Research
Geodynamics
Research in geodynamics is focussed on understanding the dynamic processes in the Earth's mantle and lithosphere and their relation to geologic events observable at the surface. Geodynamics is an inherently multidisciplinary subject. This means that we cooperate with researchers world-wide and use data from related geoscientific fields such as seismology, geodesy, geology, and geochemistry as constraints for our models.
Tidal Heating Susceptibility in Terrestrial Exoplanets
Considering the rapid rate of discovery of new planets outside our solar system,
we are attempting to model some of the more exotic terrestrial planet
possibilities that may soon be observed. In our own solar system, tidal
friction plays and important role in shaping and smoothing all orbits. In rare
cases, such as Jupiter's moon Io, the heating lasts indefinitely, utilizing
orbital energy from the rest of the complex moon system to power tremendous
volcanoes that dominate Io's surface. In exosystems, we expect the situation to
be similar, with rare cases of orbital resonance powering planetary volcanic
systems unlike anything seen before on earthlike worlds. Such tidal heating has
the potential to shift habitable zones around stars, to disrupt orbital spin
synchronization, and to send planets through multiple hundred-million year long
episodes of sudden heating or nearly halted secular cooling.Perhaps the most
powerful of these tides can even generate magma oceans.
To examine all these
possibilities, we are analyzing the general tidal heating of terrestrial class
exoplanets to identify first the orbital range and parameter spaces where tides
dominate the planetary heat flux, and also to work out the melt and advection
based limiting mechanisms to extreme tidal solutions. The tidal heating of
planets matters most in short period orbits (roughly 1 to 20 days), similar to
those of the newly discovered Hot Jupiter class. These Hot Jupiters are
believed to migrate significantly inwards to reach their observed positions.
During such migration they would potentially sweep up terrestrial inner planets
into a highly stable 2:1 orbital mean-motion resonances, that would provide the
eccentricity and thus energy transport link to maintain very long term extreme
tides. To calculate tides, we utilize various methods, from classical frequency
independent equations to the viscoelastic Maxwell and Burgers temperature
dependent rock rheologies. We examine heating in the context both of
equilibrium temperature solutions with depth, as well as for global
nonequilibrum behaviors, following the gradual changing of equilibrium points
with orbit migration. We use our results to make predictions about the
observability of tidal features on exoplanets using planned NASA missions such
as TPFi and TPFc, and to model internal heating in known terrestrial exoplanets
such as GJ876d, GJ581c and GJ581d.
Parameterized convection with a phase transition
Thermal histories of the Earth and other planets often rely on parameterized convection for describing heat transport.
Parameterized convection offers a simple relationship that omits the details and complexities of the fluid flow and relates
the heat flux from a convecting fluid layer to the fluid layer’s thermal and mechanical properties. Simple parameterized convection
for a fluid layer with uniform properties throughout is often applied to situations where the basic assumptions of the parameterization
no longer hold.
The presence of a phase boundary creates a two-layer system with each fluid layer having its own mechanical and thermal properties.
When thermal convection occurs, the two fluid layers are coupled and the classical parameterized convection scheme no longer applies.
We attempt to construct a more generalized parameterization for thermal convection by first beginning with a simple two-layer (single phase transition)
fluid that is heated from below, cooled from above, and has stress free boundaries (free slip). A simple model is developed based on the physical properties
of the fluid interface, steady state boundary layer theory, and on the balance of mass, heat, and energy.
The model predicts heat transport through the fluid, the degree of mixing between the two layers, and approximate temperature and velocity profiles throughout the convecting cells.
A numerical 2D thermal convection code is also developed and tested. Predictions from the new two-layer parameterized model are found to show reasonable agreement
with simulation results from the 2D convection code.
Planetary Magnetism and Core Dynamics
The focus of our research is on understanding the generation of planetary magnetic fields. The Earth's magnetic field has been used for navigation
for centuries or even millenia; the first scientific investigations of the field date back to the 13th century; yet today, we still do not understand
the details of the mechanism by which the field is generated.
Short-period signals in geomagnetic secular variation
In this project we investigate whether the short-period signal observed in geomagnetic secular
variation is of external or internal origin by studying time-dependent flow at the core’s
surface at subdecadal timescales. This is an interesting and important period to study for
a number of reasons (e.g. Bloxham, 1998).
First, oscillations at this period can tell us more
about the nature of the core-mantle coupling mechanism, specifically whether the dominant
coupling mechanism is gravitational, topographical or electromagnetic. Second, if subdecadal
variations of geomagnetic secular variation are in fact of internal origin, then they can place
constraints on the conductance of the mantle (Runcorn, 1955). Third, we will able to learn
about subdecadal flows in the core and their effect on the geodynamo. In addition to these
compelling reasons, studies of the underlying dynamics of short-period secular variation will
add to the existing knowledge of dynamics at decadal timescales.
The magnetic field of Saturn
The magnetic field of Saturn however is rather special, as compared to that of other planets, as Saturn is one of two gas giants in our solar system, Uranus and Neptune considered to be ice-giants.
It is a fast rotator and as such has a corotation dominated magnetosphere with its plasma concentrated near the equatorial plane rigidly co-rotating in its inner to
middle magnetosphere. Most importantly, its planetary magnetic field is spin-axisymmetric. This remains unexplained. It also means, given its gaseous nature,
that its interior rate of rotation cannot be unambiguously determined. Another reason why Saturn is of importance at this moment is because it is under current investigation.
Instruments collecting data on virtually every aspect of Saturn are currently in orbit around the planet, providing a wonderful opportunity to study the planet.
We are investigating the magnetic field of Saturn in order to determine whether we can find non-axisymmetric magnetic field components, the pursuit of which will help us
to determine Saturn’s interior rate of rotation as well as characterize externally generated magnetic fields in Saturn’s magnetosphere. We also wish to understand what type of dynamo
could yield a magnetic field such as Saturn’s and what role, if any, Saturn’s zonal flows may play in this. By developing parameter estimation algorithms for the inverse problem of finding a magnetic field model using magnetic field data obtained by the Cassini magnetometer instrument currently in orbit around Saturn, we seek to find answers to these questions.
Mercury's Magnetic Field
Variations in Earth's Rotation Rate at Millenial Timescales
High Resolution Maps of the Magnetic Field at the Core-Mantle Boundary
