Mercury's magnetic field is almost 4 billion years old

Brett Smith for redOrbit.com – @ParkstBrett

Before slamming violently into the surface of Mercury (RIP), NASA’s MESSENGER probe sent back valuable data on the planet’s magnetic field–namely that it’s almost 4 billion years old.

The revelation appears in a new study, published in the journal Science, and it helps planetary scientists flesh out even more details about Mercury, the only other planet in the inner Solar System besides Earth to have a magnetic field.

The MESSENGER probe left Earth more than a decade ago and has orbited the planet since 2011, gathering valuable data the entire time. The new study is based on data collected in the fall of 2014 and early 2015 when MESSENGER flew as low as 9.4 miles above the planet’s surface. Earlier, MESSENGER’s lowest altitudes were between 130 and 310 miles.

“The mission was originally planned to last one year; no one expected it to go for four,” said study author Catherine Johnson, a University of British Columbia planetary scientist. “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.”

Waiting to tell us its story

Caused by the liquid iron sloshing around inside the planet’s core, Mercury’s magnetic field is considerably weaker than Earth’s. Incidentally, Mars appears to have had a magnetic field once upon a time, but it seems to have vanished around 3 billion years ago.

The data gathered by MESSENGER on the magnetism of rocks in Mercury’s surface showed that the planet’s magnetic field is very old, a minimum of 3.7 to 3.9 billion years old. Mercury itself formed about the same time as Earth, more than 4.5 billion years ago.

“If we didn’t have these recent observations, we would never have known how Mercury’s magnetic field evolved over time,” Johnson said. “It’s just been waiting to tell us its story.”

In a statement, the study team noted that one of the biggest challenges faced by NASA on the MESSENGER mission was getting the probe into orbit around Mercury. The team had to avoid slipping the probe into an orbit around the Sun.

Also, once MESSENGER had established its orbit around Mercury, it had to remain cool enough to function. NASA engineers were able to develop a protective sunshield that safeguarded MESSENGER from the Sun’s radiation at such close range. The mission team also had the probe on a highly-elliptical orbit that allowed it to periodically swing out away from the Sun and cool off.

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