The magnetic field of Mercury, closest planet to the sun, is almost 4 billion years old and may once have been almost as strong as Earth's, scientists say.

That discovery by NASA's Messenger spacecraft, which studied Mercury from orbit for 4 years before crashing into the planet's surface last week, will help in understanding more about how Mercury has evolved over time, they say.

It's proof that Mercury is the only planet in the solar system other than Earth that possesses a global magnetic field generated by a shifting molten core of liquid iron, researchers say.

Scientists have seen evidence suggesting Mars once possessed a magnetic field, but it apparently disappeared more the 3 billion years ago.

Venus has either never had such a field or it is so weak it has remained undetected by planetary scientists.

Mercury formed about the same time as the Earth, and at 3.9 billion years and still detectable, its magnetic field may be the longest-lived inside any planet of the inner solar system, the researchers say.

"This means Mercury's core has to be at least [still] partially liquid," says study lead author Catherine Johnson, a planetary geophysicist at the University of British Columbia.

That finding is somewhat surprising, she says, since Mercury's small size suggests it would have cooled quickly after forming and become totally solid.

Data from Messenger gathered in 2014 and early this year, when it approached to within 90 miles of Mercury's surface, indicated trace signs of magnetization of the planet's crust, scientists reported in the journal Science.

The weak signals reveal the magnetic field is somewhere between 3.7 and 3.9 billion years old.

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

Although relatively weak now, Mercury's magnetic field may have once been as much as 100 times as strong, equivalent to the Earth's magnetic field today, she says.

The slow evolution and weakening of the field can help in understanding Mercury's structure and composition and how they changed over time as the planet cooled after forming, the researchers say.

"Being able to pin down how long Mercury has had a magnetic field helps us narrow down scenarios for the early history of Mercury and how it has changed over time," Johnson says. "This in turn helps us understand more about planetary evolution in general."

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