Mercury’s Internal Magnetic Field: Results from MESSENGER’s Search for Remanent Crustal Magnetization Associated with Impact Basins

Tuesday, 16 December 2014
Michael E Purucker1, Catherine L Johnson2, Joseph B Nicholas3, Lydia C Philpott2, Haje Korth4, Brian J Anderson5, James W Head III6, Roger J Phillips7 and Sean C Solomon8, (1)Organization Not Listed, Washington, DC, United States, (2)University of British Columbia, Department of Earth, Ocean and Atmospheric Sciences, Vancouver, BC, Canada, (3)Emergent Space Technologies, Greenbelt, MD, United States, (4)Johns Hopkins Univ/APL, Laurel, MD, United States, (5)Johns Hopkins University, Baltimore, MD, United States, (6)Brown University, Providence, RI, United States, (7)Southwest Research Institute, Boulder, CO, United States, (8)Columbia University of New York, Palisades, NY, United States
Magnetic field measurements obtained by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft in orbit around Mercury have entered a new phase since April 2014, with periapsis altitudes below 200 km. MESSENGER is now obtaining magnetic profiles across large impact features at altitudes less than the horizontal scale of those features. We use data from this latest phase to investigate evidence for remanent crustal magnetization specifically associated with impact basins and large craters. The spatial resolution of magnetic field measurements for investigating crustal magnetization is approximately equal to the altitude of the observations. We focus on large impact features because their relative ages provide a powerful chronological tool for interpreting any associated magnetic signatures. We examine profiles across large impact basins such as Caloris, Shakespeare, Budh-Sobkou and Goethe. For example, coverage over Caloris during the last year of the mission will be largely at night and will comprise 18 profiles with altitudes between 125 and 200 km and 12 profiles with altitudes between 50 and 125 km over the northern part of the basin. We use large-scale magnetospheric models developed with MESSENGER data to remove contributions from the offset axial dipole, magnetopause, and magnetotail. The residual magnetic fields above 200 km are still dominated by poorly understood magnetospheric fields such as those from the cusp and from Birkeland currents. We empirically average, or exclude observations from these local times, in order to search for repeatable internal field signals. We use local basis functions such as equivalent source dipoles, applied with regularization tools, in order to map the altitude-normalized magnetic field from internal sources. These internal sources may comprise both crustal and core contributions, and we use the information from the along-track magnetic gradient in order to separate these contributions.