For billions of years, scientists have been fascinated by the phenomenon of Mercury’s gradual shrinking. As the planet closest to the Sun, its interior has been cooling down over time, causing its rock and metal composition to contract. This shrinkage has given rise to intriguing geological features known as “thrust faults,” which are a response to the planet’s diminishing surface area. In a recent study published in Nature Geoscience, researchers delved deeper into the extent of Mercury’s shrinkage and shed light on the age and activity of these thrust faults.
Just like wrinkles on an aging apple, Mercury’s lobate scarps are the visible evidence of its shrinking. These scarps were first observed in 1974 during the Mariner 10 mission, which transmitted images of kilometers-high slopes stretching across the planet’s terrain. The Messenger mission, which orbited Mercury from 2011 to 2015, further confirmed the existence of these scarps, now known as “lobate scarps.” These scarps are geological faults where one tract of terrain is pushed over an adjacent tract due to the planet’s ongoing contraction.
Determining the age of Mercury’s scarps has been a challenging task for scientists. Typically, the density of impact craters is used to estimate the age of a planetary surface, with older surfaces exhibiting more craters. However, this method becomes complicated when dealing with Mercury, as the rate of crater formation varied significantly in the planet’s ancient history. Despite these difficulties, researchers have concluded that Mercury’s scarps are mostly around 3 billion years old. Nevertheless, questions arise concerning the age of individual scarps and whether they are still active today.
It is important to note that the fault below each scarp is not a result of a single movement but rather a cumulative effect of multiple events. To put this into perspective, even the largest earthquake on Earth in recent history, the magnitude 9 Tohoku earthquake in 2011, which caused the Fukushima disaster, was the outcome of a sudden 20-meter jump along a 100-kilometer fault. Similarly, Mercury’s “earthquakes” are likely to be smaller in scale. Accumulating the 2-3 kilometers of total shortening observed on a typical scarp would require hundreds of magnitude 9 earthquakes or millions of smaller events occurring over billions of years.
While evidence of recent fault movements on Mercury has been scarce, a breakthrough came when a PhD student at the Open University in the UK, Ben Man, noticed small fractures accompanying some scarps. These fractures, known as “grabens,” occur when the crust is stretched, even within a predominantly compressional environment like Mercury. The presence of these grabens suggests that recent movement had occurred. By analyzing the detailed images provided by the MESSENGER mission, Man identified 48 large lobate scarps with definite grabens and an additional 244 scarps with probable grabens. Confirmation of these findings is eagerly awaited from the imaging system of the joint European/Japanese BepiColombo mission scheduled to begin operation in 2026.
Mercury is not the only celestial body that experiences shrinkage and ongoing fault movements. The Moon, which has also cooled and contracted, exhibits smaller and less dramatic lobate scarps than Mercury. However, recent analysis of moonquake data collected by seismometers left on the Moon’s surface by the Apollo missions has revealed that moonquakes are clustered near these scarps. Furthermore, detailed orbital images of the Moon’s surface have captured the tracks left by bouncing boulders dislodged by moonquakes, supporting the notion of recent activity. While BepiColombo is not equipped to collect seismic data like the Apollo missions did on the Moon, it is hoped that the mission’s high-resolution images will provide additional evidence of recent quakes in the form of boulder tracks.
The study of Mercury’s shrinking and the geological features it gives rise to continues to captivate scientists. The ongoing advancements in planetary missions and image resolution hold the promise of uncovering more secrets hidden within the scarps and grabens of Mercury. By unraveling the age and activity of these structures, scientists hope to gain a more comprehensive understanding of the planet’s thermal contraction and its future evolution. Through persistence and the pursuit of knowledge, humanity continues to peel back the layers of mystery surrounding our cosmic neighbors.