The magnetic field of Mercury first formed nearly 4 billion years ago, according to new findings recorded by the Messenger spacecraft. This highly unusual discovery was among the last new findings about the planet by an observatory that far outlived its planned lifespan.

The MErcury Surface, Space ENvironment, GEochemistry and Ranging (Messenger) spacecraft was launched to Mercury in August 2004, arriving at the planet seven years later. The observatory carried out a study of the planet for four years, three years longer than its planned mission life of 12 months.

Like the Earth, Mercury was found to possess a magnetic field, albeit a much weaker one than that which surrounds our own planet. Astronomers believe this electromagnetic feature is generated by the movement of liquid iron in the core of the innermost planet, similar to the process that occurs within the Earth.

Mercury formed, along with the sun and the rest of the solar system, roughly 4.4 billion years ago. The data from Messenger suggests the magnetic field of that planet first formed somewhere between 3.7 and 3.9 billion years in the past. The feature was found by a magnetometer aboard the long-lived observatory, as it got closer and closer to the surface of its adopted world during the end of 2014 and the beginning of 2015. Magnetism recorded in rocks on the crust of the planet was too weak to record from a higher orbit.

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

Neither Venus nor Mars has a magnetic field, although research suggests the Red Planet may have possessed one over 3 billion years in the past.

"Magnetized rocks record the history of the magnetic field of a planet, a key ingredient in understanding its evolution. We already know that around 3.7 to 3.9 billion years ago Mercury was volcanically and tectonically active. We now know that it had a magnetic field at around that time," Johnson said.

Messenger was just the second spacecraft from Earth to visit the tiny planet. Mariner 10, launched in 1973, flew past Mercury but never orbited the planet, unlike the most recent mission. Between 2001 and 2015, Messenger completed over 4,000 orbits of the planet before crashing on April 30, 2015. The first visit to the innermost world of the solar system was the last flight of the Mariner program that explored the inner solar system from 1962 to 1973.

Discovery and analysis of the ancient magnetic field of Mercury was detailed in the journal Science.