Planetary scientists have long been fascinated by the enigmatic planet Mercury. As the closest planet to the Sun, it has long been known that Mercury has been shrinking for billions of years. However, the extent to which it is still contracting and the implications for its geological features have remained unclear. In this article, we delve into the newly published research that sheds light on Mercury’s shrinking and the resulting surface features known as “thrust faults.”
Mercury’s interior cooling has caused the planet to gradually contract over time. This contraction leads to the development of thrust faults, where one area of terrain is pushed over another. These faults are akin to the wrinkles that form on aging apples, but in Mercury’s case, the shrinkage is due to the thermal contraction of its interior. The first evidence of Mercury’s shrinkage was observed in 1974 when the Mariner 10 mission captured images of kilometers-high scarps, or ramp-like slopes, spanning hundreds of kilometers. Additional observations by the Messenger mission confirmed the presence of these lobate scarps all over the planet.
To estimate the age of Mercury’s surface, scientists typically count the density of impact craters. Older surfaces tend to have more craters. While this method provides some insight, it is complicated by the fact that the rate of impacts that produce craters was much higher in the planet’s distant past. Nonetheless, it has been widely accepted that Mercury’s scarps are approximately 3 billion years old. However, uncertainty remains as to whether all scarps are that old and whether they are still active today.
A breakthrough in understanding the ongoing movements of Mercury’s scarps came when a PhD student at Open University in the UK noticed small fractures accompanying the stretched upper surfaces of some scarps. These fractures, known as “grabens,” occur when the crust is stretched. Although stretching may seem contradictory to the overall compression of the crust on Mercury, it makes sense if a thrust slice of crust bends as it moves over adjacent terrain. This bending is similar to cracking a piece of toast. The grabens, which are less than 1km wide and about 100 meters deep, are significantly younger than the ancient structures on which they sit. Their relatively small size suggests they are less than 300 million years old, indicating recent movement.
Through detailed analysis of images provided by the Messenger mission, 48 large lobate scarps with grabens were identified, along with 244 scarps that likely have grabens, but they were not clearly visible. The upcoming BepiColombo mission, a joint European and Japanese mission, will provide even better imaging capabilities and is expected to confirm these findings. By studying the small grabens more clearly and potentially identifying boulder tracks resulting from recent quakes, BepiColombo can offer further evidence of Mercury’s ongoing geological activity.
The Moon, like Mercury, has also cooled and contracted over time. Although its lobate scarps are smaller and less spectacular, recent reanalysis of moonquake data has shown that moonquakes are clustered close to the scarps. Additionally, detailed orbital images of the Moon’s surface reveal boulder tracks made by rocks dislodged during moonquakes. These smaller features, like Mercury’s grabens, would be erased from visibility within a few million years, indicating recent activity.
The new research on Mercury’s shrinking and the evidence of recent movements in its scarps shed light on the planet’s geological history. As future missions like BepiColombo continue to explore and gather data, our understanding of Mercury’s fascinating geological features will undoubtedly grow. The mysteries of this shrinking planet are slowly being unraveled, offering us a glimpse into the dynamic processes shaping our solar system.