Epistemic bimodality and kinetic hypersensitivity in photochemical models of Titan's atmosphere     

M. Dobrijevic  N. Carrasco  E. Hébrard  P. Pernot 《Planetary and Space Science》2008,56(12):1630-1643

We show that photochemical models of Titan's atmosphere can give rise to bimodal distributions in the abundances of some major compounds, like C₂H₂ and C₂H₄. Sensitivity analysis enabled us to identify the causes and conditions of this bimodality. We propose several methods to control this behavior in photochemical models. In particular, we point out the importance of two key reactions and the needs for a critical evaluation of the kinetic data. We also show that the abundances of some compounds are hypersensitive to the ratio [CH₄]/[H], suggesting that a time-dependent variation of this ratio might lead to a real bistability in the high atmosphere of Titan.  相似文献   

Cooling of Accretion-Heated Neutron Stars     

Rudy Wijnands  Nathalie Degenaar  Dany Page 《Journal of Astrophysics and Astronomy》2017,38(3):49

We present a brief, observational review about the study of the cooling behaviour of accretion-heated neutron stars and the inferences about the neutron-star crust and core that have been obtained from these studies. Accretion of matter during outbursts can heat the crust out of thermal equilibrium with the core and after the accretion episodes are over, the crust will cool down until crust-core equilibrium is restored. We discuss the observed properties of the crust cooling sources and what has been learned about the physics of neutron-star crusts. We also briefly discuss those systems that have been observed long after their outbursts were over, i.e, during times when the crust and core are expected to be in thermal equilibrium. The surface temperature is then a direct probe for the core temperature. By comparing the expected temperatures based on estimates of the accretion history of the targets with the observed ones, the physics of neutron-star cores can be investigated. Finally, we discuss similar studies performed for strongly magnetized neutron stars in which the magnetic field might play an important role in the heating and cooling of the neutron stars.  相似文献   

The planktonic food web of the Bizerte lagoon (south-western Mediterranean) during summer: I. Spatial distribution under different anthropogenic pressures     

Asma Sakka Hlaili  Boutheina Grami  Nathalie Niquil  Michel Gosselin  Dominique Hamel  Marc Troussellier  Hassine Hadj Mabrouk 《Estuarine, Coastal and Shelf Science》2008

The structure and the trophic interactions of the planktonic food web were investigated during summer 2004 in a coastal lagoon of south-western Mediterranean Sea. Biomasses of planktonic components as well as bacterial and phytoplankton production and grazing by microzooplankton were quantified at four stations (MA, MB, MJ and R) inside the lagoon. Station MA was impacted by urban discharge, station MB was influenced by industrial activity, station MJ was located in a shellfish farming sector, while station R represented the lagoon central area. Biomasses and production rates of bacteria (7–33 mg C m⁻³; 17.5–35 mg C m⁻³ d⁻¹) and phytoplankton (80–299 mg C m⁻³; 34–210 mg C m⁻³ d⁻¹) showed high values at station MJ, where substantial concentrations of nutrients (NO₃⁻ and Si(OH)₄) were found. Microphytoplankton, which dominated the total algal biomass and production (>82%), were characterized by the proliferation of several chain-forming diatoms. Microzooplankton was mainly composed of dinoflagellates (Torodinium, Protoperidinium and Dinophysis) and aloricate (Lohmaniellea and Strombidium) and tintinnid (Tintinnopsis, Tintinnus, Favella and Eutintinnus) ciliates. Higher biomass of these protozoa (359 mg C m⁻³) was observed at station MB, where large tintinnids were encountered. Mesozooplankton mainly represented by Calanoida (Acartia, Temora, Calanus, Eucalanus, Paracalanus and Centropages) and Cyclopoida (Oithona) copepods, exhibited higher and lower biomasses at stations MA/MJ and MB, respectively. Bacterivory represented only 35% of bacterial production at stations MB and R, but higher fractions (65–70%) were observed at stations MA and MJ. Small heterotrophic flagellates and aloricate ciliates seemed to be the main controllers of bacteria. Pico- and nanophytoplankton represented a significant alternative carbon pool for micrograzers, which grazing represented 67–90% of pico- and nano-algal production in all stations. Microzooplankton has, however, a relatively low impact on microphytoplankton, as ≤45% of microalgal production was consumed in all stations. This implies that an important fraction of diatom production would be channelled by herbivorous meso-grazers to higher consumers at stations MA and MJ where copepods were numerous. Most of the microalgal production would, however, sink particularly at station MB where copepods were scare. These different trophic interactions suggest different food web structures between stations. A multivorous food web seemed to prevail in stations MJ and MA, whereas microbial web was dominant in the other stations.  相似文献   

36.5万年以来西边界底流冰期-间冰期不稳定性与ODP 646站位(拉布拉多海)黏土级沉积碎屑Sm/Nd比率的关系     

Nathalie Fagel  刘锐 《海洋地质前沿》2007,23(10):22-24

1区域概况随着气候—海洋系统模拟工作力度的加大,相关古气候改变北大西洋深层水(NADW)生产力速率的论著十分亟需。大多数作者认为整体的NADW生产力在冰期降低,他们的推断范围不完全一致。冰期更大量的南大洋水体涌入北大西洋,而且北大西洋深部对流现在已被更浅的和更南部的对流所取代。有关更新世末次冰期—间冰期旋回NADW演变,主要研究成果如下:(1)沿冰岛—苏格兰溢流水团路径从冰岛盆地采集柱状样,对其进行磁各向异性测试。变化结果同过去20万a水体可变强度相关联。冰期氧同位素2期、4期、6期较之间冰期1期、3期、5期有30%的强度衰…  相似文献   

Visible and near-infrared spectrophotometry of the Deep Impact ejecta of Comet 9P/Tempel 1     

Klaus W. Hodapp  Greg Aldering  Anita L. Cochran  Emmanuel Pécontal  Nathalie Blanc  David K. Lynch  S. Mazuk  Richard C. Puetter 《Icarus》2007,187(1):185-198

We have obtained optical spectrophotometry of the evolution of Comet 9P/Tempel 1 after the impact of the Deep Impact probe, using the Supernova Integral Field Spectrograph (SNIFS) at the UH 2.2-m telescope, as well as simultaneous optical and infrared spectra using the Lick Visible-to-Near-Infrared Imaging Spectrograph (VNIRIS). The spatial distribution and temporal evolution of the “violet band” CN (0-0) emission and of the 630 nm [OI] emission was studied. We found that CN emission centered on the nucleus increased in the 2 h after impact, but that this CN emission was delayed compared to the light curve of dust-scattered sunlight. The CN emission also expanded faster than the cloud of scattering dust. The emission of [OI] at 630 nm rose similarly to the scattered light, but then remained nearly constant for several hours after impact. On the day following the impact, both CN and [OI] emission concentrated on the comet nucleus had returned nearly to pre-impact levels. We have also searched for differences in the scattering properties of the dust ejected by the impact compared to the dust released under normal conditions. Compared to the pre-impact state of the comet, we find evidence that the color of the comet was slightly bluer during the post-impact rise in brightness. Long after the impact, in the following nights, the comet colors returned to their pre-impact values. This can be explained by postulating a change to a smaller particle size distribution in the ejecta cloud, in agreement with the findings from mid-infrared observations, or by postulating a large fraction of clean ice particles, or by a combination of these two.  相似文献   

Near-infrared surface photometry of a sample of barred galaxies     

D. A. Gadotti  E. Athanassoula  L. Carrasco  A. Bosma  R. E. de Souza  E. Recillas 《Monthly notices of the Royal Astronomical Society》2007,381(3):943-961

  相似文献   

Sensitivity of simulated Asian and African summer monsoons to orbitally induced variations in insolation 126, 115 and 6 kBP   总被引：1，自引：1，他引：0  

Nathalie de Noblet  Pascale Braconnot  Sylvie Joussaume  Valérie Masson 《Climate Dynamics》1996,12(9):589-603

We have conducted four numerical experiments with an atmospheric general circulation model (AGCM) to investigate the sensitivity of Asian and African monsoons to small changes (–5 to +12%), with respect to present-day, in incoming solar radiation at the top of the atmosphere. We show that, during the mid-Holocene (6 kBP where kBP means thousands of years before present-day) and the last interglacial (126 kBP), the Northern Hemisphere seasonal contrast was increased, with warmer summers and colder winters. At the time of glacial inception (115 kBP) however, summers were cooler and winters milder. As a consequence, Asia and tropical North Africa experienced stronger (weaker) summer monsoons 6 and 126 kBP (115 kBP), in agreement with previous numerical studies. This present study shows that summer warming/cooling of Eurasia and North Africa induced a shift of the main low-level convergence cell along a northwest/southeast transect. When land was warmer (during the summer months 6 and 126 kBP), the monsoon winds converged further inland bringing more moisture into northern India, western China and the southern Sahara. The southern tips of India, Indochina and southeastern China, as well as equatorial North Africa became drier. When land was cooler (during the summer 115 kBP), the main convergence zone was located over the west Pacific and the wet (dry) areas were those that were dry (wet) 6 and 126 kBP. The location and intensity of the simulated precipitation maxima were therefore very sensitive to changes in insolation. However the total amount of monsoon rain in Asia as well as in Africa remained remarkably stable through the time periods studied. These simulated migrations of convective activities were accompanied by changes in the nature of precipitation events: increased monsoon rains in these experiments were always associated with more high precipitation events (> 5 mm day ^–1), and fewer light showers (1 mm day^–). Rainy days with rates between 1 and 5 mm day^–1 were almost unchanged.  相似文献   

Ecophysiological responses to elevated CO<Subscript>2</Subscript> and temperature in <Emphasis Type="Italic">Cystoseira tamariscifolia</Emphasis> (Phaeophyceae)     

Paula S. M. Celis-Plá  Brezo Martínez  Nathalie Korbee  Jason M. Hall-Spencer  Félix L. Figueroa 《Climatic change》2017,142(1-2):67-81

Ocean acidification increases the amount of dissolved inorganic carbon (DIC) available in seawater which can benefit photosynthesis in those algae that are currently carbon limited, leading to shifts in the structure and function of seaweed communities. Recent studies have shown that ocean acidification-driven shifts in seaweed community dominance will depend on interactions with other factors such as light and nutrients. The study of interactive effects of ocean acidification and warming can help elucidate the likely effects of climate change on marine primary producers. In this study, we investigated the ecophysiological responses of Cystoseira tamariscifolia (Hudson) Papenfuss. This large brown macroalga plays an important structural role in coastal Mediterranean communities. Algae were collected from both oligotrophic and ultraoligotrophic waters in southern Spain. They were then incubated in tanks at ambient (ca. 400–500 ppm) and high CO₂ (ca. 1200–1300 ppm), and at 20 °C (ambient temperature) and 24 °C (ambient temperature +4 °C). Increased CO₂ levels benefited the algae from both origins. Biomass increased in elevated CO₂ treatments and was similar in algae from both origins. The maximal electron transport rate (ETR_max), used to estimate photosynthetic capacity, increased in ambient temperature/high CO₂ treatments. The highest polyphenol content and antioxidant activity were observed in ambient temperature/high CO₂ conditions in algae from both origins; phenol content was higher in algae from ultraoligotrophic waters (1.5–3.0%) than that from oligotrophic waters (1.0–2.2%). Our study shows that ongoing ocean acidification can be expected to increase algal productivity (ETR_max), boost antioxidant activity (EC ₅₀ ), and increase production of photoprotective phenols. Cystoseira tamariscifolia collected from oligotrophic and ultraoligotrophic waters were able to benefit from increases in DIC at ambient temperatures. Warming, not acidification, may be the key stressor for this habitat as CO₂ levels continue to rise.  相似文献   

Limnologic Analysis of Gusev Crater Paleolake, Mars     

Edmond A. Grin  Nathalie A. Cabrol 《Icarus》1997,130(2):461-474

The survey of the hydrogeologic system formed by Gusev crater and Ma'adim Vallis (Aeolis subquadrangle of Mars) points out evidence for the existence of an ice-covered lake in Gusev crater. A first lake was formed by the drainage of the aquifer in the region surrounding Gusev before the entry of Ma'adim Vallis in the crater. The existence of a former lake in Gusev is deduced from the morphology of the Ma'adim delta. Its comparison with terrestrial Antarctic analogs argues for the presence of an ice-covered lake in Gusev at the time that the southern part of the crater's rampart was breached by Ma'adim first release, and for a subice–lacustrine construction of the valley's delta. Our survey shows that Ma'adim Vallis may have entered Gusev crater as late as Late Hesperian/Early Amazonian as part of a second lake episode. The relationship between the variation of the Gusev lake water-level, the volume of the lake, and the surface of the lake bed is established by our bathymetric model. The elevation of the former lake is deduced from the elevation of the mesa-like structures in the delta of Ma'adim Vallis. Furthermore, the correlation of the crater frequency of Gusev rampart with Mars' stratigraphic age shows that lakes may have occupied Gusev crater over a period of time covering 2 Gyrs., from the formation of the crater to the last episode of water release from Ma'adim Vallis. Though it is most likely that the lake was episodical, the recurrence of abundant water in Gusev crater makes this site a high priority for missions, either for martian resource exploration, or for the search of life.  相似文献   

Structure of Saturn's Mesosphere from the 28 Sgr Occultations     

W.B. Hubbard  C.C. Porco  D.M. Hunten  G.H. Rieke  M.J. Rieke  D.W. McCarthy  V. Haemmerle  J. Haller  B. McLeod  L.A. Lebofsky  R. Marcialis  J.B. Holberg  R. Landau  L. Carrasco  J. Elias  M.W. Buie  E.W. Dunham  S.E. Persson  T. Boroson  S. West  R.G. French  J. Harrington  J.L. Elliot  W.J. Forrest  J.L. Pipher  R.J. Stover  A. Brahic  I. Grenier 《Icarus》1997,130(2):404-425

We analyze an extensive data set of immersion and emersion lightcurves of the occultation of 28 Sgr by Saturn's atmosphere on 3 July 1989. The data give profiles of number density as a function of altitude at a variety of latitudes, at pressures ranging from about 0.5 to about 20 μbar. The atmosphere is essentially isothermal in this range, with a temperature close to 140 K for an assumed mean molecular weight of 2.135. Owing to favorable ring geometry, an accurate radial scale is available for all observations, and we confirm the substantial equatorial bulge produced by zonal winds of ∼450 m/s first observed in the Voyager radio-occultation experiments. The fact that the bulge is still present at microbar pressures suggests that the equatorial winds persist to high altitudes. According to our radial scale, the 2.4-μbar level, which corresponds to half-flux in the stellar occultations, is at an equatorial radius of 60,960 km. This radial scale is in good agreement with the Voyager radio-occultation data at mbar pressures and allows smooth interpolation of the isothermal structure between the stellar-occultation and radio-occultation regions. We do not have such a smooth interpolation between our data and Voyager ultraviolet occultation data, unless we discard the lowest 200 km of Voyager ultraviolet data. When this is done, we obtain a complete atmospheric model from an equatorial radius of 61,500 km down to an equatorial radius of 60,500 km. This model gives excellent agreement between all 28 Sgr, Voyager, and Pioneer 11 data.  相似文献