Mercury’s magnetic field has been up and running for roughly 3.8 billion years, researchers report online May 7 in Science. The finding provides new insight into how the innermost planet evolved during its more vibrant youth.
Magnetized rocks scattered across ancient volcanic plains preserve signs of the planet’s magnetic field from about 800 million years after the birth of the solar system, geophysicist Catherine Johnson and colleagues report. The fossilized magnetic field was detected by the MESSENGER spacecraft, which orbited the planet for four years before crashing April 30 (SN Online: 4/30/15).
The ancient magnetic field, generated by underground currents of molten metal powered by a rapidly cooling core, is a key piece to understanding the history of the sun-scorched world, says Johnson, of the University of British Columbia in Vancouver. Until now, researchers didn’t know when the field became active.
The new data indicate that the magnet was switched on at a time when volcanoes still sculpted the surface. “Mercury had to be cooling fast enough to generate a magnetic field,” Johnson says, “while at the same time staying warm enough to drive volcanic activity.”
MESSENGER teased out the relic field by swooping in low during its final months of orbit, coming within 25 kilometers of Mercury’s surface. By extending its magnetometer, the spacecraft could pick up magnetic signals from the planet. “The signal was there the whole time,” Johnson says. Without the risky low-altitude flyovers, “we could have missed them.”
An early magnetic field is at odds with how little the planet has shrunk since it formed, “which is what makes this so interesting,” says Jean-Pierre Williams, a planetary scientist at UCLA. The new findings indicate that Mercury was forming a solid inner core quite early. However, Williams says, the planet’s modest shrinkage — just 7 kilometers (SN Online: 3/16/14) — suggests that the inner core formed much later, which would create a small core that leads to less contraction.