I:  Long-term Geomagnetic Field Behavior

A long-standing question in geomagnetism has been whether spatial variations in the physical and chemical properties of the lowermost mantle affect outer core dynamics in a way that has an observable surface expression.  For example, there has been ongoing debate as to whether the magnetic field in the Pacific region is anomalous since the paleomagnetic work of Allan Cox and colleagues in the 1970s. My research in this area includes field work, laboratory measurements, data assimilation from the literature, and the development and application of regularized non-linear inversion techniques and statistical approaches to model the time-averaged field and its temporal variations.  My PhD thesis work motivated the need for significant new paleomagnetic data sets, and a multi-institutional data collection effort, the “Time Averaged Field Investigations” project (TAFI) was funded by NSF.  The TAFI project has resulted in new data comprising a global data set almost an order of magnitude larger in number than previously, and with greatly improved geographical and temporal coverage.

II:  Lunar Seismicity

Together with my former PhD student, Reneé Bulow and colleagues Peter Shearer and Philippe Lognonné, I have ventured into a new (to me!) area.  We have been the first group to examine the complete, continuous waveform data set returned as part of the Apollo Passive Seismic Experiment.  This has led to the discovery of previously unidentified deep moonquakes [see New York Times, Feb 15, 2005], and also to a re-examination of lunar seismic velocity models and their implications for the thermal and compositional structure of the moon.  We have investigated the roles of stress and stress-rate in deep moonquake occurrence times and whether failure on a plane is a plausible description of deep moonquake source regions.   Most recently, we have been modeling the lunar seismic noise due to surface impacts, both to better understand the Apollo data and to provide some estimate of the impact flux that might be detected with future broadband seismometers on the Moon.

III:  Lunar Magnetism

I am interested in determining if and when the moon might have had a core dynamo. The proposed 3.9-3.6 Ga dynamo period on the Moon deserves re-examination for several reasons including limitations of the existing paleointensity measurements, and the stringent constraints such a dynamo places on core energetics and mantle thermal evolution.  My former PhD student, Kristin Lawrence, and I conducted a re-evaluation of published paleointensity data and made new measurements of absolute paleointensity on Apollo samples using modern lab protocols adapted specifically to deal with lunar samples. Our results indicate that the samples do not record a primary thermo-remanent magnetization acquired by cooling in the presence of a strong ambient field, but instead record a complex, multi-component magnetization history, possibly involving shock remanent magnetization. These results no longer demand a 3.9-3.6 Ga dynamo field. We are currently pursuing further analyses of Apollo samples, in particular to investigate whether an earlier or later dynamo era is recorded.  Also, together with postdoc, Surdas Mohit, we are investigating possible magnetic field chronologies consistent with the record of crustal magnetization seen global models based on the Lunar Prospector magnetometer and Electron Reflectometer data.

IV:  Mercury’s Magnetic Field 

As a participating scientist in the MESSENGER mission, I am investigating the nature and origin of Mercury’s internal magnetic field.  This is an emerging area of study: my Masters student and I already have a publication in review and have contributed to the Science paper this year led by the Magnetometer team PI.

V:  Mars

I am interested in the relationships among Mars’ magnetic field history, magmatism, heat flow and atmospheric evolution.  In a paper with Roger Phillips I examined the magnetic field anomalies in the Tharsis region of Mars, to investigate the temporal relationships of earliest Tharsis magmatism and the martian dynamo.  This work has prompted new investigations by other groups.  Mark Jellinek, Jerry Schubert and myself have used topography and magnetic field observations to place constraints on early heat flow and melt production in the Tharsis region. Kristin Lawrence, Carol Paty and I are investigating whether a crustal magnetic field on Mars could have helped protect an early atmosphere – most studies to date have investigated only the role of a global dynamo field, however uncertainties in the timing of the martian dynamo and the release of volatiles to the atmosphere from volcanism warrant consideration of the role a crustal magnetic field could have played, if any.