Petrogenesis of lunar impact melt rock meteorite Oued Awlitis 001

Oued Awlitis 001 is a highly feldspathic, moderately equilibrated, clast‐rich, poikilitic impact melt rock lunar meteorite that was recovered in 2014. Its poikilitic texture formed due to moderately slow cooling, which judging from textures of rocks in melt sheets of terrestrial impact structures, is observed in impact melt volumes at least 100 m thick. Such coherent impact melt volumes occur in lunar craters larger than ~50 km in diameter. The composition of Oued Awlitis 001 points toward a crustal origin distant from incompatible‐element‐rich regions. Comparison of the bulk composition of Oued Awlitis 001 with Lunar Prospector 5° γ‐ray spectrometer data indicates a limited region of matches on the lunar farside. After its initial formation in an impact crater larger than ~50 km in diameter, Oued Awlitis 001 was excavated from a depth greater than ~50 m. The cosmogenic nuclide inventory of Oued Awlitis 001 records ejection from the Moon 0.3 Ma ago from a depth of at least 4 m and little mass loss due to ablation during its passage through Earth's atmosphere. The terrestrial residence time must have been very short, probably less than a few hundred years; its exact determination was precluded by a high concentration of solar cosmic ray‐produced ¹⁴C. If the impact that excavated Oued Awlitis 001 also launched it, this event likely produced an impact crater >10 km in diameter. Using petrologic constraints and Lunar Reconnaissance Orbiter Camera and Diviner data, we test Giordano Bruno and Pierazzo as possible launch craters for Oued Awlitis 001.     

Evidence for multiple 4.0–3.7 Ga impact events within the Apollo 16 collection

In a histogram of lunar impact ages from the Apollo 16 site, there is a spike circa 3.9 Ga that has been interpreted to represent either a large number of nearly synchronous events or an abundance of samples that were affected slightly differently by the event that produced the Imbrium basin. To further scrutinize those age relationships, we extracted six centimeter‐sized clasts of impact melt from ancient regolith breccia 60016 and performed petrological and geochronological (⁴⁰Ar‐³⁹Ar) analyses. Three clasts have similar poikilitic textures, while others have porphyritic, aphanitic, or intergranular textures. Compositions and abundances of relict minerals are different in all six clasts and variously imply Mg‐suite and ferroan anorthosite target sequences. Estimated bulk compositions of four clasts are similar to previously defined group 1 Apollo 16 impact melt rocks, while the other two have higher Al₂O₃ and lower FeO+MgO compositions. All six clasts have similar K₂O and P₂O₅ concentrations, which could have been derived from a KREEP‐bearing component among target sequences. Eighteen ⁴⁰Ar/³⁹Ar analyses of the six clasts produced an age range from 3823 ± 75 to 4000 ± 23 Ma, consistent with estimates for the proposed late heavy bombardment. Four clasts have multiple temperature steps that define plateau ages. These ages are distinct, so they cannot be explained by a single impact event, such as the one that produced the Imbrium impact basin. The conclusion that these represent distinct ages remains after considering the possibility of artifacts in defining plateaus.     

The age of Wolfe Creek meteorite crater (Kandimalal), Western Australia

Wolfe Creek crater lies in northwestern Australia at the edge of the Great Sandy Desert. Together with Meteor Crater, it is one of the two largest craters on Earth from which meteorite fragments have been recovered. The age of the impact is poorly constrained and unpublished data places the event at about 300,000 years ago. In comparison, Meteor Crater is well constrained by exposure dating. In this paper, we present new ages for Wolfe Creek Crater from exposure dating using the cosmogenic nuclides ¹⁰Be and ²⁶Al, together with optically stimulated luminescence ages (OSL) on sand from a site created by the impact. We also present a new topographic survey of the crater using photogrammetry. The exposure ages range from ~86 to 128 ka. The OSL ages indicate that the age of the impact is most likely to be ~120 ka with a maximum age of 137 ka. Considering the geomorphic setting, the most likely age of the crater is 120 ± 9 ka. Last, we review the age of Meteor Crater in Arizona. Changes in production rates and scaling factors since the original dating work revise the impact age to 61.1 ± 4.8 ka, or ~20% older than previously reported.     

Challenges in detecting olivine on the surface of 4 Vesta

Identifying and mapping olivine on asteroid 4 Vesta are important components to understanding differentiation on that body, which is one of the objectives of the Dawn mission. Harzburgitic diogenites are the main olivine‐bearing lithology in the howardite‐eucrite‐diogenite (HED) meteorites, a group of samples thought to originate from Vesta. Here, we examine all the Antarctic harzburgites and estimate that, on scales resolvable by Dawn, olivine abundances in putative harzburgite exposures on the surface of Vesta are likely at best in the 10–30% range, but probably lower due to impact mixing. We examine the visible/near‐infrared spectra of two harzburgitic diogenites representative of the 10–30% olivine range and demonstrate that they are spectrally indistinguishable from orthopyroxenitic diogenites, the dominant diogenitic lithology in the HED group. This suggests that the visible/near‐infrared spectrometer onboard Dawn (VIR) will be unable to resolve harzburgites from orthopyroxenites on the surface of Vesta, which may explain the current lack of identification of harzburgitic diogenite on Vesta.     

The host galaxies of active galactic nuclei

We examine the properties of the host galaxies of 22 623 narrow-line active galactic nuclei (AGN) with  0.02 < z < 0.3  selected from a complete sample of 122 808 galaxies from the Sloan Digital Sky Survey. We focus on the luminosity of the [O  iii ]λ5007 emission line as a tracer of the strength of activity in the nucleus. We study how AGN host properties compare with those of normal galaxies and how they depend on L [O  iii ]. We find that AGN of all luminosities reside almost exclusively in massive galaxies and have distributions of sizes, stellar surface mass densities and concentrations that are similar to those of ordinary early-type galaxies in our sample. The host galaxies of low-luminosity AGN have stellar populations similar to normal early types. The hosts of high-luminosity AGN have much younger mean stellar ages. The young stars are not preferentially located near the nucleus of the galaxy, but are spread out over scales of at least several kiloparsecs. A significant fraction of high-luminosity AGN have strong Hδ absorption-line equivalent widths, indicating that they experienced a burst of star formation in the recent past. We have also examined the stellar populations of the host galaxies of a sample of broad-line AGN. We conclude that there is no significant difference in stellar content between type 2 Seyfert hosts and quasars (QSOs) with the same [O  iii ] luminosity and redshift. This establishes that a young stellar population is a general property of AGN with high [O  iii ] luminosities.     

Incorporation of argon, krypton and xenon into clathrates on Mars

Calculations of the trapping of heavy noble gases within multiple guest clathrates under Mars-like conditions show that a substantial fraction of the martian Xe, perhaps even the vast majority, could be in clathrates. In addition, the Xe/Kr ratio in the clathrates would probably be much higher than in the atmosphere, so the formation or dissociation of a relatively small amount of clathrate could measurably change the atmospheric ratio. Relatively crude (factor of 2) measurements of the seasonal variability in that ratio by in situ spacecraft would be sensitive to ∼10% of the seasonal atmospheric CO₂ variability being a result of clathrates, rather than pure CO₂ frost. In addition, sequestration of Xe in clathrates remains a viable mechanism for explaining the variable Xe/Kr ratios seen in different suites of martian meteorites.     

A reanalysis of the Apollo light scattering observations, and implications for lunar exospheric dust

Conspicuous excess brightness, exceeding that expected from coronal and zodiacal light (CZL), was observed above the lunar horizon in the Apollo 15 coronal photographic sequence acquired immediately after orbital sunset (surface sunrise). This excess brightness systematically faded as the Command Module moved farther into shadow, eventually becoming indistinguishable from the CZL background. These observations have previously been attributed to scattering by ultrafine dust grains (radius ∼0.1 microns) in the lunar exosphere, and used to obtain coarse estimates of dust concentration at several altitudes and an order-of-magnitude estimate of ∼10⁻⁹ g cm⁻² for the column mass of dust near the terminator, collectively referred to as model “0”.We have reanalyzed the Apollo 15 orbital sunset sequence by incorporating the known sightline geometries in a Mie-scattering simulation code, and then inverting the measured intensities to retrieve exospheric dust concentration as a function of altitude and distance from the terminator. Results are presented in terms of monodisperse (single grain size) dust distributions. For a grain radius of 0.10 microns, our retrieved dust concentration near the terminator (∼0.010 cm⁻³) is in agreement with model “0” at z=10 km, as is the dust column mass (∼3–6×10⁻¹⁰ g cm⁻²), but the present results indicate generally larger dust scale heights, and much lower concentrations near 1 km (<0.08 cm⁻³ vs. a few times 0.1 cm⁻³ for model “0"). The concentration of dust at high altitudes (z>50 km) is virtually unconstrained by the measurements. The dust exosphere extends into shadow a distance somewhere between 100 and 200 km from the terminator, depending on the uncertain contribution of CZL to the total brightness. These refined estimates of the distribution and concentration of exospheric dust above the lunar sunrise terminator should place new and more rigorous constraints on exospheric dust transport models, as well as provide valuable support for upcoming missions such as the Lunar Atmosphere and Dust Environment Explorer (LADEE).     

Radio-continuum study of the Nearby sculptor group galaxies. Part 1: NGC 300 at λ=20 cm

A series of new radio-continuum (λ=20 cm) mosaic images focused on the NGC?300 galactic system were produced using archived observational data from the VLA and/or ATCA. These new images are both very sensitive (rms?=60 μJy) and feature high angular resolution (<10?″). The most prominent new feature is the galaxy’s extended radio-continuum emission, which does not match its optical appearance. Using these newly created images a number of previously unidentified discrete sources have been discovered. Furthermore, we demonstrate that a joint deconvolution approach to imaging this complete data-set is inferior when compared to an immerge approach.     

Particle fabric in a small, type-2 ignimbrite flow unit (Laacher See, Germany) and implications for emplacement dynamics

 Ignimbrites of the 13-ka Upper Laacher See Tephra were deposited from small, highly concentrated, moderately fluidized pyroclastic flows. Their unconsolidated nature, and the prominence of accidental Devonian slate fragments, make these ignimbrites ideal for clast fabric studies. The upper flow unit of ignimbrite M14 has characteristics typical of a type-2 ignimbrite. Layer 2a and the lower part of layer 2b of the flow unit have strong, upstream-inclined a[p] fabrics (a[p] means long particle axes parallel to flow direction). Only clasts with a/b axial ratios of 2.5 or greater preserve good a[p] fabrics, whereas the a–b planes of flat fragments dip upstream irrespective of axial ratio. The a-axis fabric becomes weaker, flatter, and more girdle-like in the upper half of layer 2b. At one locality the a-axis fabric appears to rotate 40° up through the flow unit, suggesting either shear decoupling of different levels in the moving flow or unsteadiness effects in a flow depositing progressively at its base. The existence of similarly strong a[p] fabrics in layer 2a and the lower half of layer 2b appears inconsistent with the common interpretation that ignimbrite flow units are emplaced as a plug of essentially non-shearing material (layer 2b) on a thin shear layer (layer 2a), and that the entire flow freezes en masse to form the deposit. The data suggest that, if the flow froze en masse, it was shearing pervasively through at least half its thickness. Another possibility is that the flow unit aggraded progressively from the base up, and that the fabrics record the integrated history of shear directions and intensities immediately above the bed throughout the duration of deposition. Received: 13 February 1997 / Accepted: 4 April 1998