Mineralogy and petrology of comet 81P/wild 2 nucleus samples

The bulk of the comet 81P/Wild 2 (hereafter Wild 2) samples returned to Earth by the Stardust spacecraft appear to be weakly constructed mixtures of nanometer-scale grains, with occasional much larger (over 1 micrometer) ferromagnesian silicates, Fe-Ni sulfides, Fe-Ni metal, and accessory phases. The very wide range of olivine and low-Ca pyroxene compositions in comet Wild 2 requires a wide range of formation conditions, probably reflecting very different formation locations in the protoplanetary disk. The restricted compositional ranges of Fe-Ni sulfides, the wide range for silicates, and the absence of hydrous phases indicate that comet Wild 2 experienced little or no aqueous alteration. Less abundant Wild 2 materials include a refractory particle, whose presence appears to require radial transport in the early protoplanetary disk

Mineralogy and Petrology of Comet Wild 2 Nucleus Samples

The bulk of the Wild 2 samples appear to be weakly-constructed mixtures of nanometerscale grains with occasional much larger (>1{micro}m) ferromagnesian silicates, Fe-Ni sulfides, Fe-Ni metal and accessory phases. The very wide range of olivine and low-Ca pyroxene compositions in Wild 2 require a wide range of formation conditions, probably reflecting different formation locations in the protoplanetary disk. The restricted compositional ranges of Fe-Ni sulfides, the wide range for silicates, and absence of hydrous phases indicate that Wild 2 experienced little or no aqueous alteration. Less abundant Wild 2 materials include a refractory particle, whose presence appears to require large-scale radial transport in the early protoplanetary disk. The nature of cometary solids is of fundamental importance to our understanding of the early solar nebula and protoplanetary history. Until now we have had to study comets from afar using spectroscopy, or settle for analyses of interplanetary dust particles (IDPs) of uncertain provenance. We report here mineralogical and petrographic analyses of particles derived directly from Comet Wild 2. All of the Wild 2 particles we have thus far examined have been modified in various ways by the capture process. All particles that may have been loose aggregates, ''traveling sand piles'', disaggregated into individual components with the larger, denser components penetrating more deeply into the aerogel. Individual grains experienced a wide range of heating effects that range from excellent preservation to melting (Fig. 1); such behavior was expected (1, 2 ,3). What is remarkable is the extreme variability of these modifications and the fact that severely modified and unmodified materials can be found within a micrometer of each other, requiring tremendous local temperature gradients. Fortunately, we have an internal gauge of impact collection heating. Fe-Ni sulfides are ubiquitous in the Wild 2 samples, are very sensitive indicators of heating, and accurate chemical analyses can reveal which have lost S, and which have not (and are therefore stoichiometric) (Fig. 2). Our surveys show that crystalline grains are found along the entire lengths of tracks, not just at track termini

Activation of group III metabotropic glutamate receptors in selected regions of the basal ganglia alleviates akinesia in the reserpine-treated rat

1. This study examined whether group III metabotropic glutamate (mGlu) receptor agonists injected into the globus pallidus (GP), substantia nigra pars reticulata (SNr) or intracerebroventricularly (i.c.v.) could reverse reserpine-induced akinesia in the rat. 2. Male Sprague–Dawley rats, cannulated above the GP, SNr or third ventricle, were rendered akinetic with reserpine (5 mg kg(−1) s.c.). 18 h later, behavioural effects of the group III mGlu receptor agonists L-serine-O-phosphate (L-SOP) or L-(+)-2-amino-4-phosphonobutyric acid (L-AP4) were examined. 3. In reserpine-treated rats, unilateral injection of L-SOP (2000 and 2500 nmol in 2.5 μl) into the GP produced a significant increase in net contraversive rotations compared to vehicle, reaching a maximum of 83±21 rotations 120 min(−1) (n=8). Pretreatment with the group III mGlu receptor antagonist methyl-serine-O-phosphate (M-SOP; 250 nmol in 2.5 μl) inhibited the response to L-SOP (2000 nmol) by 77%. 4. Unilateral injection of L-SOP (250-1000 nmol in 2.5 μl) into the SNr of reserpine-treated rats produced a dose-dependent increase in net contraversive rotations, reaching a maximum of 47±6 rotations 30 min(−1) (n=6). M-SOP (50 nmol in 2.5 μl) inhibited the response to L-SOP (500 nmol) by 78%. 5. Following i.c.v. injection, L-SOP (2000–2500 nmol in 2.5 μl) or L-AP4 (0.5–100 nmol in 2 μl) produced a dose-dependent reversal of akinesia, attaining a maximum of 45±17 (n=8) and 72±3 (n=9) arbitrary locomotor units 30 min(−1), respectively. 6. These studies indicate that injection of group III mGlu receptor agonists into the GP, SNr or cerebral ventricles reverses reserpine-induced akinesia, the mechanism for which remains to be established

Effects of dynamical evolution of giant planets on the delivery of atmophile elements during terrestrial planet formation

Recent observations started revealing the compositions of protostellar discs and planets beyond the Solar System. In this paper, we explore how the compositions of terrestrial planets are affected by dynamical evolution of giant planets. We estimate the initial compositions of building blocks of these rocky planets by using a simple condensation model, and numerically study the compositions of planets formed in a few different formation models of the Solar System. We find that the abundances of refractory and moderately volatile elements are nearly independent of formation models, and that all the models could reproduce the abundances of these elements of the Earth. The abundances of atmophile elements, on the other hand, depend on the scattering rate of icy planetesimals into the inner disc as well as the mixing rate of the inner planetesimal disc. For the classical formation model, neither of these mechanisms are efficient and the accretion of atmophile elements during the final assembly of terrestrial planets appears to be difficult. For the Grand Tack model, both of these mechanisms are efficient, which leads to a relatively uniform accretion of atmophile elements in the inner disc. It is also possible to have a "hybrid" scenario where the mixing is not very efficient but the scattering is efficient. The abundances of atmophile elements in this case increases with orbital radii. Such a scenario may occur in some of the extrasolar planetary systems which are not accompanied by giant planets or those without strong perturbations from giants. We also confirm that the Grand Tack scenario leads to the distribution of asteroid analogues where rocky planetesimals tend to exist interior to icy ones, and show that their overall compositions are consistent with S-type and C-type chondrites, respectively.Comment: 21 pages, 16 figures, 6 tables, accepted for publication in Ap