The evolutionary histories of Earth and Mercury differ fundamentally. The smallest planet, closest to the sun, likely began cooling early on, much like a setting pudding: As early as about 1 billion years after its formation, Mercury's volcanic activity came to a halt, and a solid, continuous "rock skin" formed on the planet's surface. Earth's crust, on the other hand, is still in motion: Volcanism and plate tectonics are constantly stirring up the "Earth pudding." Exactly what Mercury's unique volcanic past looked like and how it shaped its current appearance remain unclear. However, researchers can gain important clues from the composition of its surface. ...read more read less