Only radar telescopes gave any hint of Mercury's surface conditions prior to the voyage of Mariner 10. The photographs Mariner 10 radioed back to Earth revealed an ancient, heavily cratered surface, closely resembling our own Moon. The pictures also showed huge cliffs crisscrossing the planet. These apparently were created when Mercury's interior cooled and shrank, buckling the planet's crust. Instruments on Mariner 10 discovered that Mercury has a weak magnetic field and a trace of atmosphere - a trillionth the density of Earth's atmosphere and composed chiefly of argon, neon and helium.
Mercury literally bakes and freezes at the same time.
Days and nights are long on Mercury. Mercury appears to have a crust of light silicate rock like that of Earth. Scientists believe Mercury has a heavy iron rich core making up slightly less than half of its volume. That would make Mercury's core larger, proportionally, than the Moon's core or those of any of the planets.