Geology features on Mercury, interpreted using the solar nebular model (formation theory for how our planets formed) and geology theory, are said to show that Mercury is 'rapidly' shrinking.
But this surprise contracting is even more surprising as it is not uniform and appears to be making one side of the planet Mercury smaller.
When Mariner 10 flew by Mercury, it spotted geological formations such as scarps and cliffs. These are signs that the planet is shrinking — due to the steady cooling of its core over time — forcing the crust to buckle in on itself.
But MESSENGER's comprehensive survey of Mercury's surface has shown that these features are more predominant than previously estimated — and that the planet is shrinking about five times faster than scientists thought.
"All the planets are losing heat," Solomon says, "but Mercury alone has cooled so much that its contraction has become the dominant deformational process on its surface." In total, it's estimated that Mercury's diameter has shrunk by about 8.5 miles since the planet was first formed.
Mercury is shrinking — and 4 other facts we've learned from NASA's MESSENGER probe
Geology is just a theory. A good theory predicts and most geology predictions are shown to be wrong or need massive modifications so they hardly resemble the original models.
But what seems strange about Mercury is science and geology usually use planetary geomorphology to explain and form theories of how space bodies and their geological features are formed. Massive scarps, fault lines, mountains, hills, are formed by moving and colliding tectonic plates activity.
But MESSENGER's measurements of volatiles like potassium, sulfur, sodium, and chlorine on Mercury's surface showed surprisingly high levels of them — as high or higher than on the other inner planets. "This means that all those ideas for how Mercury became so iron-rich can be rejected," Solomon says. "And it forces us to reconsider how the entire inner solar system was assembled."
Scientists are now trying to figure out alternate ways of explaining how Mercury could've lost so much outer material while retaining these volatiles.
We can't quite figure out how Mercury formed | Vox
Yet Mercury is supposed to only have one plate covering the whole surface of the planet.
Similar to a smaller planet Earth with all its supposed tectonic plates fitting together perfectly on a small planet?
As for when all this shrinking happened, that's harder to figure out, said Byrne. Craters now seen on Mercury correspond to what scientists call the Late Heavy Bombardment -- a violent time in the history of the solar system that ended about 3.8 billion years ago. Any shrinkage features that happened before that were likely wiped out by asteroid impacts during that time.
"So Mercury surface has taken a beating," said Byrne. "It's possible that Mercury started contracting before that but we don't expect to see it."
But there are signs that some shrinking happened later. One example is a giant cliff face at what's called Carnegie Rupes that runs right through a large crater. It implies that the cliff came first, was smashed by an asteroid impact, and then the cliff continued to grow after the impact.
"We don't know if it's slowly slipping or Mercury-quakes," said Bryne. But it does feed into the idea that Mercury's shrinking has been going on a long time.
The Incredible Shrinking Mercury | Discovery
The reason that planetary geomorphology seems to have been ignored for Mercury is that what is observed does not fit the theory. This is because the planetary geomorphology theory and geology theory is based on planet Earth with all its theory of multiple tectonic plates, water and wind erosion etc.
The most widely accepted model of the origin of the fault scarps is global contraction resulting from cooling of Mercury’s still-hot interior. Based on this model, global contraction should generate a planetwide array of thrust faults that are uniformly distributed on the surface, with no particular pattern in the orientations of the faults.
The new research contradicts this model: MESSENGER has found distinct patterns in the orientations of the fault scarps and clear concentrations in the number of scarps. Many scarps are concentrated in two broad longitudinal bands.
Even more surprising is that the majority of the largest fault scarps, those with lengths greater than a hundred kilometers, are found in one hemisphere of Mercury. “There are almost three times more large faults in the southern hemisphere than in the northern hemisphere. This is pretty lopsided and not what you’d expect from a uniformly contracting body,” said lead author Thomas Watters, senior scientist at the Center for Earth and Planetary Studies at the Smithsonian’s National Air and Space Museum. “The pattern in orientations and the hemispheric dichotomy in the number of large faults suggest we’re missing something.”
A shrinking Mercury is contracting in unexpected ways | American Geophysical Union
Earth is the most unusual of all the planets in our solar system. Perhaps it is not actually the best choice to use to compare to how other space bodies features were formed. For example because of planetary geomorphology we have water erosion on asteroid Vesta!