Primordial metamorphism of asteroids via electrical induction in A T Tauri-like solar wind

Simple evolutionary models of asteroids of various sizes and solar distances have been constructed assuming unipolar electrical induction heating due to passage of the Sun through a T Tauri phase with an increased magnetic field. Typical T Tauri conditions and an elementary solar wind model were used to calculate induced currents in modlels assuming electrical conductivities appropriate for carbonaceous material. Two restrictions with opposite dependence upon radius dominate the results. The electrical insulating tendency of a cold surface favors heating of larger bodies. The current-limiting backpressure of the induced magnetic field favors heating of smaller bodies. Thus it is found that maximum heating, in some cases sufficient for melting, occurs for model asteroids at the inner edge of the belt and with (model-dependent) radii from 25 to 250 km. This effect, if operant, would have produced a primordial distribution of metamorphosed asteroids primarily occurring at small solar distance and intermediate size. The observational evidence for such a distribution is unclear because the primordial distribution has likely been considerably modified by collisions, particularly at smaller sizes. There does seem to be some consistency with the model in the distribution of the largest asteroids, though data are sparse. In particular, this model seems relevant to the well-known dichotomy between Ceres and Vesta.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30 May, 1978Also Dept of Planetary Sciences     

Alternative approaches for analyses of a 100,000 m<Superscript>3</Superscript> rock slide based on Barton–Bandis shear strength criterion

A data set was derived for the Åknes rock slope, Norway, with the main focus on deriving input parameters for the Barton–Bandis shear strength criterion. Back-calculations of a 100,000 m³ rock slide were performed for evaluation of the data set. The limit equilibrium analysis showed that the joint roughness coefficient (JRC) has the greatest effect on the calculated safety factor of the slide. Probabilistic computations showed that the JRC stands out as the most important contributor to the total uncertainty over the whole set of variables and that the computed failure probability of the 1960 slide was very high, which may be interpreted that the input variables and the Barton–Bandis shear strength criterion are reasonable for the slide. JRC was measured on 0.25 m scale and on 1 m scale. The results from the two scales were different.     

A laboratory simulation of pyroclastic flows down slopes

Laboratory experiments are described which explore the dynamical consequences of buoyant convective upflow observed above hot pyroclastic flows. In nature, the convection is produced by the hot ash particles exchanging heat with air mixed into the front and top of the pyroclastic flow. This effect on the buoyancy due to the mixing of air and ash has been modelled in the laboratory using mixtures of methanol and ethylene glycol (MEG), which have a nonlinear density behaviour when mixed with water. Intermediate mixtures of these fluids can be denser than either initial component, and so the laboratory experiments were inverted models of the natural situation. We studied MEG flowing up under a sloping roof in a tank filled with water. The experiments were performed both in a narrow channel and on a laterally unconfined slope. The flow patterns were also compared with those of conventional gravity currents formed using fresh and salt water. The presence of the region of reversed buoyancy outside the layer flowing along the slope had two significant effects. First, it periodically protected the flow from direct mixing with the environment, resulting in pulses of relatively undiluted fluid moving out intermittently ahead of the main flow. Second, it produced a lateral inflow towards the axis of the current which kept the current confined to a narrow tongue, even on a wide slope.In pyroclastic flows the basal avalanche portion has a much larger density contrast with its surroundings than the laboratory flows. Calculations show that mixing of air into the dense part of a pyroclastic flow cannot generate a mixture that is buoyant in the atmosphere. However, the overlying dilute ash cloud can behave as a gravity current comparable in density contrast to the laboratory flows and can become buoyant, depending on the temperature and ash content. In the August 7th pyroclastic flow of Mount St. Helens, Hoblitt (1986) describes pulsations in the flow front, which are reminiscent of those observed in the experiments. As proposed by Hoblitt, the pulsations are caused by the ash cloud accelerating away from the front of the dense avalanche as a density current. The ash cloud then mixes with more air, becomes buoyant and lifts off the ground, allowing the avalanche to catch up with and move ahead of the cloud. The pulsing behaviour at the fronts of pyroclastic flows could account for the occurrence of cross-bedded layer 1 deposits which occur beneath layer 2 deposits in many sequences.     

Pogo and Pangaea

Long wavelength magnetic anomalies of crustal origin derived from the POGO and MAGSAT satellite data often display a strong continuity across the now-rifted continental margins when the continents are reassembled into Pangaea. These anomalies predate the breakup of the supercontinent and represent major blocks whose crustal properties are broadly similar even though those blocks are no longer contiguous.     

Identification of the projectile at the Brent crater,and further considerations of projectile types at terrestrial craters

Impact melt samples from drill hole B1-59 at the 3.8 km diameter Brent crater (Ontario) have been analysed for siderophile trace elements indicative of meteoritic contamination. Samples from the basal melt zone at 823–857 m depth are enriched in Ir, Os, Pd, Ni, Co, Cr and Se over basement, with the abundance pattern suggesting a chondritic projectile for Brent. From a Ni-Cr correlation of 10 melt samples an L or LL chondrite is inferred. The contribution of an ultramafic country rock (alnoite) in the melt is too small to significantly influence its $\text{Ni}\text{Cr}$ ratio. Glass-rich breccias from the allochthonous breccias filling the crater also contain a meteoritic component. Interelement ratios (e.g. $\text{Ni}\text{Cr}$) are, however, fractionated relative to the melt zone samples. This, as well as the low Au content of all Brent samples, is probably a product of alteration.Additional data on impact melts from the 65 km diameter crater Manicouagan still did not reveal a meteoritic component, as also for the Mistastin crater (28 km diameter) where Cr analyses set an upper limit of 1% of an achondritic projectile component in the melt. Irghizites (tektite like glasses) from the Zhamanshin impact structure have been found to contain high Ni and Co concentrations, and our data show that Ir is also enriched. It is however not possible to define the projectile-type. Enrichment of an Ivory Coast tektite in Ir is confirmed. There are large differences in siderophile element concentrations among tektites, with otherwise similar chemical composition.There are now four known craters formed by chondrites (Clearwater East, Lapparjärvi, Wanapitei, and Brent), with Brent being the smallest of these. For smaller craters the projectiles appear to be limited to iron or stony-iron meteorites, because of atmospheric destruction of relatively small stony meteorites. It appears, however, that all major classes of meteorites are represented among the projectiles at terrestrial impact craters.     

Increased photosynthesis compensates for shorter growing season in subarctic tundra—8 years of snow accumulation manipulations

This study was initiated to analyze the effect of increased snow cover on plant photosynthesis in subarctic mires underlain by permafrost. Snow fences were used to increase the accumulation of snow on a subarctic permafrost mire in northern Sweden. By measuring reflected photosynthetic active radiation (PAR) the effect of snow thickness and associated delay of the start of the growing season was assessed in terms of absorbed PAR and estimated gross primary production (GPP). Six plots experienced increased snow accumulation and six plots were untreated. Incoming and reflected PAR was logged hourly from August 2010 to October 2013. In 2010 PAR measurements were coupled with flux chamber measurements to assess GPP and light use efficiency of the plots. The increased snow thickness prolonged the duration of the snow cover in spring. The delay of the growing season start in the treated plots was 18 days in 2011, 3 days in 2012 and 22 days in 2013. Results show higher PAR absorption, together with almost 35 % higher light use efficiency, in treated plots compared to untreated plots. Estimations of GPP suggest that the loss in early season photosynthesis, due to the shortening of the growing season in the treatment plots, is well compensated for by the increased absorption of PAR and higher light use efficiency throughout the whole growing seasons. This compensation is likely to be explained by increased soil moisture and nutrients together with a shift in vegetation composition associated with the accelerated permafrost thaw in the treatment plots.