410 research outputs found
Simulation of stellar instabilities with vastly different timescales using domain decomposition
Strange mode instabilities in the envelopes of massive stars lead to shock
waves, which can oscillate on a much shorter timescale than that associated
with the primary instability. The phenomenon is studied by direct numerical
simulation using a, with respect to time, implicit Lagrangian scheme, which
allows for the variation by several orders of magnitude of the dependent
variables. The timestep for the simulation of the system is reduced appreciably
by the shock oscillations and prevents its long term study. A procedure based
on domain decomposition is proposed to surmount the difficulty of vastly
different timescales in various regions of the stellar envelope and thus to
enable the desired long term simulations. Criteria for domain decomposition are
derived and the proper treatment of the resulting inner boundaries is
discussed. Tests of the approach are presented and its viability is
demonstrated by application to a model for the star P Cygni. In this
investigation primarily the feasibility of domain decomposition for the problem
considered is studied. We intend to use the results as the basis of an
extension to two dimensional simulations.Comment: 15 pages, 10 figures, published in MNRA
How large are present-day heat flux variations across the surface of Mars?
©2016. American Geophysical UnionThe first in situ Martian heat flux measurement to be carried out by the InSight Discovery‐class mission will provide an important baseline to constrain the present‐day heat budget of the planet and, in turn, the thermochemical evolution of its interior. In this study, we estimate the magnitude of surface heat flux heterogeneities in order to assess how the heat flux at the InSight landing site relates to the average heat flux of Mars. To this end, we model the thermal evolution of Mars in a 3‐D spherical geometry and investigate the resulting surface spatial variations of heat flux at the present day. Our models assume a fixed crust with a variable thickness as inferred from gravity and topography data and with radiogenic heat sources as obtained from gamma ray measurements of the surface. We test several mantle parameters and show that the present‐day surface heat flux pattern is dominated by the imposed crustal structure. The largest surface heat flux peak‐to peak variations lie between 17.2 and 49.9 mW m−2, with the highest values being associated with the occurrence of prominent mantle plumes. However, strong spatial variations introduced by such plumes remain narrowly confined to a few geographical regions and are unlikely to bias the InSight heat flux measurement. We estimated that the average surface heat flux varies between 23.2 and 27.3 mW m−2, while at the InSight location it lies between 18.8 and 24.2 mW m−2. In most models, elastic lithosphere thickness values exceed 250 km at the north pole, while the south pole values lie well above 110 km
Instabilities of captured shocks in the envelopes of massive stars
The evolution of strange mode instabilities into the non linear regime has
been followed by numerical simulation for an envelope model of a massive star
having solar chemical composition, M=50M_sun, T_eff=10^4K and L=1.17*10^6
L_sun. Contrary to previously studied models, for these parameters shocks are
captured in the H-ionisation zone and perform rapid oscillations within the
latter. A linear stability analysis is performed to verify that this behaviour
is physical. The origin of an instability discovered in this way is identified
by construction of an analytical model. As a result, the stratification turns
out to be essential for instability. The difference to common stratification
instabilities, e.g., convective instabilities, is discussed.Comment: 16 pages, 6 figures, accepted for publication in MNRA
Mercury's low‐degree geoid and topography controlled by insolation‐driven elastic deformation
©2015. American Geophysical UnionMercury experiences an uneven insolation that leads to significant latitudinal and longitudinal variations of its surface temperature. These variations, which are predominantly of spherical harmonic degrees 2 and 4, propagate to depth, imposing a long‐wavelength thermal perturbation throughout the mantle. We computed the accompanying density distribution and used it to calculate the mechanical and gravitational response of a spherical elastic shell overlying a quasi‐hydrostatic mantle. We then compared the resulting geoid and surface deformation at degrees 2 and 4 with Mercury's geoid and topography derived from the MErcury, Surface, Space ENvironment, GEochemistry, and Ranging spacecraft. More than 95% of the data can be accounted for if the thickness of the elastic lithosphere were between 110 and 180 km when the thermal anomaly was imposed. The obtained elastic thickness implies that Mercury became locked into its present 3:2 spin orbit resonance later than about 1 Gyr after planetary formation
Present-day Mars' seismicity predicted from 3-D thermal evolution models of interior dynamics
©2018. American Geophysical UnionThe Interior Exploration using Seismic Investigations, Geodesy and Heat Transport mission, to be launched in 2018, will perform a comprehensive geophysical investigation of Mars in situ. The Seismic Experiment for Interior Structure package aims to detect global and regional seismic events and in turn offer constraints on core size, crustal thickness, and core, mantle, and crustal composition. In this study, we estimate the present‐day amount and distribution of seismicity using 3‐D numerical thermal evolution models of Mars, taking into account contributions from convective stresses as well as from stresses associated with cooling and planetary contraction. Defining the seismogenic lithosphere by an isotherm and assuming two end‐member cases of 573 K and the 1073 K, we determine the seismogenic lithosphere thickness. Assuming a seismic efficiency between 0.025 and 1, this thickness is used to estimate the total annual seismic moment budget, and our models show values between 5.7 × 1016 and 3.9 × 1019 Nm
Thermophysical modelling and parameter estimation of small solar system bodies via data assimilation
Deriving thermophysical properties such as thermal inertia from thermal
infrared observations provides useful insights into the structure of the
surface material on planetary bodies. The estimation of these properties is
usually done by fitting temperature variations calculated by thermophysical
models to infrared observations. For multiple free model parameters,
traditional methods such as Least-Squares fitting or Markov-Chain Monte-Carlo
methods become computationally too expensive. Consequently, the simultaneous
estimation of several thermophysical parameters together with their
corresponding uncertainties and correlations is often not computationally
feasible and the analysis is usually reduced to fitting one or two parameters.
Data assimilation methods have been shown to be robust while sufficiently
accurate and computationally affordable even for a large number of parameters.
This paper will introduce a standard sequential data assimilation method, the
Ensemble Square Root Filter, to thermophysical modelling of asteroid surfaces.
This method is used to re-analyse infrared observations of the MARA instrument,
which measured the diurnal temperature variation of a single boulder on the
surface of near-Earth asteroid (162173) Ryugu. The thermal inertia is estimated
to be , while all five free
parameters of the initial analysis are varied and estimated simultaneously.
Based on this thermal inertia estimate the thermal conductivity of the boulder
is estimated to be between 0.07 and 0.12 and the
porosity to be between 0.30 and 0.52. For the first time in thermophysical
parameter derivation, correlations and uncertainties of all free model
parameters are incorporated in the estimation procedure which is more than 5000
times more efficient than a comparable parameter sweep
The thermal state and interior structure of Mars
©2018. American Geophysical UnionThe present‐day thermal state, interior structure, composition, and rheology of Mars can be constrained by comparing the results of thermal history calculations with geophysical, petrological, and geological observations. Using the largest‐to‐date set of 3‐D thermal evolution models, we find that a limited set of models can satisfy all available constraints simultaneously. These models require a core radius strictly larger than 1,800 km, a crust with an average thickness between 48.8 and 87.1 km containing more than half of the planet's bulk abundance of heat producing elements, and a dry mantle rheology. A strong pressure dependence of the viscosity leads to the formation of prominent mantle plumes producing melt underneath Tharsis up to the present time. Heat flow and core size estimates derived from the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission will increase the set of constraining data and help to confine the range of admissible models.DFG, 280637173, FOR 2440: Materie im Inneren von Planeten - Hochdruck-, Planeten- und Plasmaphysi
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