Mercury is the smallest planet in the Solar System and closest to the Sun, making it difficult to observe from Earth. It has a surface resembling the Moon, with many impact craters and nearly zero gravity. Mercury's surface temperatures range greatly from -183°C to 427°C. It is one of the four terrestrial planets but has a large iron core generating a weak magnetic field compared to Earth. Mercury has no natural satellites and its plains-dominated surface closely resembles the lunar mare of the Moon.