Local subgiants and time-scales of disc formation

This paper concentrates on the relationship between the rate of gas emission from the nucleus of Comet 9P/Tempel 1, the fraction f of the nucleus that is active, and the crater damage inflicted by the recent 2005 July 4 Deep Impact space mission. The cometary nucleus has a surface area of about  1.7 × 10⁸ m²  and a mean radius of about 3700 m. Before the impact it is estimated that only a fraction f = 0.0056 of the nucleus surface was actively producing gas and dust. The active area was about  9.4 × 10⁵ m²  . Absolute magnitudes obtained at recent perihelion passages of this comet indicate that variations in the 0.0074 > f > 0.0039 range can occur from apparition to apparition. Because of the low size of the original active area, the production of a new impact crater in the diameter range 40 to 300 m would lead to a long-term change in the cometary visual magnitude in the range 0.0018 to 0.098 respectively. This is below the limit of detectability. It has been suggested that the cometary dust is in the form of 'talcum powder' not 'beach sand'. We suggest that the dust ejected from the impact site has been broken up by the energetic impact process and thus has a different size distribution from dust locked in the snowy matrix of the nucleus and normally lifted off the nucleus by gentle sublimation processes.     

Late accretion and lithification of chondritic parent bodies: Mg isotope studies on fragments from primitive chondrites and chondritic breccias

Abstract— Recent results of isotopic dating studies (¹⁸²Hf‐¹⁸²W, ²⁶Al‐²⁶Mg) and the increasing number of observed igneous and metamorphosed fragments in (primitive) chondrites provide strong evidence that accretion of differentiated planetesimals predates that of primitive chondrite parent bodies. The primitive chondrites Adrar 003 and Acfer 094 contain some unusual fragments that seem to have undergone recrystallization. Magnesium isotope analyses reveal no detectable radiogenic ²⁶Mg in any of the studied fragments. The possibility that evidence for ²⁶Al was destroyed by parent body metamorphism after formation is not likely because several other constituents of these chondrites do not show any metamorphic features. Since final accretion of a planetesimal must have occurred after formation of its youngest components, formation of these parent bodies must thus have been relatively late (i.e., after most ²⁶Al had decayed). Al‐Mg isotope data for some igneous‐textured clasts (granitoids and andesitic fragments) within the two chondrite regolith breccias Adzhi‐Bogdo and Study Butte reveal also no evidence for radiogenic ²⁶Mg. As calculated from the upper limits, the formation of these igneous clasts, the incorporation into the parent body regolith, and the lithification must have occurred at least 3.8 Myr (andesite in Study Butte) and 4.7 Myr (granitoids in Adzhi‐Bogdo) after calcium‐aluminum‐rich inclusions (CAI) formation. The absence of ²⁶Mg excess in the igneous inclusions does not exclude ²⁶Al from being a heat source for planetary melting. In large, early formed planetesimals, cooling below the closure temperature of the Al‐Mg system may be too late for any evidence for live ²⁶Al (in the form of ²⁶Mg excess) to be preserved. Thus, growing evidence exists that chondritic meteorites represent the products of a complex, multi‐stage history of accretion, parent body modification, disruption and re‐accretion.     

A stochastic inversion workflow for monitoring the distribution of CO<Subscript>2</Subscript> injected into deep saline aquifers

We present a two-step stochastic inversion approach for monitoring the distribution of CO₂ injected into deep saline aquifers for the typical scenario of one single injection well and a database comprising a common suite of well logs as well as time-lapse vertical seismic profiling (VSP) data. In the first step, we compute several sets of stochastic models of the elastic properties using conventional sequential Gaussian co-simulations (SGCS) representing the considered reservoir before CO₂ injection. All realizations within a set of models are then iteratively combined using a modified gradual deformation algorithm aiming at reducing the mismatch between the observed and simulated VSP data. In the second step, these optimal static models then serve as input for a history matching approach using the same modified gradual deformation algorithm for minimizing the mismatch between the observed and simulated VSP data following the injection of CO₂. At each gradual deformation step, the injection and migration of CO₂ is simulated and the corresponding seismic traces are computed and compared with the observed ones. The proposed stochastic inversion approach has been tested for a realistic, and arguably particularly challenging, synthetic case study mimicking the geological environment of a potential CO₂ injection site in the Cambrian-Ordivician sedimentary sequence of the St. Lawrence platform in Southern Québec. The results demonstrate that the proposed two-step reservoir characterization approach is capable of adequately resolving and monitoring the distribution of the injected CO₂. This finds its expression in optimized models of P- and S-wave velocities, density, and porosity, which, compared to conventional stochastic reservoir models, exhibit a significantly improved structural similarity with regard to the corresponding reference models. The proposed approach is therefore expected to allow for an optimal injection forecast by using a quantitative assimilation of all available data from the appraisal stage of a CO₂ injection site.     

Petrogenesis of anomalous Queen Alexandra Range enstatite meteorites and their relation to enstatite chondrites,primitive enstatite achondrites,and aubrites

Queen Alexandra Range (QUE) meteorite 94204 is an anomalous enstatite meteorite whose petrogenesis has been ascribed to either partial melting or impact melting. We studied the meteorite pairs QUE 94204, 97289/97348, 99059/99122/99157/99158/99387, and Yamato (Y)‐793225; these were previously suggested to represent a new grouplet. We present new data for mineral abundances, mineral chemistries, and siderophile trace element compositions (of Fe,Ni metal) in these meteorites. We find that the texture and composition of Y‐793225 are related to EL6, and that this meteorite is unrelated to the QUEs. The mineralogy and siderophile element compositions of the QUEs are consistent with petrogenesis from an enstatite chondrite precursor. We caution that potential re‐equilibration during melting and recrystallization of enstatite chondrite melt‐rocks make it unreliable to use mineral chemistries to assign a specific parent body affinity (i.e., EH or EL). The QUEs have similar mineral chemistries among themselves, while slight variations in texture and modal abundances exist between them. They are dominated by inclusion‐bearing millimeter‐sized enstatite (average En_99.1–99.5) with interstitial spaces filled predominantly by oligoclase feldspar (sometimes zoned), kamacite (Si approximately 2.4 wt%), troilite (≤2.4 wt% Ti), and cristobalite. Siderophile elements that partition compatibly between solid metal and liquid metal are not enriched like in partial melt residues Itqiy and Northwest Africa (NWA) 2526. We find that the modal compositions of the QUEs are broadly unfractionated with respect to enstatite chondrites. We conclude that a petrogenesis by impact melting, not partial melting, is most consistent with our observations.