Phreatomagmatic volcanic hazards where rift-systems meet the sea, a study from Ambae Island, Vanuatu

Uncorrected proofAmbae Island is a mafic stratovolcano located in the northern Vanuatu volcanic arc and has a NE–SW rift-controlled elongated shape. Several hundred scoria cones and fissure-fed lava fields occur along its long axis. After many decades of quiescence, Ambae Island erupted on the 28th of November 2005, disrupting the lives of its 10,000 inhabitants. Its activity remained focused at the central (crater-lake filled) vent and this is where hazard-assessments were focused. These assessments initially neglected that maars, tephra cones and rings occur at each tip of the island where the eruptive activity occurred b500 and b300 yr B.P. The products of this explosive phreatomagmatic activity are located where the rift axis meets the sea. At the NE edge of the island five tephra rings occur, each comparable in size to those on the summit of Ambae. Along the NE coastline, a near-continuous cliff section exposes an up to 25 m thick succession of near-vent phreatomagmatic tephra units derived from closely spaced vents. This can be subdivided into two major lithofacies associations. The first association represents when the locus of explosions was below sea level and comprises matrix-supported, massive to weakly stratified beds of coarse ash and lapilli. These are dominant in the lowermost part of the sequence and commonly contain coral fragments, indicating that the loci of explosion were located within a reef or coral sediment near the syn-eruptive shoreline. The second type indicate more stable vent conditions and rapidly repeating explosions of high intensity, producing fine-grained tephra with undulatory bedding and cross-lamination as well as megaripple bedforms. These surge and fall beds are more common in the uppermost part of the succession and form a few-m-thick pile. An older tephra succession of similar character occurs below, and buried trees in growth position, as well as those flattened within base surge beds. This implies that the centre of this eruption was very near the coastline. The processes implied by these deposits are amongst the most violent forms of volcanism on this island. In addition, the lowland and coastal areas affected by these events are the most heavily populated. This circumstance is mirrored on many similar volcanic islands, including the nearby SW Pacific examples of Taveuni (Fiji), Upolu and Savai'i (Samoa), and Ambrym (Vanuatu). These locations are paradoxically often considered safe areas during summit/central-vent eruptions, simply because they are farthest from the 34 central sources of ash-fall and lahar hazard. The observations presented here necessitate a revision of this view

Human-chimpanzee alignment: Ortholog Exponentials and Paralog Power Laws

Genomic subsequences conserved between closely related species such as human and chimpanzee exhibit an exponential length distribution, in contrast to the algebraic length distribution observed for sequences shared between distantly related genomes. We find that the former exponential can be further decomposed into an exponential component primarily composed of orthologous sequences, and a truncated algebraic component primarily composed of paralogous sequences.Comment: Main text: 31 pages, 13 figures, 1 table; Supplementary materials: 9 pages, 9 figures, 1 tabl

A hypothesis for the color bimodality of Jupiter Trojans

One of the most enigmatic and hitherto unexplained properties of Jupiter Trojans is their bimodal color distribution. This bimodality is indicative of two sub-populations within the Trojans, which have distinct size distributions. In this paper, we present a simple, plausible hypothesis for the origin and evolution of the two Trojan color sub-populations. In the framework of dynamical instability models of early Solar System evolution, which suggest a common primordial progenitor population for both Trojans and Kuiper belt objects, we use observational constraints to assert that the color bimodalities evident in both minor body populations developed within the primordial population prior to the onset of instability. We show that, beginning with an initial composition of rock and ices, location-dependent volatile loss through sublimation in this primordial population could have led to sharp changes in the surface composition with heliocentric distance. We propose that the depletion or retention of H$_{2}$S ice on the surface of these objects was the key factor in creating an initial color bimodality. Objects that retained H$_{2}$S on their surfaces developed characteristically redder colors upon irradiation than those that did not. After the bodies from the primordial population were scattered and emplaced into their current positions, they preserved this primordial color bimodality to the present day. We explore predictions of the volatile loss model - in particular, the effect of collisions within the Trojan population on the size distributions of the two sub-populations - and propose further experimental and observational tests of our hypothesisComment: 8 pages, 3 figures, accepted for publication in A

Signatures of massive collisions in debris discs

Violent stochastic collisional events have been invoked as a possible explanation for some debris discs displaying pronounced asymmetries or having a great luminosity excess. So far, no thorough modelling of the consequences of such events has been carried out, mainly because of the extreme numerical challenge of coupling the dynamical and collisional evolution of dust. We perform the first fully self-consistent modelling of the aftermath of massive breakups in debris discs. We follow the collisional and dynamical evolution of dust released after the breakup of a Ceres-sized body at 6 AU from its central star. We investigate the duration, magnitude and spatial structure of the signature left by such a violent event, as well as its observational detectability. We use the recently developed LIDT-DD code (Kral et al., 2013), which handles the coupled collisional and dynamical evolution of debris discs. The main focus is placed on the complex interplay between destructive collisions, Keplerian dynamics and radiation pressure forces. We use the GRaTer package to estimate the system's luminosity at different wavelengths. The breakup of a Ceres-sized body at 6 AU creates an asymmetric dust disc that is homogenized, by the coupled action of collisions and dynamics, on a timescale of a few $10^5$ years. The luminosity excess in the breakup's aftermath should be detectable by mid-IR photometry, from a 30 pc distance, over a period of $\sim 10^6$ years that exceeds the duration of the asymmetric phase of the disc (a few $10^5$ years). As for the asymmetric structures, we derive synthetic images for the SPHERE/VLT and MIRI/JWST instruments, showing that they should be clearly visible and resolved from a 10 pc distance. Images at 1.6$\mu$m (marginally), 11.4 and 15.5$\mu$m would show the inner disc structures while 23$\mu$m images would display the outer disc asymmetries.Comment: 16 pages, 14 figures, abstract shortened, accepted for publication in A&

Origin of the Near-Ecliptic Circumsolar Dust Band

The zodiacal dust bands are bright infrared (IR) strips produced by thermal emission from circumsolar rings of particles. Two of the three principal dust bands, known as β and γ, were previously linked to the recent asteroid collisions that produced groups of fragments, so-called asteroid families, near the orbits of (832) Karin and (490) Veritas. The origin of the third, near-ecliptic α band has been unknown until now. Here we report the discovery of a recent breakup of a >20 km diameter asteroid near α's originally suspected source location in the Themis family. Numerical modeling and observations of the α-band thermal emission from the Spitzer Space Telescope indicate that the discovered breakup is the source of α-band particles. The recent formation of all principal dust bands implies a significant time variability of the circumstellar debris disks

Tidal Downsizing model. I. Numerical methods: saving giant planets from tidal disruptions

Tidal Downsizing (TD) is a recently developed planet formation theory that supplements the classical Gravitational disc Instability (GI) model with planet migration inward and tidal disruptions of GI fragments in the inner regions of the disc. Numerical methods for a detailed population synthesis of TD planets are presented here. As an example application, the conditions under which GI fragments collapse faster than they migrate into the inner $a\sim$ few AU disc are considered. It is found that most gas fragments are tidally or thermally disrupted unless (a) their opacity is $\sim 3$ orders of magnitude less than the interstellar dust opacity at metallicities typical of the observed giant planets, or (b) the opacity is high but the fragments accrete large dust grains (pebbles) from the disc. Case (a) models produce very low mass solid cores ($M_{\rm core} < 0.1$ Earth masses) and follow a negative correlation of giant planet frequency with host star metallicity. In contrast, case (b) models produce massive solid cores, correlate positively with host metallicity and explain naturally while giant gas planets are over-abundant in metals.Comment: Submitted to MNRAS November 19 2014. Comments welcom

A new code to study structures in collisionally active, perturbed debris discs. Application to binaries

Debris discs are traditionally studied using two distinct types of numerical models: statistical particle-in-a-box codes to study their collisional and size distribution evolution, and dynamical N-body models to study their spatial structure. The absence of collisions from N-body codes is in particular a major shortcoming, as collisional processes are expected to significantly alter the results obtained from pure N-body runs. We present a new numerical model, to study the spatial structure of perturbed debris discs at dynamical and collisional steady-state. We focus on the competing effects between gravitational perturbations by a massive body (planet or star), collisional production of small grains, and radiation pressure placing these grains in possibly dynamically unstable regions. We consider a disc of parent bodies at dynamical steady-state, from which small radiation-pressure-affected grains are released in a series of runs, each corresponding to a different orbital position of the perturber, where particles are assigned a collisional destruction probability. These collisional runs produce successive position maps that are then recombined, following a complex procedure, to produce surface density profiles for each orbital position of the perturbing body. We apply our code to the case of a circumprimary disc in a binary. We find pronounced structures inside and outside the dynamical stability regions. For low $e_B$, the disc's structure is time varying, with spiral arms in the dynamically "forbidden" region precessing with the companion star. For high $e_B$, the disc is strongly asymmetric but time invariant, with a pronounced density drop in the binary's periastron direction. (better resolution version of the paper at http://lesia.obspm.fr/perso/philippe-thebault/theb2011.pdf)Comment: accepted for publication in Astronomy and Astrophysics /// There is a problem with the way Fig.1 looks on the astro-ph file. You can retrieve a correct version of the full paper at http://lesia.obspm.fr/perso/philippe-thebault/theb2011.pd

Plate motions recorded in tectonostratigraphic terranes of the Franciscan Complex and evolution of the Mendocino triple junction, northwestern California

The Mendocino triple junction area of northern California is underlain by the Coastal belt of the Franciscan complex, flanked on the east by the Central and Eastern belts of the Franciscan Complex. The coastal belt is further divided into three tectonostratigraphic terranes. Upper Cretaceous through middle Miocene rocks included in these terranes were accreted to the North American plate margin partly during normal convergence with the Farallon plate between 49 and 25 Ma at poleward rates of 2 to 5cm /yr, and partly during translation with the Pacific plate between 14 and 2 Ma at poleward rates of 3 to 6cm/yr. The evolution of the triple junction is discussed. -from Author

Planet Signatures in Collisionally Active Debris Discs: scattered light images

Planet perturbations are often invoked as a potential explanation for many spatial structures that have been imaged in debris discs. So far this issue has been mostly investigated with collisionless N-body numerical models. We numerically investigate how the coupled effect of collisions and radiation pressure can affect the formation and survival of radial and azimutal structures in a disc perturbed by a planet. We consider two set-ups: a planet embedded within an extended disc and a planet exterior to an inner debris ring. We use the DyCoSS code of Thebault(2012) and derive synthetic images of the system in scattered light. The planet's mass and orbit, as well as the disc's collisional activity are explored as free parameters. We find that collisions always significantly damp planet-induced structures. For the case of an embedded planet, the planet's signature, mostly a density gap around its radial position, should remain detectable in head-on images if M_planet > M_Saturn. If the system is seen edge-on, however, inferring the presence of the planet is much more difficult, although some planet-induced signatures might be observable under favourable conditions. For the inner-ring/external-planet case, planetary perturbations cannot prevent collision-produced small fragments from populating the regions beyond the ring: The radial luminosity profile exterior to the ring is close to the one it should have in the absence of the planet. However, a Jovian planet on a circular orbit leaves precessing azimutal structures that can be used to indirectly infer its presence. For a planet on an eccentric orbit, the ring is elliptic and the pericentre glow effect is visible despite of collisions and radiation pressure, but detecting such features in real discs is not an unambiguous indicator of the presence of an outer planet.Comment: Accepted for Publication in A&A (NOTE: Abridged abstract and (very)LowRes Figures. Better version, with High Res figures and full abstract can be found at http://lesia.obspm.fr/perso/philippe-thebault/planpapph.pdf