Chapter 21, Problem 4P

Why do small planets cool faster than large planets? Choose any two of the five Terrestrial worlds and calculate for each one the ratio of its surface area to its volume. Why is this ratio important? (Hint: Does this ratio have anything to do with the ability of a planet to lose internal heat?) (Note: The surface area of a sphere is 4πr², and the volume of a sphere is $\frac{4}{3}\pi r^3$.)