Mercury is already the smallest planet in the solar system (unless you still include Pluto, which some of us are still bitter about). While it is already the smallest, it is actually shrinking even more. This was first verified back in 1974 when NASA’s Mariner 10 mission flew by the planet. A new study published in AGU Advances, however, gives a more precise measurement of just how much it is shrinking, and why.

The reason that it is shrinking is because it is getting colder from the inside out. David Rothery is a Professor of Planetary Geosciences at the Open University and the author of the paper. He explained to The Conversation:

“Because Mercury’s interior is shrinking, its surface (crust) has progressively less area to cover. It responds to this by developing ‘thrust faults’ – where one tract of terrain gets pushed over the adjacent terrain. This is like the wrinkles that form on an apple as it ages, except that an apple shrinks because it is drying out, whereas Mercury shrinks because of thermal contraction of its interior.”

Back in 2014 it was estimated that the planet had shrunk by around 7km (4.4 miles). This was done by looking at the surface of the planet, and specifically the impact craters. By analyzing these craters they can see whether they were there before, during, or after the surface did its shrinking. By running these models, the team can come up with a timeline for the shrinkage that occurred.

For the study, the team of researchers looked at more than 6000 faults on the planet to come up with their estimates. In the paper, the team explains:

“Multiple studies use the uplifted topography that overlie these faults to estimate the amount Mercury has shrunk. However, these studies are in disagreement with one another due to the number of landforms that each study attributes to global contraction. Here, we use an alternate approach of calculating the change in volume caused by a population of faults by statistically scaling the change from the largest fault to the entire fault population.”

Based on these calculations, the team says that the shrinking that took place was between 2.3 and 3.5km (1.4 and 2.2 miles). They clarify:

“Accounting for the additional component of global contraction that could have taken place, 0.4–2.1 km [0.25–1.30 miles] of radial contraction may be considered to be added to any radial contraction estimates from faulting. Adding these two components yields a range of radial contraction of 2.7–5.6 km that is likely to have occurred on Mercury. This result is in reasonable agreement with many thermal evolution modeling efforts and, in combination with timing and strain rate estimates of global contraction, may be used to tightly constrain future thermal evolution modeling.”

This new measurement is believed to be much more accurate and should be the model that is used going forward for Mercury. Their method can also be used on other planets in the solar system that are shrinking due to the same processes.

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