1,577 research outputs found
Numerical solution of a non-linear conservation law applicable to the interior dynamics of partially molten planets
The energy balance of a partially molten rocky planet can be expressed as a
non-linear diffusion equation using mixing length theory to quantify heat
transport by both convection and mixing of the melt and solid phases. In this
formulation the effective or eddy diffusivity depends on the entropy gradient,
, as well as entropy. First we present a simplified
model with semi-analytical solutions, highlighting the large dynamic range of
, around 12 orders of magnitude, for physically-relevant
parameters. It also elucidates the thermal structure of a magma ocean during
the earliest stage of crystal formation. This motivates the development of a
simple, stable numerical scheme able to capture the large dynamic range of
and provide a flexible and robust method for
time-integrating the energy equation.
We then consider a full model including energy fluxes associated with
convection, mixing, gravitational separation, and conduction that all depend on
the thermophysical properties of the melt and solid phases. This model is
discretised and evolved by applying the finite volume method (FVM), allowing
for extended precision calculations and using as the
solution variable. The FVM is well-suited to this problem since it is naturally
energy conserving, flexible, and intuitive to incorporate arbitrary non-linear
fluxes that rely on lookup data. Special attention is given to the numerically
challenging scenario in which crystals first form in the centre of a magma
ocean.
Our computational framework is immediately applicable to modelling high melt
fraction phenomena in Earth and planetary science research. Furthermore, it
provides a template for solving similar non-linear diffusion equations arising
in other disciplines, particularly for non-linear functional forms of the
diffusion coefficient
Linking the evolution of terrestrial interiors and an early outgassed atmosphere to astrophysical observations
A terrestrial planet is molten during formation and may remain so if subject
to intense insolation or tidal forces. Observations continue to favour the
detection and characterisation of hot planets, potentially with large outgassed
atmospheres. We aim to determine the radius of hot Earth-like planets with
large outgassed atmospheres and explore differences between molten and solid
silicate planets and their influence on the mass-radius relationship and
transmission and emission spectra. An interior-atmosphere model, combined with
static structure calculations, tracks the evolving radius of a rocky mantle
that is outgassing CO and HO. Synthetic emission and transmission
spectra are generated for CO and HO dominated atmospheres. Atmospheres
dominated by CO suppress the outgassing of HO to a greater extent than
previously realised, as previous studies have applied an erroneous relationship
between volatile mass and partial pressure. We therefore predict more HO
can be retained by the interior during the later stages of magma ocean
crystallisation. Furthermore, formation of a lid at the surface can tie
outgassing of HO to the efficiency of heat transport through the lid,
rather than the atmosphere's radiative timescale. Contraction of the mantle as
it solidifies gives radius decrease, which can partly be offset by
addition of a relatively light species to the atmosphere. We conclude that a
molten silicate mantle can increase the radius of a terrestrial planet by
around compared to its solid counterpart, or equivalently account for a
decrease in bulk density. An outgassing atmosphere can perturb the total
radius according to its speciation. Atmospheres of terrestrial planets around
M-stars that are dominated by CO or HO can be distinguished by
observing facilities with extended wavelength coverage (e.g., JWST).Comment: 19 pages, published in A&A, abstract shortene
A determination of the spin-orbit alignment of the anomalously dense planet orbiting HD 149026
We report 35 radial velocity measurements of HD 149026 taken with the Keck Telescope. Of these measurements, 15
were made during the transit of the companion planet HD 149026b, which occurred on 2005 June 25. These velocities
provide a high-cadence observation of the Rossiter-McLaughlin effect, the shifting of photospheric line profiles that occurs when a planet occults a portion of the rotating stellar surface. We combine these radial velocities with previously published radial velocity and photometric data sets and derive a composite best-fit model for the star-planet system. This model confirms and improves previously published orbital parameters, including the remarkably small planetary radius, the planetary mass, and the orbital inclination, found to be Rp/RJup = 0.718 ± 0.065, Mp/MJup = 0.352 ± 0.025, and I = 86.1° ± 1.4°, respectively. Together the planetary mass and radius determinations imply a mean planetary density
of 1.18(-0.30)(+0.38)g cm(-3). The new data also allow for the determination of the angle between the apparent stellar equator and the orbital plane, which we constrain to be λ = -12° ± 15°
VapoRock: Thermodynamics of vaporized silicate melts for modeling volcanic outgassing and magma ocean atmospheres
Silicate vapors play a key role in planetary evolution, especially dominating
early stages of rocky planet formation through outgassed magma ocean
atmospheres. Our open-source thermodynamic modeling software "VapoRock"
combines the MELTS liquid model (Ghiorso et al., 1995) with gas-species
properties from multiple thermochemistry tables (e.g., Chase et al., 1998).
VapoRock calculates the partial pressures of 34 gaseous species in equilibrium
with magmatic liquid in the system Si-Mg-Fe-Al-Ca-Na-K-Ti-Cr-O at desired
temperatures and oxygen fugacities (fO2, or partial pressure of O2). Comparison
with experiments shows that pressures and melt-oxide activities (which vary
over many orders of magnitude) are reproduced to within a factor of ~3,
consistent with measurement uncertainties. We also benchmark the model against
a wide selection of igneous rock compositions including bulk silicate Earth,
predicting elemental vapor abundances that are comparable (Na, Ca, & Al) or
more realistic (K, Si, Mg, Fe, & Ti) than those of the closed-source MAGMA code
(with maximum deviations by factors of 10-300 for K & Si). Vapor abundances
depend critically on the activities of liquid components. The MELTS model
underpinning VapoRock was calibrated and extensively tested on natural igneous
liquids. In contrast, MAGMA's liquid model assumes ideal mixtures of a limited
set of chemically simplified pseudo-species, which only roughly approximates
the non-ideal compositional interactions typical of many-component natural
silicate melts. Finally, we explore how relative abundances of SiO and SiO2
provide a spectroscopically measurable proxy for oxygen fugacity in
devolatilized exoplanetary atmospheres, potentially constraining fO2 in
outgassed exoplanetary mantles
The thermal equation of state of (Mg, Fe)SiO3 bridgmanite (perovskite) and implications for lower mantle structures
The highâpressure/highâtemperature equation of state (EOS) of synthetic 13% Feâbearing bridgmanite (Mg silicate perovskite) is measured using powder Xâray diffraction in a laserâheated diamond anvil cell with a quasiâhydrostatic neon pressure medium. We compare these results, which are consistent with previous 300Â K sound speed and compression studies, with a reanalysis of Feâfree Mg endâmember data from Tange et al. (2012) to determine the effect of iron on bridgmaniteâs thermoelastic properties. EOS parameters are incorporated into an ideal lattice mixing model to probe the behavior of bridgmanite at deep mantle conditions. With this model, a nearly pure bridgmanite mantle composition is shown to be inconsistent with density and compressibility profiles of the lower mantle. We also explore the buoyant stability of bridgmanite over a range of temperatures and compositions expected for Large LowâShear Velocity Provinces, concluding that bridgmaniteâdominated thermochemical piles are more likely to be passive dense layers externally supported by convection, rather than internally supported metastable domes. The metastable dome scenario is estimated to have a relative likelihood of only 4â7%, given the narrow range of compositions and temperatures consistent with seismic constraints. If buoyantly supported, such structures could not have remained stable with greater thermal contrast early in Earthâs history, ruling out formation scenarios involving a large concentration of heat producing elements.Key PointsHigh PâT equation of state of 13% and 0% Fe bridgmanite (perovksite) is obtainedPure bridgmanite mantle is inconsistent with PREM at any Fe contentBuoyant stability of LLSVPs favors passive chemical piles over metastable domesPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/141021/1/jgrb51327.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/141021/2/jgrb51327_am.pd
Spectropolarimetry and Modeling of the Eclipsing T Tauri Star KH 15D
KH 15D is a strongly variable T Tauri star in the young star cluster NGC 2264
that shows a decrease in flux of 3.5 magnitudes lasting for 18 days and
repeating every 48 days. The eclipsing material is likely due to orbiting dust
or rocky bodies in a partial ring or warped disk that periodically occults the
star. We measured the polarized spectrum in and out of eclipse at the Keck and
Palomar observatories. Outside of the eclipse, the star exhibited low
polarization consistent with zero. During eclipse, the polarization increased
dramatically to ~2% across the optical spectrum, while the spectrum had the
same continuum shape as outside of eclipse and exhibited emission lines of much
larger equivalent width, as previously seen. From the data, we conclude that
(a) the scattering region is uneclipsed; (b) the scattering is nearly
achromatic; (c) the star is likely completely eclipsed so that the flux during
eclipse is entirely due to scattered light, a conclusion also argued for by the
shape of the ingress and egress. We argue that the scattering is not due to
electrons, but may be due to large dust grains of size ~10 micron, similar to
the interplanetary grains which scatter the zodiacal light. We construct a
warped-disk model with an extended dusty atmosphere which reproduces the main
features of the lightcurve, namely (a) a gradual decrease before ingress due to
extinction in the atmosphere (similar for egress); (b) a sharper decrease
within ingress due to the optically-thick base of the atmosphere; (c) a
polarized flux during eclipse which is 0.1% of the total flux outside of
eclipse, which requires no fine-tuning of the model. (abridged)Comment: 9 pages, 7 figures, accepted for publication in ApJ, MPEG simulation
available at http://www.astro.washington.edu/agol/scatter2.mp
Venus: Interpreting the spatial distribution of volcanically modified craters
To understand the impact cratering record on Venus, we investigate two distinct resurfacing styles: localized, thin flows and large shield volcanoes. We statistically analyze the size-frequency distribution of volcanically modified craters and, using Monte Carlo simulations, their spatial distribution. Lava flows partially fill most craters, darkening their floors in radar images. We find that a model featuring localized, thin flows occurring throughout geologic time predicts their observed distribution. Individual flows may be morphologically indistinguishable, but, combined, they cover large provinces. Recent mantle plumes may drive a small amount of hot spot magmatism that produces the observed clusters of large shield volcanoes and obviously embayed craters. Ultimately, our analysis demonstrates that two styles of volcanism are needed to explain the observed properties of impact craters and that catastrophic resurfacing is not required
Data from: Genotyping-by-Sequencing for Populus Population Genomics: An Assessment of Genome Sampling Patterns and Filtering Approaches
Continuing advances in nucleotide sequencing technology are inspiring a suite of genomic approaches in studies of natural populations. Researchers are faced with data management and analytical scales that are increasing by orders of magnitude. With such dramatic advances comes a need to understand biases and error rates, which can be propagated and magnified in large-scale data acquisition and processing. Here we assess genomic sampling biases and the effects of various population-level data filtering strategies in a genotyping-by-sequencing (GBS) protocol. We focus on data from two species of Populus, because this genus has a relatively small genome and is emerging as a target for population genomic studies. We estimate the proportions and patterns of genomic sampling by examining the Populus trichocarpa genome (Nisqually-1), and demonstrate a pronounced bias towards coding regions when using the methylation-sensitive ApeKI restriction enzyme in this species. Using population-level data from a closely related species (P. tremuloides), we also investigate various approaches for filtering GBS data to retain high-depth, informative SNPs that can be used for population genetic analyses. We find a data filter that includes the designation of ambiguous alleles resulted in metrics of population structure and Hardy-Weinberg equilibrium that were most consistent with previous studies of the same populations based on other genetic markers. Analyses of the filtered data (27,910 SNPs) also resulted in patterns of heterozygosity and population structure similar to a previous study using microsatellites. Our application demonstrates that technically and analytically simple approaches can readily be developed for population genomics of natural populations
Separating the influence of temperature, drought, and fire on interannual variability in atmospheric CO 2
The response of the carbon cycle in prognostic Earth system models (ESMs) contributes significant uncertainty to projections of global climate change. Quantifying contributions of known drivers of interannual variability in the growth rate of atmospheric carbon dioxide (CO 2 ) is important for improving the representation of terrestrial ecosystem processes in these ESMs. Several recent studies have identified the temperature dependence of tropical net ecosystem exchange (NEE) as a primary driver of this variability by analyzing a single, globally averaged time series of CO 2 anomalies. Here we examined how the temporal evolution of CO 2 in different latitude bands may be used to separate contributions from temperature stress, drought stress, and fire emissions to CO 2 variability. We developed atmospheric CO 2 patterns from each of these mechanisms during 1997â2011 using an atmospheric transport model. NEE responses to temperature, NEE responses to drought, and fire emissions all contributed significantly to CO 2 variability in each latitude band, suggesting that no single mechanism was the dominant driver. We found that the sum of drought and fire contributions to CO 2 variability exceeded direct NEE responses to temperature in both the Northern and Southern Hemispheres. Additional sensitivity tests revealed that these contributions are masked by temporal and spatial smoothing of CO 2 observations. Accounting for fires, the sensitivity of tropical NEE to temperature stress decreased by 25% to 2.9â±â0.4 Pg C yr â1 âK â1 . These results underscore the need for accurate attribution of the drivers of CO 2 variability prior to using contemporary observations to constrain longâterm ESM responses. Key Points Accurate attribution of CO 2 variability is required to constrain coupled models Combined influence of drought and fire exceed ecosystem responses to temperature Temporal and spatial smoothing of CO 2 observations masks variability from firePeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/109962/1/gbc20215.pd
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