“Somehow, Pluto is generating heat. And that has huge implications for the solar system at large.”
So writes Josh Gelernter in Pluto’s Astonishing News:
During the autumn, you can estimate how recently a lawn has been raked by how many leaves are on it. In the same way, you can estimate how recently a planet’s surface formed by how many objects have had a chance to crash into it. When things crash into a planet, they leave craters. Not one single crater on 150,000 square miles of Pluto means its surface is brand new.
That doesn’t mean Pluto itself is brand new — it means that Pluto was recently, or still is, geologically active; that its surface is being churned up and changed and covered by new material. That’s an enormous breakthrough.
Geological activity requires energy. Earth’s geologic activity — earthquakes, volcanic eruptions, and so forth — is powered by its internal heat, which comes from the friction of Earth’s innards moving around, and from the decay of radioactive elements. The heat in the Earth’s interior is proportional to its immense size; if our planet were smaller — Pluto-sized — it would be colder, and thus, it had been thought, inactive.
There are geologically active bodies that are much smaller than Earth, but they don’t generate their own heat. The most active body in the solar system is Io, the innermost of Jupiter’s four big moons; its energy comes from constantly being pulled back and forth by Jupiter and the other three moons. But nothing is pulling on Pluto. It has a big moon of its own — Charon, whose mass is about 10 percent that of Pluto — but Pluto and Charon orbit each other in perfect synchronization. Something else is powering Pluto’s surface changes; somehow, Pluto is generating heat. And that has huge implications for the solar system at large.
Also a good article in the Weekend Journal, The Man Who Flew Mankind to Pluto:
But geological processes require heat, and therein lies the riddle. “There’s no really good model for how these small planets can have their engines running after four billion years. As planets get smaller, the ratio of surface area compared to their mass goes up. That means that they can’t trap the heat inside very long. They cool off.”
He gestures to the paper cup of coffee on the table in front of him. A small cup of coffee will go cold faster than a large one; a large cup of coffee will go cold faster than a big pot. The point is that without an outside source of energy, such as tidal forces from a nearby gas giant, a planet like Pluto, which is one-sixth the size of our moon and circles the outer reaches of our solar system, should be long dead.