New data from MESSENGER, the spacecraft that orbited Mercury for four years before crashing into the planet a week ago, reveals Mercury's magnetic field is almost four billion years old. The discovery helps scientists piece together the history of Mercury, the closest planet to the sun and one about which we knew very little before MESSENGER.

NASA's MESSENGER probe left Earth in 2004, reached Mercury in 2008 and has orbited the planet since 2011, sending valuable data back to scientists. A study detailing the planet's ancient magnetic field was published in Science Express. Researchers used data obtained by MESSENGER in the fall of 2014 and 2015 when the probe flew incredibly close to the planet's surface - at altitudes as low as 15 kilometers. In the years prior, MESSENGER's lowest altitudes were between 200 and 400 kilometers.

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

Scientists have known for some time that Mercury has a magnetic field similar to Earth's, but much weaker. The motion of liquid iron deep inside the planet's core generates the field.

Mercury is the only other planet besides Earth in the inner solar system with such a magnetic field. There is evidence that Mars once had a magnetic field but it disappeared at some point over 3 billion years ago.

When MESSENGER flew close to the planet, its magnetometer collected data on the magnetism of rocks in Mercury's surface. Those tiny signals revealed that Mercury's magnetic field is very ancient, between 3.7 and 3.9 billion years old. The planet itself formed around the same time as Earth, just over 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," said Johnson, also a scientist at the Planetary Science Institute. "It's just been waiting to tell us its story."