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.
Caption: Above, in this perspective view, we look 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). The impact crater Kosho, 65 km in diameter, is seen in the center of the image (deep blue floor), and part of the crater Strindberg, 190 km in diameter, is seen in the lower left at the edge of the image. The background image is Mercury Dual Imaging System global mosaic, colored by surface elevation measured by the Mercury Laser Altimeter (MLA), both draped over a digital elevation model derived from MLA data. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

The MESSENGER spacecraft crashed onto Mercury last week after running out of fuel, but the mission provided a trove of new information on the planet closest to the Sun.

“From MESSENGER and Mariner 10 observations we already knew that Mercury has had a global magnetic field today and 40 years ago,” said Johnson, a senior scientist at the Planetary Science Institute and lead author of “Low-altitude Magnetic Field Measurements by MESSENGER Reveal Mercury’s Ancient Crustal Field,” published today in the journal Science.

With MESSENGER orbiting Mercury closer than 100 kilometers from the planet’s surface, the spacecraft’s Magnetometer instrument that measures magnetic field strength and detection was able to resolve signals too small to be detected earlier at higher altitudes. The observed decrease in signal strength measured with changes in altitude from 15 to 80 kilometers confirms that the signals are due to the presence of magnetized crustal rocks, Johnson said.


Caption: 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

Mercury is the only inner solar system body other than Earth that currently possesses a global magnetic field generated by a dynamo in a fluid metallic outer core. In Mercury, as in Earth, the outer core is molten iron.

“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.”

“If we didn’t have the recent very low-altitude observations, we would never have been able to discover these signals,” said Johnson. “Mercury has just been waiting to tell us its story.”

Visit http://www.psi.edu/news/mercurymagnetism for some images relating to Johnson’s research.

Johnson is a Participating Scientist on the MESSENGER mission, and is also a Professor of Geophysics at the University of British Columbia, Vancouver.