Mercury is the planet closest to the sun in our solar system. This small, rocky planet has almost no atmosphere. Mercury has a very elliptical orbit and a huge range in temperature. During the long daytime (which lasts 88 Earth days or an entire Mercurian year), the temperature is hotter than an oven; during the long night (the same length), the temperature is colder than a freezer.

Mercury is so close to the Sun that you can only see it near sunrise or sunset.

Mercury is a heavily cratered planet; its surface is similar to the surface of our Moon. Cratering on Mercury triggered volcanic eruptions that filled much of the surrounding area. Mercury does have a magnetic field (probably generated by a partly-liquid iron core).

MASS AND GRAVITY
Mercury's mass is about 3.3 x 1023 kg. This is about 1/20th of the mass of the Earth.

The gravity on Mercury is 38% of the gravity on Earth. A 100 pound person on Mercury would weigh 38 pounds. To calculate your weight on Mercury, just multiply your weight by 0.38
SIZE
Mercury is about 3,031 miles (4,878 km) in diameter. It is the second-smallest planet in our Solar System. Mercury is a bit over one third of the diameter of the Earth. Mercury is only slightly larger than the Earth's Moon.
ATMOSPHERE
Mercury's thin atmosphere consist of trace amounts of Hydrogen and Helium.  The atmospheric pressure is only about 1 x 10-9 millibars; this is a tiny fraction (about 2 trillionths) of the atmospheric pressure on Earth.

Since the atmosphere is so slight, the sky would appear pitch black (except for the sun, stars, and other planets, when visible), even during the day. Also, there is no "greenhouse effect" on Mercury. When the sun sets, the temperature drops very quickly since the atmosphere does not help retain the heat.
TEMPERATURE RANGE
Mercury has a huge range in temperatures. Its surface ranges in temperature from -270°F to 800°F (-168°C to 427°C). During the very long daytime (88 Earth-days long), the temperatures are very high (the second-highest in the Solar System - only Venus is hotter); during the long night, the thin atmosphere lets the heat dissipate, and the temperature drops quickly.