Tectonics on Mercury
Mercury is Tectonically Active, NASA
“Images obtained by NASA’s MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft reveal previously undetected small fault scarps– cliff-like landforms that resemble stair steps. These scarps are small enough that scientists believe they must be geologically young, which means Mercury is still contracting and that Earth is not the only tectonically active planet in our solar system, as previously thought.”
Why is this surprising? Granted the size of Mercury led scientists to conclude that it could be fully cooled. But given the proximity to sol perhaps there’s a somewhat different mechanism (compared to Earth): the sun heats the surface to the necessary depth but the core of the planet remains relatively cool.
We have a decent idea how far down solar heat would be conducted. Annual variations damp out with depth (which is why terrestrial caves tend to have the same temperature all year long.) I don’t think any plausible surface composition would make that deep enough to drive geology.
But I agree the result isn’t too surprising. The models of how quickly Mercury would cool depend on many things we don’t really know. For example the viscosity of the mantle and core. Worse, that sort of thing is extremely sensity to composition and temperature. I suspect someone, very soon, will rerun those models using different assumptions, and publish a paper saying, “It all makes sense if the iron-to-sulfur ratio in the core is X ℅ greater than terrestrial. Now what does that tell us about how Mercury formed?” or something similar.