January 14 and October 6, 2008
The top figure shows a view of Mercury from above its north pole and the trajectories along which Magnetometer observations were made by the Mariner 10 mission (blue) and the MESSENGER spacecraft (tan). The MESSENGER data from the mission's second Mercury flyby provide the only data to date from the planet's western hemisphere and are therefore key to constraining the geometry of the planet's internal magnetic field. The bottom figure graphs the magnetic field strengths measured during MESSENGER's first (blue) and second (orange) Mercury flybys, with a striking similarity in the maximum field strength measured during both encounters. The observations are displayed versus distance along the planet-Sun line; closest approach (CA) occurred at about three-fourths of a Mercury radius to the night side of the planet. The magnetopause and bow shock crossings occurred where they were expected, so for this comparison the distance scale for flyby 1 has been stretched so that these boundaries are coincident. Near CA, the flyby 2 data yield a field strength that is only a few percent lower than that obtained from flyby 1 observations. This remarkably close agreement means that the planetary magnetic moment is very nearly centered and is strongly aligned with the rotation axis, to within a tilt of 2°. This result favors models for Mercury's magnetic field generation that predict a magnetic moment aligned with the rotation axis.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
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