Mercury is mostly known for being small and impossibly hot thanks to its proximity to the Sun. But that can’t be all, right? After all, you don’t just name planets for the Roman god of war without good reason. As it turns out, Mercury is indeed more than a fiery little ball of rock: Before crashing into its surface, NASA’s Messenger spacecraft revealed that not only does it have a liquid core, but it has a magnetic field that’s much older than any scientists previously thought. Given the similarities between Mercury and Earth, the finding could tell scientists more about our own planet as well.
“From MESSENGER and Mariner 10 observations we already knew that Mercury has had a global magnetic field today and 40 years ago,” said Johnson, a senior scientist at the Planetary Science Institute and lead author of “Low-altitude Magnetic Field Measurements by MESSENGER Reveal Mercury’s Ancient Crustal Field,” published today in the journal Science. “Magnetized rocks record the history of the magnetic field of a planet, a key ingredient in understanding its evolution,” Johnson said. “We already know that around 3.7 to 3.9 billions years ago Mercury was volcanically and tectonically active. We now know that it also had a magnetic field at around that time.”
Messenger first left Earth in 2004, and first approached Mercury in 2008. Full-time orbit began in 2011, and continued until last week, when a loss of fuel sent the probe plummeting to the planet’s surface. The data used for the study were obtained late last year and early this year, when Messenger flew at it’s lowest altitudes – just 15 km above the surface, in some cases. When it fell, it went out with a bang – it crashed at a speed of about 8,700 mph, leaving a crater 16m in diameter.
“If we didn’t have the recent very low-altitude observations, we would never have been able to discover these signals,” said Johnson. “Mercury has just been waiting to tell us its story.” Indeed, at this point Mercury’s magnetic field is so weak that only the closest approaches allowed Messenger to pick up the signal.
Mercury is the only other planet in the solar system besides Earth with such a magnetic field, though Mars may have had one at one time. Since Mercury and Earth share similar magnetic field characteristics and were formed at about the same time (about 4.5 billion years ago), it behooves scientists to learn all they can about it: Clues to Mercury’s early years may well be clues to our own here on Earth.
Messenger revealed that Mercury’s magnetic field, though now weak, is much older than they thought and was probably much stronger nearly 4 billion years ago. They also discovered that Mercury has a magnetic core, and it’s exceptionally large compared to our own – it accounts for about 85% of Mercury’s mass, NASA estimates.
Sadly, Mercury’s liquid core is not made of mercury – just plain old iron, similar to Earth’s core.
Though Messenger is no more, NASA scientists are confident that the bevy of data returned by it will provide more findings in the future.