Deep inside polar craters on Mercury, sunlight has never once fallen — and NASA's MESSENGER spacecraft confirmed in 2012 that those permanently shadowed floors hold ancient water ice, sitting metres from terrain hot enough to melt lead

Deep inside polar craters on Mercury, sunlight has never once fallen — and NASA's MESSENGER spacecraft confirmed in 2012 that those permanently shadowed floors hold ancient water ice, sitting metres from terrain hot enough to melt lead

Mercury is the closest planet to the Sun. At its equator in daytime, the surface can reach about 430 degrees Celsius, well above the melting point of lead. Near the poles, however, the floor of a deep crater can remain below minus 170 degrees Celsius. Water ice lies at the surface in the coldest of these places and beneath a dark insulating layer in others. I find the contrast easier to understand once the word “near” is handled carefully. Mercu…