28 research outputs found
The effects of short-lived radionuclides and porosity on the early thermo-mechanical evolution of planetesimals
The thermal history and internal structure of chondritic planetesimals,
assembled before the giant impact phase of chaotic growth, potentially yield
important implications for the final composition and evolution of terrestrial
planets. These parameters critically depend on the internal balance of heating
versus cooling, which is mostly determined by the presence of short-lived
radionuclides (SLRs), such as aluminum-26 and iron-60, as well as the heat
conductivity of the material. The heating by SLRs depends on their initial
abundances, the formation time of the planetesimal and its size. It has been
argued that the cooling history is determined by the porosity of the granular
material, which undergoes dramatic changes via compaction processes and tends
to decrease with time. In this study we assess the influence of these
parameters on the thermo-mechanical evolution of young planetesimals with both
2D and 3D simulations. Using the code family I2ELVIS/I3ELVIS we have run
numerous 2D and 3D numerical finite-difference fluid dynamic models with
varying planetesimal radius, formation time and initial porosity. Our results
indicate that powdery materials lowered the threshold for melting and
convection in planetesimals, depending on the amount of SLRs present. A subset
of planetesimals retained a powdery surface layer which lowered the thermal
conductivity and hindered cooling. The effect of initial porosity was small,
however, compared to those of planetesimal size and formation time, which
dominated the thermo-mechanical evolution and were the primary factors for the
onset of melting and differentiation. We comment on the implications of this
work concerning the structure and evolution of these planetesimals, as well as
their behavior as possible building blocks of terrestrial planets.Comment: 19 pages, 11 figures, 5 tables; accepted for publication in Icarus;
for associated video files, see http://timlichtenberg.net/2015_porosity.html
or http://dx.doi.org/10.1016/j.icarus.2016.03.00
Ferropericlase Control of Lower Mantle Rheology : Impact of Phase Morphology
Abstract The rheological properties of Earth's lower mantle play a key role for global mantle dynamics. The mineralogy of the lower mantle can be approximated as a bridgmaniteâferropericlase mixture. Previous work has suggested that the deformation of this mixture might be dramatically affected by the large differences in viscosity between bridgmanite and ferropericlase. Here, we employ numerical models to establish a connection between ferropericlase morphology and the effective rheology of the Earth's lower mantle using a numericalâstatistical approach. Using this approach, we link the statistical properties of the twoâphase composite to its effective viscosity tensor using analytical approximations. We find that ferropericlase develops elongated structures within the bridgmanite matrix that result in significantly lowered viscosity. While our findings confirm previous endmember models that suggested a change of mantle viscosity due to the formation of interconnected weak layers, we show that significant rheological weakening can thus be already achieved even when ferropericlase does not form an interconnected network. Additionally, the alignment of weak ferropericlase leads to a pronounced viscous anisotropy that develops with total strain, which may have implications for understanding the viscosity structure of Earth's lower mantle as well as for modeling the behavior of subducting slabs. We show that to capture the effect of ferropericlase elongation on the effective viscosity tensor (and its anisotropy) in largeâscale mantle convection models, the analytical approximations that have been derived to describe the evolution of the effective viscosity of a twoâphase medium with aligned elliptical inclusions can be used
Coupling SPH and thermochemical models of planets: Methodology and example of a Mars-sized body
Giant impacts have been suggested to explain various characteristics of
terrestrial planets and their moons. However, so far in most models only the
immediate effects of the collisions have been considered, while the long-term
interior evolution of the impacted planets was not studied. Here we present a
new approach, combining 3-D shock physics collision calculations with 3-D
thermochemical interior evolution models. We apply the combined methods to a
demonstration example of a giant impact on a Mars-sized body, using typical
collisional parameters from previous studies. While the material parameters
(equation of state, rheology model) used in the impact simulations can have
some effect on the long-term evolution, we find that the impact angle is the
most crucial parameter for the resulting spatial distribution of the newly
formed crust. The results indicate that a dichotomous crustal pattern can form
after a head-on collision, while this is not the case when considering a more
likely grazing collision. Our results underline that end-to-end 3-D
calculations of the entire process are required to study in the future the
effects of large-scale impacts on the evolution of planetary interiors.Comment: 29 pages, 10 figures, accepted for publication in Icaru
Impact splash chondrule formation during planetesimal recycling
Chondrules are the dominant bulk silicate constituent of chondritic
meteorites and originate from highly energetic, local processes during the
first million years after the birth of the Sun. So far, an astrophysically
consistent chondrule formation scenario, explaining major chemical, isotopic
and textural features, remains elusive. Here, we examine the prospect of
forming chondrules from planetesimal collisions. We show that intensely melted
bodies with interior magma oceans became rapidly chemically equilibrated and
physically differentiated. Therefore, collisional interactions among such
bodies would have resulted in chondrule-like but basaltic spherules, which are
not observed in the meteoritic record. This inconsistency with the expected
dynamical interactions hints at an incomplete understanding of the planetary
growth regime during the protoplanetary disk phase. To resolve this conundrum,
we examine how the observed chemical and isotopic features of chondrules
constrain the dynamical environment of accreting chondrite parent bodies by
interpreting the meteoritic record as an impact-generated proxy of
planetesimals that underwent repeated collision and reaccretion cycles. Using a
coupled evolution-collision model we demonstrate that the vast majority of
collisional debris feeding the asteroid main belt must be derived from
planetesimals which were partially molten at maximum. Therefore, the precursors
of chondrite parent bodies either formed primarily small, from sub-canonical
aluminum-26 reservoirs, or collisional destruction mechanisms were efficient
enough to shatter planetesimals before they reached the magma ocean phase.
Finally, we outline the window in parameter space for which chondrule formation
from planetesimal collisions can be reconciled with the meteoritic record and
how our results can be used to further constrain early solar system dynamics.Comment: 20 pages, 11 figures, 2 tables; accepted for publication in Icarus;
associated blog article at goo.gl/5bDqG
Late metal-silicate separation on the IAB parent asteroid: Constraints from combined W and Pt isotopes and thermal modelling
The short-lived Hf-W decay system is a powerful chronometer
for constraining the timing of metal-silicate separation and core formation in
planetesimals and planets. Neutron capture effects on W isotopes, however,
significantly hamper the application of this tool. In order to correct for
neutron capture effects, Pt isotopes have emerged as a reliable in-situ neutron
dosimeter. This study applies this method to IAB iron meteorites, in order to
constrain the timing of metal segregation on the IAB parent body. The
W values obtained for the IAB iron meteorites range from -3.61
0.10 to -2.73 0.09. Correlating Pt with
W data yields a pre-neutron capture W of -2.90 0.06. This
corresponds to a metal-silicate separation age of 6.0 0.8 Ma after CAI
for the IAB parent body, and is interpreted to represent a body-wide melting
event. Later, between 10 and 14 Ma after CAI, an impact led to a catastrophic
break-up and subsequent reassembly of the parent body. Thermal models of the
interior evolution that are consistent with these estimates suggest that the
IAB parent body underwent metal-silicate separation as a result of internal
heating by short-lived radionuclides and accreted at around 1.4 0.1 Ma
after CAIs with a radius of greater than 60 km.Comment: 11 pages, 8 figures, 2 tables; open access article under the CC
BY-NC-ND license (see http://creativecommons.org/licenses/by-nc-nd/4.0/
Bifurcation of planetary building blocks during Solar System formation
Geochemical and astronomical evidence demonstrate that planet formation
occurred in two spatially and temporally separated reservoirs. The origin of
this dichotomy is unknown. We use numerical models to investigate how the
evolution of the solar protoplanetary disk influenced the timing of protoplanet
formation and their internal evolution. Migration of the water snow line can
generate two distinct bursts of planetesimal formation that sample different
source regions. These reservoirs evolve in divergent geophysical modes and
develop distinct volatile contents, consistent with constraints from accretion
chronology, thermo-chemistry, and the mass divergence of inner and outer Solar
System. Our simulations suggest that the compositional fractionation and
isotopic dichotomy of the Solar System was initiated by the interplay between
disk dynamics, heterogeneous accretion, and internal evolution of forming
protoplanets.Comment: Published 21 January 2021; authors' version; 30 pages, 18 figures;
summary available at http://bit.ly/BifurcationBlog (blog) and
https://bit.ly/BifurcationVideo (video
A water budget dichotomy of rocky protoplanets from Al-heating
In contrast to the water-poor inner solar system planets, stochasticity
during planetary formation and order of magnitude deviations in exoplanet
volatile contents suggest that rocky worlds engulfed in thick volatile ice
layers are the dominant family of terrestrial analogues among the extrasolar
planet population. However, the distribution of compositionally Earth-like
planets remains insufficiently constrained, and it is not clear whether the
solar system is a statistical outlier or can be explained by more general
planetary formation processes. Here we employ numerical models of planet
formation, evolution, and interior structure, to show that a planet's bulk
water fraction and radius are anti-correlated with initial Al levels in
the planetesimal-based accretion framework. The heat generated by this
short-lived radionuclide rapidly dehydrates planetesimals prior to accretion
onto larger protoplanets and yields a system-wide correlation of planet bulk
abundances, which, for instance, can explain the lack of a clear orbital trend
in the water budgets of the TRAPPIST-1 planets. Qualitatively, our models
suggest two main scenarios of planetary systems' formation: high-Al
systems, like our solar system, form small, water-depleted planets, whereas
those devoid of Al predominantly form ocean worlds, where the mean
planet radii between both scenarios deviate by up to about 10%.Comment: Preprint version; free-to-read journal version at
https://rdcu.be/bmdlw; blog article at https://t.co/p6SValG1i