Investigating the ancient Martian magnetic field using microwaves

The new microwave palaeointensity technique has been used to investigate samples from the Martian meteorite Nakhla. This technique is a promising new way to obtain absolute palaeointensity information regarding the ancient Martian magnetic field as recorded by the Martian meteorites. Assuming that a part of the magnetic remanence is of thermal origin and originating on Mars the two samples studied yield estimates of 4 μT for the Martian magnetic field at 1.35 Ga.     

Mineral chemistry of a zircon-bearing, composite, veined and metasomatised upper-mantle peridotite xenolith from kimberlite

Zircon-bearing veins in a harzburgite xenolith from kimberlite have imposed Ca-metasomatism on the harzburgite wall rock, in addition to adding K, Fe, Ti and OH. The zircon, previously dated to have an age similar to that of the xenolith-hosting kimberlite, shows higher Y, Nb, Ba, REE, Th and U contents than other mantle-derived zircons. Peripheral alteration of the zircon to baddeleyite and zirconolite, and alteration of vein ilmenite to perovskite suggest reaction with an evolving carbonatitic kimberlite melt. The high Cr₂O₃ content (0.77 wt%) of the zirconolite extends the compositional range of terrestrial zirconolite.     

Development of a risk-hedging CO2-emission policy, part I: Risks of unrestrained emissions

A rational global strategy with respect to greenhouse-gas emissions would seek to minimize total risk, which is the sum of the risk of negative impacts due to climatic change associated with a given level of emissions, and the risks associated with the process of achieving that emission level. Given the existence of reducible uncertainties in estimating these risks, and the possibility that an emission target thought to minimize total risk is later found to be not strict enough, a risk-hedging strategy is a more realistic policy objective. This paper is Part I of a two-part series in which these risks are reviewed and an interim risk-hedging emission level is proposed. Here, the risks associated with unrestrained greenhouse-gas emissions are reviewed. In particular, the carbon-cycle response to continuing CO₂ emissions; the heat trapping of projected greenhouse gas increases in comparison to other anthropogenic and natural heating or cooling perturbations; the climatic response to heating perturbations; and the impacts of projected climatic change on global agriculture, forests, coastal regions, coral reefs, water resources, terrestrial species, stratospheric and tropospheric ozone, and human comfort and welfare are critically examined. It is concluded that unrestrained emissions of greenhouse gases pose real and substantial risks to human societies and to ecosystems, and that these risks are likely to grow substantially if the climate warms beyond that associated with a CO₂ doubling. These risks clearly justify some action to limit emissions. The magnitude of emission restraint that is justified depends not only on the risks reviewed here, but also on the risks associated with measures to limit greenhouse-gas emissions, which are reviewed in Part II.     

Poikilitic chromfite in komatiitic cumulates

Summary Chromite is a widespread accessory mineral in olivine-rich cumulates derived from komatiitic lavas. The distribution and crystal habit of chromfite is related to the degree of differentiation of the parent magma as reflected in the composition of cumulus olivine. Cumulates with olivine forsterite content greater than 93 mol percent typically contain no chromfite at all, while chromfite forms clusters of disseminated euhedral grains in cumulates with forsterite less than 91 mol percent. In the forsterite 91–93 interval, chromite may develop lobate interstitial habits. In the Six Mile Well ultramafic complex in the Yakabindie region of the Agnew-Wiluna greenstone belt, and in other olivine-rich komatiitic units within this belt, chromfite shows well developed poikilitic textures enclosing olivine.The absence of intercumulus silicate phases in these rocks and the low normative chromfite content of the parent magma make it impossible for these chromfite grains to have crystallised from intercumulus trapped liquid. These rocks must be cotectic chromite-olivine adcumulates, owing their unusual texture to differing relative rates of nucleation and growth of chromfite and olivine, crystallising togetherin situ. This observation provides further evidence for a cumulus origin for oikocrysts in layered intrusions, and casts doubt on the usefulness of cumulus terminology.

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Poikilitischer Chromit in komatiitischen Kumulaten

Zusammenfassung Chromit ist in Olivin-reichen Kumulaten, die von komatiitischen Laven abstammen, ein häufiges akzessorisches Mineral. Die Verteilung und der Kristallhabitus von Chromit hängt vom Grad der Differentiation des Stammagmas, der in der Zusammensetzung der Kumulus-Olivine zum Ausdruck kommt, ab. Für Kumulate mit Olivinen mit einem Forsterit-Gehalt von über 93 mol.% ist es charakteristisch, daß sie überhaupt keinen Chromit enthalten, während Chromit in Kumulaten mit Olivinen mit weniger als 90 mol.% Forsterit Cluster aus fein verteilten, idiomorphem Körnern bildet. Im Intervall Forsterit 90–93 kann Chromit lobate, interstitielle Formen ausbilden. Im ultramafischen Komplex Six Mile Well in der Yakabindie-Region des Agnew-Wiluna-Grünsteingürtels und in anderen Olivin-reichen komatiitischen Einheiten in diesem Gürtel zeigen die Chromite gut entwickelte poikilitische Texturen mit eingeschlossenen Olivinen.Da silikatische Interkumulus-Phasen in diesen Gesteinen fehlen und der normative Chromitgehalt des Stammagmas niedrig ist, können diese Chromitkörner nicht aus einer Interkumulus-Schmelze auskristallisiert sein. Diese Gesteine müssen daher kotektische Chromit-Olivin-Adkumulate sein, die ihre ungewöhnliche Textur den variierenden relativen Keimbildungs- und Wachstumsraten von Chromit und Olivin, die gemeinsam in situ kristallisierten, verdanken. Diese Beobachtung stellt einen weiteren Beweis für die Entstehung von Oikokristallen als Kumulusphase in geschichteten Intrusionen dar und läßt die Zweckmäßigkeit der Kumulus-Terminologie als zweifelhaft erscheinen.

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With 5 Figures     

Sakmarian geography

The known palaeontological and stratigraphical evidence is used as a basis for the construction of maps of the continents showing the extent of their inundation by the sea in Sakmarian time in the Upper Palaeozoic. In the northern hemisphere apart from India the evidence is sufficiently reliable to give reasonable maps, and the great extent of the inundations suggests that the climate would be considerably modified from that of today; no undoubted Sakmarian glacials occur there. In Southern continents and India, the Gondwana biogeographical province has made correlation with the northern continents controversial, but reasons are given for assuming that Gondwana glacial deposits were at least in part Sakmarian; the resultant maps show that the Gondwana land surfaces were but little reduced in area, and that the main glacials (except for India) lie within a belt between 40 S and 20 S. Present lack of knowledge of Sakmarian conditions in Antarctica makes reconstructions of climatic belts too hazardous for possible use in enunciating or checking hypotheses of continental drift and polar wandering.     

Theory of the electromagnetic transient response of the ionosphere for plane wave excitation

Summary The reflection and transmission of a transient plane wave by an ionosphere with a linear electron density profile and constant collision frequency are calculated. The exact frequency domain solution is inverted numerically, and the resultant reflected and transmitted waveforms are found to satisfy causality. A linearly varying, nondispersive dielectric half-space is also considered, but the nonphysical nature of the model causes the reflected waveform to be noncausal. Using convolution the waveforms for a step modulated sinusoid excitation are also obtained from the impulse response of the ionosphere. The results provide insight into the physical mechanism involved in the ionospheric propagation of HF electromagnetic pulsed signals.     

Subsurface electric fields of a grounded cable of finite length for both frequency and time domain

Summary The subsurface electric fields of a current-carrying cable are examined for both steady-state and transient situations. Closed form expressions are obtained for two of the electric field components, and a numerical integration is used to obtain the third component. Waveforms for a stepfunction current excitation are displayed. The results have possible application to pulsed downlink communication in mine rescue operations.     

ISO Far-IR spectroscopy of IR-bright galaxies and ULIRGs

Based on far-infrared spectroscopy of a small sample of nearbyinfrared-bright and ultraluminous infrared galaxies (ULIRGs) with theISO Long Wavelength Spectrometer we find adramatic progression in ionic/atomic fine-structure emission line andmolecular/atomic absorption line characteristics in these galaxiesextending from strong [O III]52,88 m and [N III]57 m lineemission to detection of only faint [C II]158 m line emissionfrom gas in photodissociation regions in the ULIRGs. The molecularabsorption spectra show varying excitation as well, extending fromgalaxies in which the molecular population mainly occupies the groundstate to galaxies in which there is significant population in higherlevels. In the case of the prototypical ULIRG, the merger galaxy Arp220, the spectrum is dominated by absorption lines of OH, H₂O, CH,and [O I]. Low [O III]88 m line flux relative to the integratedfar-infrared flux correlates with low excitation and does not appear tobe due to far-infrared extinction or to density effects. A progressiontoward soft radiation fields or very dusty H II regions may explainthese effects.     

Meteorite stranding surfaces and the Greenland icesheet

Abstract— Thirty years of recoveries in East Antarctica have led to significant understanding of the regional characteristics associated with meteorite stranding surfaces. In Antarctica these sites are characterized by patches of snow‐free blue ice at high altitude on the icesheet in regions where iceflow is highly restricted. Melting is extremely rare or absent and sublimation rates are high, even though meteorite stranding surfaces are predominantly found within regions where accumulation typically dominates. Localized environmental conditions that persist for thousands of years or longer appear to be the dominant factor rather than shorter‐term or seasonal cycles. In this paper we describe our discovery of regions in Northeast Greenland with blue ice areas that exhibit many of the requisite characteristics, suggesting that they are excellent prospects for future meteorite recovery efforts.     

Observations of sunspot umbral velocity oscillations

Sunspot umbral molecular lines have been used to look for the oscillatory velocities in the umbra. Power spectrum analysis showed conspicuous power for periods in the range between 448 and 310 s. The maximum peak-to-peak amplitude of the umbral oscillatory velocity component is observed to be in the order of 0.5 km s^–1.Visiting Astronomer, Solar Division, Kitt Peak National ObservatoryOperated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.