Astronomers are getting a deeper understanding of one of the most historically mysterious planets in the solar system, its magnetic field in particular.

According to Space.com, Mercury once had a rich magnetic field that is likely billions of years old. In a new study, published in the journal Science, researchers suggest it was at least as strong as Earth's at one point.

NASA's MESSENGER spacecraft recently crashed into Mercury's surface after running out of fuel. Arriving in 2011, it was the first spacecraft to ever orbit the smallest, innermost planet in the solar system.

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

Mariner 10 once gathered information on Mercury's magnetic field, but that was from a series of flybys some 40 years ago. The spacecraft learned the field was far weaker than Earth's, but also that Mercury was the only other rocky planet in the solar system that had one.

"This means Mercury's core has to be at least partially liquid," Johnson told Space.com. "This was a surprise at first, because Mercury is very small, so you would expect it to cool quickly after it formed and be completely solid. Scientists later realized if there was a little bit of nonmetallic stuff in Mercury's core, that'd lower its freezing point and make it hard to be completely solid."

She indicated humans' understanding of Mercury would have been incomplete without the MESSENGER spacecraft.

"If we didn't have these recent observations, we would never have known how Mercury's magnetic field evolved over time," Johnson said in the release. "It's just been waiting to tell us its story."