Most of Mercury's crustal magnetic field can be explained by iron-bearing rocks responding to the planet's weak present-day core field, according to a new study that challenges the long-standing interpretation that an ancient dynamo is required. The research, led by Catherine Johnson at the University of British Columbia in Vancouver, used observations from NASA's MESSENGER mission to build a magnetization model of the planet and identify the sources of its crustal magnetism.
Mercury is the only planet in the solar system besides Earth that possesses both a global magnetic field generated by its core and a crustal magnetic field produced by magnetized rocks. Present-day, or induced, magnetization reveals where iron is located in the crust and how much is there. Ancient, or remanent, magnetization records the history of the planet's dynamo. Separating the two contributions has been a central challenge for planetary scientists.
NASA's MErcury, Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) mission observed Mercury from 2011 to 2015. A last-minute decision during the mission to lower the spacecraft's orbit allowed crustal fields to be detected, said Lon Hood of the Lunar and Planetary Laboratory at the University of Arizona. Hood has worked extensively on Mercury's crustal magnetism using MESSENGER data but was not involved in the new study.
The new analysis calculated the expected strength of induced magnetization using low-altitude observations, together with assumptions about crustal thickness, magnetic mineralogy, iron content, and the present-day core field. Consistent with earlier work, the researchers found the largest magnetization strengths in the Caloris region, along with localized signals elsewhere.
However, the spatial pattern of magnetization strength does not consistently match variations in near-surface iron content seen in MESSENGER spectroscopic data. Instead, magnetization strength correlates with crustal thickness up to about 30 kilometers, suggesting that magnetized rocks are concentrated in the upper part of the crust.
Induced magnetization is relatively small given the planet's low crustal iron content and its weak core field, which is about a hundred times weaker than Earth's. Even so, the team estimates that induced magnetization can fully account for the calculated magnetization strength across more than 85% of the area north of 38 degrees north latitude, assuming it extends through the crustal column to a depth of about 30 kilometers. That means no remanent magnetization is required to explain the observed crustal field over the majority of the mapped region.
Areas where induced magnetization does not explain the observed fields may result from iron delivered by ancient asteroid impacts, which would locally increase induced magnetization. Elsewhere, a contribution from remanent magnetization may still be needed.
On Earth, separating induced and remanent magnetization reliably requires laboratory measurements of rock samples. No such samples exist for Mercury, so the magnetic properties of its crust must be inferred from satellite observations constrained by assumptions about the composition and magnetic behavior of its rocks. Hood noted that the delineation in the new study depends on assumptions about magnetic mineralogy that cannot be verified without returned samples.
Better characterization of the crust's low-field magnetic susceptibility, currently inferred from a limited number of laboratory measurements on meteorites and minerals believed to resemble Mercury's crust, could improve understanding of the planet's crustal fields. Better core-field models and higher-resolution iron maps from the European-Japanese BepiColombo mission could also help. That mission comprises two spacecraft that launched in 2018 and is expected to run until 2029.
Hood cautioned, however, that there are no current plans for either spacecraft to approach near enough to the planet to detect or map more of the crustal magnetic field. The spacecraft will eventually descend below 100 kilometers, providing a first look at the southern hemisphere. There are also no plans to obtain samples of Mercury's crustal rocks, which would be needed to definitively determine the origin of the crustal magnetization. The researchers report their findings in PNAS.
