How much oxygen is too much? Constraining Saturn's ring atmosphere     

Alison J. Farmer  Peter Goldreich 《Icarus》2007,188(1):108-119

The discovery of a molecular oxygen atmosphere around Saturn's rings has important implications for the electrodynamics of the ring system. Its existence was inferred from the Cassini in situ detection of molecular oxygen ions above the rings during Saturn Orbit Insertion in 2004. Molecular oxygen is difficult to observe remotely, and theoretical estimates have yielded only a lower limit (Nn?¹⁰¹³ cm⁻²) to the O₂ column density. Comparison with observations has previously concerned matching ion densities at spacecraft altitudes far larger than the scale height of the neutral atmosphere. This is further complicated by charged particle propagation effects in Saturn's offset magnetic field. In this study we adopt a complementary approach, by focusing on bulk atmospheric properties and using additional aspects of the Cassini observations to place an upper limit on the column density. We develop a simple analytic model of the molecular atmosphere and its photo-ionization and dissociation products, with Nn a free parameter. Heating of the neutrals by viscous stirring, cooling by collisions with the rings, and torquing by collisions with pickup ions are all included in the model. We limit the neutral scale height to h?3000 km using the INMS neutral density nondetection over the A ring. A first upper limit to the neutral column is derived by using our model to reassess O₂ production and loss rates. Two further limits are then obtained from Cassini observations: corotation of the observed ions with the planet implies that the height-integrated conductivity of the ring atmosphere is less than that of Saturn's ionosphere; and the nondetection of fluorescent atomic oxygen over the rings constrains the molecular column from which it is produced via photo-dissociation. These latter limits are independent of production and loss rates and are only weakly dependent on temperature. From the three independent methods described, we obtain similar limits: Nn?2×¹⁰¹⁵ cm⁻². The mean free path for collisions between neutrals thus cannot be very much smaller than the scale height.  相似文献   

High-resolution simulations of stellar collisions between equal-mass main-sequence stars in globular clusters     

Alison Sills †  Tim Adams  Melvyn B. Davies  Matthew R. Bate 《Monthly notices of the Royal Astronomical Society》2002,332(1):49-54

We performed high-resolution simulations of two stellar collisions relevant for stars in globular clusters. We considered one head-on collision and one off-axis collision between two 0.6-M_⊙ main-sequence stars. We show that a resolution of about 100 000 particles is sufficient for most studies of the structure and evolution of blue stragglers. We demonstrate conclusively that collision products between main-sequence stars in globular clusters do not have surface convection zones larger than 0.004 M_⊙ after the collision, nor do they develop convection zones during the 'pre-main-sequence' thermal relaxation phase of their post-collision evolution. Therefore, any mechanism which requires a surface convection zone (i.e. chemical mixing or angular momentum loss via a magnetic wind) cannot operate in these stars. We show that no disc of material surrounding the collision product is produced in off-axis collisions. The lack of both a convection zone and a disc proves a continuing problem for the angular momentum evolution of blue stragglers in globular clusters.  相似文献   

A crisis in aerial sovereignty? Considering the implications of recent military violations of national airspace     

Alison J Williams 《Area》2010,42(1):51-59

The implementation of policies of pre-emption and securitisation by a number of states has led to an increase in the number of aerial incursions by one state's air force into another state's territory in recent years, often occurring before and, indeed, instead of ground incursions. This paper argues that it is vital that we conceptualise territory as a three-dimensional volume, rather than simply a flat area, in order to enable an analysis of how these events impact state sovereignty. The central contention of the paper is to extend recent work on territorial integrity and contingent sovereignty into this aerial dimension. A number of brief case studies are provided to illustrate how different incursion practices actively violate territorial integrity or render state sovereignty contingent. The conclusion seeks to answer the question of whether these incidents imply a crisis in aerial sovereignty or whether they confirm the chronic decline of this norm of international law.  相似文献   

The photokinetics of thermo‐tolerance in Symbiodinium     

Alison Jones  Ray Berkelmans 《Marine Ecology》2012,33(4):490-498

There is currently much debate about the ecological advantages for reef corals of hosting multiple types of the symbiotic dinoflagellate Symbiodinium. Amongst these is their apparent capacity to tolerate higher than normal water temperatures. There is strong photokinetic evidence that the trait of heat‐tolerance in plants is accompanied by energetic tradeoffs but little such evidence yet exists for corals. We use rapid light curves (RLCs) to investigate the photokinetic basis for thermo‐tolerance in the reef coral Acropora millepora with symbionts of contrasting thermal tolerance for which there are measured differences in energetics. Our results show that under non‐stressful temperatures, corals with heat‐tolerant type D Symbiodinium had a 41% lower maximum relative electron transport rate (rETRmax) and lower light absorption efficiency (α) due to lower cell Chl a content compared with corals with heat‐sensitive type C2 symbionts. Our results provide support for a photokinetic link between heat tolerance and deficits in holobiont (coral + symbiont) growth, lipid stores and reproduction. Reduced electron transport rate and light absorption capacity may be genotype‐specific attributes that enable clade D symbionts and their cnidarian hosts to cope with temperature stress but they inherently influence the photosynthetic function of the symbionts and thus have negative downstream effects on the coral.  相似文献   

The DEEP2 Galaxy Redshift Survey: the role of galaxy environment in the cosmic star formation history     

Michael C. Cooper  Jeffrey A. Newman  Benjamin J. Weiner  Renbin Yan  Christopher N. A. Willmer  Kevin Bundy  Alison L. Coil †  Christopher J. Conselice  Marc Davis  S. M. Faber  Brian F. Gerke  Puragra Guhathakurta  David C. Koo  Kai G. Noeske 《Monthly notices of the Royal Astronomical Society》2008,383(3):1058-1078

Using galaxy samples drawn from the Sloan Digital Sky Survey and the DEEP2 Galaxy Redshift Survey, we study the relationship between star formation and environment at   z ∼ 0.1  and 1. We estimate the total star formation rate (SFR) and specific star formation rate (sSFR) for each galaxy according to the measured [O  ii ]λ 3727 Å nebular line luminosity, corrected using empirical calibrations to match more robust SFR indicators. Echoing previous results, we find that in the local Universe star formation depends on environment such that galaxies in regions of higher overdensity, on average, have lower SFRs and longer star formation time-scales than their counterparts in lower density regions. At   z ∼ 1  , we show that the relationship between sSFR and environment mirrors that found locally. However, we discover that the relationship between total SFR and overdensity at   z ∼ 1  is inverted relative to the local relation. This observed evolution in the SFR–density relation is driven, in part, by a population of bright, blue galaxies in dense environments at   z ∼ 1  . This population, which lacks a counterpart at   z ∼ 0  , is thought to evolve into members of the red sequence from   z ∼ 1  to ∼0. Finally, we conclude that environment does not play a dominant role in the cosmic star formation history at   z < 1  : the dependence of the mean galaxy SFR on local galaxy density at constant redshift is small compared to the decline in the global SFR space density over the last 7 Gyr.  相似文献   

On the case? Dilemmas of collaborative research     

Rob Macmillan  Alison Scott 《Area》2003,35(1):101-105

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Tracing Ni, Cu, and Zn kinetics and equilibrium partitioning between dissolved and particulate phases in South San Francisco Bay, California, using stable isotopes and high-resolution inductively coupled plasma mass spectrometry     

Alison K. Gee  Kenneth W. Bruland 《Geochimica et cosmochimica acta》2002,66(17):3063-3083

Additions of the low occurrence stable isotopes ⁶¹Ni, ⁶⁵Cu, and ⁶⁸Zn were used as tracers to determine the exchange kinetics of metals between dissolved and particulate forms in laboratory studies of natural water and suspended sediments from South San Francisco Bay, CA. Dissolved metal isotope additions were made so that the isotope ratios (rather than total metal partitioning) were significantly altered from initial ambient conditions. Dissolved metal concentrations were determined using an organic ligand sequential extraction technique followed by analysis with high-resolution inductively coupled plasma mass spectrometry (HR-ICPMS). Exchangeable particulate concentrations were extracted using a 20% acetic acid leach followed by determination using HR-ICPMS. Equilibrium and kinetic sorption parameters were quantified according to a general model for trace metal partitioning assuming pseudo-first-order kinetics. Partition coefficients (K_D) were tracked as a function of time over the fortnight experiment. For Ni, Cu, and Zn the initial ambient K_D values were found to be 10^3.65, 10^3.88, and 10^4.52 L kg⁻¹, respectively. As a result of the dissolved metal isotope additions, the partition coefficients for all three metals dropped and then increased back to near ambient K_D values after 14 days. Curve-fitting concentration versus time profiles from both dissolved and exchangeable particulate data sets allowed determination of kinetic rate constants. The best estimates of forward and backward kinetic rate constants for Ni, Cu, and Zn respectively are k′_f = 0.03, 0.07, 0.12 d⁻¹ and k_b = 0.13, 0.12, 0.15 d⁻¹. These results predict that sorption equilibria in South Bay should be reached on the order of a month for Ni, on the order of 3 weeks for Cu, and on the order of 2 weeks for Zn. Together, the dissolved and exchangeable particulate data indicate more sluggish sorption kinetics for Ni than for Cu and Zn and suggest that different chemical forms control the speciation of these three metals in South Bay. Order of magnitude metal sorption exchange rates were estimated using these kinetic results. These calculations indicate that sorption exchange between dissolved and suspended particulate phases can cause dynamic internal cycling of these metals in South San Francisco Bay.  相似文献   

Household vulnerability to famine: Survival and recovery strategies among Berti and Zaghawa migrants in Northern Dafur,Sudan, 1982–1989     

Alison S. Pyle  Omer Abdel Gabbar 《GeoJournal》1993,30(2):141-146

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Block-effective macrodispersion for numerical simulations of sorbing solute transport in heterogeneous porous formations     

Alison E. Lawrence  Yoram Rubin 《Advances in water resources》2007

Heterogeneity is prevalent in aquifers and has an enormous impact on contaminant transport in groundwater. Numerical simulations are an effective way to deal with heterogeneity directly by assigning different hydraulic property values to each numerical grid block. Because hydraulic properties vary on different scales, but they cannot be sampled exhaustively and the number of numerical grid blocks is limited by computational considerations, the dispersive effects of unmodeled heterogeneity need to be accounted for. Dispersion tensors can be used to model the dispersion caused by unmodeled heterogeneity. The concept of block-effective macrodispersion tensors for modeling the effects of small-scale variability on solute transport introduced by Rubin et al. [Rubin Y, Sun A, Maxwell R, Bellin A. The concept of block-effective macrodispersivity and a unified approach for grid-scale- and plume-scale-dependent transport. J Fluid Mech 1999;395:161–80] is extended in this paper for use with reactive solutes. The tensors are derived for reactive solutes with spatially variable retardation factors and for solutes experiencing spatially uniform rate-limited sorption. The longitudinal block-effective macrodispersion coefficient is largest for perfect negative correlation between the log-hydraulic conductivity and the retardation factor. Because dispersion tensors, as they are usually implemented in numerical simulations, produce symmetric spreading, the applicability of the concept depends on the portion of the plume asymmetry caused by small-scale variability. The presented results show that the concept is applicable for rate-limited sorption for block sizes of one and two integral scales.  相似文献   

Rb‐Sr and Sm‐Nd isotopic and REE studies of igneous components in the bulk matrix domain of Martian breccia Northwest Africa 7034          下载免费PDF全文

Laurence E. Nyquist  Chi‐Yu Shih  Francis M. McCubbin  Alison R. Santos  Charles K. Shearer  Zhan X. Peng  Paul V. Burger  Carl B. Agee 《Meteoritics & planetary science》2016,51(3):483-498

The bulk matrix domain of the Martian breccia NWA 7034 was examined petrographically and isotopically to better understand the provenance and age of the source material that make up the breccia. Both ¹⁴⁷Sm‐¹⁴³Nd and ¹⁴⁶Sm‐¹⁴²Nd age results for mineral separates from the bulk matrix portion of breccia NWA 7034 suggest that various lithological components in the breccia probably formed contemporaneously ~4.44 Ga ago. This old age is in excellent agreement with the upper intersection ages (4.35–4.45 Ga) for U‐Pb discordia and also concordia defined by zircon and baddeleyite grains in matrix and igneous‐textured clasts. Consequently, we confirm an ancient age for the igneous components that make up the NWA 7034 breccia. Substantial disturbance in the Rb‐Sr system was detected, and no age significance could be gleaned from our Rb‐Sr data. The disturbance to the Rb‐Sr system may be due to a thermal event recorded by bulk‐rock K‐Ar ages of 1.56 Ga and U‐Pb ages of phosphates at about 1.35–1.5 Ga, which suggest partial resetting from an unknown thermal event(s), possibly accompanying breccia formation. The NWA 7034 bulk rock is LREE enriched and similar to KREEP‐rich lunar rocks, which indicates that the earliest Martian crust was geochemically enriched. This enrichment supports the idea that the crust is one of the enriched geochemical reservoirs on Mars that have been detected in studies of other Martian meteorites.  相似文献