Why is Mercury so extreme?

The answer

Mercury has the most extreme temperature range of any planet because it has virtually no atmosphere to trap or distribute heat. Its dayside, baked by a Sun that appears three times larger than on Earth, reaches 430°C - hot enough to melt lead. At night, with no atmospheric blanket, temperatures plunge to −180°C. Remarkably, despite this heat, permanently shadowed craters at Mercury's poles contain water ice, confirmed by NASA's MESSENGER mission. Mercury's enormous iron core (about 85% of the planet's radius) also makes it the densest planet relative to its size.