Compositional variability on the surface of 1 Ceres revealed through GRaND measurements of high‐energy gamma rays

High‐energy gamma rays (HEGRs) from Ceres’s surface were measured using Dawn's Gamma Ray and Neutron Detector (GRaND). Models of cosmic‐ray‐initiated gamma ray production predict that the HEGR flux will inversely vary with single‐layer hydrogen concentrations for Ceres‐like compositions. The measured data confirm this prediction. The hydrogen‐induced variations in HEGR rates were decoupled from the measurements by detrending the HEGR data with Ceres single‐layer hydrogen concentrations determined by GRaND neutron measurements. Models indicate that hydrogen‐detrended HEGR counting rates correlate with water‐free average atomic mass, which is denoted as <A>*. HEGR variations across Ceres’s surface are consistent with <A>* variations of ±0.5 atomic mass units. Chemical variations in the CM and CI chondrites, our closest analogs to Ceres’s surface, suggest that <A>* variations on Ceres are primarily driven by variations in the concentration of Fe, although other elements such as Mg and S could contribute. Dawn observations have shown that Ceres’s interior structure and surface composition have been modified by some combination of physical (i.e., ice‐rock fractionation) and/or chemical (i.e., alteration) processes that has led to variations in bulk surface chemistry. Locations of the highest inferred <A>* values, and thus possibly the highest Fe and least altered materials, tend to be younger, less cratered surfaces that are broadly associated with the impact ejecta of Ceres’s largest craters.     

Coronal heating events in high-cadence TRACE data

On March 23rd 1999 a set of TRACE (Transition Region and Coronal Explorer) observations were taken in support of Solar and Heliospheric Observatory (SOHO) / TRACE Joint Observing Program 83. The program is designed to look for coronal heating mechanisms operating at high cadence and to observe their dynamical effects on coronal loop density, temperature and magnetic structure. We present here a study of 27 small, dynamic brightening events seen in this data. These events are seen in the quiet-Sun areas surrounding the active region. The data itself consists of 157 171 Å 512×512 arc sec images at 1 arc sec resolution and 9 second cadence, a previously unavailable combination of cadence, resolution and image size. Three classes of events are introduced: complex, intermediate and simple. All three classes share the property of being dynamic on small time and length scales but differ in the complexity of their behavior. We find that the brightenings across all classes exhibit variations as part of a distribution of time scales (90–400 s) peaked around 228 s. The brightenings are no more than 5 arc sec in diameter. Motions between brightenings occur on time scales from 9 to 90 s and on length scales no greater than 10 arc sec. These motions have velocities estimated to be in the range 89–174 km s^?1. The position of these events in the spectrum of previously observed coronal heating events is discussed.

     

The use of sediments to detect human impact on the fluvial system

Sediments have been used to detect sources of contamination in a catchment of the Port Jackson (Sydney Harbour) estuary and to evaluate the effects of different land-use practices on the fluvial environment. Mean enrichment (mean concentrations over pre-anthropogenic background) of size-normalized (<62.5 μm) aquatic sediment is 10 × for Cu, 20× for Pb and 90× for Zn adjacent to industrialized areas and 2×, 7× and 7×, respectively for these metals in highly urbanized subcatchments. Diffuse sources contribute minor metals to fluvial sediment even in the most underdeveloped subcatchment (2×, 3× and 3× for Cu, Pb and Zn respectively). Organochlorine pesticide residue concentrations parallel heavy-metal trends due to a common mixed industrial base. Effects-based sediment criteria suggest that some adverse biological impacts are probably occurring in streams flowing through the industrial areas. This interpretation is supported by sequential extraction data which show that a moderate proportion of total heavy metals, especially Zn, is associated with the more bioavailable exchangeable/adsorbed phases in these aquatic sediments. High total suspended solid loads in water downstream of one of the industrial centers, and high particle-bound Cu and Pb concentrations, suggest that most contaminants exiting the catchment do so in association with the solid phase. Received: 29 March 1999 · Accepted: 30 August 1999     

Pesticides in Sediments From Queensland Irrigation Channels and Drains

Pesticide concentration in sediment from irrigation areas can provide information required to assess exposure and fate of these chemicals in freshwater ecosystems and their likely impacts to the marine environment. In this study, 103 sediment samples collected from irrigation channels and drains in 11 agricultural areas of Queensland were analysed for a series of past and presently used pesticides including various organochlorines, synthetic pyrethroids, benzoyl ureas, triazines and organophosphates. The most often detected compounds were endosulphans (, β and/or endosulphan sulphate) which were detectable in 78 of the 103 samples and levels ranged from below the limit of quantification (0.1 ng g⁻¹ dw) up to 840 ng g⁻¹ dw. DDT and its metabolites were the second most often detected pesticide investigated (74 of the 103 samples) with concentrations up to 240 ng g⁻¹ dw of ∑DDTs. Mean ∑endosulphan and ∑DDT concentrations were 1–2 orders of magnitude higher in sediments from the irrigation areas which are dominated by cotton cultivation compared to those which are dominated by sugarcane cultivation. In contrast to these insecticides, the herbicides diuron, atrazine and ametryn were the compounds which were most often detected in sediments from irrigation drains in sugarcane areas with maximum concentrations in areas of 120, 70 and 130 ng g⁻¹ dw, respectively. In particular during flood events, when light is limiting, transport of these photosynthesis inhibiting herbicides from the sugarcane cultivation areas to the marine environment may result in additional stress of marine plants.     

致命的海洋"打嗝"

大洋底部形成的有害气体会不会以气泡的形式冒出洋面,继而毁灭地球上的绝大多数生命呢?一些研究人员证明,洋底的有害气体已经将地球生命毁灭过好几次了。科学家们担心,全球变暖正在使得另外一次致命的海洋“打嗝”条件趋于成熟。科学家们通常同意这个观点:6 500万年前,来自太空     

MM5 simulations of interannual change and the diurnal cycle of southern African regional climate

Summary Two cumulus convection and two planetary boundary layer schemes are used to investigate the climate of southern Africa using the MM5 regional climate model. Both a wet (1988/89) and a dry (1991/92) summer (December–February, DJF) rainfall season are simulated and the results compared with three different observational sources: Climate Research Unit seasonal data (precipitation, 2 m surface temperature, number of rain days), satellite-derived diurnal precipitation and the Surface Radiation Budget diurnal short-wave fluxes and optical depth. Using the ETA model boundary layer in MM5 simulates too much incident short-wave radiation at the surface at 12 UTC, whereas the medium range forecast model boundary layer yields a diurnal cycle of short-wave radiation closer to the observed. The Betts-Miller convection scheme in MM5 simulates peak rainfall later in the day and less rain days than observed, whereas when using the Kain-Fritsch convection scheme a peak rainfall earlier in the day and more rain days than observed are simulated. The intensity of the hydrological cycle is therefore dependent on the choice of convection scheme, which in turn is further modified by the boundary layer scheme. Precipitation during the wet 1988/89 season is reasonably captured by most simulations, though using the Betts-Miller scheme more accurately simulates rainfall during the dry 1991/92 season. Mean DJF biases in the surface temperature and diurnal temperature range are consistent with biases in the number of rain days and the diurnal cycles of surface moisture and energy.     

High-resolution simulations for Vietnam - methodology and evaluation of current climate

To assist the government of Vietnam in its efforts to better understand the impacts of climate change and prioritise its adaptation measures, dynamically downscaled climate change projections were produced across Vietnam. Two Regional Climate Models (RCMs) were used: CSIRO’s variable-resolution Conformal-Cubic Atmospheric Model (CCAM) and the limited-area model Regional Climate Model system version 4.2 (RegCM4.2). First, global CCAM simulations were completed using bias- and variance-corrected sea surface temperatures as well as sea ice concentrations from six Coupled Model Intercomparison Project Phase 5 (CMIP5) global climate models. This approach is different from other downscaling approaches as it does not use any atmospheric fields from the GCMs. The global CCAM simulations were then further downscaled to 10 km using CCAM and to 20 km using RegCM4.2. Evaluations of temperature and precipitation for the current climate (1980-2000) were completed using station data as well as various gridded observational datasets. The RCMs were able to reproduce reasonably well most of the important characteristics of observed spatial patterns and annual cycles of temperature. Average and minimum temperatures were well simulated (biases generally less than 1oC), while maximum temperatures had biases of around 1oC. For precipitation, although the RCMs captured the annual cycle, RegCM4.2 was too dry in Oct.-Nov. (-60% bias), while CCAM was too wet in Dec.- Mar. (130% bias). Both models were too dry in summer and too wet in winter (especially in northern Vietnam). The ability of the ensemble simulations to capture current climate increases confidence in the simulations of future climate.