Around one week after it smashed spectacularly into Mercury, NASA's MESSENGER probe has posthumously turned up valuable observations revealing some of the planet's long-held secrets. Using data gleaned from the spacecraft's closest pass of the planet earlier this year, scientists have established its magnetic field to be almost 4 billion years old, shedding new light on how Mercury has evolved over its 4.5 billion year lifespan.

During a flyby of Mercury in 1974, the Mariner 10 space probe first detected the planet's global magnetic field and a large, iron-rich core. At the time, Mariner 10 measured the strength of this field to be around one hundred times weaker than Earth's. Not in the four decades since have we so closely studied the makeup of our solar system's smallest planet.

Since it first entered orbit in 2011, MESSENGER had made passes of Mercury as close as 200 km (124 mi). But in late 2014 and early 2015, the spacecraft really went in for the kill, skipping past the planet's surface at an altitude of just 15 km (9 mi). It was during this pass that MESSENGER's magnetometer examined the magnetism of rocks on the surface, gathering data that would reveal for the first time the age of the global magnetic field to be 3.7 to 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," says Catherine Johnson, planetary scientist at the University of British Columbia and lead author of the study. "It’s just been waiting to tell us its story."

It is the motion of the liquid iron inside the planet's core that produces the magnetic field. Deducing how long it has existed could help scientists form a more complete picture of the planet's history, geology and chemical makeup.

The findings of the study were published in the journal Science Express.