10 September 2019
By studying rocks within impact craters like this one, Joana S. Oliveira, an ESA researcher, found that the location of Mercury's magnetic field has changed over time in surprising ways.
Like Earth, Mercury has a liquid metallic core, with internal motions generating the magnetic field. On Earth, our north and south magnetic poles fluctuate between 10 and 60 km per year, with the orientation of our planet's magnetic field reversed more than 100 times over its 4.5 billion years.
Joana used mission data MESSENGER from NASA, which orbited Mercury between 2011 and 2015, to try to better understand the magnetic history of the innermost planet. The results of the study will help the investigations to be conducted by the joint ESA / JAXA BepiColombo mission, which is on its way to the planet, arriving in 2025.
Scientists use rocks to study how the magnetic fields of the planets evolve. Volcanic rocks created from lava cooling or rocks that have merged into major impact events are particularly useful tools. As the rocks cool down, any magnetic material contained in them aligns with the current field, preserving the direction and position of the planet's magnetic field as a picture in time.
Joana and her colleagues used observations of five-crater spacecraft with magnetic irregularities. One of the craters, called Rustaveli and found in the northern hemisphere, is shown here. It was suspected that the craters formed for a time with a different central magnetic field orientation than the current one. The researchers modeled the ancient magnetic field of Mercury based on crater data to estimate possible pole locations in the past.
They found that they were far from the current position and could have changed over time. They expected the poles to cluster at two points closest to Mercury's axis of rotation in the geographic north and south of the planet. However, the poles were randomly distributed and were all found in the southern hemisphere. The ancient poles do not align with the current magnetic north or south geographic pole of Mercury, indicating that the planet's dipolar magnetic field has moved. The results also suggest that the planet may have shifted along its axis in an event called true polar walk, where the geographic locations of the north and south poles change.
While it is not uncommon for a planet's field to change, the new results reinforce the idea that Mercury's magnetic evolution was very different from Earth's. The BepiColombo mission's two scientific probes will bring together unique magnetic field data and potentially constrain the study's findings, as well as help us contextualize the magnetic evolution of our own planet.
THE new search is published in the AGU Journal of Geophysical Research.
Read a review of the article on the AGU blog.