the rate at which heat escapes from a planet is proportional to its surface area; $\displaystyle 4 \pi R^2$. The amount of heat stored during the formation of a planet is proportional to its volume; $\displaystyle \frac{4 \pi R^3}{3}$. The radius R of Earth is twice of Mars. It takes Earth 5 billion years to lose all of its original heart of formation. How long will it take Mars to do so?

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