For billions of years, scientists have been aware that Mercury, the planet closest to the Sun, has been gradually shrinking. The cooling down of its interior has caused the rock and metal of the planet to contract, resulting in a reduction in volume. However, the extent to which Mercury is still undergoing this process remains uncertain. In a recent study published in Nature Geoscience, researchers have shed new light on this phenomenon and its implications for the planet’s surface.
As Mercury’s interior continues to shrink, its surface, or crust, has progressively less area to cover. The planet responds to this contraction by developing “thrust faults,” where one tract of terrain gets pushed over the adjacent terrain. This can be compared to the wrinkles that form on an apple as it ages, except in Mercury’s case, the shrinking is due to the thermal contraction of its interior. In 1974, the Mariner 10 mission provided the first evidence of Mercury’s shrinkage, capturing images of kilometers-high scarps snaking across the planet’s terrain. Subsequent missions, such as Messenger, revealed even more “lobate scarps” in various parts of the globe.
To determine the age of Mercury’s surface, planetary scientists rely on the density of impact craters. The more craters present, the older the surface. However, this method is challenging due to the varying rate of impacts throughout history. Despite these difficulties, it has been clear that Mercury’s scarps are relatively ancient, as they cut through older craters while also being overlain by younger ones. The consensus view is that most of Mercury’s scarps are approximately 3 billion years old. Nevertheless, questions remain about the specific ages of these scarps and whether they are still active today.
It is unlikely that the thrust fault below each scarp has moved only once. To put this into perspective, the magnitude 9 Tohoku earthquake that occurred off the coast of Japan in 2011, causing the Fukushima disaster, resulted from a sudden displacement of 20 meters along a 100-kilometer length of the responsible thrust fault. In comparison, Mercury’s “earthquakes” are likely to be smaller. The accumulation of approximately 2-3 kilometers of total shortening across a typical scarp would require hundreds of magnitude 9 earthquakes or millions of smaller events over billions of years. Understanding the scale and duration of fault movements on Mercury is crucial since the planet’s thermal contraction is unlikely to have completely ceased, although it may have slowed down.
Previously, evidence of continued movement on Mercury has been scarce. However, a breakthrough came when a PhD student at the Open University in the UK, Ben Man, noticed small fractures known as “grabens” on the stretched upper surfaces of some scarps. Grabens form when the crust is stretched, which appeared counterintuitive considering that Mercury’s crust, as a whole, is under compression. Nevertheless, Man realized that these grabens would occur if a thrust slice of crust was bent as it was pushed over adjacent terrain. This bending and subsequent fracturing can be likened to cracking a piece of toast. The grabens observed on Mercury are less than 1 kilometer wide and approximately 100 meters deep, indicating that they are much younger than the ancient structures on which they sit.
The majority of grabens on Mercury are estimated to be less than about 300 million years old based on careful calculations. This suggests that the latest movement must have occurred relatively recently. The research team, working with detailed images provided by MESSENGER, identified 48 large lobate scarps with definite grabens and an additional 244 scarps with probable grabens. These findings present prime targets for confirmation by the imaging system of the joint European/Japanese BepiColombo mission, set to begin operating in orbit around Mercury in early 2026.
The Moon, also subject to cooling and contraction, has its own lobate scarps, albeit smaller and less prominent than those on Mercury. Recent analysis of moonquake locations suggests that some of the Moon’s scarps are still active today. Seismometers left on the Moon’s surface by Apollo missions recorded moonquakes clustered near the lobate scarps. Detailed images of the Moon’s surface from orbit have also revealed boulder tracks resulting from rock dislodgment during moonquakes. These boulder tracks, which are similar in scale to Mercury’s grabens, would have been erased within a few million years if they were not relatively young.
Although BepiColombo will not provide seismic data like its counterparts on the Moon, it can offer enhanced insights into Mercury’s geology. The mission’s detailed imagery may reveal boulder tracks that serve as additional evidence of recent quakes. Confirmation of the grabens observed by MESSENGER and further exploration of the scarps on Mercury will contribute to a deeper understanding of the planet’s geological activity.
The continuous shrinking of Mercury’s interior has resulted in the development of thrust faults, causing the planet’s surface to exhibit lobate scarps. Recent research suggests that many of these scarps have experienced movement even in geologically recent times. The discovery of grabens on the upper surfaces of scarps indicates that some of the thrust fault movements are likely to be relatively young, with the majority estimated to be less than 300 million years old. Further investigation through the BepiColombo mission holds the potential to uncover additional evidence of recent quakes and shed further light on Mercury’s geology.