The formation of Ganymede's grooved terrain: Numerical modeling of extensional necking instabilities     

Michael T. Bland  Adam P. Showman 《Icarus》2007,189(2):439-456

Ganymede's grooved terrain likely formed during an epoch of global expansion, when unstable extension of the lithosphere resulted in the development of periodic necking instabilities. Linear, infinitesimal-strain models of extensional necking support this model of groove formation, finding that the fastest growing modes of an instability have wavelengths and growth rates consistent with Ganymede's grooves. However, several questions remain unanswered, including how nonlinearities affect instability growth at large strains, and what role instabilities play in tectonically resurfacing preexisting terrain. To address these questions we numerically model the extension of an icy lithosphere to examine the growth of periodic necking instabilities over a broad range of strain rates and temperature gradients. We explored thermal gradients up to 45 K km⁻¹ and found that, at infinitesimal strain, maximum growth rates occur at high temperature gradients (45 K km⁻¹) and moderate strain rates (10⁻¹³ s⁻¹). Dominant wavelengths range from 1.8 to 16.4 km (post extension). Our infinitesimal growth rates are qualitatively consistent with, but an order of magnitude lower than, previous linearized calculations. When strain exceeds ∼10% growth rates decrease, limiting the total amount of amplification that can result from unstable extension. This fall-off in growth occurs at lower groove amplitudes for high-temperature-gradient, thin-lithosphere simulations than for low-temperature-gradient, thick-lithosphere simulations. At large strains, this shifts the ideal conditions for producing large amplitude grooves from high temperature gradients to more moderate temperature gradients (15 K km⁻¹). We find that the formation of periodic necking instabilities can modify preexisting terrain, replacing semi-random topography up to 100 m in amplitude with periodic ridges and troughs, assisting the tectonic resurfacing process. Despite this success, the small topographic amplification produced by our model presents a formidable challenge to the necking instability mechanism for groove formation. Success of the necking instability mechanism may require rheological weakening or strain localization by faulting, effects not included in our analysis.  相似文献   

Propagation of magnetoacoustic-gravity waves in a horizontally-stratified medium: IV. kinematics     

Iain McKaig  John A. Adam 《Astrophysics and Space Science》1993,202(2):259-271

A model magneto-atmosphere is used to discuss the behavior of the group velocity for magnetoacoustic-gravity (MAG) waves as a function of their angular frequency. The qualitative frequency/time history of a signal generated by a source differs significantly from the case where the magnetic field is zero, for which a clear distinction may be made between the acoustic and gravity components of the signal.  相似文献   

Contact Partial Melting of Granitic Country Rock, Melt Segregation, and Re-injection as Dikes into Ferrar Dolerite Sills, McMurdo Dry Valleys, Antarctica     

Hersum  Taber G.; Marsh  Bruce D.; Simon  Adam C. 《Journal of Petrology》2007,48(11):2125-2148

Numerous, interconnected, granitic dikes (<30 cm in widthand hundeds of meters in length) cut Ferrar dolerite sills ofthe McMurdo Dry Valleys, Antarctica. The source of the graniticdikes is partial melting of granitic country rock, which tookplace in the crust at a depth of about 2–3 km adjacentto contacts with dolerite sills. Sustained flow of doleriticmagma through the sill generated a partial melting front thatpropagated into the granitic country rock. Granitic partialmelts segregated and collected at the contact in a melt-rich,nearly crystal-free reservoir adjacent to the initial doleritechilled margin. This dolerite chilled margin was subsequentlyfractured open in the fashion of a trapdoor by the graniticmelt, evacuating the reservoir to form an extensive complexof granitic dikes within the dolerite sills. At the time ofdike injection the dolerite was nearly solidified. Unusuallycomplete exposures allow the full physical and chemical processesof partial melting, segregation, and dike formation to be examinedin great detail. The compositions of the granitic dikes andthe textures of partially melted granitic wall rock suggestthat partial melting was characterized by disequilibrium mineraldissolution of dominantly quartz and alkali feldspar ratherthan by equilibrium melting. It is also unlikely that meltingoccurred under water-saturated conditions. The protolith granitecontains only 7 vol.% biotite and estimated contact temperaturesof 900–950°C suggest that melting was possible ina dry system. Granite partial melting, under closed conditions,extended tens of meters away from the dolerite sill, yet meltsegregation occurred only over less than one-half a meter fromthe dolerite chilled margin where the degree of partial meltingwas of the order of 50 vol.%. This segregation distance is consistentwith calculated length scales expected in a compaction-drivenprocess. We suggest that the driving force for compaction wasdifferential stress generated by a combination of volume expansionas a result of granite partial melting, contraction during doleritesolidification, and relaxation of the overpressure driving doleriteemplacement. On a purely chemical basis, the extent of meltsegregation necessary under fractional and batch melting tomatch the Rb concentrations between melt and parent rock isa maximum of 48 and 83 vol.% melt, respectively. KEY WORDS: Antarctica; dike injection; disequilibrium; granite partial melting; silicic melt segregation  相似文献   

Geothermometric and isotopic studies of dehydration waters: implications for thermal conditions in the Central Carpathian Synclinorium,SE Poland     

Adam Porowski 《Environmental Earth Sciences》2015,74(12):7539-7553

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Unveiling the atmospheres of giant exoplanets with an EChO-class mission     

Vivien Parmentier  Adam P. Showman  Julien de Wit 《Experimental Astronomy》2015,40(2-3):481-500

More than a thousand exoplanets have been discovered over the last decade. Perhaps more excitingly, probing their atmospheres has become possible. With current data we have glimpsed the diversity of exoplanet atmospheres that will be revealed over the coming decade. However, numerous questions concerning their chemical composition, thermal structure, and atmospheric dynamics remain to be answered. More observations of higher quality are needed. In the next years, the selection of a space-based mission dedicated to the spectroscopic characterization of exoplanets would revolutionize our understanding of the physics of planetary atmospheres. Such a mission was proposed to the ESA cosmic vision program in 2014. Our paper is therefore based on the planned capabilities of the Exoplanet Characterization Observatory (EChO), but it should equally apply to any future mission with similar characteristics. With its large spectral coverage (0.4 ? 16 μm), high spectral resolution (λ/Δλ > 300 below 5 μm and λ/Δλ > 30 above 5 μm) and 1.5m mirror, a future mission such as EChO will provide spectrally resolved transit lightcurves, secondary eclipses lightcurves, and full phase curves of numerous exoplanets with an unprecedented signal-to-noise ratio. In this paper, we review some of today’s main scientific questions about gas giant exoplanets atmospheres, for which a future mission such as EChO will bring a decisive contribution.  相似文献   

Predicting June Mean Rainfall in the Middle/Lower Yangtze River Basin     

Gill M. MARTIN  Nick J. DUNSTONE  Adam A. SCAIFE  Philip E. BETT 《大气科学进展》2020,37(1):29-41

We demonstrate that there is significant skill in the GloSea5 operational seasonal forecasting system for predicting June mean rainfall in the middle/lower Yangtze River basin up to four months in advance.Much of the rainfall in this region during June is contributed by the mei-yu rain band.We find that similar skill exists for predicting the East Asian summer monsoon index(EASMI)on monthly time scales,and that the latter could be used as a proxy to predict the regional rainfall.However,there appears to be little to be gained from using the predicted EASMI as a proxy for regional rainfall on monthly time scales compared with predicting the rainfall directly.Although interannual variability of the June mean rainfall is affected by synoptic and intraseasonal variations,which may be inherently unpredictable on the seasonal forecasting time scale,the major influence of equatorial Pacific sea surface temperatures from the preceding winter on the June mean rainfall is captured by the model through their influence on the western North Pacific subtropical high.The ability to predict the June mean rainfall in the middle and lower Yangtze River basin at a lead time of up to 4 months suggests the potential for providing early information to contingency planners on the availability of water during the summer season.  相似文献   

Real-time multi-model decadal climate predictions   总被引：1，自引：1，他引：0  

Doug M. Smith  Adam A. Scaife  George J. Boer  Mihaela Caian  Francisco J. Doblas-Reyes  Virginie Guemas  Ed Hawkins  Wilco Hazeleger  Leon Hermanson  Chun Kit Ho  Masayoshi Ishii  Viatcheslav Kharin  Masahide Kimoto  Ben Kirtman  Judith Lean  Daniela Matei  William J. Merryfield  Wolfgang A. Müller  Holger Pohlmann  Anthony Rosati  Bert Wouters  Klaus Wyser 《Climate Dynamics》2013,41(11-12):2875-2888

We present the first climate prediction of the coming decade made with multiple models, initialized with prior observations. This prediction accrues from an international activity to exchange decadal predictions in near real-time, in order to assess differences and similarities, provide a consensus view to prevent over-confidence in forecasts from any single model, and establish current collective capability. We stress that the forecast is experimental, since the skill of the multi-model system is as yet unknown. Nevertheless, the forecast systems used here are based on models that have undergone rigorous evaluation and individually have been evaluated for forecast skill. Moreover, it is important to publish forecasts to enable open evaluation, and to provide a focus on climate change in the coming decade. Initialized forecasts of the year 2011 agree well with observations, with a pattern correlation of 0.62 compared to 0.31 for uninitialized projections. In particular, the forecast correctly predicted La Niña in the Pacific, and warm conditions in the north Atlantic and USA. A similar pattern is predicted for 2012 but with a weaker La Niña. Indices of Atlantic multi-decadal variability and Pacific decadal variability show no signal beyond climatology after 2015, while temperature in the Niño3 region is predicted to warm slightly by about 0.5 °C over the coming decade. However, uncertainties are large for individual years and initialization has little impact beyond the first 4 years in most regions. Relative to uninitialized forecasts, initialized forecasts are significantly warmer in the north Atlantic sub-polar gyre and cooler in the north Pacific throughout the decade. They are also significantly cooler in the global average and over most land and ocean regions out to several years ahead. However, in the absence of volcanic eruptions, global temperature is predicted to continue to rise, with each year from 2013 onwards having a 50 % chance of exceeding the current observed record. Verification of these forecasts will provide an important opportunity to test the performance of models and our understanding and knowledge of the drivers of climate change.  相似文献   

Lost in translation? Interpretations of the probability phrases used by the Intergovernmental Panel on Climate Change in China and the UK     

Adam J. L. Harris  Adam Corner  Juemin Xu  Xiufang Du 《Climatic change》2013,121(2):415-425

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Convective-conductive transitions and sensitivity of a convecting ice shell to perturbations in heat flux and tidal-heating rate: Implications for Europa     

Giuseppe Mitri  Adam P. Showman 《Icarus》2005,177(2):447-460

We investigate the response of conductive and convective ice shells on Europa to variations of heat flux and interior tidal-heating rate. We present numerical simulations of convection in Europa's ice shell with Newtonian, temperature-dependent viscosity and tidal heating. Modest variations in the heat flux supplied to the base of a convective ice shell, ΔF, can cause large variations of the ice-shell thickness Δδ. In contrast, for a conductive ice shell, large ΔF involves relatively small Δδ. We demonstrate that, for a fluid with temperature-dependent viscosity, the heat flux undergoes a finite-amplitude jump at the critical Rayleigh number Racr. This jump implies that, for a range of heat fluxes relevant to Europa, two equilibrium states—corresponding to a thin, conductive shell and a thick, convective shell—exist for a given heat flux. We show that, as a result, modest variations in heat flux near the critical Rayleigh number can force the ice shell to switch between the thin, conductive and thick, convective configurations over a ∼10⁷-year interval, with thickness changes of up to ∼10-30 km. Depending on the orbital and thermal history, such switches might occur repeatedly. However, existing evolution models based on parameterized-convection schemes have to date not allowed these transitions to occur. Rapid thickening of the ice shell would cause radial expansion of Europa, which could produce extensional tectonic features such as fractures or bands. Furthermore, based on interpretations for how features such as chaos and ridges are formed, several authors have suggested that Europa's ice shell has recently undergone changes in thickness. Our model provides a mechanism for such changes to occur.  相似文献   

A 2dF survey of the Small Magellanic Cloud     

Christopher J. Evans  Ian D. Howarth  Michael J. Irwin  Adam W. Burnley  Timothy J. Harries 《Monthly notices of the Royal Astronomical Society》2004,353(2):601-623

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