It’s been long thought that of the Fab (Terrestrial) Five, only Earth remains geologically active. However, recent evidence shows that little Mercury, long thought to be tectonically dead, is actually shrinking!
The evidence comes in the form of small troughs (upper arrows in the picture below) and scarps (lower arrows) astronomers were able to photograph on the rocky world’s surface.
But how do scientists know that these markings make for evidence that Mercury is shrinking? Well, they’ve also found similar geological features on the Moon, who also happens to be a metaphorical “twin” of Mercury’s, that show its own shrinkage.
Why are these worlds shrinking though? As the planet cools (yes, cooling! Even despite Mercury being the closest world to the Sun), its interior is condensing, almost as if the planet is crumpling up on itself. That’s why we see these sharp ridges and mountains emerge.
Cecily and the Solar System 
Interesting post. I wasn’t aware that Mercury and the Moon were actually shrinking. If shrinking is a product of a celestial body’s molten core cooling does this mean that Mars too will eventually show signs of shrinking as well?
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It’s pretty cool to consider that the only reason we can say this now is because the telescope that was taking the pictures was actually orbiting closer than usual. Since it was closer, better resolution images could be taken of the surface. It makes me wonder how much detail we miss because of resolution limits? Great post! Thanks!
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Mercury is already small enough! No, but in all seriousness, it is cool to see that it’s not as boring as we once thought. I wonder how long we have known that has been shrinking or when did these geological features start to form. Mercury finally getting some love.
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