Mercury has a tough time. It is not only the smallest planet in the Solar System, but also the closest to our Sun. This unfortunate location has led to cracks and fractures on the surface of Mercury, and its crust has been subjected to significant stress, a new study has shown.
Mercury is dry, harsh, and heavily cratered; the planet appears deformed with high cliffs and ridges, as well as fault lines running along its surface. The origin of Mercury’s scars has long remained a mystery: how did the planet cool and shrink in such an unusual way billions of years ago after its formation? It turns out that the answer may be related to its uncomfortable proximity to the Sun. A team of researchers from the University of Bern created physical models of Mercury to see how the tidal forces of the Sun affect the small planet and found that the star could influence the development and orientation of tectonic features on its surface over long periods of time. The results of the study are described in detail in an article published in the Journal of Geophysical Research: Planets.
Planets are formed from hot molten material left over from the birth of a star. Over time, these objects cool and their internal materials compress, causing their crust to wrinkle and crack. Evidence shows that Mercury, on the other hand, has not only shrunk – its surface has also shifted sideways. Cracks and faults have also formed in its rocky crust. Scientists had assumed that the process that formed Mercury’s outer layer was the result of this cooling and compression, but the study suggests that it could be the planet’s cozy orbit around the Sun.
Mercury has one of the most unique orbits in the solar system. One revolution around the Sun takes about 88 Earth days, during which the planet rotates on its axis three times for every two orbits. Its orbit is also highly elliptical and tilted about 7 degrees compared to the plane of the Earth’s orbit, its eccentricity meaning that the tidal forces Mercury experiences from the Sun vary greatly. “These orbital characteristics create tidal stresses that can leave a mark on the planet’s surface,” said Lilian Burkhard, a researcher at the Department of Space Research and Planetary Sciences at the Institute of Physics at the University of Bern and lead author of the study, in a statement. “We see tectonic structures on Mercury that indicate that something more than just global cooling and contraction is happening.”
The study team sought to find out how these tidal forces contribute to the formation of Mercury’s crust. They used physical models of Mercury over the past 4 billion years to calculate how the Sun’s tidal forces might have affected its surface tension. The results showed that changes in the Sun’s gravitational pull have affected Mercury’s tectonic features over time.
“Until now, tidal stresses have been largely ignored because they were thought to be too small to play a significant role,” Burkhardt said. “Our results show that although the magnitude of these stresses is not sufficient to generate faults, the direction of the tidally induced shear stresses is consistent with the observed orientations of fault-shear faults on the surface of Mercury.”
The recent findings can also be applied to other planets, illustrating how subtle forces other than tectonics can have a long-term impact on their surfaces. “Understanding how a planet like Mercury deforms helps us understand how planetary bodies evolve over billions of years,” Burkhardt says.
The scientists behind the new study hope to gather more clues about Mercury’s deformed surface with the BepiColombo mission, which was launched in October 2018 as a joint venture between the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA). BepiColombo is only the third spacecraft to visit Mercury; the elusive planet is difficult to reach due to the Sun’s powerful gravitational pull, which may have damaged the planet’s surface.
