Mars’s dynamo shut down early in the planet’s history, but it’s hard to tell when. On one hand, many Martian impact basins formed between 4.1-3.7 Ga look completely nonmagnetic at spacecraft altitudes. Because each basin-forming event heats a massive volume of material, which then acquires magnetization in the ambient field as it cools, the simplest interpretation of the weak magnetism of these basins is that Mars’s dynamo was inactive by ~4.1 Ga. However, magnetized volcanic features have been identified that are substantially younger, potentially requiring an active dynamo as late as 3.6 Ga. This potential 500 Myr difference in the dynamo’s estimated longevity could have major implications for Mars’s climate history and the properties of its deep interior!
During my PhD, I have been focused on taking a multidisciplinary approach to better resolving the dynamo’s cessation age. I use a combination of paleomagnetic work and modeling to both place new constraints on Mars’s magnetic history and resolve apparent contradictions in different age constraints.
Martian meteorite paleomagnetism
A magnetic map of part of a thick section of ALH 84001, produced with the quantum diamond microscope at the Harvard Paleomagnetics Lab, overlaid on a reflected light image of the same region.
In the absence of returned samples from Mars, Martian meteorites represent the only direct source of information about Mars’s magnetic field. Although most meteorites are either too young or too shocked to retain a magnetic record of the dynamo, the meteorite ALH 84001 is both old enough (4.1 billion years!) and relatively lightly shocked.
Taking advantage of new high-resolution magnetic imaging techniques to characterized the paleomagnetic record of ancient chromite-sulfide assemblages within an ALH 84001 sample. We found two distinct grain populations magnetized in near-opposite directions, likely reflecting magnetization acquired in two different impact events. Using the ages of known impact heating events experienced by ALH 84001, we were able to date one of these magnetizations to ~3.9 Ga. This requires the Martian dynamo to have been active after 4.1 Ga, and may represent the first paleomagnetic evidence of dynamo reversals on another planet!
Read the paper here!
Large impact basin magnetism
Simulated net magnetization intensity for a model martian impact basin cooled in a reversing dynamo field.
The main roadblock to a longer-lived Martian dynamo is the observation of tens of large impact basins, most formed between 4.1-3.7 Ga, which appear demagnetized in satellite magnetic field measurements. However, if the Martian dynamo was reversing during the slow cooling of these large impact basins, they may appear demagnetized even if the dynamo was active when they formed.
To test this hypothesis, we used finite element methods to model the cooling and magnetization acquisition of large martian impact basins. We found that Earth-like or faster reversal rates could likely produce large basins that appear demagnetized today, especially if excavation and late remagnetization processes were efficient.
Read the LPSC abstract here, and keep an eye out for the paper!
High-pressure magnetic imaging
A pyrrhotite grain from EET A79001 loaded in a QDM-DAC, including a transmitted light image of the source (A) and the corresponding magnetic field map (B).
A key question for meteorite paleomagnetism–especially for Martian meteorites–is the extent to which shock can permanently reset remanence.
We are currently working on using the recently developed hybrid quantum diamond microscope-diamond anvil cell (QDM-DAC) to magnetically image Martian materials under high pressures.
Read the LPSC abstract here.
Quantum diamond microscopy
A magnetic map of a slice of ALH 84001 produced with the quantum diamond microscope at the Harvard Paleomagnetics Lab.
My paleomagnetic work is almost entirely performed using the quantum diamond microscope (QDM) and QDM-DAC in the Harvard Paleomagnetics Lab, which use NV fluorescence in diamond to magnetically image samples at micron-scale resolution. I’m interested in developing procedures for analyzing complex magnetic data, with a particular interest in inversion techniques.