Mercury is the smallest planet in the solar system and is the closest planet to the sun and has the fastest orbit around the sun, completing one orbit every 88 Earth days. Much of what we know about Mercury comes from Earth-based observations and spacecrafts which have been sent there. At first glance, Mercury appears very similar to our moon, with huge impact craters and evidence of early volcanic activity. Mercury is only 30% larger than our moon, but it is much heavier. It's gravity however, is still too weak to hold onto any kind of atmosphere, and because of this temperatures on the surface can be as hot as 427°C (801°F), and can drop to as low as -183°C (-297°F). Mercury's large mass is a result of the planet having a strangely large iron core, which occupies 40% of Mercury's interior. Astronomers believe that Mercury may have been larger in its past, but likely lost most of its outer mantle and crust in a collision in its very early history. Scientist are unsure about why it doesn't have a moon in its hill sphere. A possible explanation could be its closeness to the Sun, due to the gravitational influence of the sun in its hill sphere.
While Mercury has a very fast orbit, completing one full orbit every 88 Earth days, a day on Mercury is actually quite long, around 58.6 Earth days. However, there are some regions on Mercury which never see sunlight at all. Mercury is probably the last place you would expect to find ice, yet astronomers have found observational evidence in the north and south poles that ice may exist within some craters. Because Mercury orbits completely upright, the sun barely peeks over the horizon at the north and south poles. Some craters remain in perpetual darkness. Astronomers have found evidence, in the form of bright radar reflections, that some of these craters may contain ice, likely deposited by earlier impacts from comets.
Image credit: NASA/ESA