Titan’s cloud seasonal activity from winter to spring with Cassini/VIMS     

S. Rodriguez  S. Le Mouélic  P. Rannou  C. Sotin  R.H. Brown  J.W. Barnes  C.A. Griffith  J. Burgalat  K.H. Baines  B.J. Buratti  R.N. Clark  P.D. Nicholson 《Icarus》2011,216(1):89-110

Since Saturn orbital insertion in July 2004, the Cassini orbiter has been observing Titan throughout most of the northern winter season (October 2002–August 2009) and the beginning of spring, allowing a detailed monitoring of Titan’s cloud coverage at high spatial resolution with close flybys on a monthly basis. This study reports on the analysis of all the near-infrared images of Titan’s clouds acquired by the Visual and Infrared Mapping Spectrometer (VIMS) during 67 targeted flybys of Titan between July 2004 and April 2010.The VIMS observations show numerous sporadic clouds at southern high and mid-latitudes, rare clouds in the equatorial region, and reveal a long-lived cloud cap above the north pole, ubiquitous poleward of 60°N. These observations allow us to follow the evolution of the cloud coverage during almost a 6-year period including the equinox, and greatly help to further constrain global circulation models (GCMs). After 4 years of regular outbursts observed by Cassini between 2004 and 2008, southern polar cloud activity started declining, and completely ceased 1 year before spring equinox. The extensive cloud system over the north pole, stable between 2004 and 2008, progressively fractionated and vanished as Titan entered into northern spring. At southern mid-latitudes, clouds were continuously observed throughout the VIMS observing period, even after equinox, in a latitude band between 30°S and 60°S. During the whole period of observation, only a dozen clouds were observed closer to the equator, though they were slightly more frequent as equinox approached.We also investigated the distribution of clouds with longitude. We found that southern polar clouds, before disappearing in mid-2008, were systematically concentrated in the leading hemisphere of Titan, in particular above and to the east of Ontario Lacus, the largest reservoir of hydrocarbons in the area. Clouds are also non-homogeneously distributed with longitude at southern mid-latitudes. The n = 2-mode wave pattern of the distribution, observed since 2003 by Earth-based telescopes and confirmed by our Cassini observations, may be attributed to Saturn’s tides.Although the latitudinal distribution of clouds is now relatively well reproduced and understood by the GCMs, the non-homogeneous longitudinal distributions and the evolution of the cloud coverage with seasons still need investigation. If the observation of a few single clouds at the tropics and at northern mid-latitudes late in winter and at the start of spring cannot be further interpreted for the moment, the obvious shutdown of the cloud activity at Titan’s poles provides clear signs of the onset of the general circulation turnover that is expected to accompany the beginning of Titan’s northern spring. According to our GCM, the persistence of clouds at certain latitudes rather suggests a ‘sudden’ shift in near future of the meteorology into the more illuminated hemisphere. Finally, the observed seasonal change in cloud activity occurred with a significant time lag that is not predicted by our model. This may be due to an overall methane humidity at Titan’s surface higher than previously expected.  相似文献   

Organic sedimentary deposits in Titan’s dry lakebeds: Probable evaporite     

Jason W. Barnes  Jacob Bow  Jacob Schwartz  Robert H. Brown  Jason M. Soderblom  Alexander G. Hayes  Graham Vixie  Stéphane Le Mouélic  Sebastien Rodriguez  Christophe Sotin  Ralf Jaumann  Katrin Stephan  Laurence A. Soderblom  Roger N. Clark  Bonnie J. Buratti  Kevin H. Baines  Philip D. Nicholson 《Icarus》2011,216(1):136-140

We report the discovery of organic sedimentary deposits at the bottom of dry lakebeds near Titan’s north pole in observations from the Cassini Visual and Infrared Mapping Spectrometer (VIMS). We show evidence that the deposits are evaporitic, making Titan just the third known planetary body with evaporitic processes after Earth and Mars, and is the first that uses a solvent other than water.  相似文献   

Saturn’s F ring grains: Aggregates made of crystalline water ice     

Sanaz Vahidinia  Jeffrey N. Cuzzi  Matt Hedman  Bruce Draine  Roger N. Clark  Ted Roush  Gianrico Filacchione  Philip D. Nicholson  Robert H. Brown  Bonnie Buratti  Christophe Sotin 《Icarus》2011,215(2):682-694

We present models of the near-infrared (1-5 μm) spectra of Saturn’s F ring obtained by Cassini’s Visual and Infrared Mapping Spectrometer (VIMS) at ultra-high phase angles (177.4-178.5°). Modeling this spectrum constrains the size distribution, composition, and structure of F ring particles in the 0.1-100 μm size range. These spectra are very different from those obtained at lower phase angles; they lack the familiar 1.5 and 2 μm absorption bands, and the expected 3 μm water ice primary absorption appears as an unusually narrow dip at 2.87 μm. We have modeled these data using multiple approaches. First, we use a simple Mie scattering model to constrain the size distribution and composition of the particles. The Mie model allows us to understand the overall shapes of the spectra in terms of dominance by diffraction at these ultra-high phase angles, and also to demonstrate that the 2.87 μm dip is associated with the Christiansen frequency of water ice (where the real refractive index passes unity). Second, we use a combination of Mie scattering with Effective Medium Theory to probe the effect of porous (but structureless) particles on the overall shape of the spectrum and depth of the 2.87 μm band. Such simple models are not able to capture the shape of this absorption feature well. Finally, we model each particle as an aggregate of discrete monomers, using the Discrete Dipole Approximation (DDA) model, and find a better fit for the depth of the 2.87 μm feature. The DDA models imply a slightly different overall size distribution. We present a simple heuristic model which explains the differences between the Mie and DDA model results. We conclude that the F ring contains aggregate particles with a size distribution that is distinctly narrower than a typical power law, and that the particles are predominantly crystalline water ice.  相似文献   

Simplified Approach for Calculating Building-Specific Attenuation Factors and Vapor Intrusion Mitigation System Flux-Based Radius of Influence     

Paul Nicholson B.A.Sc.  Darius Mali B.A.Sc  Todd A. McAlary Ph.D. 《Ground Water Monitoring & Remediation》2021,41(2):122-131

This article describes a simplified method to calculate a building-specific subslab to indoor air attenuation factor using data collected during pressure-field extension testing similar to industry standards for radon mitigation. It also describes a simplified method to calculate the radius of influence for a conventional suction point using a mass flux-balance model. The analysis is based on three simple measurements: (1) the extraction flow rate, (2) cross-slab applied vacuum at a radial distance of 3 feet, and (3) cross-slab applied vacuum at a radial distance of 10 feet. The intent is to provide a practitioner with a rapid and useful screening-level assessment of whether the benefits of reduced mitigation system costs warrant an investment in a more detailed mathematical analysis of the flow and vacuum data. In addition, this may also help a practitioner to make real-time decisions regarding placement of communication test points during pressure-field extension testing.  相似文献   

ESP: a tool to estimate scale parameter for multiresolution image segmentation of remotely sensed data     

Lucian Drǎguţ  Dirk Tiede  Shaun R. Levick 《International journal of geographical information science》2013,27(6):859-871

The spatial resolution of imaging sensors has increased dramatically in recent years, and so too have the challenges associated with extracting meaningful information from their data products. Object-based image analysis (OBIA) is gaining rapid popularity in remote sensing science as a means of bridging very high spatial resolution (VHSR) imagery and GIS. Multiscalar image segmentation is a fundamental step in OBIA, yet there is currently no tool available to objectively guide the selection of appropriate scales for segmentation. We present a technique for estimating the scale parameter in image segmentation of remotely sensed data with Definiens Developer®. The degree of heterogeneity within an image-object is controlled by a subjective measure called the ‘scale parameter’, as implemented in the mentioned software. We propose a tool, called estimation of scale parameter (ESP), that builds on the idea of local variance (LV) of object heterogeneity within a scene. The ESP tool iteratively generates image-objects at multiple scale levels in a bottom-up approach and calculates the LV for each scale. Variation in heterogeneity is explored by evaluating LV plotted against the corresponding scale. The thresholds in rates of change of LV (ROC-LV) indicate the scale levels at which the image can be segmented in the most appropriate manner, relative to the data properties at the scene level. Our tests on different types of imagery indicated fast processing times and accurate results. The simple yet robust ESP tool enables fast and objective parametrization when performing image segmentation and holds great potential for OBIA applications.  相似文献   

Reordering China,Respacing the World: Belt and Road Initiative (一带一路) as an Emergent Geopolitical Culture     

Shaun Lin  James D. Sidaway  Chih Yuan Woon 《The Professional geographer》2013,65(3):507-522

Since it was set out in speeches by China’s Premier Xi Jinping in September and October 2013, discussion of the Belt and Road Initiative (BRI) has proliferated in China. We argue that the scale and density of this discussion can usefully be conceptualized as an emergent geopolitical culture, reworking geopolitical narratives and spatial policies established in China in the second half of the twentieth century. In charting this emergence, we examine the ways in which the BRI links the reordering of China’s economic geography with articulations of China and the world. Surveying BRI as an emergent geopolitical culture, we draw on the example of the narratives of familiarity, partnership, and location vis-à-vis China and Singapore. Our conclusions reflect on how the BRI is appropriated to visions of China’s destiny. Key Words: Belt and Road Initiative, China, geopolitical culture, Singapore.  相似文献   

Making journal articles ‘live’: turning academic writing into scientific dialog     

Ola Ahlqvist  Francis Harvey  Hyowon Ban  Wei Chen  Shaun Fontanella  Meng Guo  Nisheet Singh 《GeoJournal》2013,78(1):61-68

Web 2.0 services and social networking offer possibilities to transform academic publishing to facilitate scientific dialogue. We present ‘The Live Paper’ in this editorial as a concept to consider how to leverage existing web technologies to produce rich, omnidirectional, and interactive narratives on research. Following the format of a traditional research report we describe and discuss how to enrich research reports utilizing various types of analytical tools, data, and web services in a published paper embedded in evolving scientific discourse. We also consider how the live papers would influence the practice, culture, and economy of academic research. We conclude that geospatial researchers have an opportunity to lead this development towards richer and more open ways of communicating our findings. The technology and infrastructure to do so exist today, but several barriers remain.  相似文献   

Evaluating TCE Abiotic and Biotic Degradation Pathways in a Permeable Reactive Barrier Using Compound Specific Isotope Analysis     

Paulo Lojkasek‐Lima  Ramon Aravena  Orfan Shouakar‐Stash  Shaun K. Frape  Massimo Marchesi  Stephanie Fiorenza  John Vogan 《Ground Water Monitoring & Remediation》2012,32(4):53-62

A pilot‐scale zero valent iron (ZVI) Permeable Reactive Barrier (PRB) was installed using an azimuth‐controlled ‐vertical hydrofracturing at an industrial facility to treat a chlorinated Volatile Organic Compound (VOC) plume. Following ZVI injection, no significant reduction in concentration was observed to occur with the exception of some multilevel monitoring wells, which also showed high levels of total organic carbon (TOC). These patterns suggested that the zero valent iron was not well distributed in the PRB creating leaky conditions. The geochemical data indicated reducing conditions in these areas where VOC reduction was observed, suggesting that biotic processes, associated to the guar used in the injection of the iron, could be a major mechanism of VOC degradation. Compound‐Specific Isotope Analysis (CSIA) using both carbon and chlorine stable isotopes were used as a complementary tool for evaluating the contribution of abiotic and biotic processes to VOC trends in the vicinity of the PRB. The isotopic data showed enriched isotope values around the PRB compared to the isotope composition of the VOC source confirming that VOC degradation is occurring along the PRB. A batch experiment using site groundwater collected near the VOC source and the ZVI used in the PRB was performed to evaluate the site‐specific abiotic isotopic fractionation patterns. Field isotopic trends, typical of biodegradations were observed at the site and were different from those obtained during the batch abiotic experiment. These differences in isotopic trends combined with changes in VOC concentrations and redox parameters suggested that biotic processes are the predominant pathways involved in the degradation of VOCs in the vicinity of the PRB.  相似文献   

The Geographies of the Justice for Janitors     

Luis L.M. Aguiar  Shaun Ryan 《Geoforum》2009,(6):949-958

On the heels of the successes of its Justice for Janitors (J4J) model to organize cleaners in the United States, the Service Employees International Union is exporting this model to Canada and Australia. In this article we examine the geographies of the implementation of the J4J model in these two contexts. And while the “ramping up” of the J4J to the globe makes sense to organize an increasingly globalizing cleaning industry, the model must nonetheless pay attention to the local scale and histories of existing organizations.  相似文献   

A transmission electron microscopy (TEM) study of silver nanoparticles associated with mine waste from New Caledonian nickel deposits: potential origins of silver toxicity in a World Heritage Site     

Brittany A. Cymes  Mark P. S. Krekeler  Kirsten N. Nicholson  Jeffry D. Grigsby 《Environmental Earth Sciences》2017,76(18):640

The nature of silver nanoparticles associated with nickel-laterite ore deposits in New Caledonia (SW Pacific Ocean) is explored using transmission electron microscopy. The silver nanoparticles, dispersed in a matrix of amorphous silica, display complex textures such as linear aggregation and bimodal diameter distributions of 3–12 and 20–60 nm for one sample and 2–30 and 150–650 nm for the other, indicative of Ostwald ripening. These features as well as chemical heterogeneity (31–82% Ag—normalized X-ray energy dispersive spectrum), destructive assemblage interfaces, and amorphous and nanoparticle silica phases are consistent with those observed in epithermal gold deposits, potentially indicating a similar origin. Silver is highly toxic to a wide variety of organisms, and therefore, its significant presence in material directly related to mine runoff is a major threat to the New Caledonian environment, particularly the lagoons, which are listed as a World Heritage Site. Potential exists that the silver nanoparticles observed here provide the key to the origin of the toxic silver accumulation observed in New Caledonian reef organisms.  相似文献