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MESSENGER Finds Evidence of Ancient Magnetic Field on Mercury

Mercury’s magnetic field, generated by a dynamo process in its outer core, has been in place far longer than previously known, research by PSI’s Catherine Louise Johnson found. 
About 4 billion years ago, Mercury’s magnetic field could have been much stronger than today, as indicated by low-altitude observations made by NASA’s MESSENGER spacecraft that revealed evidence of magnetization of ancient crustal rocks on Mercury.  
“Magnetized rocks record the history of the magnetic field of a planet, a key ingredient in understanding its evolution,” Johnson said. “We already know that around 3.7 to 3.9 billion years ago Mercury was volcanically and tectonically active.  We now know that it had a magnetic field at around that time.”
Above, in this cartoon of field lines and observed signal, the white lines show schematic magnetic field lines above the surface of Mercury from magnetized crustal rocks. The MESSENGER spacecraft passed over a region of crustal magnetization and the Magnetometer instrument measured small variations in the magnetic field (illustrated by the blue wiggly line). Because the signals are small they were only observed when MESSENGER was very close to the planet.
Credit:  NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Below, this perspective view looks west across Suisei Planitia (blue colors), the site of some of the crustal magnetic signals. The plains are comprised of volcanic lava flows that erupted and solidified several billion years ago, filling the low areas between the higher topography (red colors).
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

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