I'm wondering if some of this has something to do with the warming sun thought to be significantly lower in output long ago compared to now. Most of our understanding has already been posted here. It's really hot deep down - like surface of the Sun hot, 6000K. Dirt/rocks are a major insulator keeping us from being evaporated by the heat. Much of this is the heat of formation of the planet. Best scenario for the moon is we were slammed by a Mars sized object quite some time back. That added a lot of energy to the Earth and evidently created the debris that combined into our moon. Thermal energy leakage is not enough to keep the surface anywhere close to warm. That's all solar radiation doing that and even this keeps us above freezing at least in some areas only because of our atmosphere.