Courtesy: NASA/GSFC
My Ph.D. work focussed on Martian Geodynamics. I find this field so interesting, because we can answer important first-order questions about the shape of Mars. Why is there a hemispherical dichotomy? Why is it oriented north-south? Why is Tharsis so big? How active was the planet in the past? Topography and gravity data from the many spacecraft missions are the primary observations we can use to study these problems. I developed theoretical and computational models to explain these observations. Just by looking at the MOLA topography map below, you can tell that something is up.
Courtesy: NASA/GSFC
I got my Ph.D. from the Department of Astrophysical and Planetary Sciences at the University of Colorado in Boulder. My thesis advisor, Shijie Zhong is a professor in the Department of Physics.
Tharsis is a broad volcanic region on Mars, comprising up to one quarter of the planet's surface area, and representing a substantial long-wavelength topography and geoid anomaly. One possibility is that Tharsis is dynamically supported by a long-wavelength mantle plume pushing up the surface from below. Another is that Tharsis is simply a large volume of volcanic material piled up on the surface. While both scenarios can account for the observed topography, they will have very different signatures in the geoid (equipotential surface).
Using a finite-element code, Citcom, we ran several numerical models of convection in the Martian mantle, with temperature- and depth-dependent viscosity. We found that we can generate long-wavelength plume structures in the Martian mantle with this kind of formulation. We then treated the lithosphere as an elastic shell and calculated the deformation that stresses from the plume buoyancy would produce to find the topography. We calculated the geoid anomalies produced both by the dynamic topography and the mass anomaly of the plume itself. Since the geoid anomalies calculated from this model do not match the MGS observations, we concluded that Tharsis is not dynamically supported and is probably some kind of a volcanic construction.
I have investigated the origin of the hemispheric dichotomy on Mars. Currently, there is little consensus among the Mars community as to whether the dichotomy has an exogenic origin (one or more giant impacts excavating what is now the northern lowlands), or an endogenic origin (degree-1 mantle convection, degree-1 magma ocean overturn, plate tectonics). Shijie and I suggest that it's likely that this degree-1 feature is somehow related to the mantle dynamics. I've run a series of 3D convection models to see under what conditions a degree-1 pattern may develop, and on what timescale. Using a layered viscosity structure that may be appropriate to early Mars, I've been able to develop a single plume structure in under 100 My, which is fast enough to predate the dichotomy. Click on the image to the right for an animation. Warning: it's big.
A possible future direction for this project is to demonstrate that the single plume structure in the mantle can cause the crust to assume a similar shape. During the Noachian, the Martian mantle was much warmer than it is today, and the lithosphere was quite thin. The crust, however, appears to be relatively thick, possibly 50 km. In this situation, the lower crust much be mobile. A degree-1 convection pattern could erode the lower crust from the area above the plume and deposit it at the bottom of the crust over the upwelling. Alternaltely, a degree-1 plume may generate substantial melt which may form a thick crust above the upwelling.
Another curious characteristic of the hemispheric dichotomy is that it's pretty much north-south and not east-west or some intermediate orientation. Did the dichotomy form in this rather specific orientation, or has the planet shifted since its formation?
I've investigated the possiblity of True Polar Wander on early Mars, that is a wholesale shift of the planets orientation due to the location of mass anomalies. Simply put, planets are in stable rotation states when positive mass anomalies (such as large mountain ranges or volcanoes) are on the equator and negative mass anomalies (such as depressions or buoyant mantle plumes) are at the pole.
It's been suggested that Tharsis, a large positive mass anomaly may have reoriented the planet such that Tharsis moved to the equator. However, that doesn't explain the orientation of the dichotomy. Tharsis is on the dichotomy boundary and so the dichotomy could have any orientation. Furthermore, if the planet reoriented, the readjustment of the rotational bulge should have resulted in a rather characteristic pattern of fractures which has not been observed.
I've found that the dichotomy itself represents a negative long-wavlength geoid anomaly. The northern lowlands are essentially a giant hole in the ground. Because the dichotomy predates Tharsis, the dichotomy itself is sufficient to drive True Polar Wander, resulting in lowlands that are centered on the pole. When Tharsis subsequently formed on the dichotomy boundary it was already close to the equator. Thus, there was no need for Tharsis to drive a great deal of additional reorientation as it formed close to a rotatinally stable position.
The relationship between the dichotomy and Tharsis is still curious. These two large-scale structures are 90 degrees apart. But why? This remains an active area of research. I'll hope to add some images later to illustrate this a bit better.