New Numerical Model Illuminates Mercury's Early Tectonic, Volcanic, and Magnetic Field History
Understanding a planet’s thermal history—how it cools over time—helps scientists determine how its surface, interior and atmosphere evolved. Mercury is among the most enigmatic of the inner solar system planets in terms of its evolution. The Mariner 10 and MESSENGER missions have revealed that the planet has undergone substantial contraction, that it has today and had in the past a global magnetic field, and that it had extensive volcanic activity early in its history. At about one third the radius of Earth, how such a small planet came to be characterized by processes most scientists ascribe to larger planets like Earth has been a mystery.
Mercury is notoriously difficult to observe from Earth, and hard for spacecraft to reach, due to its fast orbital speed and proximity to the Sun. Despite this, some major milestones in the planet’s history are known – revealed by data collected by the only two spacecraft to have visited the planet. The reverse faults that dominate Mercury’s surface are evidence for the first milestone, known since the Mariner 10 mission in the 1970s. These faults indicate that Mercury began to radially contract around 3.9 billion years ago, shrinking due to interior secular cooling, leaving the entire planet marred by giant, step-like scarps. The MESSENGER mission from 2008 to 2015 confirmed these observations and added two new major discoveries to them. Images of the surface from cameras onboard the MESSENGER spacecraft revealed evidence for a second milestone: planet-wide volcanic activity, voluminous enough to have formed a 15-60 km-thick crust that was later deformed as the planet contracted. The magnetic field recorded in this crust is evidence for the third of these milestones: it indicates that Mercury’s core produced an electromagnetic dynamo (i.e., a global magnetic field) from roughly 4 to 3.5 billion years ago. For all three of these events—volcanism, global contraction and an active dynamo—to have occurred, Mercury had to have cooled immensely in the earliest stages of its life as a planet. Until now, no thermal evolution model incorporates this early strong cooling.
In a new approach, PhD Candidate Georgia Peterson and Dr’s Mark Jellinek and Catherine Johnson have created a new set of thermal models for Mercury. Peterson posits that the early ‘volcanic resurfacing’ acted as a major pathway for heat to travel from the interior of the planet to the crust and then out into space, providing the large early cooling event missing in other models. Incorporating this cooling event into their model accounts for the major milestones in the planet’s history: Mercury’s early cooling both sustained its core dynamo by driving convection in the planet’s molten interior, and accounts for the planet’s crustal contraction. The outputs of their model not only predict the total amount of radial contraction, but match the geological evidence that this contraction occurred more rapidly early in Mercury’s history than it does today.
Along with Mercury’s extensive volcanic past, Peterson’s model also takes into account finer details, such as how mantle concentrations of H2O and heat-producing radioactive elements change during volcanic resurfacing, a new approach to calculate the amount of global contraction, and a more rigorous method of determining dynamo generation in a planetary core. Incorporating these new aspects into her modeling approach has had impressive results; 36% of her models can account for Mercury’s early contraction and dynamo milestones, compared to only 2% when the approach in previous studies is used. Their work, recently published in the October issue of Science Advances, opens the door to future studies on Mercury’s modern-day core dynamo and could help reconcile tectonics and dynamo generation on planets like Mars, Venus and Earth that also underwent early global volcanic resurfacing.