On The 53Mn Heterogeneity In The Early Solar System

It is well established that the prolonged and thorough mixing of numerous nucleosynthetic components that constitutes the matter in the solar nebula resulted in an essential isotopic homogeneity of the solar system material. This may or may not be true for the short-lived radionuclides which were injected into or formed within the solar nebula just prior to or during solar system formation. Distinguishing between their heterogeneous or homogeneous distribution is important because the short- lived radionuclides are now widely used for the relative chronology of various objects and processes in the early solar system and as constraints for models of nucleosynthesis. The recent studies of the ⁵³Mn-⁵³Cr isotope system (half life of ⁵³Mn is 3.7 Ma) in various solar system objects have shown that the relative abundance of radiogenic ⁵³Cr is consistent with essentially homogeneous distribution of ⁵³Mn in the asteroid belt. Thus, the relative ⁵³Mn-⁵³Cr chronometer can be directly used for dating samples which originated in the asteroid belt. Importantly, however, all meteorite groups studied so far indicate a clear excess of ⁵³Cr as compared to Earth and to a lunar sample, which exhibits also a terrestrial ⁵³Cr/⁵²Cr ratio. The results from the Martian (SNC) meteorites show that their ⁵³Cr excesses are less than half of those found in the asteroid belt bodies. Thus, the characteristic ⁵³Cr/⁵²Cr ratio of Mars is intermediate between that of the Earth-Moon system and those of the other meteorites. If these ⁵³Cr variations are viewed as a function of the heliocentric distance, the radial dependence of the relative abundances of radiogenic ⁵³Cr is indicated. This observed gradient can be explained by either an early, volatility controlled, Mn/Cr fractionation within the nebula or by an initial radial heterogeneous distribution of ⁵³Mn. Although model calculations of the Mn/Cr ratios in the bulk terrestrial planets seem to be inconsistent with the volatility driven scenario, the precision of these calculations is inadequate for eliminating this possibility. In contrast, recent studies of the ⁵³Mn-⁵³Cr system in the enstatite chondrites indicate that, while their bulk Mn/Cr ratios are essentially the same as in ordinary chondrites, the ⁵³Cr excess in bulk enstatite chondrites is three times lower than that in the bulk ordinary chondrites. This difference cannot be explained by a Mn/Cr fractionation and, thus, strongly suggests that a radial heterogeneous distribution of ⁵³Mn must have existed in at least the early inner solar system. Using the observed gradient and the ⁵³Cr/⁵²Cr ratio of the bulk enstatite chondrites, their parent body(ies) formed at ∼1.4 AU or somewhat closer to the Sun. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Generation of Correlated Weibull Clutter for Signal Detection Problems

A method is discussed for generating Weibull vectors with a desired correlation matrix and specified parameters. Such vectors may represent samples of a correlated clutter signal. The presented method makes use of a suitable nonlinear transformation of random Gaussian vectors with correlated components. Computational aspects of the method are also discussed.     

Venus Express: Scientific goals, instrumentation, and scenario of the mission

The first European mission to Venus (Venus Express) is described. It is based on a repeated use of the Mars Express design with minor modifications dictated in the main by more severe thermal environment at Venus. The main scientific task of the mission is global exploration of the Venusian atmosphere, circumplanetary plasma, and the planet surface from an orbiting spacecraft. The Venus Express payload includes seven instruments, five of which are inherited from the missions Mars Express and Rosetta. Two instruments were specially designed for Venus Express. The advantages of Venus Express in comparison with previous missions are in using advanced instrumentation and methods of remote sounding, as well as a spacecraft with a broad spectrum of capabilities of orbital observations.     

Initial results from the ISEE-1 and -2 plasma wave investigation

In this paper we present an initial survey of results from the plasma wave experiments on the ISEE-1 and -2 spacecraft which are in nearly identical orbits passing through the Earth's magnetosphere at radial distances out to about 22.5R_e. Essentially every crossing of the Earth's bow shock can be associated with an intense burst of electrostatic and whistler-mode turbulence at the shock, with substantial wave intensities in both the upstream and downstream regions. Usually the electric and magnetic field spectrum at the shock are quite similar for both spacecraft, although small differences in the detailed structure are sometimes apparent upstream and downstream of the shock, probably due to changes in the motion of the shock or propagation effects. Upstream of the shock emissions are often observed at both the fundamental, f_-^p, and second harmonic, 2f_p^-, of the electron plasma frequency. In the magnetosphere high resolution spectrograms of the electric field show an extremely complex distribution of plasma and radio emissions, with numerous resonance and cutoff effects. Electron density profiles can be obtained from emissions near the local electron plasma frequency. Comparisons of high resolution spectrograms of whistler-mode emissions such as chorus detected by the two spacecraft usually show a good overall similarity but marked differences in detailed structure on time scales less than one minute. Other types of locally generated waves, such as the (n+1/2)f_-^gelectron cyclotron waves, show a better correspondence between the two spacecraft. High resolution spectrograms of kilometric radio emissions are also presented which show an extremely complex frequency-time structure with many closely spaced narrow-band emissions.