Planetary scientists have long known that Mercury has been contracting for billions of years. Although the planet is the closest planet to the Sun, its interior is cooled by internal heat leakage. This means that the rock (and metal included) from which it is composed must shrink slightly in volume. However, it is unknown how much the planet is still contracting today and, if so, how long this will continue. Our new article is now online Natural Geologyoffers a new perspective.

As Mercury’s interior shrinks, its surface (crust) has less and less surface area to cover. It responds to this by creating “thrust faults” when part of the land pushes against neighboring land. This is similar to the wrinkles that form on an apple as it ages; However, the apple shrinks because it dries, while Mercury shrinks due to thermal contraction in its interior.

The first evidence that Mercury was shrinking came in 1974, when the Mariner 10 mission released photographs of kilometer-long escarpments (ramp-like cliffs) meandering across the landscape for hundreds of kilometers. Orbiting Mercury from 2011 to 2015, Messenger showed many more “blade protrusions” (as they were later called) around the world.

From such observations it was concluded that slight geological faults, known as thrusts, approach the surface beneath each bulge and are the answer to the fact that Mercury’s radius has decreased by about 7 km.

So when did this happen? A generally accepted way to determine the age of Mercury’s surface is to count the density of impact craters. The older the surface, the more craters it has. However, this method is difficult because the speed of crater-forming impacts was much higher in the distant past.

However, it was always clear that Mercury’s bulges must be quite old; for although they cut some old craters, many younger craters lie on the ridges, and so the ridges must be older than these.

When was the last time this ledge moved?

The general consensus is that most of the debris on Mercury is about 3 billion years old. But are they all that old? And did the elderly stop moving a long time ago, or are they still active today?

We should not expect the driving force under each steepness to act only once. The 9-magnitude Tohoku earthquake, which occurred in 2011 off the coast of Japan, which was the world’s largest earthquake in recent years and triggered the Fukushima disaster, was the result of a sudden 20-meter jump along the 100-kilometer length of the responsible fault.

The largest “earthquakes” on Mercury are probably smaller. It would take 9 “earthquakes” of magnitudes in the hundreds, or more likely millions of smaller events that could span billions of years, to accumulate the total shortening of 2-3 km that can be measured in a typical bulge on Mercury. Determining the magnitude and duration of fault movements on Mercury is important because we do not expect Mercury’s thermal compression to end completely, although it should slow down.

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There was still little evidence. But our team found clear signs that many outcrops have continued to move geologically recently, even though they appeared billions of years ago.

This work began when Ben Mann, a PhD student at the UK’s Open University, noticed small cracks in the stressed upper surfaces of some of the ridges. He interpreted these as “grabens,” a geological word describing a strip of land plunging between two parallel faults.

This usually happens when the shell is stretched. Stretching may seem surprising on Mercury, where the crust is generally compressed, but Mankind realized that these grabens would form if a pushed piece of crust twisted as it was pushed into the surrounding terrain. If you try to bend a piece of toast, it may crack in the same way.

Grabens are less than 1 km wide and less than 100 meters deep. Such relatively small details must be much newer than the ancient structure on which they stand, or they would have already been lost to view by impacts that threw material to the surface in a process aptly called “gardening.”

Based on erosion rates from horticulture, we estimate the age of most grabens to be less than 300 million years. This suggests that the last move must have occurred “recently” as well.

Working with the most detailed images provided by MESSENGER, Meng found 48 large lobed cliffs with decidedly small grabens. The other 244 are crowned with “probable” grabens that are not clearly visible in the best MESSENGER images.

These are now primary targets to be confirmed by the imaging system of the joint European-Japanese BepiColombo mission, which is expected to enter Mercury orbit in early 2026.

Lessons of the Month

The moon also became colder and shorter. Their front ridges are much smaller and less impressive than those on Mercury, but we are certain that some on the Moon are geologically recent but still active today. This is because a recent reanalysis of moonquake locations recorded by seismometers (vibration detectors) dropped on the lunar surface by several Apollo missions showed that moonquakes clustered near the front ridges.

Additionally, the most detailed images of the Moon’s surface taken from orbit show scars formed by rocks bouncing off ledges, possibly after being moved by moonquakes. Similar logic applies to these rocks, which are much smaller in scale than Mercury’s grabens: They will disappear from view in just a few million years, so they must be young.

BepiColombo will not land, so we have no chance of collecting any seismic data on Mercury. However, in addition to showing small grabens more clearly, their most detailed images can also reveal rock scars that could be further evidence of recent earthquakes. I’m looking forward to learning. Source