Question:

Explain why convection is the most efficient way to transfer heat inside Earth, but radiation is ultimately the means by which planet Earth cools off.

Answer:

The answer has two parts:

1. Convection is most efficient inside Earth - this can be explained a couple different ways. Convection involves the motion of a fluid, which carries the heat with it, and the rock of Earth's interior acts like a fluid over geologic time. You can also say that convection is the most efficient because conduction and radiation are not efficient inside Earth - like the analogy of passing the ball from one person to the next, they work very slowly.

2. Radiation is ultimately the way Earth cools off: radiation works best in a vacuum (nothing to get in the way of it), and convection and conduction need physical matter to work, so the heat at Earth's surface cannot pass into space by conduction or convection, but can efficient pass out into space by radiation.