admin Planetary scientists have long known that Mercury has been shrinking for billions of years. Even though it is the closest planet to the sun, its interior cooled as the heat inside escaped. This means that the rock (and within it the metal) that it is composed of must contract slightly in volume.
However, it remains unclear to what extent the planet continues to shrink today – and, if so, how long this might continue. Our new paper, published in the journal Nature Geoscience, now provides new insight.
As Mercury’s interior is shrinking, its surface (crust) has less and less area to cover. It responded to this by developing a “thrust fault”, in which a large area of terrain was pushed onto adjacent terrain (see image below). It’s like the wrinkles that form on an apple as it ages, except that the apple shrinks because it dries out while Mercury shrinks because of heat contraction within it.
The first evidence of Mercury’s contraction appeared in 1974 when the Mariner 10 mission transmitted images of several kilometer-high cliffs (ramp-like slopes) zigzagging hundreds of kilometers across the terrain. Messenger, which orbited Mercury from 2011 to 2015, shows more “lobe scars” (as they are called) in all regions of the world.
From such observations it can be inferred that gently dipping geological faults, called thrusts, reach the surface beneath each cliff and are a response to the fact that Mercury’s radius has shrinks to a total of ‘about 7 km’.
But when did this happen? The accepted method for determining the age of Mercury’s surface is to calculate the density of impact craters. The older the surface, the more craters it has. But this method is complicated because crater impact rates were much higher in the distant past.
However, it is clear that Mercury’s scars must be quite old, because although they pass through some older craters, some younger craters overlap these scars and so These scars must be older than these cracks.
When was the last time this cliff moved?
The consensus view is that Mercury’s scars are mostly about 3 billion years old. But are they all that old? And did the elders stop moving a long time ago or are they still active today?
The thrust fault beneath each cliff should not have moved even once. The largest earthquake on Earth in recent years, the 2011 magnitude 9 Tohoku quake off the coast of Japan that caused the Fukushima disaster, was the result of a sudden jump 20 meters high over a length of 100 meters. km of the fault line responsible for thrusting.
Mercury’s largest “quakes” are probably smaller. To accumulate a total measurable length of 2 to 3 km on a typical Mercury cliff, hundreds of magnitude 9 “earthquakes” would be needed, or more likely millions of smaller events, which could lasting billions of years.
Understanding the magnitude and timing of fault movements on Mercury is important because we do not expect Mercury’s thermal contraction to be completely complete, although it is expected to slow.
Cracking up
Until now, evidence has been sparse. But our team found unambiguous signs that many scarps have continued to move in geologically recent times, even if they were initiated billions of years ago.
This work was triggered when a Ph.D. student at Open University in the U.K., Ben Man, noticed that some scarps have small fractures piggy-backing on their stretched upper surfaces. He interpreted these as “grabens,” the geological word to describe a strip of ground dropped down between two parallel faults.
This typically happens when the crust is stretched. Stretching may seem surprising on Mercury, where overall the crust is being compressed, but Man realized that these grabens would occur if a thrust slice of crust has been bent as it is pushed over the adjacent terrain. If you try to bend a piece of toast, it may crack in a similar way.
The grabens are less than 1km wide and less than about 100 meters deep. Such comparatively small features must be much younger than the ancient structure on which they sit, otherwise they would have already been erased from view by impacts tossing material across the surface in a process aptly named “impact gardening.”
Based on the rate of blurring resulting from impact gardening, we calculated that the majority of grabens are less than about 300 million years old. This suggests that the latest movement must have happened equally “recently.”
Working with the most detailed images provided by MESSENGER, Man found 48 large lobate scarps that definitely have small grabens. A further 244 were topped by “probable” grabens—which aren’t seen quite clearly enough on the best MESSENGER images.
These are now prime targets for confirmation by the imaging system of the joint European/Japanese BepiColombo mission, which should start operating in orbit around Mercury early in 2026.
Lessons from the moon
The moon also cools and contracts. Its lobe scars are significantly smaller and less spectacular than those on Mercury, but on the Moon we know for certain that in addition to being geologically recent, some scars remain is active to this day.
Indeed, a recent reanalysis of the locations of earthquakes recorded by seismometers (vibration detectors) left on the surface of the moon by several Apollo missions shows that The earthquake was concentrated near the lobe scars.
Additionally, the most detailed images of the Moon’s surface from orbit show traces left by rocks bouncing off steep faces, perhaps after being displaced by lunar earthquakes . Much smaller than Mercury’s grabens, a similar logic applies to these rocky tracks: they will be erased from view after only a few million years, so they must be young.
The moon has also cooled and contracted. Its lobate scarps are considerably smaller and less spectacular than those on Mercury, but on the moon we know for sure that as well as being geologically recent, some are active today.
This is because recent reanalysis of the locations of moonquakes recorded by seismometers (vibration detectors) left on the moon’s surface by several Apollo missions shows that moonquakes are clustered close to lobate scarps.
Also, the most detailed images of the moon’s surface from orbit reveal the tracks made by boulders bouncing down scarp faces, presumably after being dislodged by moonquakes. Much smaller in scale than Mercury’s grabens, similar logic applies to these boulder tracks: they would become erased from visibility after only a few million years, so they must be young.
BepiColombo will not land and therefore we have no prospect of collecting seismic data on Mercury. However, in addition to showing smaller grabens more clearly, more detailed images of it can reveal traces of rocks that could be further evidence of recent earthquakes. I can’t wait to find out.
