665 research outputs found
Regional tectonic analysis of Venus equatorial highlands and comparison with Earth-based Magellan radar images
Research on regional tectonic analysis of Venus equatorial highlands and comparison with earth-based and Magellan radar images is presented. Over the past two years, the tectonic analysis of Venus performed centered on global properties of the planet, in order to understand fundamental aspects of the dynamics of the mantle and lithosphere of Venus. These include studies pertaining to the original constitutive and thermal character of the planet, as well as the evolution of Venus through time, and the present day tectonics. Parameterized convection models of the Earth and Venus were developed. The parameterized convection code was reformulated to model Venus with an initially hydrous mantle to determine how the cold-trap could affect the evolution of the planet
Evolution of planetesimal velocities
A self-consistent set of equations for the velocity evolution of a general planetesimal population is presented. The equations are given in a form convenient for calculations of the early stages of planetary accumulation when it is necessary to model the planetesimal swarm by methods of gas dynamics, rather than follow the orbital evolution of individual bodies. Steady state velocities of a simple planetesimal population consisting of two different sizes of bodies are calculated. Dynamical friction is found to be an important mechanism for transferring kinetic energy from the larger planetesimals to the smaller ones. When the small planetesimals are relatively abundant, gas drag and inelastic collisions among the smaller bodies are of comparable importance for dissipating energy from the population
POSTNATAL BRAIN DYSMORPHOLOGY INDUCED BY PRENATAL ALCOHOL EXPOSURE: A PRECLINICAL MRI STUDY
poster abstractBrain dysmorphology is one of the most critical features of Fetal Alcohol Spectrum Disorders (FASD). This study was designed to use high resolution preclinical MRI system to compare the brain structures between alcohol exposed C57BL/6 mice with control. The objective is to examine how alcohol affects a dose- and timing-dependent brain dysmorphology during development comparable to that of human FASD. Three treated groups, ALC (pre- and pregnancy alcohol with 4.2 % (v/v) alcohol liquid), PF (pre alcohol and a calorically matched liquid pregnancy diet), and CHOW (ad lib chow/water), were examined. Mouse heads were imaged using 9.4T preclinical MRI system with 3D gradient echo (GRE) sequence to acquire volumetric images with voxel size as low as 40 microns. Whole brain, olfactory bulbs, cortex, hypothalamus, and cerebellum were segmented and the volumes were calculated. Data was examined by ANOVA followed with paired comparison between treatment groups to test the effect of prenatal alcohol exposure. ALC group had shown consistently smaller mean volumes of difference brain regions than the other two groups. Volume of total brain, olfactory bulbs and cerebellum were observed to be significantly different for ALC compared to PF pups. This indicated that prenatal alcohol exposure caused retarded fetal brain development. Comparing PF with CHOW pups, only cerebellum volume was observed to be significantly different. For cortex volume, no significant difference was shown for any pairwise comparison. These results suggest that alcohol effect contribute to brain dysmorphology, and match with our previous craniofacial dysmorphology study. This could be important to assist in the understanding of clinical variants of human FASD patients in brain dysmorphology
Collisions between equal sized ice grain agglomerates
Following the recent insight in the material structure of comets,
protoplanetesimals are assumed to have low densities and to be highly porous
agglomerates. It is still unclear if planetesimals can be formed from these
objects by collisional growth. Therefore, it is important to study numerically
the collisional outcome from low velocity impacts of equal sized porous
agglomerates which are too large to be examined in a laboratory experiment. We
use the Lagrangian particle method Smooth Particle Hydrodynamics to solve the
equations that describe the dynamics of elastic and plastic bodies.
Additionally, to account for the influence of porosity, we follow a previous
developed equation of state and certain relations between the material strength
and the relative density. Collisional growth seems possible for rather low
collision velocities and particular material strengths. The remnants of
collisions with impact parameters that are larger than 50% of the radius of the
colliding objects tend to rotate. For small impact parameters, the colliding
objects are effectively slowed down without a prominent compaction of the
porous structure, which probably increases the possibility for growth. The
protoplanetesimals, however, do not stick together for the most part of the
employed material strengths. An important issue in subsequent studies has to be
the influence of rotation to collisional growth. Moreover, for realistic
simulations of protoplanetesimals it is crucial to know the correct material
parameters in more detail.Comment: 7 pages, 11 figures, accepted by A&
Collisional Velocities and Rates in Resonant Planetesimal Belts
We consider a belt of small bodies around a star, captured in one of the
external or 1:1 mean-motion resonances with a massive perturber. The objects in
the belt collide with each other. Combining methods of celestial mechanics and
statistical physics, we calculate mean collisional velocities and collisional
rates, averaged over the belt. The results are compared to collisional
velocities and rates in a similar, but non-resonant belt, as predicted by the
particle-in-a-box method. It is found that the effect of the resonant lock on
the velocities is rather small, while on the rates more substantial. The
collisional rates between objects in an external resonance are by about a
factor of two higher than those in a similar belt of objects not locked in a
resonance. For Trojans under the same conditions, the collisional rates may be
enhanced by up to an order of magnitude. Our results imply, in particular,
shorter collisional lifetimes of resonant Kuiper belt objects in the solar
system and higher efficiency of dust production by resonant planetesimals in
debris disks around other stars.Comment: 31 pages, 11 figures (some of them heavily compressed to fit into
arxiv-maximum filesize), accepted for publication at "Celestial Mechanics and
Dynamical Astronomy
Shape models and physical properties of asteroids
Despite the large amount of high quality data generated in recent space
encounters with asteroids, the majority of our knowledge about these objects
comes from ground based observations. Asteroids travelling in orbits that are
potentially hazardous for the Earth form an especially interesting group to be
studied. In order to predict their orbital evolution, it is necessary to
investigate their physical properties. This paper briefly describes the data
requirements and different techniques used to solve the lightcurve inversion
problem. Although photometry is the most abundant type of observational data,
models of asteroids can be obtained using various data types and techniques. We
describe the potential of radar imaging and stellar occultation timings to be
combined with disk-integrated photometry in order to reveal information about
physical properties of asteroids.Comment: From Assessment and Mitigation of Asteroid Impact Hazards boo
Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets
Nature, 435, pp. 466-469, http://dx.doi.org./10.1038/nature03676International audienc
A Clash of Old and New Scientific Concepts in Toxicity, with Important Implications for Public Health
Background A core assumption of current toxicologic procedures used to establish health standards for chemical exposures is that testing the safety of chemicals at high doses can be used to predict the effects of low-dose exposures, such as those common in the general population. This assumption is based on the precept that âthe dose makes the poisonâ: higher doses will cause greater effects. Objectives We challenge the validity of assuming that high-dose testing can be used to predict low-dose effects for contaminants that behave like hormones. We review data from endocrinology and toxicology that falsify this assumption and summarize current mechanistic understanding of how low doses can lead to effects unpredictable from high-dose experiments. Discussion Falsification of this assumption raises profound issues for regulatory toxicology. Many exposure standards are based on this assumption. Rejecting the assumption will require that these standards be reevaluated and that procedures employed to set health standards be changed. The consequences of these changes may be significant for public health because of the range of health conditions now plausibly linked to exposure to endocrine-disrupting contaminants. Conclusions We recommend that procedures to establish acceptable exposure levels for endocrine-disrupting compounds incorporate the inability for high-dose tests to predict low-dose results. Setting acceptable levels of exposure must include testing for health consequences at prevalent levels of human exposure, not extrapolations from the effects observed in high-dose experiments. Scientists trained in endocrinology must be engaged systematically in standard setting for endocrine-disrupting compounds
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
On the Ionisation Fraction in Protoplanetary Disks I: Comparing Different Reaction Networks
We calculate the ionisation fraction in protostellar disk models using a
number of different chemical reaction networks, including gas-phase and
gas-grain reaction schemes. The disk models we consider are conventional
alpha-disks, which include viscous heating and radiative cooling. The primary
source of ionisation is assumed to be X-ray irradiation from the central star.
We consider a number of gas-phase chemical networks. In general we find that
the simple models predict higher fractional ionisation levels and more
extensive active zones than the more complex models. When heavy metal atoms are
included the simple models predict that the disk is magnetically active
throughout. The complex models predict that extensive regions of the disk
remain magnetically uncoupled even with a fractional abundance of magnesium of
10(-8). The addition of submicron sized grains with a concentration of 10(-12)
causes the size of the dead zone to increase dramatically for all kinetic
models considered. We find that the simple and complex gas-grain reaction
schemes agree on the size and structure of the resulting dead zone. We examine
the effects of depleting the concentration of small grains as a crude means of
modeling the growth of grains during planet formation. We find that a depletion
factor of 10(-4) causes the gas-grain chemistry to converge to the gas-phase
chemistry when heavy metals are absent. 10(-8) is required when magnesium is
included. This suggests that efficient grain growth and settling will be
required in protoplanetary disks, before a substantial fraction of the disk
mass in the planet forming zone between 1 - 10 AU becomes magnetically active
and turbulent.Comment: 21 pages, 23 figures, accepted for publication in A & A Includes
correction to our implementation of the Umebayashi-Nakano reaction networ
- âŠ