968 research outputs found
Collisional stripping of planetary crusts
Geochemical studies of planetary accretion and evolution have invoked various
degrees of collisional erosion to explain differences in bulk composition
between planets and chondrites. Here we undertake a full, dynamical evaluation
of 'crustal stripping' during accretion and its key geochemical consequences.
We present smoothed particle hydrodynamics simulations of collisions between
differentiated rocky planetesimals and planetary embryos. We find that the
crust is preferentially lost relative to the mantle during impacts, and we have
developed a scaling law that approximates the mass of crust that remains in the
largest remnant. Using this scaling law and a recent set of N-body simulations,
we have estimated the maximum effect of crustal stripping on incompatible
element abundances during the accretion of planetary embryos. We find that on
average one third of the initial crust is stripped from embryos as they
accrete, which leads to a reduction of ~20% in the budgets of the heat
producing elements if the stripped crust does not reaccrete. Erosion of crusts
can lead to non-chondritic ratios of incompatible elements, but the magnitude
of this effect depends sensitively on the details of the crust-forming melting
process. The Lu/Hf system is fractionated for a wide range of crustal formation
scenarios. Using eucrites (the products of planetesimal silicate melting,
thought to represent the crust of Vesta) as a guide to the Lu/Hf of
planetesimal crust partially lost during accretion, we predict the Earth could
evolve to a superchondritic 176-Hf/177-Hf (3-5 parts per ten thousand) at
present day. Such values are in keeping with compositional estimates of the
bulk Earth. Stripping of planetary crusts during accretion can lead to
detectable changes in bulk composition of lithophile elements, but the
fractionation is relatively subtle, and sensitive to the efficiency of
reaccretion.Comment: 15 pages, 9 figures. Accepted for publication in EPSL. Abstract
shortened. Accompanying animations can be found at
http://www.star.bris.ac.uk/pcarter/crust_strip
Did Earth eat its leftovers? Impact ejecta as a component of the late veneer
The presence of highly siderophile elements in Earth's mantle indicates that
a small percentage of Earth's mass was delivered after the last giant impact in
a stage of 'late accretion.' There is ongoing debate about the nature of
late-accreted material and the sizes of late-accreted bodies. Earth appears
isotopically most similar to enstatite chondrites and achondrites. It has been
suggested that late accretion must have been dominated by enstatite-like bodies
that originated in the inner disk, rather than ordinary or carbonaceous
chondrites. Here, we examine the provenances of 'leftover' planetesimals
present in the inner disk in the late stages of accretion simulations.
Dynamically excited planet formation produces planets and embryos with similar
provenances, suggesting that the Moon-forming impactor may have had a stable
isotope composition very similar to the proto-Earth. Commonly, some
planetesimal-sized bodies with similar provenances to the Earth-like planets
are left at the end of the main stage of growth. The most chemically-similar
planetesimals are typically fragments of proto-planets ejected millions of
years earlier. If these similar-provenance bodies are later accreted by the
planet, they will represent late-accreted mass that naturally matches Earth's
composition. The planetesimal-sized bodies that exist during the giant impact
phase can have large core mass fractions, with core provenances similar to the
proto-Earth. These bodies are an important potential source for highly
siderophile elements. The range of core fractions in leftover planetesimals
complicates simple inferences as to the mass and origin of late accretion based
on the highly siderophile elements in the mantle.Comment: 20 pages, 11 figures. To be published in PSJ. See
http://doi.org/10.5281/zenodo.6380403 for an executable version of this pape
Post-Giant Impact Planetesimals Sustaining Extreme Debris Disks
Extreme debris disks can show short term behaviour through the evolution and
clearing of small grains produced in giant impacts, and potentially a longer
period of variability caused by a planetesimal population formed from giant
impact ejecta. In this paper, we present results of numerical simulations to
explain how a planetesimal populated disk can supply an observed extreme debris
disk with small grains. We simulated a sample of giant impacts from which we
form a planetesimal population. We then use the -body code {\sc Rebound} to
evolve the planetesimals spatially and collisionally. We adopt a simplistic
collision criteria in which we define destructive collisions to be between
planetesimals with a mutual impact velocity that exceeds two times the
catastrophic disruption threshold, . We find that for some configurations,
a planetesimal populated disk can produce a substantial amount of dust to
sustain an observable disk. The semi-major axis at which the giant impact
occurs changes the mass added to the observed disk substantially while the
orientation of the impact has less of an effect. We determine how the collision
rate at the collision point changes over time and show that changes in
semi-major axis and orientation only change the initial collision rate of the
disk. Collision rates across all disks evolve at a similar rate.Comment: 20 pages, 21 figures, 3 tables, accepted by MNRA
Hiding in the Shadows II: Collisional Dust as Exoplanet Markers
Observations of the youngest planets (1-10 Myr for a transitional disk)
will increase the accuracy of our planet formation models. Unfortunately,
observations of such planets are challenging and time-consuming to undertake
even in ideal circumstances. Therefore, we propose the determination of a set
of markers that can pre-select promising exoplanet-hosting candidate disks. To
this end, N-body simulations were conducted to investigate the effect of an
embedded Jupiter mass planet on the dynamics of the surrounding planetesimal
disk and the resulting creation of second generation collisional dust. We use a
new collision model that allows fragmentation and erosion of planetesimals, and
dust-sized fragments are simulated in a post process step including
non-gravitational forces due to stellar radiation and a gaseous protoplanetary
disk. Synthetic images from our numerical simulations show a bright double ring
at 850 m for a low eccentricity planet, whereas a high eccentricity planet
would produce a characteristic inner ring with asymmetries in the disk. In the
presence of first generation primordial dust these markers would be difficult
to detect far from the orbit of the embedded planet, but would be detectable
inside a gap of planetary origin in a transitional disk.Comment: Accepted for publication in Ap
Intrinsic and extrinsic factors drive ontogeny of early-life at-sea behaviour in a marine top predator
Young animals must learn to forage effectively to survive the transition from parental provisioning to independent feeding. Rapid development of successful foraging strategies is particularly important for capital breeders that do not receive parental guidance after weaning. The intrinsic and extrinsic drivers of variation in ontogeny of foraging are poorly understood for many species. Grey seals (Halichoerus grypus) are typical capital breeders; pups are abandoned on the natal site after a brief suckling phase, and must develop foraging skills without external input. We collected location and dive data from recently-weaned grey seal pups from two regions of the United Kingdom (the North Sea and the Celtic and Irish Seas) using animal-borne telemetry devices during their first months of independence at sea. Dive duration, depth, bottom time, and benthic diving increased over the first 40 days. The shape and magnitude of changes differed between regions. Females consistently had longer bottom times, and in the Celtic and Irish Seas they used shallower water than males. Regional sex differences suggest that extrinsic factors, such as water depth, contribute to behavioural sexual segregation. We recommend that conservation strategies consider movements of young naïve animals in addition to those of adults to account for developmental behavioural changes
Explaining the variability of WD 1145+017 with simulations of asteroid tidal disruption
Post-main-sequence planetary science has been galvanised by the striking
variability, depth and shape of the photometric transit curves due to objects
orbiting white dwarf WD 1145+017, a star which also hosts a dusty debris disc
and circumstellar gas, and displays strong metal atmospheric pollution.
However, the physical properties of the likely asteroid which is discharging
disintegrating fragments remain largely unconstrained from the observations.
This process has not yet been modelled numerically. Here, we use the N-body
code PKDGRAV to compute dissipation properties for asteroids of different
spins, densities, masses, and eccentricities. We simulate both homogeneous and
differentiated asteroids, for up to two years, and find that the disruption
timescale is strongly dependent on density and eccentricity, but weakly
dependent on mass and spin. We find that primarily rocky differentiated bodies
with moderate (~3-4 g/cm^3) bulk densities on near-circular (e <~ 0.1) orbits
can remain intact while occasionally shedding mass from their mantles. These
results suggest that the asteroid orbiting WD 1145+017 is differentiated,
resides just outside of the Roche radius for bulk density but just inside the
Roche radius for mantle density, and is more akin physically to an asteroid
like Vesta instead of one like Itokawa.Comment: Accepted in MNRAS. Movies here!:
http://www.star.bris.ac.uk/pcarter/WD1145_asteroid_disruption
Tensile properties of the transverse carpal ligament and carpal tunnel complex
A new sophisticated method that uses video analysis techniques together with a Maillon Rapide Delta to determine the tensile properties of the transverse carpal ligament–carpal tunnel complex has been developed. Six embalmed cadaveric specimens amputated at the mid-forearm and aged (mean (SD)): 82 (6.29) years were tested. The six hands were from three males (four hands) and one female (two hands). Using trigonometry and geometry the elongation and strain of the transverse carpal ligament and carpal arch were calculated. The cross-sectional area of the transverse carpal ligament was determined. Tensile properties of the transverse carpal ligament–carpal tunnel complex and Load–Displacement data were also obtained. Descriptive statistics, one-way ANOVA together with a post-hoc analysis (Tukey) and t-tests were incorporated. A transverse carpal ligament–carpal tunnel complex novel testing method has been developed. The results suggest that there were no significant differences between the original transverse carpal ligament width and transverse carpal ligament at peak elongation (P= 0.108). There were significant differences between the original carpal arch width and carpal arch width at peak elongation (P=0.002). The transverse carpal ligament failed either at the mid-substance or at their bony attachments. At maximum deformation the peak load and maximum transverse carpal ligament displacements ranged from 285.74 N to 1369.66 N and 7.09 mm to 18.55 mm respectively. The transverse carpal ligament cross-sectional area mean (SD) was 27.21 (3.41)mm2. Using this method the results provide useful biomechanical information and data about the tensile properties of the transverse carpal ligament–carpal tunnel complex
Solution-Phase Synthesis of Heteroatom-Substituted Carbon Scaffolds for Hydrogen Storage
This paper reports a bottom-up solution-phase process for the preparation of pristine and heteroatom (boron, phosphorus, or nitrogen)-substituted carbon scaffolds that show good surface areas and enhanced hydrogen adsorption capacities and binding energies. The synthesis method involves heating chlorine-containing small organic molecules with metallic sodium at reflux in high-boiling solvents. For heteroatom incorporation, heteroatomic electrophiles are added to the reaction mixture. Under the reaction conditions, micrometer-sized graphitic sheets assembled by 3−5 nm-sized domains of graphene nanoflakes are formed, and when they are heteroatom-substituted, the heteroatoms are uniformly distributed. The substituted carbon scaffolds enriched with heteroatoms (boron ~7.3%, phosphorus ~8.1%, and nitrogen ~28.1%) had surface areas as high as 900 m^2 g^(−1) and enhanced reversible hydrogen physisorption capacities relative to pristine carbon scaffolds or common carbonaceous materials. In addition, the binding energies of the substituted carbon scaffolds, as measured by adsorption isotherms, were 8.6, 8.3, and 5.6 kJ mol^(−1) for the boron-, phosphorus-, and nitrogen-enriched carbon scaffolds, respectively
Compositional evolution during rocky protoplanet accretion
The Earth appears non-chondritic in its abundances of refractory lithophile
elements, posing a significant problem for our understanding of its formation
and evolution. It has been suggested that this non-chondritic composition may
be explained by collisional erosion of differentiated planetesimals of
originally chondritic composition. In this work, we present N-body simulations
of terrestrial planet formation that track the growth of planetary embryos from
planetesimals. We simulate evolution through the runaway and oligarchic growth
phases under the Grand Tack model and in the absence of giant planets. These
simulations include a state-of-the-art collision model which allows multiple
collision outcomes, such as accretion, erosion, and bouncing events, that
enables tracking of the evolving core mass fraction of accreting planetesimals.
We show that the embryos grown during this intermediate stage of planet
formation exhibit a range of core mass fractions, and that with significant
dynamical excitation, enough mantle can be stripped from growing embryos to
account for the Earth's non-chondritic Fe/Mg ratio. We also find that there is
a large diversity in the composition of remnant planetesimals, with both
iron-rich and silicate-rich fragments produced via collisions.Comment: 19 pages, 15 figures. Accepted for publication in ApJ. Accompanying
animations can be found at http://www.star.bris.ac.uk/pcarter/comp_evo_1
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