Mercury 4 Bln Years Ago: A Magnetic Field, Volcanoes and Earthquakes

Data from the Messenger spacecraft reveals that Mercury has a weak magnetic field which first switched on almost four billion years ago, but may have turned on and off during its history.

Scientists unraveling the history of Mercury using new results from the Messenger spacecraft have found that its magnetic field is between 3.7 and 3.9 billion years old, having started about 700 million years after the planet's formation.

According to low-altitude observations taken by Messenger in the fall of 2014 and 2015, when the probe flew as close as 15 kilometers from the surface of the planet, its magnetic field has been in place far longer than previously thought, and could have been 100 times more powerful than Mercury has today.

Messenger's Magnetometer instrument collected data on the magnetism of rocks in Mercury’s surface, generated by the motion of liquid iron deep inside the planet's core.

"If we didn't have the recent very low-altitude observations, we would never have been able to discover these signals," explained Catherine Johnson, a University of British Columbia planetary scientist and lead author of the study, published on Thursday in the journal Science.

"Magnetized rocks record the history of the magnetic field of a planet, a key ingredient in understanding its evolution," said Johnson. "We already know that around 3.7 to 3.9 billion years ago Mercury was volcanically and tectonically active. We now know that it also had a magnetic field at around that time." 

This diagram from NASA shows locations of energetic electron events relative to Mercury's magnetic field

© NASA

Mercury is the only other planet apart from Earth in the inner solar system with such a magnetic field, though there is evidence that Mars once had a magnetic field that disappeared more than 3 billion years ago. 

Messenger crashed into Mercury on April 30, when it ran out of fuel after spending four years orbiting the planet. “The mission was originally planned to last one year; no one expected it to go for four,” explained Johnson, adding that more data and analysis is needed to determine if the magnetic field operated continuously, or if it switched off at some point, and later restarted.

"The science from these recent observations is really interesting and what we’ve learned about the magnetic field is just the first part of it," said the researcher.