A Major Force Is Distorting Mercury's Surface.

Mercury has it rough. Not only is it the smallest planet in the solar system, it’s also the closest to our Sun. This unfortunate position has caused Mercury to develop cracks and fractures across its surface, and generate stresses to its crust, a new study has found.

Mercury is dry, rugged, and heavily cratered; the planet appears deformed with towering cliffs and ridges, as well as fracture lines that run along its surface. The origin of Mercury’s scars has long been a mystery: How did the planet cool and contract in such an unusual way billions of years ago after it formed? Turns out, the answer may be due to its uncomfortable proximity to the Sun. A team of researchers from the University of Bern created physical models of Mercury to see how much of the Sun’s tidal forces affect the small planet, revealing that the star may have influenced the development and orientation of tectonic features on its surface over long periods of time. The results are detailed in a study published in the Journal of Geophysical Research: Planets.

Planets form from the hot, molten material left over from the birth of a star. Over time, these objects cool and their internal materials shrink, causing them to contract as their crusts wrinkle and crack. Evidence has shown that Mercury, on the other hand, not only shrank—its surface also shifted laterally. Cracks and fractures also formed in its rocky crust. Scientists assumed that the process that shaped Mercury’s outer layer was a result of this cooling and contracting, but the study suggests it may be the planet’s cozy orbit around the Sun.

Mercury has one of the most unique orbits in the solar system. It takes about 88 Earth days to complete one orbit around the Sun, during which the planet rotates around its axis three times every two orbits. Its orbit is also highly elliptical and is tilted by around 7 degrees compared to Earth’s orbital plane, its eccentricity means that the tidal forces Mercury experiences from the Sun vary a lot. “These orbital characteristics create tidal stresses that may leave a mark on the planet’s surface,” Liliane Burkhard, a researcher at the Space Research and Planetary Sciences Division at the Institute of Physics at the University of Bern, and lead author of the study, said in a statement. “We can see tectonic patterns on Mercury that suggest more is going on than just global cooling and contraction.”

The team behind the study sought to investigate how these tidal forces contribute to shaping Mercury’s crust. They used physical models of Mercury over the past 4 billion years to calculate how the Sun’s tidal forces may have influenced its surface tensions. The results showed that the the changing gravitational pull of the Sun has impacted Mercury’s tectonic features over time.

“Tidal stresses have been largely overlooked until now, as they were considered to be too small to play a significant role,” Burkhard said. “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.”

The recent findings can also be applied to other planets, illustrating how subtle forces aside from tectonics can make a lasting impact on its surface. “Understanding how a planet like Mercury deforms helps us understand how planetary bodies evolve over billions of years,” according to Burkhard.

The scientists behind the new study are hoping to gather more clues about Mercury’s deformed surface through the BepiColombo mission, which 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 hard to reach due to the Sun’s powerful gravitational pull that may have maimed the planet’s surface.