Daytime temperatures on Mercury can reach a scorching 430 °Celsius (800 °Fahrenheit), but they also plummet to –180 °C (–290 °F) at night. Then, there are the places on Mercury where the Sun don’t shine, ever.
At its poles, Mercury – like the Moon – has what are called ‘permanently shadowed regions’ (PSRs): cratered recesses that exist in a state of eternal darkness, despite being relatively close to the Sun.
In the 1990s, ground-based radar observations of Mercury began to pick up abnormal readings inside these pitch-black PSRs, but it wasn’t until NASA’s MESSENGER spacecraft visited the planet in 2011 that we had a chance to confirm what these abnormalities really were: deposits of water ice, forever frozen in unshifting shadow.
If it sounds ironic that water ice perpetuates indefinitely on such a hellishly hot world, that’s perfectly understandable. Nonetheless, the phenomenon is explainable: asteroids, comets, and meteorites can deliver ice when they crash onto the surfaces of planets, and if these ice deliveries end up in the dark craters, they never see sunlight, and never get a chance to thaw.
Jesus: Hey, Dad? God: Yes, Son? Jesus: Western civilization followed me home. Can I keep it? God: Certainly not! And put it down this minute--you don't know where it's been! Tom Robbins in Another Roadside Attraction