The conditions on every planet in the solar system are either much colder or much hotter than on Earth. On one planet, they are both. Mercury is half as far from the sun as Earth, so it isn't surprising that it is hot there -- but it is also bone-chillingly cold when the sun isn't shining. There is such a large temperature difference on Mercury because it lacks an atmosphere.
Day and Night on Mercury
Scientists once believed that Mercury always presented the same face to the sun, but in 1965, they discovered that it rotates slowly -- three times for every two orbits. That makes a day slightly shorter than a year. Because Mercury has very little tilt relative to its orbital motion, its seasons are based on the eccentricity of its orbit. In summer, when it is at its closest approach to the sun, the daytime temperature can reach 465 degrees Celsius (870 degrees Fahrenheit). At night, the temperature can drop to -184 degrees Celsius (-363 degrees Fahrenheit). This happens because the planet has no atmosphere to retain heat.
Comparisons with Other Planets
The temperature on Mercury's surface fluctuates more widely than that on the surface of any other planet. It can vary by 649 degrees Celsius (1,168 degrees Fahrenheit). By comparison, the extremes on Earth and Mars are separated by 160 degrees Celsius (288 degrees Fahrenheit); and the temperature on Venus, which is almost as hot as the hottest temperature on Mercury, is constant. The outer gas giants -- Jupiter, Saturn, Uranus and Neptune -- all have surfaces that compare with Mercury at its coldest, but they become warmer deeper inside their atmospheres because they have hot cores.
Planetary Temperature Gradients
The temperature of Jupiter's core is 24,000 degrees Celsius (43,232 degrees Fahrenheit), which is hotter than the surface of the sun. Consequently, the gas giant displays a larger temperature gradient from surface to core than any other planet. By comparison, the surface-to-core gradient on Earth is approximately 5,000 degrees Celsius (9,000 degrees Fahrenheit). Mercury has a large core that is mostly solid, but molten at the center. The surface-to-core temperature gradient on that planet is more like that of Earth than that of Jupiter.
Water Ice on Mercury
In November 2012, the U.S. National Aeronautics and Space Administration's MESSENGER spacecraft observed what scientists had long suspected -- the presence of water ice on Mercury's poles. Because the planet has practically no tilt relative to its orbit, certain areas at the poles remain in permanent shadow. The temperature remains below -170 degrees Celsius (-274 degrees Fahrenheit) because there is no atmospheric warming effect. Data from the spacecraft suggests that exposed ice exists in the coldest places at both poles, but that the bulk of the ice is covered with an "unusually dark material." The data not only indicates the existence of water ice, it suggests that it is a major constituent of the northern polar region.