In the vastness of our Solar System, Mercury has always been a planet of intrigue. As the closest planet to the Sun, scientists have long been aware of its unique characteristics, one of which is its gradual shrinkage over billions of years. Mercury’s interior, despite its scorching proximity to the Sun, has been slowly cooling down, causing the planet to contract. This contraction is manifested on the surface as the development of “thrust faults” – a fascinating geological phenomenon where one section of terrain is pushed over another. These thrust faults resemble the wrinkles that form on aging fruits, except that in Mercury’s case, the planet is shrinking due to thermal contraction.
Early Discoveries and the Role of Mariner 10 and Messenger Missions
The first evidence of Mercury’s shrinkage came to light in 1974 when the Mariner 10 mission transmitted images of kilometers-high scarps across the planet’s terrain. These scarps, known as “lobate scarps,” continued to be observed in all parts of Mercury’s globe during the Messenger mission, which orbited the planet from 2011 to 2015. Through these observations, scientists were able to determine that Mercury had shrunk in radius by approximately 7km. However, the exact timeline of this shrinkage remained a mystery.
Unlocking the Age of Mercury’s Surface
To determine the age of Mercury’s surface, researchers traditionally relied on counting the density of impact craters. The older the surface, the more craters it would have. However, this method posed challenges due to the varying rate of impact events over time. Despite these challenges, scientists identified that Mercury’s scarps were relatively ancient. Although they cut through older craters, they were overlaid with several younger craters, indicating that the scarps predated the latter. The consensus among experts was that these scarps were approximately 3 billion years old. But were all the scarps that old, and are they still active today?
The Recurrence of Thrust Fault Movements on Mercury
Contrary to popular belief, thrust faults on Mercury don’t experience a singular movement. Earthquakes, which serve as a significant indicator, are recurrent events that contribute to the overall shortening of a scarp. Accumulating the 2-3km of shortening measured across a typical scarp on Mercury would require numerous magnitude 9 “earthquakes” or even millions of smaller events. Additionally, it is crucial to evaluate the scale and duration of fault movements on Mercury, as its thermal contraction is an ongoing process, even if it is gradually slowing down. Until recently, evidence of these movements has been scant.
A Breakthrough Discovery: Grabens on Mercury’s Scarps
The breakthrough in understanding Mercury’s thrust faults and their ongoing activities came from the observant eye of a PhD student at Open University, Ben Man. Man noticed small fractures, referred to as “grabens,” on the stretched upper surfaces of some scarps. These grabens are geological formations that occur when the crust is stretched. Although stretching may seem counterintuitive given the overall compression of Mercury’s crust, Man theorized that thrust slices of crust had bent as they were pushed over adjacent terrains, resulting in the formation of these grabens. Their relatively small size, less than 1km wide and about 100 meters deep, indicated that they were much younger than the ancient structures beneath them.
Age Estimation of Grabens: A Window into Recent Activity
To estimate the age of these grabens, researchers relied on the rate of blurring resulting from a process called impact gardening, where impacts toss material across the surface. Based on this calculation, the majority of grabens were determined to be less than 300 million years old. This suggests that the latest movement of these grabens occurred relatively recently. Detailed analysis using images provided by the MESSENGER mission identified 48 large lobate scarps with confirmed grabens, as well as 244 scarps with probable grabens awaiting further confirmation from the BepiColombo mission.
Interestingly, the Moon, like Mercury, has also experienced shrinkage and contraction over time. Although the lobate scarps on the Moon are smaller and less remarkable compared to those on Mercury, recent studies have shown that some scarps on the Moon remain active today. The analysis of moonquake locations in combination with images of boulder tracks suggests ongoing geological activity. While BepiColombo won’t provide seismic data like seismometers on the Moon’s surface, it may offer additional evidence of recent quakes through high-resolution imaging of Mercury’s surface.
As the joint European/Japanese BepiColombo mission prepares to explore orbit around Mercury in 2026, the confirmation and study of scarps with grabens will be a primary focus. These new findings shed light on the ongoing geological activity on Mercury and further emphasize the need to understand the scale and duration of such movements. Unveiling the secrets of Mercury’s shrinkage and its geology opens up exciting avenues for deciphering the complex history of our neighboring planet.