Tidal forces from the Sun could have deformed cliffs on Mercury

The surface of Mercury is characterized by hills and steep cliffs. Unlike Earth, Mercury has no tectonic plates that shift against each other. Instead, its crust is similar to a single solid shell. Hills and cliffs were largely formed because the planet cooled and shrank over time after its formation over 4.5 billion years ago.

As a planet cools, it usually contracts evenly with little sideways motion. However, close observations of Mercury show that its surface not only shrank but also shifted laterally, hinting that more complex internal and external forces are at play. A new study by Dr. Liliane Burkhard and Prof. Dr. Nicolas Thomas from the Space Research and Planetary Sciences Division at the Institute of Physics at the University of Bern now provides further explanations for the formation of these surface structures. In the new study, they show that Mercury's surface features were not only influenced by the planet’s cooling and contraction, but also by tidal forces related to Mercury's orbit around the Sun and its rotation around itself. The study has just been published in the Journal of Geophysical Research: Planets.

New clues from models of Mercury's orbit and rotation

Scientists have long been debating exactly how the fault patterns observed on Mercury, i.e. cracks and fracture lines in the rocky crust, were formed. Until now, it was assumed that Mercury’s tectonics were mainly a result of the cooling and contraction of the planet. But Mercury’s unique orbital behavior may also play a role.

The small planet in the immediate vicinity of the Sun is exposed to its enormous gravitational field and thus its tidal force. Mercury is locked in a 3:2 spin-orbit resonance, which means that it rotates three times around its axis for every two orbits around the Sun. In addition, its orbit is highly elliptical instead of circular, causing the tidal forces it experiences to vary periodically over time. The changing forces generate stresses within the crust, gradually deforming the surface and potentially influencing tectonic activity. "These orbital characteristics create tidal stresses that may leave a mark on the planet’s surface. We can see tectonic patterns on Mercury that suggest more is going on than just global cooling and contraction. Our goal was to investigate how tidal forces contribute to shaping Mercury’s crust," explains Liliane Burkhard, first author of the study.

Burkhard and Thomas created physical models of Mercury's interior, both present and past, and used these to calculate how the Sun's tidal forces could influence surface tensions over the last four billion years. "By changing parameters such as rotational speed and orbital eccentricity, we were able to simulate and deduce how Mercury's tectonics might have evolved," explains Burkhard.

The influence of the Sun

The research results show that the tidal forces of the Sun could have influenced the development and orientation of tectonic features on Mercury's surface over long geological periods of time. "Tidal stresses have been largely overlooked until now, as they were considered to be too small to play a significant role. Our results show that while the magnitude of these stresses is not sufficient to generate faulting alone, the direction of the tidally induced shear stresses are consistent with the observed orientations of fault-slip patterns on Mercury’s surface. This suggests that tidal stresses may have influenced the development and shear orientation of tectonic features over long geologic time periods. This is an aspect of Mercury's evolution that has not yet been explored," says Burkhard.

These results illustrate that even subtle forces such as the changing gravitational pull of the Sun can leave a lasting impression on a planet's surface. "Understanding how a planet like Mercury deforms helps us understand how planetary bodies evolve over billions of years," emphasizes Burkhard. These findings could also be applied to other planets in order to better understand their geological history and development.

Bernese participation in the BepiColombo mission

The researchers hope to gain further insights from the analysis of data from the BepiColombo mission of the European Space Agency (ESA) and the Japanese Space Agency (JAXA), which is currently on its way to Mercury. The University of Bern is involved in this mission with two instruments, including the laser altimeter BELA (BepiColombo Laser Altimeter), which was designed and built at the Physics Institute of the University of Bern with an international consortium. Nicolas Thomas is co-project leader, Liliane Burkhard instrument scientist for BELA.

In April 2018, BELA embarked on its 80 million kilometer journey to the planet Mercury on board the Mercury Planetary Orbiter, where it will arrive at the end of 2026. "BELA is a laser altimeter that will use laser pulses to measure the distance to Mercury's surface from an altitude of around 1,000 km with an accuracy of around 10 cm. Thanks to this data, we will be able to create a 3D image of Mercury's topography," says Thomas. Burkhard adds: "The data from BELA will help us refine the models for tectonic deformation and surface composition in order to draw an even more detailed picture of the geological processes on Mercury," concludes Burkhard.