Atmospheric Disturbances and Radiation Impulses Caused by Large-Meteoroid Impacts on the Surface of Mars. I. Formation and Evolution of Dust Cloud

We consider the mechanisms of the formation of dust ejected from craters produced by large-meteoroid impacts on the Martian surface, as well as the mechanisms of the elevation of dust that already existed on the surface, due to impulsed aeolian processes. Detailed numerical calculations of the dust injection, the shock wave propagation, and the formation and evolution of the dust cloud are carried out for vertical impacts of meteoroids with sizes from 1 m to 100 m. The results of these calculations show that dust raised by a 1-m impactor is sufficient to produce a local dust storm, while the mass of dust formed in impacts of large bodies is comparable to the mass of a regional or even a global dust storm. The impact detection rates for 1-, 5-, 20-, and 100-m-sized meteoroids are estimated to be a few impact events per year, one event in every 5–6 years, one event in every 300–800 years, and one event in every 5000–20000 years, respectively. In the last case, the thickness of the global layer of precipitated dust and small fragments, which has been formed through impacts over a period of 10⁷–10⁸ years, is comparable to the thickness of the global dust layer on the Martian surface. In the first case, the mass of raised dust is greater than that for typical dust devils. The speed of impulsed wind at large distances from the impact site is shown to exceed the critical speed at which the blowing-off of dust from the surface begins. Some factors that may enhance the dust ejection have been previously ignored in numerical calculations. We discuss here the role of these factors. The second part of our study deals with the determination of the impact-induced radiation impulse and the estimation of its effect on the rise of dust.     

Anisotropy of the mantle inferred from observations of P to S converted waves

Summary. Seismic anisotropy has been previously studied at depths usually not exceeding 100 or 150 km. In this paper we present a method of analysis of seismic records which is very sensitive to azimuthal anisotropy and is applicable at almost any depth range. The idea of the method is to detect and analyse the SH -component of the waves, converted from P to S in the mantle. The procedure of record processing includes frequency filtering, axis rotation, transformation of the record to a standard form, stacking the standardized SH -component records of many seismic events, and the harmonic analysis of amplitude as a function of the direction of wave propagation. When applied to the long-period records of NORSAR the procedure detected a converted wave with the properties implying the possibility of its propagation in a transversely isotropic medium with a horizontal axis of symmetry . Our preferred model postulates anisotropy of ∼ 1 per cent in a layer 50 km thick at the base of the upper mantle.     

Geodynamic conditions of formation of massive sulfide deposits in the Magnitogorsk Megazone,Southern Urals,and prospection criteria

The zoned composition changes of the massive sulfide deposits in the major massive sulfide zone of the Southern Urals such as the Magnitogorsk Megasynclinorium are considered. The zoning is expressed as the trend of Ni–Co–Cu → Zn–Cu → Cu–Zn → Au–Ba–Pb–Cu–Zn. This trend is related to two basic factors: (1) the subduction process with the slab’s eastward subsidence preconditioned the formation (from the west to the east) of the following massive sulfide zones: accretionary prism, frontal island arc, developed island arc, inter-arc spreading zone, split back arc, and back-arc spreading; (2) the longitudinal zoning of the massive sulfide paleovolcanic belts related to changes in the thickness of the crust’s basaltic layer and an inclination of the subducting plate in transverse blocks of the belt. The first factor affects the general paleovolcanic and metallogenic latitudinal zoning of the studied region, while the second factor defines the local meridional zoning. The composition of ore-bearing solutions is dependent on the formation depth of the subduction fluids, magma differentiation type, and the ratio of deep fluids to solutions of near-surface convective cells. The combination of the geodynamic factors expressed in the composition of ore-bearing volcanic complexes and the specific geological settings defines the massive sulfide mineralization composition and productivity criteria. The most productive structures include the frontal island-arc and inter-arc spreading zones and within them, the central-type volcanic edifices whose basalts are referred to as the island-arc tholeiite series and are characterized by the minimum TiO₂ and Zr content and low La/Yb ratios.