Mercury is the innermost planet in the Solar System, but surprisingly it is not the hottest planet; that would be Venus. The reason for this is that Mercury has a thin atmosphere that cannot trap solar heat. Its maximum atmospheric pressure is estimated to be 0.00000000003 pounds per square inch; on Earth, the atmospheric pressure is around 14.7 pounds per square inch. Mercury’s atmosphere is composed of very small amounts of hydrogen, oxygen, helium and sodium.
The planet still get hot on the side that faces the Sun, it experiences the most extreme temperatures, which can reach as high as 700° Kelvin (427°C). In fact, Mercury receives 6.5 times the amount of sunlight that the Earth does and every part of the surface of the planet receives direct sunlight for 59 days at a time.
On the dark side of the planet, on the other hand, temperatures can fall to as low as 100°K (-173°C) , making it the coldest of all the inner planets (which also include Earth and Mars). There are also believed to be craters dotting the north and south poles, and temperatures in them might even reach temperatures as low as 90°K. The mean temperature of Mercury averages to around 452°K.
Between the extreme temperatures and the thin atmosphere, there is almost no possibility of plant or animal life on Mercury as we know it. This is because liquid water on Mercury is impossible, as it would be boiled away by the heat of the Sun.
Mercury also has no weather due to the lack of an atmosphere. Its surface strongly resembles that of the Moon, as small comets and meteors freely crash into the planet without burning up. Just like Earth, however, Mercury is believed to have a core of solid iron which makes up some three-fourths of its radius and is surrounded by a layer of liquid iron.
The Planet Mercury