The control network of Iapetus

A control network of the Saturnian satellite Iapetus has been established photogrammetrically from pictures taken by the two Voyager spacecraft. Coordinates of 62 control points have been computed and listed; pixel measurements of these points were made on 14 Voyager 1 and 66 Voyager 2 pictures. Some of these points are identified on the preliminary U.S. Geological Survey map of Iapetus and many are identified by name. The Voyager 1 and Voyager 2 pictures covered limited regions of the satellite's surface and contained no overlapping areas. The longitude system on Iapetus is defined by the crater Almeric; the 276° meridian passes through the center of this crater. The obliquity of Iapetus has been measured as 0°.4 ± 1°.6. The mean radius of Iapetus has been determined as 718 ± 8 km.     

Einstein's greatest mistake?

Recent astronomical observations have re-opened the old paradox that the universe appears to be younger than some of the objects within it. I suggest that a natural resolution of this paradox lies with the re-introduction of Einstein's cosmological constant, and that sentiment against this constant fails to take into account its connection with quantum field theory.     

Devensian glacigenic sedimentation and landscape evolution in the Cardigan area of southwest Wales

The depositional processes associated with late Devensian ice in areas bordering the Irish Sea basin have been the subject of considerable debate. Among the key areas around the Irish Sea, southwest Wales occupies a particularly crucial position because it is here that ice flowing from the north impinged upon the coast orthogonally and encroached inland. Two main hypotheses have emerged concerning deglaciation of the Irish Sea basin. The traditional hypothesis holds that sedimentation was ice‐marginal or subglacial, whereas an alternative hypothesis that emerged in the 1980s argued that sedimentation was glaciomarine. Southwest Wales is well‐placed to contribute to this debate. However, few detailed sedimentological studies, linked to topography, have been made previously in order to reconstruct glacial environments in this area. In this paper, evidence is presented from four boreholes drilled recently in the Cardigan area, combined with data from coastal and inland exposures in the lower Teifi valley and adjacent areas. A complex history of glaciation has emerged: (i) subglacial drainage channel formation in pre‐Devensian time, (ii) deposition of iron‐cemented breccias and conglomerates possibly during the last interglacial (or in the early/mid‐Devensian interstadial), (iii) late Devensian ice advance across the region, during which a glaciolacustrine sequence over 75 m thick accumulated, within a glacial lake known as Llyn Teifi, (iv) a second high‐level glaciolacustrine succession formed near Llandudoch, (v) outside the Teifi valley, ice‐marginal, subglacial and glaciofluvial sediments were also laid down, providing a near‐continuous cover of drift throughout the area. Glacial advance was characterized by reworking, deformation and sometimes erosion of the underlying sediments. The glaciomarine hypothesis is thus rejected for southwest Wales. Copyright © 2001 John Wiley & Sons, Ltd.     

Deriving the normalised ion-neutral collision frequency from EISCAT observations

Common programme observations by the EISCAT UHF radar revealed an extended interval, post geomagnetic local noon on 03 April 1992, during which the F-region ion velocity orthogonal to the geomagnetic field was significantly enhanced, to values exceeding 2 km s⁻¹ corresponding to a perpendicular electric field of some 100 mV m⁻¹. Observations from this interval are used to illustrate a method by which estimates of the E-region ion-neutral collision frequency may be derived in the presence of enhanced electric field. From both the rotation of the ion velocity vector and the reduction in the ion velocity magnitude relative to that in the F-region, independent estimates of the normalised ion-neutral collision frequency are made at the UHF E-region tristatic altitudes; the derived values are, in general, lower than model predictions. Although initial calculations assume a stationary neutral atmosphere, first-order estimates of the E-region neutral wind are subsequently employed to calculate revised estimates of the normalised ion-neutral collision frequency; these neutral winds are derived by attributing the difference between predicted and observed enhancements in field-parallel ion temperature to thermospheric motion. The inclusion of neutral winds, which are themselves not inconsiderable, appears to have only a limited effect on the normalised collision frequencies derived.     

Interannual variability and regional climate simulations

Summary An assessment is made of a regional climate model's skill in simulating the mean climatology and the interannual variability experienced in a specific region. To this end two ensembles comprising three realizations of month-long January and July simulations are undertaken with a limited are a operational NWP model. The modelling suite is driven at its lateral boundaries by analysed meteorological fields and the computational domain covers Europe and the North-western Atlantic with a horizontal resolution of 56 km.Validation is performed against both operational ECMWF analyses and objectively analysed precipitation fields from a network of ~ 1400 SYNOP rain gauge stations. Analysis of the simulated ensemble-mean climatology indicates that the model successfully reproduces both the winter and summer distributions of the primary dynamical and thermodynamical field, and also provides a reasonable representation of the measured precipitation over most of Europe. Typically the domain averaged model-biases are below 0.5 K for temperature and 0.1 g/kg for specific humidity. Analysis of the interannual variability reveals that the model captures the wintertime changes including that of the precipitation distribution, but in contrast the summertime precipitation totals for the individual years is not simulated satisfactorily and only partially reproduces the observed regional interannual variability.The latter shortcomings are related to the following factors. Firstly the model bias in the dynamical fields is somewhat larger for summer than winter, while at the same time summertime interannual variability is associated with weaker effects in the dynamical fields. Secondly the summertime precipitation distribution is more substantially affected by small-scale moist convection and surface hydrological processes. Together these two factors suggest that summertime precipitation over continental extratropical land masses might be intrinsically less predictable than wintertime synoptic scale precipitation.With 17 Figures     

Constraints on Archaean crustal evolution of the Zimbabwe craton: a U-Pb zircon,Sm-Nd and Pb-Pb whole-rock isotope study

 The U-Pb ages of zircons from seven felsic volcanic and plutonic rocks from northern Zimbabwe combined with field data and Pb-Pb and Sm-Nd whole-rock isotope data, constrain the timespan of development of the Harare-Shamva granite-greenstone terrain and establish the relative involvement of juvenile mantle-derived and reworked crustal material. Basement-cover field relationships and isotope and geochemical data demonstrate that the greenstones were deposited onto 3.2–2.8 Ga basement gneisses, in ensialic, continental basins. Geodynamic models for the generation of the areally extensive bimodal magmatic products and growth of the pre-existing crustal nucleus consistent with our interpretations are rift-related: (1) intracontinental rifting related to mantle plume activity or; (2) rifting in a back-arc environment related to a marginal volcanic arc. The data, in conjunction with field evidence, do not indicate the presence and accretion of an older (ca. 2.70 Ga) and a younger (ca. 2.65 Ga) greenstone sequence in the Harare part of the greenstone belt, as was recently postulated on the basis of SHRIMP zircon ages. Zircon ages for basal felsic volcanics (2715±15 Ma) and a subvolcanic porphyry (2672±12 Ma) constrain the initiation and termination of deposition of the greenstone sequence. The timespan of deposition of the Upper Bulawayan part of the greenstone sequence corresponds well with radiometric ages for Upper Bulawayan greenstones in the central and southern part of the craton and supports the concept of craton-wide lithostratigraphic correlations for the late Archaean in Zimbabwe. Zircon ages for a syn-tectonic gneiss (2667±4 Ma) and a late syn-tectonic intrusive granodiorite (2664±15 Ma) pinpoint the age of deformation of the greenstone sequence and compare well with a Pb-Pb age of shear zone related gold mineralization (2659±13 Ma) associated with the latter intrusive phase. The intimate timing relation of greenstone deformation and granitoid emplacement, but also the metamorphic evidence for a thermal effect of the batholiths on the surrounding greenstone belts, and the structural and strain patterns in the greenstone sequence around and adjacent to the batholiths, imply that the intrusion of the granitoids had a significant influence on the tectono-thermal evolution of the greenstone belt. Prolonged magmatic activity is indicated by the zircon ages of small, post-tectonic plutons intrusive into the greenstone belt, with a mineralized granodiorite dated at 2649±6 Ma and an unmineralized tonalite at 2618± 6 Ma. The 2601±14 Ma crystallization age of an “external” Chilimanzi-type granite agrees well with existing radiometric ages for similar granites within the southern part of the craton, demonstrating a craton-wide event and heralding cratonization. The similarity between U-Pb zircon ages and T_DM model ages (2.65–2.62 Ga) and the positive ɛNd_T values (+3 to +2) for the late syn-tectonic and post-tectonic intrusive plutons within the greenstone belt indicate magmatism was derived directly from the mantle or by anatexis of lower crustal sources, with very short crustal residence times, and minor contamination with older crust. The rather high model μ₁ values (8.2–8.6) are unlikely to indicate the involvement of significant amounts of older crust and may be inherited from a high U/Pb mantle source, as was suggested by previous workers for the Archaean mantle beneath Southern Africa. The older T_DM ages for the felsic volcanics (3.0–2.8 Ga) and the porphyries (2.8–2.7 Ga) suggest that these felsic magmas were derived by partial melting of a source that was extracted from the mantle ca. 200 Ma prior to volcanism or may indicate interaction between depleted mantle-derived melts and older crustal material. Received: 15 August 1995 / Accepted: 12 January 1996