The orbital-thermal evolution and global expansion of Ganymede     

Michael T. Bland  Adam P. Showman 《Icarus》2009,200(1):207-221

The tectonically and cryovolcanically resurfaced terrains of Ganymede attest to the satellite's turbulent geologic history. Yet, the ultimate cause of its geologic violence remains unknown. One plausible scenario suggests that the Galilean satellites passed through one or more Laplace-like resonances before evolving into the current Laplace resonance. Passage through such a resonance can excite Ganymede's eccentricity, leading to tidal dissipation within the ice shell. To evaluate the effects of resonance passage on Ganymede's thermal history we model the coupled orbital-thermal evolution of Ganymede both with and without passage through a Laplace-like resonance. In the absence of tidal dissipation, radiogenic heating alone is capable of creating large internal oceans within Ganymede if the ice grain size is 1 mm or greater. For larger grain sizes, oceans will exist into the present epoch. The inclusion of tidal dissipation significantly alters Ganymede's thermal history, and for some parameters (e.g. ice grain size, tidal Q of Jupiter) a thin ice shell (5 to 20 km) can be maintained throughout the period of resonance passage. The pulse of tidal heating that accompanies Laplace-like resonance capture can cause up to 2.5% volumetric expansion of the satellite and contemporaneous formation of near surface partial melt. The presence of a thin ice shell and high satellite orbital eccentricity would generate moderate diurnal tidal stresses in Ganymede's ice shell. Larger stresses result if the ice shell rotates non-synchronously. The combined effects of satellite expansion, its associated tensile stress, rapid formation of near surface partial melt, and tidal stress due to an eccentric orbit may be responsible for creating Ganymede's unique surface features.  相似文献   

The XMM Cluster Survey: forecasting cosmological and cluster scaling-relation parameter constraints     

Martin Sahlén  Pedro T. P. Viana  rew R. Liddle  A. Kathy Romer  Michael Davidson  Mark Hosmer  Ed Lloyd-Davies  Kivanc Sabirli  Chris A. Collins  Peter E. Freeman  Matt Hilton  Ben Hoyle  Scott T. Kay  Robert G. Mann  Nicola Mehrtens  Christopher J. Miller  Robert C. Nichol  S. Adam Stanford  Michael J. West  † 《Monthly notices of the Royal Astronomical Society》2009,397(2):577-607

We forecast the constraints on the values of  σ₈, Ω_m  and cluster scaling-relation parameters which we expect to obtain from the XMM Cluster Survey (XCS). We assume a flat Λ cold dark matter Universe and perform a Monte Carlo Markov Chain analysis of the evolution of the number density of galaxy clusters that takes into account a detailed simulated selection function. Comparing our current observed number of clusters shows good agreement with predictions. We determine the expected degradation of the constraints as a result of self-calibrating the luminosity–temperature relation (with scatter), including temperature measurement errors, and relying on photometric methods for the estimation of galaxy cluster redshifts. We examine the effects of systematic errors in scaling relation and measurement error assumptions. Using only  ( T , z )  self-calibration, we expect to measure Ω_m to ±0.03 (and  Ω_Λ  to the same accuracy assuming flatness), and σ₈ to ±0.05, also constraining the normalization and slope of the luminosity–temperature relation to ±6 and ±13 per cent (at 1σ), respectively, in the process. Self-calibration fails to jointly constrain the scatter and redshift evolution of the luminosity–temperature relation significantly. Additional archival and/or follow-up data will improve on this. We do not expect measurement errors or imperfect knowledge of their distribution to degrade constraints significantly. Scaling-relation systematics can easily lead to cosmological constraints 2σ or more away from the fiducial model. Our treatment is the first exact treatment to this level of detail, and introduces a new 'smoothed ML' (Maximum Likelihood) estimate of expected constraints.  相似文献   

Matter temperature during cosmological recombination     

Douglas Scott  Adam Moss 《Monthly notices of the Royal Astronomical Society》2009,397(1):445-446

The temperature of the atomic matter in the Universe is held to that of the cosmic background radiation until decoupling at   z ∼ 100  . After this, it cools faster than the radiation [  ∝ (1 + z )²  rather than  (1 + z )  ] and would have fallen to about 20 mK today if astrophysical feedback processes had not heated up the interglactic medium. We show how the derivative of the Compton coupling equation helps numerically to follow the decoupling process.  相似文献   

Recent arrival of faint cluster galaxies on the red sequence: luminosity functions from 119 deg² of CFHTLS     

Ting Lu  David G. Gilbank  Michael L. Balogh  Adam Bognat 《Monthly notices of the Royal Astronomical Society》2009,399(4):1858-1876

The global star formation rate has decreased significantly since   z ∼ 1  , for reasons that are not well understood. Red-sequence galaxies, dominating in galaxy clusters, represent the population that have had their star formation shut off, and may therefore be the key to this problem. In this work, we select 127 rich galaxy clusters at  0.17 ≤ z ≤ 0.36  , from 119 deg² of the Canada–France–Hawaii Telescope Legacy Survey (CFHTLS) optical imaging data, and construct the r '-band red-sequence luminosity functions (LFs). We show that the faint end of the LF is very sensitive to how red-sequence galaxies are selected, and an optimal way to minimize the contamination from the blue cloud is to mirror galaxies on the redder side of the colour–magnitude relation. The LFs of our sample have a significant inflexion centred at     , suggesting a mixture of two populations. Combining our survey with low-redshift samples constructed from the Sloan Digital Sky Survey, we show that there is no strong evolution of the faint end of the LF (or the red-sequence dwarf-to-giant ratio) over the redshift range  0.2 ≲ z ≲ 0.4  , but from   z ∼ 0.2  to ∼0 the relative number of red-sequence dwarf galaxies has increased by a factor of ∼3, implying a significant build-up of the faint end of the cluster red sequence over the last 2.5 Gyr.  相似文献   

Effects of a large convective storm on Saturn's equatorial jet     

Kunio M. Sayanagi  Adam P. Showman 《Icarus》2007,187(2):520-539

Hubble Space Telescope observations revealed that Saturn's equatorial jet at the cloud level blows at ∼275 m s⁻¹ today, approximately half the ∼470 m s⁻¹ wind during the Voyager flybys in 1980-1981. Radiative transfer calculations estimate the clouds to be significantly higher today than in 1980. The higher clouds make it difficult to observationally isolate any true slowdown from the vertical wind shear because Voyager and Cassini observations show that the winds become slower with altitude. Here, we test the hypothesis that the large equatorial storm in 1990 called the Great White Spot (GWS) decelerated the equatorial jet. We first use order of magnitude estimates to show: (1) if the GWS triggers vertical momentum redistribution, a minor speed change in the troposphere can lead to a substantial stratospheric wind speed change; (2) storm-triggered turbulent mixing slows a prograde equatorial jet; and (3) a prograde equatorial jet inhibits turbulent mixing in latitude. To test whether a GWS-like large storm decelerates the equatorial jet, we perform numerical experiments using the Explicit Planetary Isentropic Coordinate (EPIC) atmosphere model. Our simulation results are consistent with our order of magnitude predictions. We show that the storm excites waves, and the waves transport westward momentum from the troposphere to the stratosphere and decelerate the equatorial jet by as much as ∼40 m s⁻¹ at the 10-mbar level. However, our results show that the storm's effect is too weak at the cloud levels to halve the jet's speed from ∼470 m s⁻¹. Our results suggest that a combination of higher clouds and a true slowdown is necessary to explain the apparent equatorial jet slowdown. We also analyze the effect of waves on the apparent cloud motions, and show that waves can influence cloud-tracking wind speed measurements.  相似文献   

Hydrodynamic instability of convectively unstable atmospheres in shear flow     

J. A. Adam 《Astrophysics and Space Science》1977,50(2):493-514

Results are presented concerning the interaction between regions of convectively unstable fluid, bounded above and below by stable fluid, with a basic horizontal flow field, sheared in a vertical direction. The analysis is conveniently based on the definition of the mechanical energy flux associated with wave motion in a stratified compressible fluid, and enables bounds to be placed on the real and complex phase velocities of overstable modes, in addition to some general results on the net upward wave energy flux. It is shown that purely exponentially growing modes (with horizontal wavevectors spanwise to the shear) do not exist. A known sufficient condition for the stability of stable atmospheres is reproduced here with an interesting modification, and details of energy-flux discontinuities at certain singular points of the equations are given. The work is relevant to any astrophysical and geophysical situations in which convectively unstable regions and shear flows are likely to be together present, but the special motivation here is that of describing some aspects of the interaction between supergranular flow and granular convection.  相似文献   

Non-explosive,constructional evolution of the ice-filled caldera at Volcán Sollipulli,Chile     

J. S. Gilbert  M. V. Stasiuk  S. J. Lane  C. R. Adam  M. D. Murphy  R. S. J. Sparks  J. A. Naranjo 《Bulletin of Volcanology》1996,58(1):67-83

 A radar and gravity survey of the ice-filled caldera at Volcán Sollipulli, Chile, indicates that the intra-caldera ice has a thickness of up to 650 m in its central part and that the caldera harbours a minimum of 6 km³ of ice. Reconnaissance geological observations show that the volcano has erupted compositions ranging from olivine basalt to dacite and have identified five distinct volcanic units in the caldera walls. Pre- or syn-caldera collapse deposits (the Sharkfin pyroclastic unit) comprise a sequence which evolved from subglacial to subaerial facies. Post-caldera collapse products, which crop out along 17 of the 20 km length of the caldera wall, were erupted almost exclusively along the caldera margins in the presence of a large body of intra-caldera ice. The Alpehué crater, formed by an explosive eruption between 2960 and 2780 a. BP, in the southwest part of the caldera is shown to post date formation of the caldera. Sollipulli lacks voluminous silicic pyroclastic rocks associated with caldera formation and the collapse structure does not appear to be a consequence of a large-magnitude explosive eruption. Instead, lateral magma movement at depth resulting in emptying of the magma chamber may have generated the caldera. The radar and gravity data show that the central part of the caldera floor is flat but, within a few hundred metres of the caldera walls, the floor has a stepped topography with relatively low-density rock bodies beneath the ice in this region. This, coupled with the fact that most of the post-caldera eruptions have taken place along the caldera walls, implies that the caldera has been substantially modified by subglacial marginal eruptions. Sollipulli caldera has evolved from a collapse to a constructional feature with intra-caldera ice playing a major role. The post-caldera eruptions have resulted in an increase in height of the walls and concomitant deepening of the caldera with time. Received: 12 June 1995 / Accepted: 7 December 1995  相似文献   

A linear scattering problem in magnetohydrodynamics: Transmission resonances in a magnetic slab     

John A. Adam 《Astrophysics and Space Science》1987,133(2):317-337

The reduced linearized equations of ideal magnetohydrodynamics which are highly nonlinear in the eigenvalue parameter, are linearized about a prescribed value of that parameter, enabling the equation to be expressed as a Schrödinger equation with piecewise uniform coefficients. Reflection and transmission coefficients are obtained using standard techniques, and in addition to the possibility of total transmission of an incident wave occurring (together with complex-valued resonance energies), the magnetic field introduces other total transmission energy levels which have no counterpart in the absence of a magnetic field.  相似文献   

Simultaneous inversion for 3-D variations in shear and bulk velocity in the mantle   总被引：7，自引：0，他引：7  

Wei-jia Su  Adam M. Dziewonski 《Physics of the Earth and Planetary Interiors》1997,100(1-4):135-156

Until recently, most of the seismic tomographic modeling has been addressing the question of lateral heterogeneity either in P- or S-wave velocities. The S-wave velocity variations are larger and hence provide stronger signal on long-period waveforms. The direct P travel times, being the first arrivals, on the other hand, are most frequently reported in the International Seismological Centre (ISC) Bulletins. In mineral physics experiments, the variation in bulk velocity is more often measured. To better understand the differences between δv_P and δv_P patterns and better link the results from mineral physics to those of seismic tomography, we formulate the inverse problem in terms of relative perturbations in the shear velocity v_S = (μ/)^1/2 and bulk sound velocity v_Φ = (K/)^1/2. We use a large data set which consists of waveforms, waveform-derived travel times and travel times from the ISC Bulletins. The earthquakes are relocated using corrections for lateral heterogeneity. The events which cannot be reliably determined are discarded. The model is defined as spherical harmonics to Degree 12 horizontally and as Chebyshev polynomials to order 13 radially, for both shear and bulk sound velocity. The inversion is performed under smoothness constraint. The resolution tests and bootstrapping analysis indicate that the model is well recovered, particularly at long wavelength.

The results indicate a much larger variability of shear than bulk sound velocity. The model explains observations well. The most intriguing result obtained in this study is that the variations in shear velocity and bulk sound velocity are negatively correlated in the lowermost mantle. The explanation is not very clear. From the mineral physics point of view, it is not unlikely that this could be explained in terms of thermal variation, even though we are unwilling to rule out the possibility of large wavelength compositional variations.  相似文献   

A note on σ-stability in hydromagnetics     

J. A. Adam 《Astrophysics and Space Science》1982,82(1):115-121

The concept of -stability is reviewed and applied to the case of aligned magnetoatmospheric flow. A sufficient condition for -stability is derived, and the upper bound arising from this condition is investigated as a function of .  相似文献