Current Research Projects
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Map of the magnetic field of Mars observed by the Mars Global Surveyor satellite at a nominal 400 km altitude. Where the field falls below the minimum contour a shaded MOLA topography relief map provides context. Connerney et al., (2005) Proc. Nat. Academy Sci. 102 (42) 14970-14875. |
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Martian shergottite meteorites, Gusev rocks, Pathfinder rocks, compositions consistent with TES spectra (all from McSween et al., 2009), and the starting materials used by Hammer, et al. |
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BSE images of Fe-rich, Al-poor basalt cooled at varying rates under moderately oxidizing conditions. White crystals are Fe-Ti-Mg-Al spinel carrying remanent magnetization. Titanomagnetite abundance and morphology are intimately linked with cooling rate and oxygen fugacity. |
Combined petrologic and rock-magnetism study of synthetic Martian basalts
The strong intensity of the Martian magnetic anomalies mapped by the Mars Global Surveyor (MGS) has led to considerable interest in the magnetization of the Martian crust, including the remanence acquisition process, composition of the magnetic recording assemblage, and subsequent modification of the remanence by impacts, volcanism, and near surface processes. The magnetic anomalies are more intense than the largest terrestrial anomaly, which requires a very efficient carrier of thermoremanent magnetization (TRM) or chemical remanent magnetization (CRM). On Earth, Fe-Ti spinel (titanomagnetite) is the main carrier of crustal magnetization. Our research seeks to find conditions that produce strong remanent magnetization in magmas relevant to the Mars crust.
We approach this question using a combination of experimental petrology and rock magnetism. Basalts synthesized using experimental parameters that span a range of conditions expected on Mars allow systematic investigation of the effects of chemical composition, fO2, and cooling history on the bulk mineralogy and mineral textures, magnetic mineralogy and domain state, and remanence carrying properties of our run products.
The first major thrust of this research effort, performed in close collaboration with Stefanie Brachfeld (Montclair State University) was to perform dynamic cooling experiments on basaltic starting compositions, and then chracterize splits of the resulting material both petrologically and magnetically. We found that samples synthesized under moderately oxidizing conditions acquired intense thermoremanent magnetizations (TRM), even in relatively weak applied fields. Moderate to high Curie temperatures of these materials imply stable magnetizations down to depths of several 10s of km in the Martian crust.
Julie Bowles further explored the problem by using a starting material with low Fe/Al as well as a starting material with high Fe/Al and incorporating Cr and Mn. The high Fe/Al composition is patterned after the SNC-meteorite parent melts, and the low Fe/Al material is identical except for Fe, Al, and Ca contents. It typifies terrestrial basalt and satisfies compositional constraints posed by deconvolved Thermal Emission Spectrometer (TES) data from the Mars surface (middle right). She found that for rapidly cooled magmas, the most likely set of conditions to result in an intense, stable remanence may be an Fe-rich crust crystallized under moderately oxidizing (IW+2 to QFM) conditions.
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Thermal remanent magnetization acquired in a 49 uT field (a and b), and median destructive temperature derived from thermal demagnetization of TRM (c and d). |
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A second major thrust of this effort, conducted by Lisa Tatsumi-Petrochilos, Julie Bowles, David Cuomo (Montclair), and Deepa Shah (Montclair), is examining the magnetic consequences of low-temperature (subsolidus) annealing of Fe-Ti-Al-Mg spinel. This process is deemed potentially important for Martian crust, but has not previously been examined in a forward experiment (i.e., starting from a homogeneous phase). We predicate this work on the presence of impurity cations in the titanomagnetite solution, which raises the solvus temperature so that unmixing occurs on a laboratory time scale.
Samples containing multidomain oxide grains are synthesized under controlled oxygen fugacity at magmatic temperature, and then held at subsolidus temperature for several months. In some experiments, exsolution occurs and has the expected consequence of reducing magnetic domain size. In other experiments, exsolution does not occur but samples display magnetic susceptibility, NRM, and ARM intensities comparable to or those of their rapidly-cooled counterparts. Even though the magnetic properties of these long-duration anneal samples are moving towards those of their rapidly-cooled counterparts, the rapidly cooled samples retain much higher magnetization at high AF treatments. An Fe-rich/Al-poor intrusion containing exclusively multidomain grains and residing near the Martian surface would likely be shock-demagnetized, given its very soft coercivity spectra.
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_100_c_labeled.jpg "Dynamic cooling experiment run product.")
The olivine-phyric shergottite Yamato 980459 (Y-98), widely believed to represent a primary melt of the martian mantle, is the subject of our latest investigation. Within the context of the larger study aimed at understanding magnetic anomalies in the martian crust, and taking advantage of the unique mineralogical constraints and primitive nature of Y-98, Emily First is conducting dynamic cooling experiments on synthetic Y98 in order to constrain the thermal history of this intriguing martian magma.
Funding: NASA Mars Data Analysis NAG512486 and Mars Fundamental Research Program NNG05GL92G,and NNX11AM29G