Scientists at Johns Hopkins University made a recent discovery that has once again changed our understanding of the solar system. Based on information gathered and sent back to Earth by NASA’s Messenger space probe right before it crashed into Mercury’s surface, we now know that the closest planet to the sun has had a magnetic field for much longer than previously thought.

The data collected by Messenger suggests that Mercury’s magnetic field was much stronger 4 billion years ago that it is today. Researcher Catherine Johnson and her team examined evidence of magnetization apparent in photos of the rocks on Mercury’s crust.

Right after the probe delivered the photo to NASA’s Deep Space Network, which is a collection of global radio antennae that tracks data on the agency’s robotic missions around the solar system, the signal was lost in what scientists assume was the craft’s final contact with the closest planet to the sun.

The four-year mission came to an end when the craft could no longer maintain its orbit around the solar system’s innermost planet due to lack of fuel. Mercury is just 36 miles from the sun, compared to Earth, which is 93 million miles away from the center of the solar system. Mercury is a peculiar world, with both frigid and extremely hot temperatures. Messenger also revealed that Mercury has a magnetic field similar to that of Earth’s, created by the motion of metallic fluids within the planet’s core.

The main challenge the Messenger mission faced was getting the space probe into orbit around Mercury. Due to the planet’s proximity to the sun, it was extremely difficult for flight engineers to avoid its gravitational pull. In addition to the challenge of catching Mercury’s comparatively weak gravitational force, high temperatures also made things tricky. Messenger was equipped with a sunshield designed to protect the spaceship cool on the side that faced the sun. NASA engineers also attempted to chart a long, elliptical orbit around Mercury, giving Messenger time to cool off as it rounded the backside of the planet.

The study was published by Johnson and her colleagues last Thursday, entitled “Low-altitude Magnetic Field Measurements by MESSENGER Reveal Mercury’s Ancient Crustal Field,” in the journal Science.

Messenger had to fly within 100 kilometers of Mercury’s surface in order to measure the strength and direction of the magnetic field. The recorded magnetic signal was much weaker the higher Messenger flew, suggesting that much of the magnetic strength was the result of the presence of magnetized rocks in the crust, said Johnson.

Mercury is the only other planet besides Earth in the inner solar system that boasts a global magnetic field generated by a fluid metallic outer core. Just like Earth’s core, the center of Mercury is made of molten iron.

The history of Mercury’s magnetic field is told by its rocks. The magnetic crystals formed on the planet’s crust suggest that during the time that Mercury was volcanically and tectonically active, between 3.7 and 3.9 billion years ago, an active magnetic field was present as well.

Johnson praises the efforts of NASA engineers to allow Messenger to fly close enough to Mercury’s surface to collect the data that led to this new understanding.