Click to viewEver since the International Astronomical Union demoted Pluto to a dwarf planet (does it even deserve a capital "P"???) in 2006, astronomers around the world have been at odds to describe just what they mean when they say the word "planet." For the moment, the solar system is holding steady with eight of them, but late last week evidence returned from the Mercury MESSENGER mission showed that the smallest planet left is shrinking. One has to wonder: how long will it be before Mercury gets plutoed?
Mercury is about twice as big as pluto, but still is the smallest object called a "planet" orbiting the Sun. The question is: how much smaller will it get? It will never get anywhere near as small as the former ninth planet, but will the IAU see fit to demote it too as it continues shrinking? Only time will tell.
Meanwhile, Mercury's molten iron core continues to cool, shrinking the planet from the inside. Small particles of solid iron 'snow' rain down toward the ever-widening solid core. But even as the solid grows it's denser than the liquid and so takes up less space. This has been going on probably for billions of years and over time the shrinkage has caused Mercury's crust to buckle and fold up on itself, as seen here (that y-shaped fracture in the left side of the image is a huge fracture in the rock. The whole picture is about 200 kilometers wide):
On the right hand side of the image, the craters with the soft-looking rims appear to be old impact basins that have been filled in with lava, indicating the Mercury once had some serious volcanoes exploding on its surface. Why did the volcanoes die off? Mercury cooled off. Just like on Mars and the Moon, Mercury was fiery when it first came into being, but lost its heat in the roughly 4.5 billion years since, silencing is volcanic activity. Earth is cooling in a similar way and in a few billion years it will get too cold for volcanoes too. When it does it will go quiet forever.