N-Body Simulations of Planetesimal Evolution: Effect of Varying Impactor Mass Ratio

We present results from direct N-body simulations of collisions between gravitational aggregates of varying size as part of a study to parameterize planetesimal growth in the Solar System. We find that as the ratio of projectile to target mass departs from unity, the impact angle has less effect on the outcome. At the same time, the probability of planetesimal growth increases. Conversely, for a fixed impact energy, collisions between impactors with mass ratio near unity are more dispersive than those with impactor mass ratio far from unity. We derive an expression for the accretion probability as a function of mass ratio. For an average mass ratio of 1:5, we find an accretion probability of ∼60% over all impact parameters. We also compute the critical specific dispersal energy Q*_D as a function of projectile size. Extrapolating to a projectile size of 1 m with a 1-km target, we find Q*_D=10³−10⁴ J kg⁻¹, in agreement with several other collision models that use fundamentally different techniques. Our model assumes that the components of each gravitational aggregate are identical and indestructible over the range of sampled impact speeds. In future work we hope to incorporate a simple fracture model to extend the range of applicable speeds and we plan to implement our results in a large-scale planetesimal evolution code.     

A Simplified Kinetic Element Formulation for the Rotation of a Perturbed Mass-Asymmetric Rigid Body

Euler's equations, describing the rotation of an arbitrarily torqued mass asymmetric rigid body, are scaled using linear transformations that lead to a simplified set of first order ordinary differential equations without the explicit appearance of the principal moments of inertia. These scaled differential equations provide trivial access to an analytical solution and two constants of integration for the case of torque-free motion. Two additional representations for the third constant of integration are chosen to complete two new kinetic element sets that describe an osculating solution using the variation of parameters. The elements' physical representations are amplitudes and either angular displacement or initial time constant in the torque-free solution. These new kinetic elements lead to a considerably simplified variation of parameters solution to Euler's equations. The resulting variational equations are quite compact. To investigate error propagation behaviour of these new variational formulations in computer simulations, they are compared to the unmodified equations without kinematic coupling but under the influence of simulated gravity-gradient torques.     

The Bastille day Shock and Merged Interaction Region at 63 au: Voyager 2 Observations

A transient flow system containing several streams and shocks associated with the Bastille Day 2000 solar event was observed by the WIND and ACE spacecraft at 1 AU. Voyager 2 (V2) at 63 AU observed this flow system after it moved through the interplanetary medium and into the distant heliosphere, where the interstellar pickup protons strongly influence the MHD structures and flow dynamics. We discuss the Voyager 2 magnetic and plasma observations of this event. Increases in the magnetic field strength B, density N, temperature T and speed V were observed at the front of a stream at V2, consistent with presence of a shock related to the Bastille Day shock at 1 AU. However, the jumps occurred in a 16.9-hour data gap, so that the shock was not observed directly, and the properties of the candidate shock cannot be determined precisely. The candidate shock was followed by a merged interaction region (MIR) that moved past V2 for at least 10 days. The first part of this MIR contains a structure that might be a magnetic cloud. Just ahead of the shock there was an abrupt increase in density associated with a decrease in temperature such that the solar wind thermal pressure was constant across it. Just behind the shock there was an abrupt decrease in density associated with a net increase in magnetic field strength. This appears to be a pressure balanced structure in which the interstellar pickup protons make a significant contribution.     

Field validation of antioxidant enzyme biomarkers in mussels (Perna viridis) and clams (Ruditapes philippinarum) transplanted in Hong Kong coastal waters

Green-lipped mussels, Perna viridis, and Manila clams, Ruditapes philippinarum were sourced from “clean” sites in the Hong Kong region, depurated in a laboratory using uncontaminated filtered seawater for 8 days, and transplanted to a suspected gradient of chemically polluted sites in Hong Kong. After 14- and 28-days of field exposure, several antioxidant parameters including glutathione S transferase (GST), catalase (CAT), glutathione peroxidase (GPx), and glutathione (GSH) were quantified in gill and hepatopancreas tissues. Whole body tissue concentrations of polycyclic aromatic hydrocarbons (PAHs), petroleum hydrocarbons (PHCs), polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCs) were determined in pooled site samples. Chemical analyses indicated that: (a) clams had higher levels of PAHs, PHCs, DDTs and PCBs, whereas mussels had higher hexachlorocyclohexane (HCHs) and there was no difference between species for dieldrin and remaining OCs; (b) Kat O should not be continued as a “clean” reference site for Hong Kong, because of the levels of contaminants measured and (c) PAH concentrations in the current survey were similar to those previously measured. Toxicological conclusions were: (a) antioxidant responses were different between species; (b) CAT and GST have highest utility in clams for field use in Hong Kong, whereas CAT in both gill and hepatopancreas tissue showed most potential in mussels; (c) significant induction of antioxidant responses over day 0 (excluding GPx in both tissues, and GST in mussel hepatic tissue); (d) groups of contaminants do not consistently induce antioxidant responses and (e) organochlorines and PCBs correlated significantly with CAT and GST in clam hepatopancreas and with CAT in mussel gill and hepatic tissue. Multivariate statistical techniques indicated little relationship between the site patterns for antioxidant responses and the contaminant gradients identified in body burden analysis.     

Okadaic acid, a causative toxin of diarrhetic shellfish poisoning, in green-lipped mussels Perna viridis from Hong Kong fish culture zones: method development and monitoring

Green-lipped mussels (Perna viridis) were collected from seven fish culture zones (FCZs) in Hong Kong and analyzed for okadaic acid (OA). A conventional HPLC method was modified by incorporating a proteinase K digestion step. Results suggest that a higher recovery (2.5 times higher) of OA was obtained from spiked samples after the addition of 1.08 mg proteinase K in comparison with samples incubated without the proteolytic enzyme. For the hepatopancreas (HP) of individual field-collected mussels, the additional digestion step can enhance OA extraction by 3.1 times. Spatial and temporal variations in OA concentrations in the mussels from various FCZs were investigated. The highest concentration of OA in mussel HP samples was 1164.9 ng/g HP wet wt. With respect to OA concentrations in whole mussel tissues from seven sites and four seasonal samplings, the concentrations were between 70.0 and 131.0 ng/g wet wt., which did not exceed the generally recognized international regulatory criteria (>200 ng/g) for OA.     

An Asian quandary: where have all of the PBDEs gone?

Polybrominated diphenyl ethers (PBDEs) are widely used fire retardant compounds which, like polychlorinated biphenyls (PCBs), have become ubiquitous in environmental media. Time trend analyses of PBDE levels in sediments and biota generally show an increasing trend. The lower (tetra- and penta-) PBDEs accumulate and predominate in biota, whilst the higher congeners (deca-) are prevalent in aquatic environments and sediments. PBDEs also biomagnify, with highest concentrations of the lower brominated congeners being seen in top predators including birds, marine mammals and humans. Toxicological studies of PBDEs have been limited, although effects mediated via the thyroid gland may be extremely important. Disturbingly, PBDEs may be sequestering in certain Asian environments to a higher degree than in other parts of the world, due to the unregulated disposal of computers and electronic equipment. However, this potential problem remains poorly investigated, to date.     

An analytical theory for the orbit of nereid

A complete analytical dynamic theory for the motion of Nereid has been constructed, accurate to approximately 0.01 arc second over several hundred years. The solution uses the Lie transform approach advanced by Deprit and is consistent with respect to the magnitudes of the disturbing functions, including all perturbations to an accuracy of 10^–8 relative to the two-body potential (oblateness and third-body). Multiple short-period variables in the third-body perturbations are related via the ratio of their mean motions, reducing the number of independent variables. Extensive use is made of expansions giving trigonometric functions of the true anomaly as analytical Fourier series in the mean anomaly. Initial constants and mass parameters come from the data obtained during the Voyager II encounter with Neptune in 1989.