Although Mercury is the closest planet to the sun, it isn't the hottest. That title belongs to Venus, which has a thick, heat-trapping atmosphere. Mercury's very thin atmosphere holds little heat, and temperatures on the planet vary widely between day and night, between equator and poles. The average of these extreme temperatures is 167 degrees Centigrade (332.6 degrees Fahrenheit), according to information sent back from the spacecraft Messenger, which entered into orbit around Mercury in 2011.
Hotter Than an Oven
Mercury reaches 426 degrees Centigrade (800 degrees Fahrenheit); in other words, it gets hotter than a domestic oven. The planet spins very slowly compared to Earth, one day lasting 176 Earth days and a year lasting 88 Earth days, so that Mercury travels around the sun twice in one day. The side of Mercury facing the sun grows extremely hot, but the atmosphere is too thin to spread the heat. The hottest temperatures occur at midday at the equator. Sunrise on Mercury brings rapidly rising temperatures, the sun appearing three times larger and shining 11 times more brightly than it does on Earth.
Colder Than Ice
Mercury's coldest temperature is about -223 degrees Centigrade (-370 degrees Fahrenheit). Temperatures fall rapidly as night descends and are also permanently low at the poles, which receive little sunlight. Mercury doesn't tilt on its axis as the Earth does, so its poles only ever receive low levels of light. Even before retrieving the Messenger data, scientists suspected that Mercury's poles were covered in ice, due to earlier observations that showed they were covered in a reflective substance. Tests the Messenger spacecraft performed indicated that the reflective substance is water ice, which scientists suspect arrived by comet within the last tens of millions of years. Ice also exists in deep craters, which are permanently in shadow.
Messenger discovered areas of ice on Mercury where temperatures are about -173 degrees Centigrade (-280 degrees Fahrenheit). By counting particles called neutrons flying off Mercury, Messenger was able to detect ice in nonreflective, dark areas on the surface. Neutrons are part of the debris created when cosmic rays strike planets, but hydrogen atoms stop them, and water contains two hydrogen atoms. The dark layer covering the ice is about 15 cm (6 inches) thick, and scientists speculate it's carbon-based matter remaining after surface ice has evaporated.
The most moderate temperatures on Mercury are underground and between the poles and the equator. In spite of its extreme temperatures, Mercury is capable of supporting human life in certain areas. Temperatures 90 to 120 cm (3 to 4 feet) below the surface in regions that receive average levels of sunlight are temperate enough to support a human colony, and people could use the water ice that already exists on the planet. But gravity less than half of Earth's and an unbreathable atmosphere would present problems for would-be colonists.