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- 저자 : McFadden (검색결과 3 건)
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Iron content determines how space weathering flux variations affect lunar soils
McFadden, James,Garrick-Bethell, Ian,Sim, Chae K.,Kim, Sungsoo S.,Hemingway, Doug Elsevier 2019 Icarus Vol.333 No.-
<P><B>Abstract</B></P> <P>Previous work has established that the solar wind and micrometeoroids produce spectral changes on airless silicate bodies. However, the relative importance of these two weathering agents, the timescales over which they operate, and how their effects depend on composition have not yet been well determined. To help address these questions we make use of the fact that solar wind and micrometeoroid fluxes vary with latitude on the Moon. Previous work has shown that this latitudinally varying flux leads to systematic latitudinal variations in the spectral properties of lunar soils. Here we find that the way in which a lunar soil's spectral properties vary with latitude is a function of its iron content, when we consider soils with 14–22 wt% FeO. In particular, a 50% reduction in flux corresponds to a significant increase in reflectance for 14 wt% FeO soils, while the same flux reduction on 21 wt% FeO soils is smaller by a factor of ~5, suggesting that this brightening effect saturates for high FeO soils. We propose that lower iron soils may not approach saturation because grains are destroyed or refreshed before sufficient nano- and micro-phase iron can accumulate on their rims. We compare our results to the spectral variations observed across the Reiner Gamma swirl, which lies on a high‑iron surface, and find it has anomalous brightness compared to our predictions. Swirls in Mare Marginis, which lie on a low iron surface, exhibit brightness differences that suggest reductions in solar wind flux between 20 and 40%. Our inferences suffer from the limited latitudinal extent of the maria and the convolution of micrometeoroid flux and solar wind flux changes with latitude. Superior constraints on how space weathering operates throughout the inner solar system would come from in situ measurements of the solar wind flux at lunar swirls.</P> <P><B>Highlights</B></P> <P> <UL> <LI> We find that the lunar maria brighten at higher latitudes, and the total brightening is a function of soil iron content. </LI> <LI> Above a soil iron content of ~21 wt% FeO, there is a negligible change in brightness with latitude. </LI> <LI> We use this finding to predict the brightness of the Reiner Gamma swirl, and find that it is anomalously bright. </LI> </UL> </P>
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Underwood, T S A,Sung, W,McFadden, C H,McMahon, S J,Hall, D C,McNamara, A L,Paganetti, H,Sawakuchi, G O,Schuemann, J IOP 2017 Physics in medicine & biology Vol.62 No.8
<P>Whilst Monte Carlo (MC) simulations of proton energy deposition have been well-validated at the macroscopic level, their microscopic validation remains lacking. Equally, no gold-standard yet exists for experimental metrology of individual proton tracks. In this work we compare the distributions of stochastic proton interactions simulated using the TOPAS-nBio MC platform against confocal microscope data for Al<SUB>2</SUB>O<SUB>3</SUB>:C,Mg fluorescent nuclear track detectors (FNTDs). We irradiated <img ALIGN='MIDDLE' ALT='$8\times 4\times 0.5$ ' SRC='http://ej.iop.org/images/0031-9155/62/8/3237/pmbaa6429ieqn001.gif'/> mm<SUP>3</SUP> FNTD chips inside a water phantom, positioned at seven positions along a pristine proton Bragg peak with a range in water of 12 cm. MC simulations were implemented in two stages: (1) using TOPAS to model the beam properties within a water phantom and (2) using TOPAS-nBio with Geant4-DNA physics to score particle interactions through a water surrogate of Al<SUB>2</SUB>O<SUB>3</SUB>:C,Mg. The measured median track integrated brightness (IB) was observed to be strongly correlated to both (i) voxelized track-averaged linear energy transfer (LET) and (ii) frequency mean microdosimetric lineal energy, <img ALIGN='MIDDLE' ALT='$\overline{{{y}_{F}}}$ ' SRC='http://ej.iop.org/images/0031-9155/62/8/3237/pmbaa6429ieqn002.gif'/>, both simulated in pure water. Histograms of FNTD track IB were compared against TOPAS-nBio histograms of the number of terminal electrons per proton, scored in water with mass-density scaled to mimic Al<SUB>2</SUB>O<SUB>3</SUB>:C,Mg. Trends between exposure depths observed in TOPAS-nBio simulations were experimentally replicated in the study of FNTD track IB. Our results represent an important first step towards the experimental validation of MC simulations on the sub-cellular scale and suggest that FNTDs can enable experimental study of the microdosimetric properties of individual proton tracks.</P>
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Susceptibility of amphibians to chytridiomycosis is associated with MHC class II conformation
Bataille, Arnaud,Cashins, Scott D.,Grogan, Laura,Skerratt, Lee F.,Hunter, David,McFadden, Michael,Scheele, Benjamin,Brannelly, Laura A.,Macris, Amy,Harlow, Peter S.,Bell, Sara,Berger, Lee,Waldman, Bru The Royal Society 2015 Proceedings, Biological sciences Vol.282 No.1805
<P>The pathogenic chytrid fungus <I>Batrachochytrium dendrobatidis</I> (Bd) can cause precipitous population declines in its amphibian hosts. Responses of individuals to infection vary greatly with the capacity of their immune system to respond to the pathogen. We used a combination of comparative and experimental approaches to identify major histocompatibility complex class II (MHC-II) alleles encoding molecules that foster the survival of Bd-infected amphibians. We found that Bd-resistant amphibians across four continents share common amino acids in three binding pockets of the MHC-II antigen-binding groove. Moreover, strong signals of selection acting on these specific sites were evident among all species co-existing with the pathogen. In the laboratory, we experimentally inoculated Australian tree frogs with Bd to test how each binding pocket conformation influences disease resistance. Only the conformation of MHC-II pocket 9 of surviving subjects matched those of Bd-resistant species. This MHC-II conformation thus may determine amphibian resistance to Bd, although other MHC-II binding pockets also may contribute to resistance. Rescuing amphibian biodiversity will depend on our understanding of amphibian immune defence mechanisms against Bd. The identification of adaptive genetic markers for Bd resistance represents an important step forward towards that goal.</P>
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