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A false colour image from the Messenger spacecraft shows part of a volcanic plain (blue) known as Suisei Planitia, one of the places where the probe detected traces of Mercury’s ancient magnetic field lingering in the metal-bearing minerals of the surface rocks.

NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Just one week after a NASA spacecraft ended its mission by slamming into the surface of Mercury, scientists have revealed an important discovery made during the probe's final months of operation.

As it glided low over the Mercury's cratered terrain, in a deliberate death spiral that would eventually cause it to crash, the Messenger spacecraft picked up the weak traces of an ancient magnetic field, frozen in surface rocks. The discovery confirms that the planet's core has been at least partly molten and geologically active throughout its existence and sheds light on the evolution of the solar system's rocky planets, including Earth.

"These signals are so small that we could easily have missed them if we'd never had the observations at low altitude," said Catherine Johnson, a planetary scientist at the University of British Columbia and lead author of a report describing the find, published Thursday in the journal Science.

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Messenger was launched in 2004 and became the first spacecraft ever to orbit the planet six years later. The mission was a feat of space engineering because of Mercury's close proximity to the sun. As it orbited, the spacecraft had to hide behind a ceramic heat shield to prevent its sensitive electronics from getting fried.

Last year, with Messenger's fuel running out, scientists decided to end the mission by bringing the probe as near as they could to the planet's surface.

It was during a series of close approaches over Mercury's northern hemisphere, including some as low 15 kilometres above the surface, that Messenger detected a relic magnetic field in the planet's 3.7 to 3.9 billion-year-old crust. The field is still present because metal-bearing minerals in the surface rocks were magnetized as they cooled and solidified.

To detect the relic field, Dr. Johnson and her colleagues had to first carefully subtract the much stronger signal of the planet's current magnetic field from their data. Mercury is the only rocky planet besides Earth that generates its own magnetic field today. The source is thought to be the movement of electrically charged fluids near the top of the planet's massive iron core.

The researchers estimate that Mercury's magnetic field was at least as strong in the distant past as it is now and it may have been up to 100 times stronger. The result is important for the understanding of the planet's history, because it tells scientists what the planet's core was doing at a time when volcanoes were erupting all over its surface. This shows the rate at which Mercury was cooling early in its history and further clarifies how rocky planets change over time.

"If we are interested – and we are – in [Earth's] magnetic field, then we'd better also understand what Mercury is offering here," said Karl-Heinz Glassmeier of the Braunschweig Technical University in Germany.

Dr. Glassmeier is principal investigator of the magnetometer experiment on the BepiColombo spacecraft, a joint mission of the European and Japanese space agencies that is scheduled for liftoff to Mercury in 2017.

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He added that BepiColombo should reveal even more details about Mercury's magnetism – past and present – because it will examine regions of the planet where Messenger could not provide much data.

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