Mercury’s magnetic field, generated by a dynamo process in its outer core, has been in place far longer than previously known, a paper by Planetary Science Institute’s Catherine Johnson reports.
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 magnetisation of ancient crustal rocks on Mercury.
The MESSENGER spacecraft crashed onto Mercury on 30th April 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 May 7th in the journal Science.
With MESSENGER orbiting Mercury closer than 100 kilometres (~60 miles) 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 kilometres (9 to 50 miles) confirms that the signals are due to the presence of magnetised crustal rocks, Johnson said.
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.
“Magnetised 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.”