Today, Dave and Dr. Pitts are stopping by the ice craters of Mercury, the planet closest to the Sun. Temperatures on Mercury typically hover around a balmy 800 degrees Fahrenheit, on the side facing the Sun. Yet there are at least 100 billion tons of ice on the little planet. How is this possible? The answer lies in Mercury’s lack of atmosphere. Without an atmosphere, heat is not conducted to the parts of the planet that aren’t getting direct solar radiation. The side of Mercury that doesn’t face the sun has a temperature of 300 degrees below zero. The abundance of ice can be found in craters at Mercury’s poles. The craters are always in shadow, so they receive no solar radiation.