Call for PhD Studentship Proposals for September 2014 start
PART A
Title of Project: Impact melting and vaporization of Earth
Supervisor: Lars Stixrude
Co-Supervisor: Carolina Lithgow-Bertelloni
Duration: 3 years
Sponshorship: MOLTENEARTH, Prof. Stixrude’s European Research Council Advanced
Investigator Grant
Project Description:
The goal of this project is to understand the consequences are large-scale impacts by
determining the properties of the massive quantities of silicate liquid and vapor that are expected
at extreme conditions that have not previously been explored. Predictions, based on first
principles molecular dynamics, will provide essential constraints on the amount of melting and
vaporization suffered by rocky planets as a result of large impacts. The initial target will be
MgSiO3 liquid, which serves as a first order representation of the bulk silicate Earth. The student
will determine the melting point of this material and the physical properties of the liquid, including
those that govern the response to large impacts and the release from high pressure when vapor
is formed, and those that are experimentally measurable, such as density and electrical
conductivity. Results will be compared with pioneering experimental studies as part of a
consortium at the National Ignition Facility (USA). Previous results on MgSiO3 composition at
lower pressure and temperature will serve as a point of comparison and a source of insight into
the underlying connections between structure and thermodynanic properties. The student will
also have the opportunity to explore the possible implications of their results for understanding
the interiors of super-Earth exo-planets.
Current research support
1. “The thermal conductivity of lower mantle minerals”. NERC. Stixrude is co-I. Period:
1/2/10-1/2/13. Amount: £512,906.
2. “Melting in the deep Earth”. NERC. Stixrude is co-I. Period: 1/1/12-31/12/14. Amount:
3.
£464,885.
“MoltenEarth: Fluid Silicates at Extreme Conditions and the Magma Ocean”, Stixrude is
PI. Period: 1/3/12-2/28/17. Amount: € 2,498,891.
Ph.D. Student Supervision
Name
**
Bijaya Karki
Gerd Steinle-Neumann
Boris Kiefer
M. Kathleen Davis
Sun Ni
Nico de Koker
Adam Martins
Date
Responsibility Current Position
1997
2001
2002
2005
2008
2008
2014
30%
100%
100%
100%
100%
100%
100%
*Date of degree or expected date of degree.
**University of Edinburgh
Assistant Prof., Louisiana State University
Assistant Prof., University of Bayreuth
Assistant Prof., New Mexico State Univ.
Geoscientist, Shell Explor. & Prod.
HSBC, Operations Analysis
Post-Doc, Universität Bayreuth
Ph.D. student, UCL
PART B
Title of Project: Impact melting and vaporization of planets including the proto-Earth
Supervisor: Lars Stixrude
Duration: 4 years
Sponshorship: MOLTENEARTH, Prof. Stixrude’s ERC Advanced Investigator Grant
Background
Giant impacts are likely to be a ubiquitous planetary process, resulting in large scale melting and
vaporization of target and impactor. Recent results have emphasized the importance of massive
vaporization for understanding the composition of the Earth-moon system. Earth is the product of
an evolutionary process that began with a largely molten initial condition, a magma ocean.
Melting is the engine of Earth’s chemical evolution, which makes possible the formation of core,
mantle, crust, and atmosphere. The magma ocean is central to our understanding of Earth’s
differentiation since much of it happened early: as
Earth solidified, chemical evolution slowed to the
trickle of partial melt that continues to form crust
today. Melting processes provide us with a book of
Earth’s history, but one that we cannot read,
particularly the early chapters that set the stage. For
example, the significance of much of mantle
chemical heterogeneity and its implications for Earth
evolution are still shrouded in mystery. The reason
for this is simple: we still know very little of the
melting process during Earth’s earliest history.
How much of the Earth and its ejecta melted or vaporized
during the giant impact that formed the Moon? Could a
continuous magma/vapor envelope have encompassed
the Earth and the growing Moon? Simulations of the
process (from Robin Canup) can tell us about energy
input, but extents of melting and vaporization are only
guesses. Progress depends on understanding silicate
melting at the very high pressures generated in giant impacts.
The scenario depicted, in which the moon and a terrestrial magma ocean form as the result of a
giant impact is highly uncertain because it hinges at nearly every point on the aspects of the
behavior of fluid silicates that are still poorly known. The uncertainties begin with the initial
condition: did the magma ocean encompass the entire mantle? For a given input of energy, e.g.
from a giant impact, the answer depends on the silicate melting curve, and the pressuretemperature path followed by the impacted material. How much of the ejecta was melted or
vaporized? Impacts may have generated a dense silicate atmosphere that may be important in
controlling the cooling time scale of the upper magma ocean, for understanding chemical
similarities between Earth and Moon, and for the origin of our present atmosphere.
The goal of MOLTENEARTH is the construction of HeFESTo, a comprehensive thermodynamic
model of the mantle including melting, vaporization, and core-reaction that will increase 50-fold
the pressure range of mantle melting models. HeFESTo will specify the physico-chemical
processes that drive magma ocean dynamics, and the response of the growing Earth to large
impacts thereby allowing us to test scenarios such as that represented in the figure.
MOLTENEARTH will allow us to predict the quantities central to any attempt to understand magma
ocean evolution including the freezing interval of silicate liquids, their buoyancy with respect to
coexisting crystals, and their composition. MOLTENEARTH will almost certainly lead to the
consideration of new origin scenarios as the full richness of the response to impacts is included,
along with new ways of testing these against present-day observations.
Approach
The goal of this student project is to determine the thermodynamic properties of silicate liquids
that control the response of Earth to large impacts using first principles molecular dynamics
simulations. Such simulations have already had an important impact on our views of magma
ocean evolution, for example, leading to the idea of the basal magma ocean. But they have only
scratched the surface. A coordinated suite of new simulations are needed to make progress. In
particular the pressure-temperature regime relevant to large impacts remains unexplored: a
symmetric impact at Keplerian velocity generates a peak pressure of 1000 GPa (1 TPa), 7 times
the pressure at the base of Earth’s mantle. Yet learning about material behavior at these
conditions is essential for understanding the consequences of large impacts, including the depth
of melting, and the amount and composition of vapor produced on release. The results will
provide a rich source of information on the physical properties of silicate liquids, and will constrain
the fundamental thermodynamic relation of silicate liquids over the entire pressure-temperature
regime relevant to large impacts and the magma ocean. The focus will be on simulation of
homogeneous silicate liquids with the choice of compositions guided by the abundance of
components in the Earth, and those regions of composition space that are the target of existing or
planned experiments.
The student will perform first principles molecular dynamics simulations of silicate liquids over
the pressure-temperature regime of large impacts (P<1 TPa, and T<100,000 K). The initial target
will be MgSiO3 liquid, which serves as a first order representation of the bulk silicate Earth. The
student will determine the melting point of this material and the physical properties of the liquid,
including those that govern the response to large impacts and the release from high pressure
when vapor is formed, and those that are experimentally measurable, such as density and
electrical conductivity. Results will be compared with pioneering experimental studies at TPa
pressure being performed as part of a consortium at the National Ignition Facility (USA), of which
Prof. Stixrude is a member. Previous results on MgSiO3 composition at lower pressure and
temperature will serve as a point of comparison and a source of insight into the underlying
connections between structure and thermodynanic properties.
The student will also have the opportunity to explore the possible implications of their results
for understanding the interiors of super-Earth exo-planets. These may contain large regions of
deep silicate melt due to their large thermal inertia. Moreover, they may exhibit unique
mechanisms of generating magnetic fields in deep highly conductive silicate melt layers.
Training and support
The student will be part of a team of researchers led by Prof. Stixrude as part of MOLTENEARTH,
an ambitious project to construct a comprehensive thermodynamic model of magma ocean
thermodynamics. The Department of Earth Sciences contains a strong, vibrant, and supportive
group focusing on Earth’s evolution, including eight permanent staff members. The student will
have ample opportunity to develop programming, computational, and data analysis skills, gain
unique training in thermodynamics, condensed matter theory, and fluid dynamics, participate in
international conferences and publish academic work. UCL provides a large variety of training
courses both for academic and non-academic purposes.
Student Prerequisites
The candidate will be expected to demonstrate a fluency in mathematics and physics, and some
prior familiarity with computational problems would be an advantage.
References
L. Stixrude and B. Karki, Structure and freezing of MgSiO3 liquid in Earth’s lower mantle, Science
310, 297 (2005).
L. Stixrude, N. de Koker, N. Sun, M. Mookherjee, and B. B. Karki, Thermodynamics of silicate
liquids in the deep Earth, Earth and Planetary Science Letters, 278, 226 (2009).
L. Stixrude and R. Jeanloz, Fluid helium at conditions of giant planetary interiors, PNAS, 105,
11071 (2008).