Deciphering the origin of the regular satellites of gaseous giants - Iapetus: The Rosetta ice-moon

Ever since their discovery the regular satellites of Jupiter and Saturn have held out the promise of providing an independent set of observations with which to test theories of planet formation. Yet elucidating their origins has proven elusive. Here we show that Iapetus can serve to discriminate between satellite formation models. Its accretion history can be understood in terms of a two-component gaseous subnebula, with a relatively dense inner region, and an extended tail out to the location of the irregular satellites, as in the SEMM model of Mosqueira and Estrada (2003a,b) (Mosqueira, I., Estrada, P.R. [2003a]. Icarus 163, 198-231; Mosqueira, I., Estrada, P.R. [2003b]. Icarus 163, 232-255). Following giant planet formation, planetesimals in the feeding zone of Jupiter and Saturn become dynamically excited, and undergo a collisional cascade. Ablation and capture of planetesimal fragments crossing the gaseous circumplanetary disks delivers enough collisional rubble to account for the mass budgets of the regular satellites of Jupiter and Saturn. This process can result in rock/ice fractionation as long as the make up of the population of disk crossers is non-homogeneous, thus offering a natural explanation for the marked compositional differences between outer solar nebula objects and those that accreted in the subnebulae of the giant planets. For a given size, icy objects are easier to capture and to ablate, likely resulting in an overall enrichment of ice in the subnebula. Furthermore, capture and ablation of rocky fragments become inefficient far from the planet for two reasons: the gas surface density of the subnebula is taken to drop outside the centrifugal radius, and the velocity of interlopers decreases with distance from the planet. Thus, rocky objects crossing the outer disks of Jupiter and Saturn never reach a temperature high enough to ablate either due to melting or vaporization, and capture is also greatly diminished there. In contrast, icy objects crossing the outer disks of each planet ablate due to the melting and vaporization of water-ice. Consequently, our model leads to an enhancement of the ice content of Iapetus, and to a lesser degree those of Titan, Callisto and Ganymede, and accounts for the (non-stochastic) compositions of these large, low-porosity outer regular satellites of Jupiter and Saturn. For this to work, the primordial population of planetesimals in the Jupiter-Saturn region must be partially differentiated, so that the ensuing collisional cascade produces an icy population of ?1 m size fragments to be ablated during subnebula crossing. We argue this is likely because the first generation of solar nebula ∼10 km planetesimals in the Jupiter-Saturn region incorporated significant quantities of ²⁶Al. This is the first study successfully to provide a direct connection between nebula planetesimals and subnebulae mixtures with quantifiable and observable consequences for the bulk properties of the regular satellites of Jupiter and Saturn, and the only explanation presently available for Iapetus’ low density and ice-rich composition.     

Izu-Bonin arc subduction under the Honshu island, Japan: Evidence from geological and seismological aspect

The Izu-Bonin intra-oceanic arc with 20–35 km thick continental crust is being subducted under the Honshu, presumably since 17 Ma. Tomographic image clearly demonstrates that the whole Izu-Bonin arc is subducting under the Honshu arc. Geologic cross section and the thickness of continental crust do not support the accretion of thick crust in spite of the continued subduction over 17 Ma.     

http://dx.doi.org/10.1016/j.gsf.2016.07.001

In the tandem planet formation regime,planets form at two distinct sites where solid particles are densely accumulated due to the on/off state of the magnetorotational instability(MRI).We found that tandem planet formation can reproduce the solid component distribution of the Solar System and tends to produce a smaller number of large planets through continuous pebble flow into the planet formation sites.In the present paper,we investigate the dependence of tandem planet formation on the vertical magnetic field of the protoplanetary disk.We calculated two cases of B_Z 3.4 × 10~(-3) G and B_Z = 3.4 × 10~(-5) G at 100 AU as well as the canonical case of B_Z = 3.4 × 10~(-4) G.We found that tandem planet formation holds up well in the case of the strong magnetic field(B_Z 3.4 × 10~(-3) G).On the other hand,in the case of a weak magnetic field(B_Z= 3.4 × 10~(-5) G) at 100 AU,a new regime of planetary growth is realized:the planets grow independently at different places in the dispersed area of the MRl-suppressed region of r-8-30 AU at a lower accretion rate of M 10~(-7.4)M_⊙yr~(-1).We call this the "dispersed planet formation" regime.This may lead to a system with a larger number of smaller planets that gain high eccentricity through mutual collisions.     

Some general relativistic effects on the dynamics of fluid disks rotating around a Schwarzschild black hole

In this paper we present various classes of solutions for perfect fluid disks rotating around Schwarzschild black holes. We study the profiles of pressure, density and specific angular momentum and the formation of cusp-like structures at the inner edge of the disks. Using the trial function method, we calculate the frequency of the global axi-symmetric oscillations. We compare the results with those of the corresponding Newtonian calculations to find the general relativistic effects.     

Spatial and seasonal aspects of beach stability

Recent work at three contrasting sites in England and Wales has shown characteristics atypical of those frequently reported elsewhere. These differences are:

• (a) Taking each entire beach system there is no uniform trend of erosion or accretion, nor a progressive variation in beach elevation or volume alongshore, from one survey to the next. However, for Swansea Bay the ‘long-term’ (i. e. 18 months) range in profile height along that stretch of coast where the alignment of the beach is normal to the direction of wave approach, correlates well with computed wave energy derived from relevant offshore wave directions.
• (b) While beach variability is greatest during the ‘winter’ (i. e. storm) period there is no overall tendency for a drawdown of sediment from the intertidal zone at that time. Response times are relatively short. Thus high beach levels need not necessarily be associated with ‘summer’ conditions.
• (c) Although in Swansea Bay there is a tendency for the beach height to fluctuate least at mid-tide level this is not true of the other two sites. In no area does sediment eroded from the upper exposed part of the beach regularly appear to be deposited on the lower exposed part, or vice versa.

     

Sediment accretion against a buttress beneath the Peruvian continental margin at 12° S as simulated with sandbox modeling

Reflection seismic data from the Peruvian continental margin at 12° S clearly reveal an accretionary wedge and buttress. Sandbox experiments applying the physical concept of the Coulomb theory allow the systematic investigation of the growth and deformation of such an accretionary structure. The style of deformation of the buttress and the internal structure of the wedge is observed in the sandbox models. The possibility of underplating material beneath the buttress and the amount of tectonic erosion depend on the physical properties of the materials, mainly internal friction, cohesion and basal friction. Boundary conditions such as the height of the subduction gate and the thickness of incoming sand also constrain the style of growth of the model accretionary structure.The configurations of two experiments were closely scaled to reflection seismic depth sections across the Peruvian margin. A deformable buttress constructed of compacted rock powder is introduced to replicate the basement rock which allows deformation similar to that in the seismic data. With the sandbox models it is possible to verify a proposed accretionary history derived from seismic and borehole data. The models also help in understanding the mechanisms which control the amount of accretion, subduction and underplating as a function of physical properties, boundary conditions and the duration of convergence.     

Seasonal changes in surface level of a salt marsh creek

Nineteen surveys were carried out over a two-year period to determine the surface height of a salt marsh creek located on the north side of the River Esk, Cumbria, England. An AGA 112 electromagnetic distance measurer mounted on a Wild TI theodolite was used in conjunction with acrylic reflectors to follow the form of the ground. The results showed that there were no significant net trends in surface level, although seasonal variations of the order of 2 cm occurred. Vegetated areas responded in similar fashion to bare surfaces. It is thought that the elevational changes were attributable to the swelling of clay particles during the winter months rather than the effects of erosion and accretion.     

The Sino-Korean Craton and supercontinent history: Problems and perspectives

The Sino-Korean Craton (SKC) is an enigmatic block in the history of supercontinents older than Pangea. Its accretion to Eurasia and the effect of the broad region of Mesozoic–Cenozoic extension in northern Eurasia that crosses the eastern part of the SKC are among several problems that need to be resolved in understanding the configuration of SKC and the overprinting of earlier histories in this block. We present a synopsis of these problems and perspectives for future research.