Magnetometer survey of the proposed Sirente meteorite crater field,central Italy: Evidence for uplifted crater rims and buried meteorites

Abstract— The Sirente crater field consists of a 120 m wide, rimmed main depression flanked to the northwest by about 30 smaller depressions. It has been dated to the first centuries A.D. An impact origin is suggested, but not confirmed. The small size combined with the properties of the target material (carbonate mud) would neither allow shock features diagnostic of impact, nor projectile vaporization. Consequently, a meteoritic component in the sediments would be very localized. At impacts of this size the projectile most likely is an iron meteorite. Any iron meteorites on the ground surface would, in Iron Age Europe, have been removed shortly after the event. However, if the depressions are of impact origin they should contain meteorites at great depth in analogy with known craters. The magnetic properties of iron meteorites differ distinctly from the very low magnetic sediments and sedimentary rocks of the Sirente area. We have used a proton precession magnetometer/gradiometer to produce magnetic anomaly maps over four of the smaller depressions (～8 m diameter), as well as two crossing profiles over a fifth depression (～22 m diameter). All show distinct magnetic anomalies of about 20 nT, the larger depression up to 100 nT. Magnetic modeling shows a best fit for structures with upturned strata below their rims, excluding a karstic origin but supporting an explosive formation. The 100 nT anomaly can only be explained by highly‐magnetic objects at a few meters depth. All together, the magnetic data provides a strong indication for an impact origin of the crater field.     

Geological and geochemical data from the proposed Sirente crater field: New age dating and evidence for heating of target

Abstract— The proposed Sirente crater field consists of a slightly oblong main structure (main crater) 120 m in width and about 30 smaller structures (satellite craters), all in unconsolidated but stiff carbonate mud. Here we focus on the subsurface structure of the satellite craters and compare the Sirente field with known meteorite crater fields. We present a more complete outline of the crater field than previously reported, information on the subsurface morphology of a satellite crater (C8) 8 m in width, radiocarbon and thermoluminescence (TL) ages of material from this crater, and evidence for heated material in both crater C8 and the rim of the main crater. Crater C8 has a funnel shape terminating downwards, and evidence for soil injection from the surface to a depth of 9 m. The infill contained dispersed charcoal and small, irregular, porous fragments of heated clay with a calibrated age of b.p. 1712 (¹³C‐corrected radiocarbon age: b.p. 1800 ± 100) and a TL age of b.p. 1825 (calculated error ± 274). Together with previous radiocarbon age (b.p. 1538) of the formation of the main crater (i.e., target surface below rim), a formation is suggested at the beginning of the first millennium a.d. Although projectile vaporization is not expected in Sirente‐sized craters in this type of target material, we used geochemistry in an attempt to detect a meteoritic component. The results gave no unequivocal evidence of meteoritic material. Nevertheless, the outline of the crater field, evidence of heated material within the craters, and subsurface structure are comparable with known meteorite crater fields.     

On the chronology of lunar origin and evolution

An origin of the Moon by a Giant Impact is presently the most widely accepted theory of lunar origin. It is consistent with the major lunar observations: its exceptionally large size relative to the host planet, the high angular momentum of the Earth–Moon system, the extreme depletion of volatile elements, and the delayed accretion, quickly followed by the formation of a global crust and mantle.According to this theory, an impact on Earth of a Mars-sized body set the initial conditions for the formation and evolution of the Moon. The impact produced a protolunar cloud. Fast accretion of the Moon from the dense cloud ensured an effective transformation of gravitational energy into heat and widespread melting. A “Magma Ocean” of global dimensions formed, and upon cooling, an anorthositic crust and a mafic mantle were created by gravitational separation.Several 100 million years after lunar accretion, long-lived isotopes of K, U and Th had produced enough additional heat for inducing partial melting in the mantle; lava extruded into large basins and solidified as titanium-rich mare basalt. This delayed era of extrusive rock formation began about 3.9 Ga ago and may have lasted nearly 3 Ga.A relative crater count timescale was established and calibrated by radiometric dating (i.e., dating by use of radioactive decay) of rocks returned from six Apollo landing regions and three Luna landing spots. Fairly well calibrated are the periods ≈4 Ga to ≈3 Ga BP (before present) and ≈0.8 Ga BP to the present. Crater counting and orbital chemistry (derived from remote sensing in spectral domains ranging from γ- and x-rays to the infrared) have identified mare basalt surfaces in the Oceanus Procellarum that appear to be nearly as young as 1 Ga. Samples returned from this area are needed for narrowing the gap of 2 Ga in the calibrated timescale. The lunar timescale is not only used for reconstructing lunar evolution, but it serves also as a standard for chronologies of the terrestrial planets, including Mars and possibly early Earth.The Moon holds a historic record of Galactic cosmic-ray intensity, solar wind composition and fluxes and composition of solids of any size in the region of the terrestrial planets. Some of this record has been deciphered. Secular mixing of the Sun was constrained by determining ³He/⁴He of solar wind helium stored in lunar fines and ancient breccias. For checking the presumed constancy of the impact rate over the past ≈3.1 Ga, samples of the youngest mare basalts would be needed for determining their radiometric ages.Radiometric dating and stratigraphy has revealed that many of the large basins on the near side of the Moon were created by impacts about 4.1 to 3.8 Ga ago. The apparent clustering of ages called “Late Heavy Bombardment (LHB)” is thought to result from migration of planets several 100 million years after their accretion.The bombardment, unexpectedly late in solar system history, must have had a devastating effect on the atmosphere, hydrosphere and habitability on Earth during and following this epoch, but direct traces of this bombardment have been eradicated on our planet by plate tectonics. Indirect evidence about the course of bombardment during this epoch on Earth must therefore come from the lunar record, especially from additional data on the terminal phase of the LHB. For this purpose, documented samples are required for measuring precise radiometric ages of the Orientale Basin and the Nectaris and/or Fecunditatis Basins in order to compare these ages with the time of the earliest traces of life on Earth.A crater count chronology is presently being built up for planet Mars and its surface features. The chronology is based on the established lunar chronology whereby differences between the impact rates for Moon and Mars are derived from local fluxes and impact energies of projectiles. Direct calibration of the Martian chronology will have to come from radiometric ages and cosmic-ray exposure ages measured in samples returned from the planet.     

L'assorbimento della radiazione solare da parte di uno strato di polveri vulcaniche

Riassunto Servendosi dei risultati ottenuti in precedenti lavori, si perviene alle equazioni che regolano la propagazione dei raggi solari attraverso ad uno strato di polveri vulcaniche le quali, applicate ai dati forniti dalle osservazioni effettuate nell'estate e nell'autunno 1912, dopo il violento parossismo del Katmai, hanno permesso di pervenire alle conclusioni seguenti: 1^o) lo strato di polveri dovette essere tale da contenere circa 15×10⁶ granuli in un cilindro ad asse verticale, di altezza uguale allo spessore dello strato e di sezione 1 cm², numero circa 44 volte più grande di quello determinato dall'Humphreys con altro metodo assai più semplificato applicato alle medesime osservazioni del 1912; 2^o) ammettendo come probabile uno spessore equivalente di 1 Km per lo strato, il numero di granuli per cm³ risulta di circa 150.

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Summary Using the results obtained in preceding investigations we arrived to the equations of propagation of the solar beams through a layer of volcanic dusts. These equations applied to the data furnished by the observations made in the summer and autumn of 1912 after the violent Katmai paroxysm, brought to the following conclusions: 1 ^st ) the layer of dusts had to be such to contain approximately 15×10⁶ granules in a vertical cylinder of a height equal to the thickness and with a section of 1 cm², number about 44 times higher than the one obtained byHumphreys with a much more simplified method applied to the same observations of 1912; 2 ^nd ) assuming as likely an equivalent thickness of 1 Km for the layer, the number of granules for each cm³ results to be about 150.

     

Coefficiente di diffusione per un granulo di polvere vulcanica,nel caso di energia incidente su di esso secondo tutte le direzioni

Riassunto Viene determinata l'energia diffusa da un granulo di polvere vulcanica in ciascuno dei due semispazi individuati da un piano passante per il centro di figura del granulo, nel caso in cui incida su di esso un fascio di raggi paralleli, la cui direzione formi un angolo con la normale al piano considerato, pervenendo alla conclusione che, al variare di da 0 a , l'energia diffusa nel semispazio di provenienza della luce varia dal 26 al 130 dell'energia incidente e, viceversa, quella diffusa nel semispazio in cui la luce prosegue il suo cammino varia dal 130 al 26 dell'energia incidente, e che, nel caso in cui la luce incida su un granulo secondo tutte le direzioni contenute nell'angolo solido 2 , il 38 viene diffuso nel semispazio di provenienza e il 118 nell'altro semispazio, mentre, com'è ovvio, in ogni caso il rapporto tra energia diffusa ed energia incidente si mantiene costante ed è pari a 0.156.

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Summary In the present paper is determined the energy scattered by a grain of volcanic ashes in each of the two hemispaces individuated by a plane passing through the figure centre of the grain, in the case of a beam of parallel rays incident on it, the direction of the rays forming an angle with the normal to the considered plane. One arrives to the conclusion that, varying from 0 to , the energy scattered in the hemispace of the light origin varies from 26 to 130 of the incident energy, and, vice-versa, the energy scattered in the hemispace in which the light continues its way, varies from 130 to 26 of the incident energy; and that, in the case in which the light is incident on a grain from all the directions contained in the solid angle 2, a 38 is scattered in the hemispace of the light origin and a 118 in the other hemispace, while, as it is obvious, in each case the ratio between scattered energy and incident energy is constant and equal to 0.156.

     

Evaluation of alkene degradation in the detailed tropospheric chemistry mechanism, MCM v3, using environmental chamber data

The representation of alkene degradation in version 3 of the Master Chemical Mechanism (MCM v3) has been evaluated, using environmental chamber data on the photo-oxidation of ethene, propene, 1-butene and 1-hexene in the presence of NO_x, from up to five chambers at the Statewide Air Pollution Research Center (SAPRC) at the University of California. As part of this evaluation, it was necessary to include a representation of the reactions of the alkenes with O(³P), which are significant under chamber conditions but generally insignificant under atmospheric conditions. The simulations for the ethene and propene systems, in particular, were found to be sensitive to the branching ratios assigned to molecular and free radical forming pathways of the O(³P) reactions, with the extent of radical formation required for proper fitting of the model to the chamber data being substantially lower than the reported consensus. With this constraint, the MCM v3 mechanisms for ethene and propene generally performed well. The sensitivity of the simulations to the parameters applied to a series of other radical sources and sink reactions (radical formation from the alkene ozonolysis reactions and product carbonyl photolysis; radical removal from the reaction of OH with NO₂ and β-hydroxynitrate formation) were also considered, and the implications of these results are discussed. Evaluation of the MCM v3 1-butene and 1-hexene degradation mechanisms, using a more limited dataset from only one chamber, was found to be inconclusive. The results of sensitivity studies demonstrate that it is impossible to reconcile the simulated and observed formation of ozone in these systems for ranges of parameter values which can currently be justified on the basis of the literature. As a result of this work, gaps and uncertainties in the kinetic, mechanistic and chamber database are identified and discussed, in relation to both tropospheric chemistry and chemistry important under chamber conditions which may compromise the evaluation procedure, and recommendations are made for future experimental studies. Throughout the study, the performance of the MCM v3 chemistry was also simultaneously compared with that of the corresponding chemistry in the SAPRC-99 mechanism, which was developed and optimized in conjunction with the chamber datasets.     

Gamma ray spectra of some lavas from Vesuvius

The authors have carried out the gamma-ray spectrometry of some Vesuvius lavas, by means of a S.E.L.O. equipment which consists of a 100 channels analyzer joined to a photomultiplier tube annexed to aNaI(T1) phosphor. TheU ²³⁸,Th ²³²,Ra ²²⁶ andK ₂ O contents have been determined and the following conclusions have been drawn: theU ²³⁸ seems to be present in the examined lavas and the contents are fairly equal within the errors; also theTh ²³² andK ₂ O contents show no sensible variations in the different lavas. TheRa ²²⁶ content is remarkably higher than that necessary for the equilibrium with the observedU ²³⁸ content and shows a decrease with the age of the lavas. TheRa ²²⁶ content which exceeds that necessary for the equilibrium with the observedU ²³⁸ content decreases with a law similar to the natural radioactive decay ofRa ²²⁶.     

Chemical compositions of impact melt breccias and target rocks from the Tenoumer impact crater,Mauritania

Abstract— The impact melt breccias from the Tenoumer crater (consisting of a fine‐grained intergrowth of plagioclase laths, pyroxene crystals, oxides, and glass) display a wide range of porosity and contain a large amount of target rock clasts. Analyses of major elements in impact melt rocks show lower contents of SiO₂, Al₂O₃, and Na₂O, and higher contents of MgO, Fe₂O₃, and CaO, than the felsic rocks (i.e., granites and gneisses) of the basement. In comparison with the bulk analyses of the impact melt, the glass is strongly enriched in Si‐Al, whereas it is depleted both in Mg and Fe; moreover, the impact melt rocks are variably enriched or depleted in some REE with respect to the felsic and mafic bedrock types. Gold is slightly enriched in the impact melt, and Co, Cr, and Ni abundances are possibly due to a contribution from mafic bedrock. Evidences of silicate‐carbonate liquid immiscibility, mainly as spherules and globules of calcite within the silicate glass, have been highlighted. HMX mixing calculation confirm that the impact melt rocks are derived from a mixing of at least six different target lithologies outcropping in the area of the crater. A large contribution is derived from granitoids (50%) and mica schist (17–19%), although amphibolites (?15%), cherty limestones (?10%), and ultrabasites (?6%) components are also present. The very low abundances of PGE in the melt rock seem to come mainly from some ultrabasic target rocks; therefore, the contamination from the meteoritic projectile appears to have been negligible.