The terrestrial planet that has the coldest surface is Mercury. At night, it has the coldest temperature and dips down to 100 K. However Mars has the coldest average surface temperature of anywhere between 160 and 280 K. Mercury has the coldest surface temperature because of its slow spin and short year. Because its the closest planet to the sun, it only takes 88 days to complete one revolution around the sun. Due to its slow spin and short year, it takes a long time for the sun to rise and set. Mercury has only one sunrise every 180 days. The long time spent in the darkness is the reason why it has the coldest surface temperature.
