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Big objects retain heat better than small objects do. Most of Earth's internal heat is generated by four long-lived radioisotopes--potassium 40, thorium 232, uranium 235 and uranium 238--that release energy over billions of years as they decay into stable isotopes. Earth's large size (about 12,740 kilometers across) ensures that this heat is lost relatively slowly, which explains why our planet still has a molten outer core and volcanic eruptions at its surface. Smaller bodies, however, have a larger ratio of surface area to volume, allowing them to cool down faster by radiating their heat into space. Earth's moon, for example, is only about one fourth the size of Earth, so it loses heat much more quickly. As a result, major lunar eruptions of basalt, the most common volcanic rock, ceased nearly three billion years ago.
Heat loss is even faster in the small rocky asteroids that whirl through the inner solar system, mostly between the orbits of Mars and Jupiter. Vesta, the third-largest asteroid, has a diameter of 516 kilometers, giving it a surface-to-volume ratio 25 times greater than Earth's. But a paradox arises: despite its small size, Vesta shows evidence of past geologic activity. Spectroscopic observations of Vesta's surface indicate that it is covered with volcanic basalt, leading researchers to conclude that Vesta's interior once melted. The cause of the heating cannot be long-lived radioisotopes; given the primordial concentrations of the isotopes and the expected rate of heat loss, calculations show that the radioactive decay could not have melted Vesta or any other asteroid. Another heating mechanism must therefore be responsible, but what is it? This question has dogged planetary scientists for decades.