Accretion of Saturn’s mid-sized moons during the viscous spreading of young massive rings: Solving the paradox of silicate-poor rings versus silicate-rich moons     

Sébastien Charnoz  Aurélien Crida  Julie C. Castillo-Rogez  Valery Lainey  Luke Dones  Özgür Karatekin  Gabriel Tobie  Stephane Mathis  Christophe Le Poncin-Lafitte  Julien Salmon 《Icarus》2011,216(2):535-550

The origin of Saturn’s inner mid-sized moons (Mimas, Enceladus, Tethys, Dione and Rhea) and Saturn’s rings is debated. Charnoz et al. [Charnoz, S., Salmon J., Crida A., 2010. Nature 465, 752–754] introduced the idea that the smallest inner moons could form from the spreading of the rings’ edge while Salmon et al. [Salmon, J., Charnoz, S., Crida, A., Brahic, A., 2010. Icarus 209, 771–785] showed that the rings could have been initially massive, and so was the ring’s progenitor itself. One may wonder if the mid-sized moons may have formed also from the debris of a massive ring progenitor, as also suggested by Canup [Canup, R., 2010. Nature 468, 943–946]. However, the process driving mid-sized moon accretion from the icy debris disks has not been investigated in details. In particular, Canup’s (2010) model does not seem able to explain the varying silicate contents of the mid-sized moons (from 6% to 57% in mass). Here, we explore the formation of large objects from a massive ice-rich ring (a few times Rhea’s mass) and describe the fundamental properties and implications of this new process. Using a hybrid computer model, we show that accretion within massive icy rings can form all mid-sized moons from Mimas to Rhea. However in order to explain their current locations, intense dissipation within Saturn (with Q_p < 2000) is required. Our results are consistent with a satellite origin tied to the rings formation at least 2.5 Gy ago, both compatible with either a formation concurrent to Saturn or during the Late Heavy Bombardment. Tidal heating related to high-eccentricity post-accretional episodes may induce early geological activity. If some massive irregular chunks of silicates were initially present within the rings, they would be present today inside the satellites’ cores which would have accreted icy shells while being tidally expelled from the rings (via a heterogeneous accretion process). These moons may be either mostly icy, or, if they contain a significant amount of rock, already differentiated from the ice without the need for radiogenic heating. The resulting inner mid-sized moons may be significantly younger than the Solar System and a ∼1 Gyr formation delay is possible between Mimas and Rhea. The rings resulting from this process would evolve to a state compatible with current mass estimates of Saturn’s rings, and nearly devoid of silicates, apart from isolated silicate chunks coated with ice, interpreted as today Saturn’s rings’ propellers and ring-moons (like Pan or Daphnis).  相似文献   

DIAGENETIC CONTROLS ON GROUNDWATER SAPPING AND VALLEY FORMATION,COLORADO PLATEAU,REVEALED BY OPTICAL AND ELECTRON MICROSCOPY     

Julie E. Laity 《自然地理学》2013,34(2):103-125

Investigations into processes of valley formation on the Colorado Plateau have confirmed the important role of sapping in the Navajo Sandstone. The sapping process produces drainage systems that differ uniquely from fluvially eroded networks in their valley morphology, network pattern, spatial evolution, and degree of structural, lithologic, and stratigraphic control. The Navajo Sandstone is a highly transmissive aquifer. Sapping results from groundwater emergence above a permeability boundary formed by the underlying Kayenta Formation. This discharge undercuts cliff faces, and causes massive slab failures and vertical cliff recession. The principal agent for the physical weakening of the Navajo Sandstone at a site of seepage appears to be the mechanical separation of sand grains by the deposition of calcite from saturated waters.

The control of porosity and permeability by textural and mineralogical features in the Navajo Sandstone and Kayenta Formation was studied using reflected light and cathodoluminescence examination of epoxy-impregnated, polished sections, and a scanning electron microscope study of unimpregnated, unpolished rock chips. The most significant diagenetic factor in porosity reduction in the Kayenta Formation is the abundance of clay filling the pores. Other factors incude the development of quartz overgrowths and extensive carbonate deposition.  相似文献   

“If They Only Knew”: Color Blindness and Universalism in California Alternative Food Institutions     

Julie Guthman 《The Professional geographer》2013,65(3):387-397

This article takes on the cultural politics of “if they only knew” as it relates to alternative food practice. It draws on surveys and interviews of managers of two kinds of alternative food institutions—farmers' markets and community-supported agriculture—to illustrate the color-blind mentalities and universalizing impulses of alternative food discourse. The ways in which these discourses instantiate whiteness may have a chilling effect on people of color who tend not to participate in these markets proportionate to whites. Minor exclusionary practices may have profound implications for shaping projects of agro-food transformation.  相似文献   

Updating estimates of low-streamflow statistics to account for possible trends     

Annalise G. Blum  Stacey A. Archfield  Robert M. Hirsch  Richard M. Vogel  Julie E. Kiang  Robert W. Dudley 《水文科学杂志》2013,58(12):1404-1414

ABSTRACT

Accurate estimators of streamflow statistics are critical to the design, planning, and management of water resources. Given increasing evidence of trends in low-streamflow, new approaches to estimating low-streamflow statistics are needed. Here we investigate simple approaches to select a recent subset of the low-flow record to update the commonly used statistic of 7Q10, the annual minimum 7-day streamflow exceeded in 9 out of 10 years on average. Informed by low-streamflow records at 174 US Geological Survey streamgages, Monte Carlo simulation experiments evaluate competing approaches. We find that a strategy which estimates 7Q10 using the most recent 30 years of record when a trend is detected, reduces error and bias in 7Q10 estimators compared to use of the full record. This simple rule-based approach has potential as the basis for a framework for updating frequency-based statistics in the context of possible trends.  相似文献   

Enceladus: A hypothesis for bringing both heat and chemicals to the surface     

Dennis L. Matson  Julie C. Castillo-Rogez  Ashley Gerard Davies  Torrence V. Johnson 《Icarus》2012,221(1):53-62

The eruptive plumes and large heat flow (～15 GW) observed by Cassini in the South Polar Region of Enceladus may be expressions of hydrothermal activity inside Enceladus. We hypothesize that a subsurface ocean is the heat reservoir for thermal anomalies on the surface and the source of heat and chemicals necessary for the plumes. The ocean is believed to contain dissolved gases, mostly CO₂ and is found to be relatively warm (～0 °C). Regular tidal forces open cracks in the icy crust above the ocean. Ocean water fills these fissures. There, the conditions are met for the upward movement of water and the dissolved gases to exsolve and form bubbles, lowering the bulk density of the water column and making the pressure at its bottom less than that at the top of the ocean. This pressure difference drives ocean water into and up the conduits toward the surface. This transportation mechanism supports the thermal anomalies and delivers heat and chemicals to the chambers from which the plumes erupt. Water enters these chambers and there its bubbles pop and loft an aerosol mist into the ullage. The exiting plume gas entrains some of these small droplets. Thus, nonvolatile chemical species in ocean water can be present in the plume particles. A CO₂ equivalent-gas molar fraction of ～4 × 10^?4 for the ocean is sufficient to support the circulation. A source of heat is needed to keep the ocean warm at ～0 °C (about two degrees above its freezing point). The source of heat is unknown, but our hypothesis is not dependent on any particular mechanism for producing the heat.  相似文献   

A Sense of Place in Cultural Ecosystem Services: The Case of Cornish Fishing Communities     

Julie Urquhart  Tim Acott 《社会与自然资源》2014,27(1):3-19

Fishing communities in Europe are facing significant challenges due to policy measures aimed at reducing fishing effort in order to cope with the “crisis” in key stocks. While it is imperative to ensure sustainability of the resource, such policies may overlook the contribution of fisheries to the social and cultural well-being of coastal communities. This article explores the contribution that sense of place can make to understanding the relationship between fishing and cultural ecosystem services, drawing on case studies from Cornwall, southwest England. Through semistructured interviews with fishing community stakeholders we outline how fishing contributes to sense of place in terms of individual and community identities, as well as a particular place character associated with the physical presence of fishing. We suggest that a co-constructionist account of the relationship between sea and land can provide a starting point for understanding the cultural landscape that emerges as a result of fishing.  相似文献   

Opening Up the Water Poverty Index—Co-Producing Knowledge on the Capacity for Community Water Management Using the Water Prosperity Index     

Anna C. Jonsson  Julie Wilk 《社会与自然资源》2014,27(3):265-280

  相似文献   

Seismic reflection models of continental crust based on metamorphic terrains     

David M. Fountain  Daniel T. McDonough  Julie M. Gorham 《Geophysical Journal International》1987,89(1):61-66

Summary. Laboratory seismic velocity measurements on rock samples from metamorphic terrains, coupled with geologic cross sections, provide the basis for synthetic seismic reflection profiles for various types of continental crust. Results from greenstone belts, mylonite zones and partial cross sections of continental crust indicate that lithologic heterogeneity and geometrical factors control crustal reflection characteristics.  相似文献   

A uranium-thorium dated late Eemian or early midlandian organic deposit from near Kilfenora between Spa and Fenit,Co. Kerry,Ireland     

Henk Heijnis  Julie Ruddock  Peter Coxon 《第四纪科学杂志》1993,8(1):31-43

This paper discusses the dating of stratigraphically important Quaternary sequences from a site near Fenit, Co. Kerry, which have been the subject of debate since they were first described by Mitchell in 1970. The overall stratigraphy of the Quaternary deposits have been investigated and detailed analyses of the organic material carried out. Pollen from biogenic sediments have been analysed and samples of peat dated using the uranium-thorium disequilibrium method. The pollen assemblages match no others previously recorded in Ireland and appear to represent a cool temperate phase following the last interglacial. The uranium-thorium dates of between 114000 and 123 000 yr BP indicate that the deposit dates from Oxygen Isotope Stage 5, possibly post-dating the last temperate stage (the Eemian Stage interglacial; Oxygen Isotope Substage 5e). The dating of this deposit and the realisation that it is not penultimate temperate stage (Gortian) in age invalidates much recent speculation on the age of the Gortian interglacial.  相似文献   

Subduction and melting processes inferred from U-Series, Sr-Nd-Pb isotope, and trace element data, Bicol and Bataan arcs, Philippines     

S. Andrew DuFrane  Yemane Asmerom  Julie D. Morris 《Geochimica et cosmochimica acta》2006,70(13):3401-3420

We present U-series, Sr-Nd-Pb isotope, and trace element data from the two principal volcanic chains on Luzon Island, developed over oppositely dipping subduction zones, to explore melting and mass transfer processes beneath arcs. The Bataan (western) and Bicol (eastern) arcs are currently subducting terrigenous and pelagic sediments, respectively, which have different trace element and isotopic compositions. The range of (²³⁰Th/²³⁸U) disequilibria for both arcs is 0.85-1.15; only lavas from Mt. Mayon (Bicol arc) have ²³⁰Th activity excesses. Bataan lavas have higher ⁸⁷Sr/⁸⁶Sr and lower ¹⁴³Nd/¹⁴⁴Nd than Bicol lavas (⁸⁷Sr/⁸⁶Sr = 0.7042-0.7046, ¹⁴³Nd/¹⁴⁴Nd = 0.51281-0.51290 vs. ⁸⁷Sr/⁸⁶Sr = 0.70371-0.70391, ¹⁴³Nd/¹⁴⁴Nd = 0.51295-0.51301) and both arcs show steep linear arrays towards sediment values on ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb diagrams. Analysis of incompatible element and isotopic data allows identification of a sediment component that, at least in part, was transferred as a partial melt to the mantle wedge peridotite. Between 1% and 5% sediment melt addition can explain the isotopic and trace element variability in the rocks from both arcs despite the differences in sediment supply. We therefore propose that sediment transfer to the mantle wedge is likely mechanically or thermally limited. It follows that most sediments are either accreted, reside in the sub-arc lithosphere, or are recycled into the convecting mantle. However, whole-sale sediment recycling into the upper mantle is unlikely in light of the global mid-ocean ridge basalt data. Fluid involvement is more difficult to characterize, but overall the Bicol arc appears to have more fluid influence than the Bataan arc. Rock suites from each arc can be related by a dynamic melting process that allows for ²³⁰Th ingrowth, either by dynamic or continuous flux melting, provided the initial (²³⁰Th/²³²Th) of the source is ∼0.6-0.7. The implication of either model is that inclined arrays on the U-Th equiline diagram may not have chronologic significance. Modeling also suggests that U-series disequilibria are influenced by the tectonic convergence rate, which dictates mantle matrix flow. Thus with slower matrix flow there is a greater degree of ²³⁰Th ingrowth. While other factors such as prior mantle depletion and addition of a subducted component may explain some aspects of U-series data, an overall global correlation between tectonic convergence rate and the extent of U-Th disequilibria may originate from melting processes.  相似文献