Global MHD modeling of Mercury's magnetosphere with applications to the MESSENGER mission and dynamo theory

We use a global magnetohydrodynamic (MHD) model to simulate Mercury's space environment for several solar wind and interplanetary magnetic field (IMF) conditions in anticipation of the magnetic field measurements by the MESSENGER spacecraft. The main goal of our study is to assess what characteristics of the internally generated field of Mercury can be inferred from the MESSENGER observations, and to what extent they will be able to constrain various models of Mercury's magnetic field generation. Based on the results of our simulations, we argue that it should be possible to infer not only the dipole component, but also the quadrupole and possibly even higher harmonics of the Mercury's planetary magnetic field. We furthermore expect that some of the crucial measurements for specifying the Hermean internal field will be acquired during the initial fly-bys of the planet, before MESSENGER goes into orbit around Mercury.     

北秦岭西段冥古宙锆石(4.1～3.9Ga)年代学新进展

2007年王洪亮等报道在北秦岭西段火山岩中获得一粒年龄为4079±5Ma的冥古宙捕虏锆石。之后,对这一发现开展了深入的调查研究,我们除利用SHIMP技术方法对原4079Ma的锆石进行验证外,新获得了两粒~(207)Pb/~(206)Pb年龄为4007±29Ma和3908±45Ma捕获的变质成因锆石,表明早在4.0Ga已经有变质作用的发生,这或许说明在冥古宙时期地球已经具有相当规模和厚度的地壳。同时开展的岩石学研究表明,蕴含古老锆石的母岩属于火山碎屑熔岩类而不是火山熔岩。     

Maar-diatreme phreatomagmatism at Hopi Buttes,Navajo Nation (Arizona), USA

Remnants of an extensive maar-diatreme volcanic field are magnificently exposed at various depths of erosion in the Hopi Buttes volcanic field of northeastern Arizona. Field and petrographic studies of both the maar and diatreme elements of a selection of volcanoes within the field show that: (1) lower sections of the maar rim sequences are typically rich in sandy mudrock derived from the pre-eruptive Mio-Pliocene Bidahochi Formation, and the muddy Bidahochi sediment was soft and wet at the time of maar eruptions; (2) beds higher within the rims contain generally increased proportions of sandstone clasts from the Triassic Wingate Formation. In the diatremes, late-emplaced breccia has deeper-seated lithics than more marginal breccia emplaced earlier; and (3) many vents are topped by megacryst-enriched scoria and spatter, and deep-seated xenoliths are known only from upper diatreme and craterfilling tephra. These observations show that: (1) eruptions at Hopi Buttes involved interaction of magma with unconsolidated mudrock at shallow levels, and the phreatomagmatic processes that provided the bulk of the energy involved in the violent eruptions were driven by the interaction of magma and wet sediment; (2) the locus of explosive activity migrated downward as eruptions progressed; and (3) the closing stage of many eruptions was characterized by rapid magma rise and relative depletion of water.     

MHD Simulation of Comets: The Plasma Environment of Comet Hale-Bopp

MHD simulation results of the interaction of the expanding atmosphere of comet Hale-Bopp with the magnetized solar wind are presented. At the upstream boundary a supersonic and superalfvénic solar wind enters into the simulation box 25 million km upstream of the nucleus. The solar wind is continuously mass loaded with cometary ions originating from the nucleus. The effects of photoionization, recombination and ion-neutral frictional drag are taken into account in the model. The governing equations are solved on an adaptively refined unstructured Cartesian grid using our MUSCL-type upwind numerical technique, MAUS-MHD (Multiscale Adaptive Upwind Scheme for MHD). The combination of the adaptive refinement with the MUSCL-scheme allows the entire cometary atmosphere to be modeled, while still resolving both the shock and the diamagnetic cavity of the comet. Detailed simulation results for the plasma environment of comet Hale-Bopp for slow and fast solar wind conditions are presented. We also calculate synthetic H₂O⁺, CO⁺ and soft x-ray images for observing conditions on April 11, 1997. This revised version was published online in July 2006 with corrections to the Cover Date.