and Astrophysics (with better figures) (note: full abstract in the *.pdf file

Representativeness of Eddy-Covariance flux footprints for areas surrounding AmeriFlux sites

Large datasets of greenhouse gas and energy surface-atmosphere fluxes measured with the eddy-covariance technique (e.g., FLUXNET2015, AmeriFlux BASE) are widely used to benchmark models and remote-sensing products. This study addresses one of the major challenges facing model-data integration: To what spatial extent do flux measurements taken at individual eddy-covariance sites reflect model- or satellite-based grid cells? We evaluate flux footprints—the temporally dynamic source areas that contribute to measured fluxes—and the representativeness of these footprints for target areas (e.g., within 250–3000 m radii around flux towers) that are often used in flux-data synthesis and modeling studies. We examine the land-cover composition and vegetation characteristics, represented here by the Enhanced Vegetation Index (EVI), in the flux footprints and target areas across 214 AmeriFlux sites, and evaluate potential biases as a consequence of the footprint-to-target-area mismatch. Monthly 80% footprint climatologies vary across sites and through time ranging four orders of magnitude from 103 to 107 m2 due to the measurement heights, underlying vegetation- and ground-surface characteristics, wind directions, and turbulent state of the atmosphere. Few eddy-covariance sites are located in a truly homogeneous landscape. Thus, the common model-data integration approaches that use a fixed-extent target area across sites introduce biases on the order of 4%–20% for EVI and 6%–20% for the dominant land cover percentage. These biases are site-specific functions of measurement heights, target area extents, and land-surface characteristics. We advocate that flux datasets need to be used with footprint awareness, especially in research and applications that benchmark against models and data products with explicit spatial information. We propose a simple representativeness index based on our evaluations that can be used as a guide to identify site-periods suitable for specific applications and to provide general guidance for data use

Impactor flux and cratering on Ceres and Vesta: Implications for the early Solar System

We study the impactor flux and cratering on Ceres and Vesta caused by the collisional and dynamical evolution of the asteroid Main Belt. We develop a statistical code based on a well-tested model for the simultaneous evolution of the Main Belt and NEA size distributions. This code includes catastrophic collisions and noncollisional removal processes such as the Yarkovsky effect and the orbital resonances. The model assumes that the dynamical depletion of the early Main Belt was very strong, and owing to that, most Main Belt comminution occurred when its dynamical structure was similar to the present one. Our results indicate that the number of D > 1 km Main Belt asteroids striking Ceres and Vesta over the Solar System history are approximately 4 600 and 1 100 respectively. The largest Main Belt asteroids expected to have impacted Ceres and Vesta had diameters of 71.7 km and 21.1 km. The number of D > 0.1 km craters on Ceres is \sim 3.4 \times 10^8 and 6.2 \times 10^7 on Vesta. The number of craters with D > 100 km are 47 on Ceres and 8 on Vesta. Our study indicates that the D = 460 km crater observed on Vesta had to be formed by the impact of a D \sim 66.2 km projectile, which has a probability of occurr \sim 30% over the Solar System history. If significant discrepancies between our results about the cratering on Ceres and Vesta and data obtained from the Dawn Mission were found, they should be linked to a higher degree of collisional evolution during the early Main Belt and/or the existence of the late heavy bombardment. An increase in the collisional activity in the early phase may be provided for an initial configuration of the giant planets consistent with, for example, the Nice model. From this, the Dawn Mission would be able to give us clues about the initial configuration of the early Solar System and its subsequent dynamical evolution.Comment: Accepted for publication in Astronomy and Astrophysic

A Giant Crater on 90 Antiope?

Mutual event observations between the two components of 90 Antiope were carried out in 2007-2008. The pole position was refined to lambda0 = 199.5+/-0.5 eg and beta0 = 39.8+/-5 deg in J2000 ecliptic coordinates, leaving intact the physical solution for the components, assimilated to two perfect Roche ellipsoids, and derived after the 2005 mutual event season (Descamps et al., 2007). Furthermore, a large-scale geological depression, located on one of the components, was introduced to better match the observed lightcurves. This vast geological feature of about 68 km in diameter, which could be postulated as a bowl-shaped impact crater, is indeed responsible of the photometric asymmetries seen on the "shoulders" of the lightcurves. The bulk density was then recomputed to 1.28+/-0.04 gcm-3 to take into account this large-scale non-convexity. This giant crater could be the aftermath of a tremendous collision of a 100-km sized proto-Antiope with another Themis family member. This statement is supported by the fact that Antiope is sufficiently porous (~50%) to survive such an impact without being wholly destroyed. This violent shock would have then imparted enough angular momentum for fissioning of proto-Antiope into two equisized bodies. We calculated that the impactor must have a diameter greater than ~17 km, for an impact velocity ranging between 1 and 4 km/s. With such a projectile, this event has a substantial 50% probability to have occurred over the age of the Themis family.Comment: 30 pages, 3 Tables, 8 Figures. Accepted for publication in Icaru

On the dynamics and collisional growth of planetesimals in misaligned binary systems

Context. Abridged. Many stars are members of binary systems. During early phases when the stars are surrounded by discs, the binary orbit and disc midplane may be mutually inclined. The discs around T Tauri stars will become mildly warped and undergo solid body precession around the angular momentum vector of the binary system. It is unclear how planetesimals in such a disc will evolve and affect planet formation. Aims. We investigate the dynamics of planetesimals embedded in discs that are perturbed by a binary companion on a circular, inclined orbit. We examine collisional velocities of the planetesimals to determine when they can grow through accretion. We vary the binary inclination, binary separation, D, disc mass, and planetesimal radius. Our standard model has D=60 AU, inclination=45 deg, and a disc mass equivalent to the MMSN. Methods. We use a 3D hydrodynamics code to model the disc. Planetesimals are test particles which experience gas drag, the gravitational force of the disc, the companion star gravity. Planetesimal orbit crossing events are detected and used to estimate collisional velocities. Results. For binary systems with modest inclination (25 deg), disc gravity prevents planetesimal orbits from undergoing strong differential nodal precession (which occurs in absence of the disc), and forces planetesimals to precess with the disc on average. For bodies of different size the orbit planes become modestly mutually inclined, leading to collisional velocities that inhibit growth. For larger inclinations (45 degrees), the Kozai effect operates, leading to destructively large relative velocities. Conclusions. Planet formation via planetesimal accretion is difficult in an inclined binary system with parameters similar to those considered in this paper. For systems in which the Kozai mechanism operates, the prospects for forming planets are very remote.Comment: 24 pages, 16 figures, recently published in Astronomy and Astrophysic

Dust Production from collisions in extrasolar planetary systems The inner Beta-Pictoris disc

Dust particles observed in extrasolar planetary discs originate from undetectable km-sized bodies but this valuable information remains uninteresting if the theoretical link between grains and planetesimals is not properly known. We outline in this paper a numerical approach we developed in order to address this issue for the case of dust producing collisional cascades. The model is based on a particle-in-a-box method. We follow the size distribution of particles over eight orders of magnitude in radius taking into account fragmentation and cratering according to different prescriptions. A very particular attention is paid to the smallest particles, close to the radiation pressure induced cut-off size $R_{pr}$, which are placed on highly eccentric orbits by the stellar radiation pressure....(abstract continued in the uploaded paper)Comment: A&A accepted (in press

Predicting language learners' grades in the L1, L2, L3 and L4: the effect of some psychological and sociocognitive variables

This study of 89 Flemish high-school students' grades for L1 (Dutch), L2 (French), L3 (English) and L4 (German) investigates the effects of three higher-level personality dimensions (psychoticism, extraversion, neuroticism), one lower-level personality dimension (foreign language anxiety) and sociobiographical variables (gender, social class) on the participants' language grades. Analyses of variance revealed no significant effects of the higher-level personality dimensions on grades. Participants with high levels of foreign language anxiety obtained significantly lower grades in the L2 and L3. Gender and social class had no effect. Strong positive correlations between grades in the different languages could point to an underlying sociocognitive dimension. The implications of these findings are discussed

Thermal evolution and activity of Comet 9P/Tempel 1 and simulation of a deep impact

We use a quasi 3-D thermal evolution model for a spherical comet nucleus, which takes into account the diurnal and latitudinal variation of the solar flux, but neglects lateral heat conduction. We model the thermal evolution and activity of Comet 9P/Tempel 1, in anticipation of the Deep Impact mission encounter with the comet. We also investigate the possible outcome of a projectile impact, assuming that all the energy is absorbed as thermal energy. An interesting result of this investigation, is that the estimated amount of dust ejected due to the impact is equivalent to 2--2.6 days of activity, during "quiet" conditions, at perihelion. We show that production rates of volatiles that are released in the interior of the nucleus depend strongly on the porous structure, in particular on the surface to volume ratio of the pores. We develop a more accurate model for calculating this parameter, based on a distribution of pore sizes, rather than a single, average pore size.Comment: 25 pages, 8 figures, accepted for publication in PASP (in press). For fig.xx (composite image, sec.4) and a better resolution of fig.6 see, http://geophysics.tau.ac.il/personal/gal%5Fsarid